CS 414 - WEED SCIENCE

Chapter 1

I. INTRODUCTION

Weed Science: is a discipline that investigates the biology and ecology of weeds and how to best manage these plant species for the betterment of mankind.

Before we can intelligently approach and study the Weed Science discipline, we first must determine what a weed is.

What is a weed?

A weed is often defined as “a plant out of place”, or “a plant growing where it is not wanted”. These definitions are overly simplistic and can be misleading. Designating a plant as a weed simply by its physical location does not take into account problems caused by weeds nor certain characteristics of plants that contribute to their potential to be weeds. These simple definitions do not distinguish plants that possess truly weedy characteristics from those that are only an occasional nuisance. Weeds possess certain definable characteristics that set them apart from other plant species.

The Weed Science Society of America defines a weed as “any plant that is objectionable or interferes with the activities or welfare of man.”

Weeds are perhaps best defined as “plants that are competitive, persistent, and pernicious, and are undesirable because they interfere with human activities.”

1. Weeds are a special form of vegetation that are highly successful in agricultural environments, and more broadly, they are plants that are extremely successful in environments disturbed by man.

2. In an ecological sense, weeds are pioneers of secondary succession, of which the weedy arable field is a special case. Succession - is a process where species composition of a plant community changes over time.

Climax vegetation - last stage of succession and is often considered (ideally) as a community having a constant species composition.

Plants may be called weeds according to human (anthropocentric) values:

1. Societal value: Some weeds are allowed to persist in locations because of aesthetic or other values. For example, in many developing nations, some weeds are not removed because they have medicinal, ritual, or food value (i.e. ‘good’ weeds). conversely, some plants because of ethnic values are overharvested in some regions leading to invasive exotics, i.e., in the Himalayans of Asia.

2. Regulatory concern: some weeds and seeds are prohibited by state or federal regulations due to risk of substantial economic or human injury. For example, it is illegal to transport some non-native weeds (such as tropical soda apple, witchweed, and tropical spiderwort) in North Carolina even though they are known to exist or have existed in the state. However other ornamental relatives of some very problematic weeds are not regulated, including Red Baron grass (cogongrass biotype), ornamental morningglories, and Amaranthus spp. (pigweeds), wisteria, trumpetcreeper, and bamboo.

There are approximately 350,000 plant species in the world.

Approximately 12 to 15 of these species provide 90% of the food for the world.

Approximately 250 plant species are considered weeds of some importance, the others could be. One might consider this job security.

Of the 250 weed species, approximately 70% of these species belong to the same 12 families that provide 90% of the world's food.

Important plant families, which contain both major crops and the world's worst weeds.

% Of species classified in the world's worst weeds.	Family	Crops	Weeds
44%	Gramineae	Barley, Corn, Oats, Rice, Sorghum, Wheat	Johnsongrass, Barnyardgrass, Cogongrass, Wild oats
4%	Solanaceae	White potato	Nightshades, Jimsonweed
5%	Convolvulaceae	Sweet potato	Morningglories, Field bindweed, field dodder
5%	Euphorbiaceae	Cassava	Spotted spurge, Nodding spurge
6%	Leguminosae	Soybean, Peanut	Sicklepod, Fl. beggarweed

Some plants considered weeds were once cultivated as crops: wild oats, johnsongrass, cogongrass, and common lambsquarters are a few examples.

A crop one year may become or be considered a weed the next year. Examples include transgenic glyphosate-resistant (Roundup Ready) corn in Roundup Ready cotton.

Salisbury (1961) stated "Casual weeds of today are likely to become noxious weeds tomorrow" owing to the continual modification of cultural practices and "modernization" of agriculture.

II. SUCCESS IN THE AGROECOSYSTEM

Success in an evolutionary sense is generally accepted to mean the continuation of a genetic line over time. Thus, evolutionary success is reflected in numbers of individuals, reproductive capacity, and area and range of habitats occupied. Weeds, however, are not always the most successful in this evolutionary context when compared with the native vegetation of an area. Example: very few weeds will be found in areas not disturbed by man. Annual weeds are not found in forests, especially in areas with climax vegetation (pine forests in the coastal plain, oak-hickory forests in the piedmont and mountains, tall grass prairies of the Midwest, or in short grass prairies of the Central Great Plains of the U.S.

A. Definition of success: The most successful agricultural weeds are

often those considered to be the most

troublesome among crops.

1. Successful weeds vary according to the crop and geographical location.

North Carolina:	Cotton: Peanuts: Soybeans:	nutsedges, sicklepod, Palmer amaranth, morningglories nutsedges, ragweed, lambsquarters grasses, sicklepod, dayflowers, Palmer amaranth, morningglories
Georgia:	Cotton: Peanuts: Soybeans:	nutsedges, sicklepod Florida beggarweed, Texas panicum, nutsedges, bristly starbur, wild poinsettia grasses, sicklepod
Illinois:	Corn: Soybeans:	giant foxtail, velvetleaf, waterhemp giant foxtail, waterhemp, velvetleaf

2. Success may be measured as:

a. Rapid colonization

b. Difficulty in removal and/or control.

c. Ability to suppress crop productivity.

In general, plant characteristics that contribute to colonizing ability and competitiveness are found in many of our "worst" weeds to a much greater extent than in typical crops.

Successful weeds vary according to the crop and geographical location (i.e., climate and soil type). The ability of weeds to dominate in an area depends, of course, on many factors. Obviously, a species must first arrive at a location in order to colonize it. An individual seed that germinates in a new location and survives to reproduce for several years - surviving herbivores, climate, disease, and weed management efforts (herbicides, tillage, mowing, etc.) - is said to have become “naturalized” in a location. Many weed species are “pre-adapted” to the climate and management routines (such as timing of tillage and harvesting), as they survived in similar climates or circumstances where they originated. Kudzu, for example, originated in tropical Asia but is very successful in the southern USA. Similarity in climate and a lack of limiting diseases and herbivores are believed to have allowed it to rapidly expand its geographical range in the USA. Although kudzu has reportedly spread as far north as Ithaca, NY, its growth is less rapid, and it is less successful in reproducing and spreading in the colder climate of the northeast. Species such as giant foxtail, velvetleaf, and waterhemp tend to be the most common and troublesome weeds in cropping systems in the upper Midwest. These species are typically not major problems in North Carolina. Common and troublesome weeds in North Carolina cropping systems include Palmer amaranth, morningglory species, common ragweed, and annual grasses including crabgrass species, foxtails, goosegrass, and Panicum species. Some weeds that were considered troublesome as little as 5 years ago are no longer considered problematic due to new herbicide technologies. For example, sicklepod is readily controlled by glyphosate (Roundup) and the registration and widespread use of Roundup Ready cotton and soybean in North Carolina makes sicklepod far less common and problematic.

B. Adaptive Strategies of Weeds

One commonly used approach used by ecologists to understand the relationship of organisms to the environment is to compare adaptive strategies.

Plants adapt to the environment by balancing the allocation of resources and energy among physiological processes and biomass components throughout the life cycle.

Patterns of allocation may be thought of as strategies, or sets of life history characteristics, that determines survival and reproduction, or success, in a particular environment.

1. R- and K- selection

One widely accepted theory is that of r- and K-selection:

a. r-selected organisms are adapted for colonization and reproduction in an unpredictable environment. Agricultural weeds seem to fit most closely to the characteristics of r-selected organisms.

b. K-selected organisms are adapted for persistence and reproduction in stable environments. K-selected species most often occupy the late stages of succession, while r-selected species occupy early stages of succession. A given organism may be an r- or K-strategist only in relation to another organism.

2. C, S, and R Selection.

J. P. Grime proposed that external factors that limit the amount of plant material in any environment might be classified into two categories. The first is stress, or phenomena that limit production, and the second is disturbance, the partial or total destruction of plant biomass arising from natural- or human-caused events. Three possible evolutionary strategies for adaptation have occurred.

Competitors - maximize the capture of resources in productive, undisturbed conditions, and are abundant during early and intermediate stages of succession.

Stress tolerators - are long-lived plants adapted for conditions of limited productivity, and often occupy late stages of succession.

Ruderals - are usually short-lived herbs with high seed production, which occupy the earliest stages of succession.

It appears that a number of species including many herbaceous weeds including annuals, biennials, and some perennials, as well as many crops, possess characteristics common to both competitors and ruderals, and thus be classified as competitive ruderals.

The species composition (i.e., the number or biomass of each species that occurs in an area, usually expressed as a fraction or percentage of the total number of plants or amount of biomass) might change when a different “selection pressure” is applied. For example, the species composition of weeds in many conventional agricultural fields is different today than it was 20 years ago, or 100 years ago. Why? How might the increase in “no-till” production affect the species composition in previously tilled agricultural fields? What happens in abandoned fields?

C. Characteristics of the "Ideal Weed" (from a weed viewpoint)

1. Rapid growth through vegetative phase to flowering.

2. Germination requirements fulfilled in many environments.

a. Water: First step in germination is imbibition of water. Until a critical content of water is reached, germination will not proceed.

b. Oxygen: Oxygen is essential for respiratory reactions (which are part of germination). There is less oxygen deeper in the soil.

c. Temperature: There is a minimum, a maximum, and an optimum temperature for germination. Different species of weeds (and crops) have different temperature requirements for germination. Temperature requirements explain the phenomenon of periodicity of germination.

d. Light: Some weed species require light for germination. Examples include pigweed, lambsquarters, and Virginia pepperweed. Tiny seed often have a light requirement for germination; seed must be on or near surface before germination will occur. Tillage may allow enough light to contact the seed to stimulate germination. Light response controlled by the phytochrome system.

e. Chemical germination stimulate from host plant: Parasitic weeds such as witchweed and dodder have this requirement. It is a protective mechanism to keep seed from germinating when no host plant is present.

f. Quiesence: an inactive state in which the seed is capable of germination but does not germinate because environmental conditions are not favorable. It is a survival mechanism to ensure seed germinate only when conditions are favorable for the plant to grow. Also called enforced dormancy.

Seed of some species (such as many annual grasses) have an after-ripening requirement. The seed are shed after the embryo is fully formed, but embryo needs additional time to complete physiological development. Probably associated with formation of germination-promoting hormones.

Germination inhibitors (hormones) may be present which inhibit germination. These inhibitors must be leached out of the seed before germination can proceed.

Burke et al. (Weed Sci. 51:342–347) reported effects of various environmental effects on crowfootgrass seed germination.

3. Many weeds have dual modes of reproduction. Most are angiosperms, capable of reproducing by seed, but many also reproduce vegetatively and are among our most troublesome weeds.

4. Discontinuous germination (internally controlled) and great longevity of seed.

5. Continuous seed production for as long as growing conditions permit. Often seed have some type of dormancy mechanism.

Innate dormancy - seed is dormant when shed from the plant, could be due to genetic control, immature seed embryo which is not fully developed when shed from the plant, after-ripening requirement, or dependent upon a specific environmental stimulus.

Induced dormancy - seed does not possess innate dormancy, however, seed develops dormancy after exposure to environmental stimulus, perhaps temperature extremes, drought, elevated CO₂, etc.

Results of Dormancy:

A. Beale’s study: Buried seed of 20 weed species, then dug up at intervals. After 40 years, some seed of redroot pigweed, prostrate pigweed, common ragweed, black mustard, Virginia pepperweed, evening primrose, broadleaf plantain, purslane, and curly dock were viable. After 80 years, seed of curly dock, evening primrose, and common mullein were still viable.

B. Duvel’s study: Buried seed of 101 weed species, then dug up at intervals.

Years after burial	Number of species with viable seed
1	71
6	68
10	68
20	57
30	44
38	36

Species with viable seed after 38 years of burial:
Species	% germination
Jimsonweed	91
Common mullein	48
Velvetleaf	38
Lambsquarters	7
Green foxtail	1

C. Implications of dormancy: Longevity in burial studies enhanced due to deep burial where oxygen supply is limited. If brought to surface and environmental conditions are favorable, many of the seed would germinate sooner. Repeated tillage for several years without reinfestation will reduce weed seed population in soil. However, weeds produce many seed and it is extremely difficult to completely avoid reinfestation. Conclusion: allowing weeds to go to seed increases potential problems for many years to come.

6. Self-compatibility but not complete autogamy or apomixy.

Apomixy - reproduction without meiosis; vegetative reproduction.

7. Weeds often produce seeds or other propagules that are often the same size and shape as crop seeds, making physical separation difficult and facilitating spread by man.

rice -	red rice
sorghum -	johnsongrass, shattercane
soybean -	showy crotalaria, balloonvine (seed)
peanuts-	nutsedge tubers (not seeds)

8. Cross-pollination by unspecialized visitors or wind.

9. Very high seed output in favorable environmental circumstances.

Weed	Seed production/plant
Wild oat Purslane Barnyardgrass Goosegrass Common lambsquarters Common ragweed Smartweed	110 - 450 10,000 2000 - 40,000 50,000 - 135,000 13,000 - 500,000 3,400 3,200

Methods of Weed Propagation (reproduction)

A. Sexual reproduction: requires pollination of a flower, leading to seed production. Weeds are very prolific seed producers (see table). This ability contributes to “weediness” of species and difficulty of control. Annual and biennial weeds and simple perennials reproduce sexually. Creeping perennials may reproduce sexually or asexually.

B. Asexual reproduction: production of new plants form vegetative reproductive structures. Does not require flowers and pollination, seed are not produced. Process confined to creeping perennials. Also called vegetative reproduction. Creeping perennials have various vegetative organs for reproduction. See the definitions that follow.

10. Many weeds have special adapted seed dispersal mechanisms. Adaptations for short-distance and long-distance dispersal.

A. Natural dispersal:

1. Wind

- Light seed can become wind-borne. Examples are witchweed and horseweed. The spread of glyphosate-resistant horseweed is exasperated by this mechanism of dispersal.

- Seed can be blown along over soil surface or on top of crusted snow. Example is common ragweed.

- Mature plant may be moved by wind, with seed being dispersed as the plant is moved. Examples are tumbling pigweed and Russian thistle.

- Some species have special adaptations on seed to aid in wind dispersal, such as “wings” (example is milkweed) or “parachutes” (example is dandelion).

2. Water: Seeds can be moved in streams and drainage canals. Seed deposited downstream by flooding or irrigation. Seed can also be moved by soil erosion.

3. Animals

- Mucilaginous seed coats: some weed seed have sticky seed coverings, like “glue”. Seed will stick to animal’s coat and fall off later. Example is plantains.

- Hooks and barbs: some species have hooks or barbs on seed that stick to animal’s coat. Example is cocklebur.

- Digestive tract: animals such as cows and birds eat seed, and some seed remain viable as they pass through digestive tract. Seed then deposited in excrement.

4. Forceful dehiscence: In some species, seed pod bursts open suddenly and seed are “shot” out 20 feet or more. Example is Oxalis. This method does not distribute seed as far as other natural dispersal methods. (hairy cress and creeping woodsorrel can discharge seed 3 m!)

- Wild oats have tension-released mechanisms of dispersal of seed through the air and also bury fallen seeds in the ground through coiling and uncoiling of parts of the fruit in response to humidity changes.

B. Artificial Dispersal: A result of man’s activities.

2. Artificial dispersal: basically a result of man’s activities

a. Machinery: weed seed can be moved on equipment

b. Crop seed: very common method of spread. Many of our most widespread weeds were brought to this country from Europe and elsewhere as seed contaminants with our colonial and pioneer ancestors. Weed seeds that mature near the time of crop harvest can be included in the harvested grain. Weed seeds that mimic the crop in shape, color, or size are more difficult to separate from the crop seed and might be planted along with the crop seed. Seed laws and seed certification standards cover presence of weed seed in addition to things such as percent germination and varietal purity.

The Federal Seed Act of 1939 defines a noxious weed as "any weed or plant that is so declared by an authoritative group, with the legal power to make such a declaration, to be harmful or possess noxious characteristics." This act regulates interstate and foreign commerce in seeds. Its purpose is to protect purchasers from mislabeled or contaminated crop seed and is administered by the USDA.

The Federal Seed Act requires, in part, that the following information be provided on seed labels in interstate commerce:

1. Percentage of pure seed of the named crop.

2. Percentage of other crop seeds.

3. Percentage of weed seeds.

4. The names of noxious weed seeds present and the rate of their occurrence

The North Carolina Seed Law of 1963: The purpose is to “regulate labeling, possessing for sale, sale and offering or exposing for sale or otherwise providing for planting purposes of agricultural seeds, vegetable seeds and screenings; to prevent misrepresentation thereof”.

The North Carolina Seed Law requires, in part, that the following information be provided on seed labels for seed offered for sale within the state:

1. Commonly accepted name of the kind and variety of seed

2. Percentage by weight of inert matter

3. Percentage by weight of agricultural seeds other than those named on label

4. Percentage by weight of all weed seeds, including noxious weed seeds

5. Percentage of germination, exclusive of hard seed, for the named agricultural seed

6. Percentage of hard seed, if present

7. Name and number per pound of each kind of restricted noxious weed seed present

The North Carolina Seed Law further states that it is unlawful to transport or offer for sale agricultural seeds containing:

1. prohibited noxious weed seeds

2. restricted noxious weed seeds, except as allowed by the law

3. weed seeds in excess of 2% by weight unless otherwise prohibited by other parts of the Law

The North Carolina Seed Law lists the following as prohibited noxious weeds:

1. Balloonvine, Cardiospermum halicacabum L.

2. Showy crotalaria, Crotalaria spectabilis Roth

3. Smooth crotalaria, Crotalaria pallida Ait.

4. Itchgrass, Rottboellia cochinchinensis (Lour.) W. Clayton

5. Jimsonweed_Datura stramonium L.

6. Johnsongrass, Sorghum halepense (L.) Pers.

7. Serrated tussock, Nassella trichotoma (Nees) Hack.

8. Witchweed, Striga asiatica (L.) Ktze.

No seed of prohibited noxious weeds can be present in seed offered for sale in NC. The North Carolina Seed Law lists the following as restricted noxious weeds, along with the allowable limitations (27 species):

Common name	Latin binomial	Limit/lb of seed
Anoda, spurred	Anoda cristata (L.)Schlecht.	4 seeds
Bermudagrass	Cynodon dactylon (L.) Pers.	27 seeds
Bindweed, field	Convolvulus arvensis L.	27 seeds
Bindweed, hedge	Calystegia sepium (L.) R.Br.	27 seeds
Cockle, corn	Agrostemma githago L.	10 seeds
Cornflower	Centaurea cyanus L.	27 seeds
Dock, broadleaf	Rumex obtusifolius L.	54 seeds
Dock, curly	Rumex crispus L.	54 seeds
Dodder	Cuscuta spp.	54 seeds
Foxtail, giant	Setaria faberi Herrm.	54 seeds
Garlic, wild	Allium spp.
	Small grains or larger seeds	4 bulblets
	Grasses and small seeded legumes	27 bulblets
Horsenettle	Solanum carolinense L.	54 seeds
Morningglory	Ipomoea spp.	8 seeds
Mustard, wild et al.	Brassica spp.	54 seeds
Nutsedge, purple	Cyperus rotundus L.	2 tubers or 27 seeds
Nutsedge, yellow	Cyperus esculentus L.	2 tubers or 27 seeds
Onion, wild	Allium spp.
	Small grains or larger seeds	4 bulblets
	Grasses and small seeded legumes	27 bulblets
Panicum, Texas	Panicum texanum Buckl.	27 seeds
Plantain, bracted	Plantago aristata Michx.	54 seeds
Plantain, buckhorn	Plantago lanceolata L.	54 seeds
Quackgrass	Elytrigia repens (L.) Nevski	54 seeds
Radish, wild	Raphanus raphanistrum L.	12 seeds
Sandbur	Cenchrus spp.	4 seeds
Sicklepod	Senna obtusifolia L.	4 seeds
Thistle, blessed	Cnicus benedictus L.	4 seeds
Thistle, Canada	Cirsium arvense (L.) Scop.	27 seeds
Velvetleaf	Abutilon theophrasti Medicus	4 seeds

With restricted noxious weeds, some seed are allowed in certified crop seed, but the amount depends upon the certification standards for the particular crop. See below for NC certification standards for soybeans.

Certification Standards for Soybeans - NC
	Certified 1	Certified 2
Pure seed (min)	98%	96%
Inert matter (max)	2%	4%
Weed seeds^1(max)	0.02%	0.05%
Restricted noxious weeds (max)	none	1 seed/lb
Other crop seed (max) other kinds other varieties	3 seed/lb^2 0.4%	5 seed/lb^3 0.4%
Germination (min)	80%	70%
^1Shall not exceed 10/lb. ^2Prohibits corn and cowpea. ^3Permit one corn and one cowpea per lb.

In 1974, the Federal Noxious Weed Act (Public Law 93-629) was enacted to control the spread of noxious weeds. The Act gave the Secretary of Agriculture the authority to designate plants as noxious weeds by regulation, and the movement of all such weeds in interstate or foreign commerce was prohibited except under permit.

The Federal Noxious Weed Act defines a noxious weed as “any living stage (including, but not limited to, seeds and reproductive parts) of any parasitic or other plant, of a kind or subdivision of a kind, which of foreign origin, is new or not widely prevalent in the United States, and can directly or indirectly injure crops, other useful plants, livestock, poultry, or other interests of agriculture, including irrigation, or navigation, or the fish, or wildlife resources of the United States, or the public health.”

c. Livestock feed, hay and straw, manure

d. Other methods related to man’s activities, such as moving soil for construction and landscaping, use of weedy plants in floral arrangements, movement of aquatic weeds on boat propellers, etc.

11. Production of some seed in wide range of environmental conditions; tolerance and plasticity.

12. If a perennial, vigorous vegetative reproduction or regeneration from fragments. Roots have the ability to penetrate deep into the soil. Perennial organs (rhizomes, corms, etc.) have the ability to send shoots up from deep in the soil. Large food reserves can be stored in rhizomes, corms, tubers, etc.

Ramet - a single unit of clonal growth.

Genet - genetically distinct individuals.

TYPES OF VEGETATIVE REPRODUCTION

· Stolons and runners - long slender stems that grow along the soil surface and produce adventitious roots and new shoots. Examples - bermudagrass, large crabgrass.

· Rhizomes - underground stems that produce adventitious roots and shoots. Examples - johnsongrass, quackgrass, purple and yellow nutsedge.

· Tubers - enlarged terminal portions of rhizomes. They possess extensive storage tissue and axillary buds. Examples - yellow and purple nutsedge.

· Bulbs - underground modified buds consisting of a stem and fleshy scale leaves. Food storage is in the leaves. Example - wild garlic.

· Corms - enlarged, vertical underground stems covered with one or more layers of leaf bases. Food storage is in the stem. Example - bulbous buttercup, a perennial herb.

· Roots - many species produce long horizontal roots that give rise to shoots. Example - Canada thistle.

· Stems - some species produce adventitious roots and new shoots near the tips of branches. Example - dandelion.

· Fragmentation - spread and establishment of ramet by various plant parts, such as excised leaves or stems. Example - bermudagrass.

The Hutchinson Encyclopedia. 2000

13. If a perennial, brittleness, so as not to be drawn from the ground easily.

14. Parasitic weeds. Parasitism is a process by which one plant lives on a plant host and derives nutrients or energy from that host via a living linkage, typically through a haustoria.

Examples: witchweed - corn, sorghum, millet, cowpeas

mistletoe - pine trees, hardwoods

dodders - numerous crops and trees

broomrapes - carrots, tomatoes, sunflowers

15. Many weeds have adaptations that repel grazing such as spines, chemicals that impart a bad taste or odor to herbivores.

16. Weeds are ubiquitous, they exist everywhere that man has been or is at presently.

17. Weeds are resilient and are difficult to control.

Major characteristics of successful weeds.

1. Physiological:

a. High relative growth rate of seedlings

b. High rates of photosynthesis

c. Rapid leaf and root development

d. Rapid transition from vegetative to reproductive growth

e. High capacity for acclimation to a changing environment

2. Reproductive:

a. Largely self-fertilized, some outcrossing

b. Copious seed production

c. Will set seed under a wide range of conditions

d. Pollination by wind or by insects in general

3. Agronomic:

a. Weed and crop may share many morphological and physiological similarities

b. Seed maturity coincides with crop harvest

c. Resistance or tolerance to herbicides

d. Can overcome mechanical control by vegetative regeneration

e. Prolonged seed viability

f. Discontinuous germination over prolonged periods

CHARACTERISTICS OF SUCCESSFUL WEEDS - ESTABLISHED PHASE

1. Growth and Resource Capture. Success of plants in isolation and in mixture is associated with early and rapid establishment, rapid canopy development, and rapid root growth. In general, a species that grows faster than its neighbors will use a disproportionate share of the available resources, to the detriment of the neighbors.

a. Leaf area partitioning (rate of expansion of new leaf area) is highly correlated with rapid growth. Several studies have shown that growth parameters related to plant size and leaf area are the best predictors of competitiveness in mixtures of plant species.

2. Photosynthetic Pathways. Based on photosynthetic pathways, plants can be divided into three major groups. These groups include C₃ (Calvin cycle), C₄ (C₄-dicarboxylic acid), and CAM (crassulacean acid metabolism) plants.

While each of these three groups includes weed species, the C₄ pathway is highly represented in agricultural weeds.

Surveys of weed and crop species have reported that only about 0.2% of the world's flora possess this pathway, yet it is found in many of the major weeds of the world. Among the world's 76 worst weeds, 42% employ C₄ photosynthesis, and 78% of the 18 worst weeds are C₄ plants. Of the world's top 16 crops, most are C₃.

Photosynthetic pathway of the worlds worst weeds.

Common name	Pathway	# of countries found as a weed.
1. Purple nutsedge	C4	91
2. Bermudagrass	C4	90
3. Barnyardgrass	C4	65
4. Jungle rice	C4	64
5. Goosegrass	C4	64
6. Johnsongrass	C4	51
7. Cogongrass	C4	49
8. Water hyacinth	C3	50
9. Common purslane	C4	78
10. Common lambsquarters	C3	58

C₃ photosynthesis - the primary carboxylator is ribulose-1,5-biphosate carboxylase/oxygenase (RuBisCO), and the first stable product of carbon reduction is the 3-carbon acid (3-phosphoglycerate).

C₄ photosynthesis - the primary carboxylator is phosphoenol-pyruvate carboxylase (PEPC), and the initial detectable products are the 4-carbon acids, oxaloacetate, malate, and aspartate. These acids are transferred from the leaf mesophyll cells to the adjacent bundle sheath cells where they are decarboxylated, and the CO₂ that is generated is recaptured by RuBisCO. Since PEPC is a far more efficient carboxylator than RuBisCO, it serves to trap CO₂ from low ambient concentrations (micromolar in air) and to provide an efficiently high CO₂ concentration (Millimolar) in the vicinity of the poorer carboxylase (RuBisCO). In this way, C₄ plants can reduce CO₂ at high rates and are often perceived as being more efficient than C₃ plants. In addition, because of their more effective reduction of CO₂, they can operate at much lower CO₂ concentrations, such that stomatal apertures may be reduced and so water is conserved.

The C₄ pathway is often regarded as an 'optional extra' to the C₃ system, and offers a clear photosynthetic advantage under conditions of relatively high photon flux density, temperature, and limited water availability, i.e. in tropical and mainly subtropical environments.

Conversely, plants solely possessing the C₃ pathway are more advantaged in relatively temperate conditions of lower temperatures and photon flux density, and an assumed less limiting water supply.

SOME PHYSIOLOGICAL AND PERFORMANCE CHARACTERISTICS ASSOCIATED WITH THE C4 PATHWAY.
Characteristic	Approximate quantitative relationship compared to C3 species.
High temperature optimum for Photosynthesis	30-45 degrees vs. 15-30 degrees
High light optimum for Photosynthesis	Full sunlight vs. 30% full.
High photosynthesis rates per unit leaf area	About twice as much under optimal conditions.
High growth rates under optimum conditions for photosynthesis	About twice as much.
High dry matter production per unit of water used.	Two to three times as much.

Table. Grams of water required to produce 1 lb of dry matter for C₄ and C₃ weeds and crops.

Species	G H20/lb dry matter of dry matter
C4 pathway
Prostrate pigweed	260
Common purslane	281
Foxtail millet	285
Sorghum	304
Corn	349
Average	296
C3 pathway
Wheat	557
Cotton	568
Cowpea	569
Common lambsquarters	658
Prostrate knotweed	678
Rice	682
Beans	700
Prostrate verain	702
Smooth brome	977
Average	667

In competitive mixtures grown at different temperatures, the species with the higher photosynthetic rate showed faster growth and subsequent shading of the other species.

It has been concluded that C₄ photosynthesis does not confer an intrinsic advantage to the plant in competition with a C₃ species; rather, the advantage depends heavily upon environmental conditions.

Thus, in agricultural environments characterized by water and heat stress, many of the worst summer annual and perennial weeds will be C₄, while the winter annual weeds are most often C₃ species.

3. Water Uptake.

In addition to seasonal water supply, a number of plant factors regulate the availability of water in any environment. These factors include: root development, structure, and distribution; tolerance of low water potential in plant tissues; control of transpirational water loss, and water use efficiency. Information in this area is very limited. The limited research that has been conducted indicates that weeds have more rapid root elongation and/or deeper, more extensive root systems than crops. At 20 days after emergence, most weeds in this study had larger root systems and greater absorbing surfaces than any of the cereals tested.

Soil water 'feeding' habits of common weeds in summer fallow.

Species	Rooting depth	Feeding depth area/plant	Feeding consume	Plants to consume water/acre
	(ft)	(ft)	(sq. ft)	(number)
Cocklebur	9.6	14	154	285
Puncturevine	8.4	11	95	460
Russ. thistle	5.9	8	50	870
Pigweed	7.8	6	28	1560
Kochia	7.3	11	95	460
Grain sorghum	5.6	7	38	1150

The physiological control of water availability by a plant may be expressed as water use efficiency (WUE), which is the amount of CO₂ fixed or dry matter produced per unit of water lost in transpiration. The highest WUE values are found in C₄ species, while C₃ species have relatively lower WUE values. Higher WUE in C₄ relative to C₃ species is due to the CO₂-conserving mechanisms of the C₄ pathway.

The competitive advantage to a plant of higher WUE due to the C₄ pathway is not always clear-cut. It has been shown in some situations, that a plant that maintains open stomata and high rates of transpiration under water stress may control water availability to a greater extent than a plant with high WUE and stomata more responsive to stress.

This scenario was found to be the case in mixtures of a C₃ weed (common lambsquarters) and the C₄ weed (redroot pigweed), where high WUE of redroot pigweed did not confer a competitive advantage. Thus in many agricultural fields, rapid root development coupled with excessive use of water may contribute to 'weediness' as well as to competitiveness. The advantage conferred by C₄ photosynthesis and its high WUE, found in many weeds, would be most apparent in hot, dry, high light environments, where water stress is often a problem.

WEED LIFE CYCLES

Another way to classify weeds is based on their life cycles.

A. Annuals. An annual is a plant that completes its life cycle from seed to seed in less than one year or in one growing season. Most agricultural weeds are annuals.

Summer annuals. Complete life cycle during period from spring to fall. Seed germinate in spring, plant flowers and produces seed in mid- to late summer, and die in the fall. Examples include large crabgrass, sicklepod, and Ipomoea morningglories.

Winter annuals. Complete life cycle during period from fall to spring. Seed germinate in late summer, fall, and/or throughout winter months, plant flowers and produce seed in mid- to late spring, and die in early summer. Examples: common chickweed, henbit, and horseweed.

Weed management of annuals.

Annuals respond well to cultural, mechanical, and chemical management. Cultivation, mowing, hoeing, and hand pulling are generally effective for the control of annual weeds, especially if applied during the vegetative phase of growth.

Since these weeds reproduce by seeds, they can often be controlled with herbicides, especially soil-applied herbicides. They also can be controlled with postemergence-applied (herbicides applied directly to the weed foliage) herbicides. However, because of their often rapid growth during the vegetative phase, chemical control efforts must be undertaken in a timely fashion to be effective. In general, older and larger annual weeds are more difficult to control with herbicides than are younger and smaller weeds.

Cultural control of annual weeds is an integral component of weed management. Because annuals rely exclusively on seed for perpetuation of the species within a habitat, it is particularly important to minimize seed production within the habitat and to prevent seed introduction into the habitat. Regardless of the control tactic, efficacy in controlling annual weeds decreases with size and age.

Bull thistle as a rosette above and to the right bull thistle bolting. Courtesy Jack Dekker, Iowa State.

B. Biennials. Biennials are plants that live for more than 1 year, but less than 2 years. They germinate, emerge, and grow vegetatively, forming a basal rosette with a thick storage root during the first year. After a cold period (vernalization), they typically undergo floral initiation, flower, and produce seed during the second year of the life cycle. The terminal event for biennial species is flowering. If flowering is prevented during the second year, some biennials may continue to live and flower in a subsequent year. However, they are not regarded as perennials because once they flower, their life cycle is complete. They should not be confused with winter annuals, which live during two calendar years, but don't live for more than 1 year. Musk thistle, wild carrot, bull thistle, and common mullein are biennials. Relative to annuals and perennials, few weed species are biennials.

Weed management of biennials.

Tillage or other forms of disturbance often interrupt the life cycle of these plants. Therefore, tillage and cultural practices are effective methods for managing biennial weeds. However, regardless of the control tactic, biennial weeds are best controlled when they are in the seedling or rosette stage.

C. Perennials. Perennials are plants in which the vegetative structures live for more than 2 years, typically by renewed growth from the same root system.

Another distinctive feature is the mechanism(s) by which the species perpetuates itself in a habitat. Perennials are often capable of vegetative perpetuation (increases the population), which offers a means of spread and propagation without having to have reproductive processes that involve flowering.

Simple herbaceous perennials normally reproduce by seed and do not naturally spread vegetatively. However, if a portion of the plant is damaged or removed, regeneration may occur. Simple herbaceous perennials survive unfavorable seasons and resume growth via dormant roots, rhizomes, bulbs, tubers, and crowns. They may or may not produce seed during the year of establishment. Examples: dandelions, curly dock, buckhorn plantain.

Creeping herbaceous perennials in addition to producing seed may also spread via a variety of organs: roots, stems, stolons, rhizomes, or tubers. Examples include johnsongrass, purple and yellow nutsedge, and common bermudagrass.

Woody perennials, including shrubs and trees, undergo secondary thickening, which is expressed in incremental growth. These plants often flower only after reaching maturity, which may occur shortly after establishment or which may take many years. Woody perennials often reproduce by seed, but they may also regenerate from buds and spread by means of root sprouts.

Weed management of perennials.

Perennial weeds are among the most difficult to manage, because of the multitude of mechanisms by which they propagate, especially since many perennial weeds spread and regenerate via specialized vegetative organs (see earlier lecture for a description of these organs).

Timing of control efforts depends largely on the control strategy or strategies. Perennial weeds, like annuals and biennials, that establish from seed are easiest to control in the seedling stage. However, control often is much more difficult if the perennial plant propagates principally by vegetative means or has passed beyond the seedling stage. In fact, integrated strategies that capitalize on the collective efficacy of mechanical, cultural, and chemical tactics generally are the most effective for managing perennial weeds.

Difficulty in control is the rule with perennial weeds, not the exception. Concerning classifying weeds according to their life cycle, it is noteworthy that individual plants of given species may not necessarily behave consistently with their specified life cycle. Life cycle is greatly influenced by environment and expression is often very plastic. For example, plants belonging to species normally regarded as annuals may function as perennials, and vice versa, depending on the environmental condition to which they are exposed.

GROWTH FORM AND HABIT

Weeds are often classified on the basis of their growth form. They may be divided into herbaceous and woody species, a division that often reflects an aspect of their life cycle.

Herbaceous species may be subdivided into 1) vines, 2) those with prostrate growth, and 3) those with upright growth.

Woody species may be subdivided into 1) trees, 2) shrubs, and 3) woody vines.

BINOMIAL NOMENCLATURE

Caspar Bauhin (1560-1624) devised a system where every living organism would be named with only two words.

The Swedish botanist/naturalist Carl Linnaeus (1707-1778) undertook the task of naming and classifying the whole living world and published this classification in 1753 in a book called the Species Plantarum. This publication is regarded as the beginning of the modern system of plant nomenclature.

In the Linnaean system the first word of the two-part name is always the genus and the first letter of the genus is always capitalized. The second word is the species epithet and is never capitalized (both names are often italicized). For a scientific name to be correct, it must 1) be spelled correctly, 2) genus should be capitalized [first letter only], 3) species is never capitalized, and 4) since I can't tell positively if you are printing in italics, you must underline the scientific name.

Frequently or always, you will note some designation (called the authority) after a scientific name. This designation usually denotes who named or classified it, or someone who studied the organism in great detail. Many times the authority will be "L." after Linnaeus. Other frequently encountered authorities are "Pers." after the German botanist Christian Hendrik Persoon (1761-1865), "Hook." after the British botanist William Jackson Hooker (1785-1865), and "Gray" after the American botanist Asa Gray (1810-1888).

Technically speaking, no scientific name is complete without the authority. This inclusion allows one to trace names and plants through the botanical literature. For this class, you do not need to know the authority.

The classifying of biological organisms is a dynamic process. Changes are still occurring.

Why do I need to know and use scientific names?

1. Accuracy, many plants, especially weeds have more than one common name.

a. Abutilon theophrasti - wild cotton, buttonweed, velvetleaf.

b. Panicum texanum - buffalograss, Texas panicum.

c. Sida spinosa - iron weed, tea weed, prickly sida.

d. Cassia obtusifolia (L.) now called Senna obtusifolia (L.) Irwin and Barneby, but it will now be back to Cassia obtusifolia (L.).

e. Conyza canadensis (L.) Cronq. – know as horseweed or mare’s tail.

f. Ambrosia trifida – giant ragweed or horseweed.

2. If working in another region of the country or internationally - the genus name will tell you a lot about the weed in terms of morphological and growth characteristics, and perhaps a good idea on management plans.

The seed producing plants (Spermatophytes) comprise two classes, the Angiosperms and the Gymnosperms.

Angiosperms: are plants where the seeds are borne within a mature ovary (fruit).

Gymnosperms: are plants whose seeds are not borne in an ovary, the conifers are an example.

Most weeds are Angiosperms.

The Angiosperms are further divided into two subclasses: the Monocotyledoneae (Monocots) and the Dicotyledoneae (Dicots).

Monocots and Dicots

This classification scheme is widely used (especially by agronomists) and is based on the number of cotyledons present.

A cotyledon: is a seed leaf and generally stores food in dicotyledons and absorbs food in monocotyledons.

The Monocotyledoneae are often referred to as the Monocots and include the:

grasses (Family Gramineae)

sedges (Family Cyperaceae)

lillies (Family Liliaceae)

cattails (Family Typhaceae)

Many people will collectively refer to these 'Monocots' as "grassy" or "grass-like weeds" because of their leaf shape and form. Be very careful doing this. Control from most herbicides varies dramatically between the true grasses and the sedges and lillies.

Characteristics of monocots:

-embryo with a single cotyledon.

-early leaves always alternate.

-leaves mostly parallel-veined.

-flower parts in threes or sixes, never in fives.

-vascular cambium absent.

-scattered primary vascular bundles in the stem.

Monocots: plants whose seedlings bear only one cotyledon (seed leaf). Typified by parallel leaf venation. Includes grasses (Gramineae)and sedges (Cyperaceae). Other monocot families which contain problem weeds include Juncaceae (rush family), Liliaceae (lily family), Commelinaceae (spiderwort family), Dioscoreaceae (yam family), Typhaceae (cattail family), Najadaceae (pondweed family), Lemnaceae (duckweed family), Alismaceae (water plantain family), and Pontederiaceae (pickerel-weed family).

The Dicotyledoneae are often referred to as the 'Dicots', include many of the weeds that are often referred to as "broadleaf weeds" or simply broad-leaves. Not all plants that have broad leaves are dicots.

Characteristics of dicots:

-embryo with a pair of opposite cotyledons.

-leaves usually net veined.

-flower parts mostly in fours and fives.

-vascular cambium generally present.

-primary vascular bundles in a ring.

Classifying weeds as monocots versus dicots, or grassy weeds versus broad-leaves is well entrenched in the plant science literature. This classification scheme is an integral part of much of the literature on the technologies of weed control, weed management, and many herbicide labels. This widespread use does not mean that everything is accurately described.

WEED INTERFERENCE

Plant interactions can be positive or negative. The combined effect of all negative plant interactions is called interference. Interference between crops and weeds includes competition, allelopathy, and parasitism.

Weed Competition. Competition is a process that occurs when the combined resource demands of plants within a given area exceeds the available supply.

Plants may compete for:

1). water (increases as the season progresses).

2). nutrients (increases as the season progresses).

Nitrogen is one of the most limiting nutrients followed by phosphorous (P) and potassium (K). Competition for nitrogen generally occurs about 4 to 6 weeks into the season while competition for P and K probably occurs later in the season when root systems start overlapping.

Essential Elements (nutrients) for Plant Growth

Three criteria must be met to determine if an element is essential for plant growth.

a. An element is essential if the plant cannot complete its life cycle in the absence of that element.

b. An element is essential if it forms part of any molecule or constituent of the plant that is itself essential in the plant.

c. The element must be acting directly inside the plant and not causing some other element to be more readily available or antagonizing the effect of another element.

ESSENTIAL ELEMENTS, SOURCE, AND POTENTIAL FOR

HAVING LIMITED AVAILABILITY.

Element	Source	Limiting?
C carbon	air	very seldom if ever
H hydrogen	water	if water is limited
O oxygen	air	very seldom
P phosphorus	soil	likely
K potassium	soil	likely
N nitrogen	soil	yes
S sulfur	soil	potentially
Ca calcium	soil	potentially
Fe iron	soil	potentially
Mg magnesium	soil	potentially
B boron	soil	potentially
Cl chlorine	soil	potentially
Cu copper	soil	potentially
Mo molybdenum	soil	potentially
Mn manganese	soil	potentially
Zn zinc	soil	Potentially
may be essential elements in some plants.
Na sodium	soil	potentially
Co cobalt	soil	potentially

Competition for nutrients would vary by species. For example, some plant species are responsive to potassium fertilizer, others to phosphorus.

If a weed responds to a nutrient and a crop does not respond, then there may exist an opportunity to enhance crop competitiveness by managing or limiting the nutrient availability.

This area has not been extensively studied. However, research has been initiated at NCSU to investigate competitive interactions as influenced by various nutrients.

3). light (starts about 6 weeks into the season).

4). CO₂ (very seldom occurs).

5). O₂ (very seldom occurs).

6). Space (depends on plant densities and interacts with competition for nutrients, water, and light. Starts about 6 weeks into the season.

Generally plants do not compete for CO₂ or O₂. Under certain environmental conditions a shortage of carbon dioxide could occur, but I am not aware of any such situation in the field. Competition for oxygen could occur in flooded fields, but this situation would obviously be the exception, rather than the rule.

TYPES OF COMPETITION

Competition is expressed as altered growth and development of one or both species.

Intraspecific competition: occurs when two or more plants of the same species coexist in time and space and simultaneously demand a limited resource. Weeds of the same species compete with each other, same for the crop.

Interspecific competition: occurs when two or more species coexist in time and space and simultaneously demand a limited resource.

Allelopathy: is a type of negative interference that occurs when one plant produces and releases chemicals into the environment (air or soil) which are deleterious to the growth and development of another plant. The effects of allelopathy have been widely established and are known to play an important role in some ecosystems. However, the effects are not well understood and the practical application of the process has been limited.

However, some crops are thought to provide allelopathic suppression of weeds including rye, hairy vetch, crown vetch, subterranean clover, and sunflowers. A lot of research is currently being conducted to further assess the effectiveness in cropping systems. This type of research has brought about the use of cover and "smother" crops, and living mulches.

Crop yield decreases nonlinearly to successive increases in weed density.

As weed density increases, loss per weed decreases. For example, Askew et al. (Weed Science 2002) examined the effect of Pennsylvania smartweed on cotton yield and other growth parameters.

Length and duration of competition.

With respect to time or period of competition, weeds that emerge before or at the time of crop emergence generally reduce crop yield more than weeds that emerge after crop emergence.

The longer weeds are permitted to compete with a crop, the greater the crop yield loss will be.

Weed-free requirement: is the minimum period following crop emergence that the crop must be maintained free of weeds in order to prevent crop yield loss.

Weed competition period: the maximum amount of time that weeds can be allowed to compete with the crop and not result in yield loss.

Critical period for weed competition: the time interval between the weed-free requirement and the weed competition period is referred to as the critical period of weed control. More precisely, it is the period of time between that period after crop seeding when weed competition does not reduce yield, and the time period after which weed presence does not reduce yield.

Figure 2.The influence of weed-free period and weed competition period on crop yield and critical period of weed control.

(Radosevich, Holt and Ghersa. 1997. Weed Ecology.)

Critical period for weed competition with various crops.

Crop	Weed-free weeks required	Weeks of weed competition tolerated
Soybean	5	8
Corn	3	6
Cotton	6	8
Peanuts	4	8
Potato	6	9
Paddy rice	3	9
Soybeans	3	8-9

Figure 1. This figure shows the importance of early-season weed removal to ensure cotton lint potential. Data averaged over 4 locations in NC from 2003 - 2004 (Wilcut et al. unpublished data).

Figure 2. This figure shows the importance of an early-season weed-free window for maximizing cotton lint yield potential. Data averaged over 4 locations in NC from 2003 - 2004 (Wilcut et al. unpublished data).

Figure 3. This figure shows the critical period for weed control to avoid a 5% yield loss. Data averaged over 4 locations in NC from 2003 - 2004 (Wilcut et al. unpublished data).

Figure 4. This figure shows the yield production for various weed competition intervals throughout a growing season. Data averaged over 4 locations in NC from 2003 - 2004 (Wilcut et al. unpublished data).

THRESHOLDS

Threshold: is defined as a point at which a stimulus is just strong enough to produce a response.

Damage threshold: used to define the weed population at which a negative yield

response is detected.

The economic threshold: is the weed density at which the value of the loss due to

weed competition exceeds the cost of control.

Action threshold: may include consideration of other factors such as the effect of

allowing weed to set seed (seed rain), which may potentially

affect weed management in succeeding crops. Action

threshold is the point at which some control action is initiated.

Types of Losses. Weeds Cause:

A. Direct Losses

1. Reduced Crop Yields

a. Competition - Weeds compete with crops for light, water, nutrients, and possibly essential gasses and space. Utilization of these growth inputs by weeds results in less being available to the crop. As the weed population increases, a point is reached where weeds utilize enough of the growth inputs to limit the amount available to the crop. When growth inputs to the crop are limited, crop growth and development are adversely affected and a yield reduction occurs. Light is typically the growth input for which there is greatest competition between crop plants and weeds. However, competition for moisture and nutrients can also be significant.

Competition can be used as a weed management tool. Cultural practices that promote uniform stands of healthy, vigorously growing crop plants whose canopy closes quickly gives the crop a competitive advantage over weeds.

b. Allelopathy - Process by which a plant releases into the environment an organic chemical that affects the growth and development of other plants.

Allelopathy can result from root exudates or leaf leachates from living weeds or from compounds released from dead weeds as they decay in the soil. Johnsongrass rhizomes, for example, release chemical(s) that inhibit growth of soybeans.

Allelopathy may be useful in weed management. Crop plants (or their residues) may release chemicals that are inhibitory to weeds. For example, chemicals released from wheat straw inhibit germination and growth of certain broadleaf weeds. A heavy straw mulch helps suppress weeds in no-till environments. Some efforts have been made to breed for crop plants with greater allelopathic effects, but success to date has been limited.

The effects of competition and allelopathy are difficult to separate under field conditions. "Interference" is a term sometimes used to describe the effects of weeds on crops irregardless of the cause (competition and/or allelopathy). Both the species of weed and the species of crop can influence the extent of interference. Additionally, the length of time the weeds are present can influence the extent of interference.

Example of effect of crop species on season-long interference; NC data.
Crop	Fall panicum density	% yield loss
Corn	1 per 16 ft of row 1 per 2 ft of row	0 10
Soybeans	1 per 16 ft of row 1 per 2 ft of row	0 15
Peanuts	1 per 16 ft of row 1 per 2 ft of row	25 65

Example of effect of weed species on season-long weed interference. NC data.
Crop	Weed	Weed density	% yield loss
Peanuts	broadleaf signalgrass	1 per 2 ft of row 1 per 16 ft of row	28 7
Peanuts	fall panicum	1 per 2 ft of row 1 per 16 ft of row	65 25
Peanuts	Palmer amaranth	1 per 2 ft of row 1 per 16 ft of row	36 2
Peanuts	jimsonweed	1 per 2 ft of row 1 per 16 ft of row	51 12
Cotton	pale smartweed	1 per 2 ft of row 1 per 16 ft of row	26 5
Cotton	ladysthumb smartweed	1 per 2 ft of row 1 per 16 ft of row	25 2
Cotton	Pennsylvania smartweed	1 per 2 ft of row 1 per 16 ft of row	40 10

Example of weed density effects on crop yield. Scott et al. 2000.

Effect of Datura stramonium density on Gossypium hirsutum yield loss in 1998 and 1999. Percent yield loss was fitted to the rectangular hyperbola equation: y = (I × x)/(1 + (I × x/100)), where y is percent yield loss, x is weed density, and I is percent yield loss as weed density approaches zero.

Example of length of time of weed interference. Monks and Schultheis, 1998.
Weeks of interference by large crabgrass	Marketable yield of watermelon (kg/ha)
0	61,880
2	50,720
4	39,550
6	28,390
10	6,060

Example of length of weed control. Monks and Schultheis, 1998.
Weeks after planting when large crabgrass was seeded	Marketable yield of watermelon (kg/ha)
0	6,940
2	34,830
4	49,280
6	59,890
10	60,000

ECONOMIC LOSSES FROM WEEDS

Losses from weeds in the United States are in excess of $8.0 billion annually. The estimated average annual monetary loss caused by weeds with current control strategies in 46 crops was estimated at $4.1 billion. If no herbicides were used, this loss was estimated to be $19.6 billion.

Weeds pose one of the most important threats to our supplies of food and fiber. Losses in both yield and quality of crops due to weeds, as well as costs of weed control, constitute an enormous economic problem in all areas. Weeds have a major influence on the production decisions made by producers. Additional land, livestock, labor, equipment, fuel, herbicides, insecticides, fungicides, fertilizer, and irrigation water may be required to maintain economic production when weeds are present. In addition, allergic reactions to weeds directly affect the health and well-being of many people in rural and urban populations.

Economic losses due to weeds may include one or more of the following:

yield loss quality loss cost of control

harvest losses equipment costs crop restrictions

lower farm value consumer losses governmental cost

Environmental cost

Reduced Harvesting Efficiency:

a. Harvest of the crop may be delayed while waiting for weeds to dry down.

b. Presence of weeds, especially high populations, often means a slower harvest speed.

c. Weeds can cause increase harvesting losses.

d. High populations of weeds may cause additional wear on harvesting equipment.

Reduced Quality of Harvested Crop:

a. Trash and weed seed may lower the price received for crop. Examples: cotton classed as "grassy"; price dockage for foreign matter in soybeans.

b. Weeds may impart off-flavors in products made from crop. Example: wild garlic bulblets in wheat.

c. Seed of some weeds are toxic. Buyer may refuse delivery of crop if certain toxic weed seed are present. Example: crotalaria.

d. Weeds may increase moisture content of harvested crop, resulting in greater drying costs or a price dockage for excess moisture.

e. There may be a loss of crop quality (crop deterioration or weathering) while waiting for weeds to dry down enough to harvest.

Indirect Losses in Crops:

1. Increased Crop Production Costs

a. Cost of control (herbicides, herbicide application, cultivation, additional land preparation, equipment costs, cost of manager’s time).

2. Crop Damage

a. Herbicide damage to crops (damage to treated crop, drift to another crop, or carryover to rotational crops).

b. Certain weed control practices create conditions favorable for other pests; example is cultivating peanuts.

c. Mechanical damage from cultivation or other control measures.

d. Moisture loss from cultivation.

3. Weeds may limit rotational choices

a. There may not be effective management options for some weed species in a particular crop. To be able to manage those weeds, the grower may have to plant a less profitable crop in that field. This may be the case with high-value crops, such as tobacco and various vegetable crops, where herbicide options are limited.

b. Some herbicides may persist long enough to damage certain crops planted the following year. Herbicides with long persistence may be helpful in control of certain weeds in a given crop, but the grower may have to plant a less profitable rotational crop on that field in the following year because other crops may be sensitive to the herbicide residues. For example, Cadre is very effective on a number of problem weeds in peanuts, but Cadre will carry over and damage cotton the following year. If Cadre is necessary to manage weeds in peanuts, the grower will have to plant a potentially less profitable crop, such as corn, the following year rather than cotton.

4. Weeds may be alternate hosts for insects, diseases, or nematodes that attack crops.

Examples: horsenettle is an alternate host for tobacco mosaic virus; common ragweed is an alternate host for granville wilt; johnsongrass is an alternate host for maize chlorotic dwarf virus; wild mustard is an alternate host for cabbage root maggot; sicklepod and crotolaria are alternate hosts for the soybean cyst nematode.

Human health considerations: poison ivy, pollen, poisonous weeds

1. Health Hazards to Humans and Livestock

a. Poisons

1. Dermal poisons - cause skin irritations. Poison ivy is example.

2. Internal poisons - cause sickness or death if eaten. There are numerous poisonous weeds. Examples include jimsonweed, crotalaria, bracken fern, black nightshade, sicklepod.

2. Cover and breeding sites for rodents, snakes, etc - This is mainly a problem in non-cropland and residential areas.

Increased pest populations: resulting from weeds serving as hosts to nematodes, disease causing organisms, insects, etc.

Reduced aesthetic quality (lawns, golf courses, landscaped areas).

Safety: reduced vision at intersections, rights-of-ways such as trees close to roads, railroad tracks

: is also an issue on home lawns and especially athletic fields, etc. Anything that reduces sod strength and root structure could lead to more injuries.

Economic threshold for safety related issues may be zero.

Rights-of-way: for example utility lines, weedy vines growing on power poles, etc. Major costs associated from weed control in rights-of-way, storage areas, railroads, etc.

Waterways: reduced navigation, reduced water flow.

Livestock: reduced weight gains in weedy nonproductive fields, also danger from poisonous plants.

Invasive Weeds. May lead to extinction of native species (not just plants) and a loss in biodiversity

Peer pressure.