Introduction to Weeds and Herbicides

Weeds are plants whose undesirable qualities outweigh their good points.

What Are Weeds and Their Impacts?

There are numerous definitions of a weed. Some common definitions include:

  • a plant that is out of place and not intentionally sown
  • a plant that grows where it is not wanted or welcomed
  • a plant whose virtues have not yet been discovered
  • a plant that is competitive, persistent, pernicious, and interferes negatively with human activity

No matter which definition is used, weeds are plants whose undesirable qualities outweigh their good points, at least according to humans. Human activities create weed problems since no plant is a weed in nature. Though we may try to manipulate nature for our own good, nature is persistent. Through manipulation, we control certain weeds, while other more serious weeds may thrive due to favorable growing conditions. Weeds are naturally strong competitors, and those weeds that can best compete always tend to dominate.

Both humans and nature are involved in plant-breeding programs. The main difference between the two programs is that humans breed plants for yield, while nature breeds plants for survival.

Characteristics of Weeds

There are approximately 250,000 species of plants worldwide; of those, about 3 percent, or 8,000 species, behave as weeds. Of those 8,000, only 200 to 250 are major problems in worldwide cropping systems. A plant is considered a weed if it has certain characteristics that set it apart from other plant species. Weeds possess one or more of the following characteristics that allow them to survive and increase in nature:

  • abundant seed production
  • rapid population establishment
  • seed dormancy
  • long-term survival of buried seed
  • adaptation for spread
  • presence of vegetative reproductive structures
  • ability to occupy sites disturbed by humans

Abundant Seed Production

Weeds can produce tens or hundreds of thousands of seeds per plant, while most crop plants only produce several hundred seeds per plant. The following are some examples of approximate numbers of seeds produced per weed:

  • giant foxtail—10,000
  • common ragweed—15,000
  • purslane—52,000
  • lambsquarters—72,000
  • pigweed—117,000

Since most weeds deposit their seeds back to the soil, seed numbers in the soil increase rapidly from year to year if the weeds are not managed. Despite that many weed seeds are either not viable, eaten by animals or insects, or decompose within several months after they are deposited, hundreds of millions of viable weed seeds per acre can still be present and waiting to germinate.

Rapid Population Establishment

Most weeds can germinate and become established relatively quickly. They also produce viable seeds even under environmental and soil conditions that are not favorable for most crop plants. Under ideal conditions, dense weed populations can thrive and easily outcompete a crop if left unchecked. Under poor conditions, certain weeds can adapt and produce some viable seeds in a relatively short time period (6 to 8 weeks).

Seed Dormancy

Dormancy is basically a resting stage or a temporary state in which the weed seeds do not germinate because of certain factors. Dormancy is a survival mechanism that prevents germination when conditions for survival are poor. For example, seeds of summer annual weeds will generally not germinate in the fall, preventing them from being killed by cold winter conditions. The various factors that affect dormancy are temperature, moisture, oxygen, light, the presence of chemical inhibitors, tough seed coat, and immature embryos. There are several kinds of dormancy, but the most commonly used terms to describe dormancy are innate, induced, and enforced.

Innate or primary dormancy inhibits germination at the time seeds are shed from the plant. After the seed shatters from the parent plant, time is required for immature embryos to develop, natural inhibitors to leach out, or extremes of temperature to crack hard seed coats and allow germination to occur. These conditions cause innate dormancy, and, once lost, this type of dormancy cannot reoccur.

Induced dormancy is a temporary dormancy that occurs when a seed is exposed to hot or cold temperatures. It continues after temperatures change and prevents germination during the wrong time of year. The dormancy is broken by temperatures opposite of those that induced it.

Summer heat induces dormancy in summer annual weeds such as yellow foxtail and pigweed, preventing germination in the fall. Cold temperatures in fall and winter break this dormancy (usually by mid-winter), and the seeds germinate in spring when conditions are right. In winter annual weeds, the process is reversed.

Dormancy can be induced in many weed seeds when a crop canopy filters sunlight, shading the ground and reducing germination. Dormancy can be induced over and over again for as long as the seeds remain viable.

Enforced dormancy takes place when environmental conditions—cold temperatures, lack of moisture or oxygen, and occasionally a high salt concentration in the soil—are unfavorable. When limitations are removed, seeds germinate freely. Summer annual weed seeds lose their induced dormancy by mid-winter and, if not for the cold temperatures, would germinate at that time.

Seeds of different weed species have various temperature requirements for germination. Common chickweed can germinate under snow cover, while common purslane will not germinate until the soil temperature reaches 70 to 75°F. Crop seeds are generally planted at or near the optimum soil temperature needed for quick germination—a temperature that is also ideal for some weed seeds.

Seeds require water for germination. Seeds in dry soils may remain dormant even when all other factors promoting germination are favorable.

Oxygen availability also influences a seed’s ability to germinate. Water may fill soil pores and exclude air, limiting germination in very wet soils. Soil compaction also may reduce the oxygen supply and prevent seeds from germinating. Deep plowing, tillage, or hoeing can bring buried seeds to the surface, where they readily germinate upon exposure to oxygen.

Long-Term Survival of Buried Seed

If conditions are adequate, buried weed seeds have the potential to remain viable for 40 years or more. Broadleaf weed seeds tend to last longer in the soil than grassy weed seed since they usually have tougher seed coats. In most cases, the majority of seeds only exist in the soil for a few years due to germination, decomposition, predator feeding, or other factors. However, with the large number of seeds produced, a small percentage may remain viable for long-term survival.

Adaptation for Spread

Weeds have certain mechanisms for easy dispersal of seeds. Most seeds or seed pods have special structures that allow them to cling, fly, or float. Common cocklebur and burdock seed pods have hooks that attach to animal fur or feathers; curly dock seeds have bladder-like structures that allow them to float; and milkweed, dandelion, and thistle seeds have a feathery pappus that allows them to be carried by the wind. Other weeds, such as jewelweed or snapweed, have pods that “explode” when the seeds are mature, projecting them several feet from the parent plant. Weeds can also be spread when animals or birds eat their fruit and deposit the seeds with their droppings. Weed seeds can be widely spread through crop seeds, grains, feed hay, and straw. These and other human activities probably account for the long-distance spreading of weeds.

Vegetative Reproductive Structures

Most perennial weeds possess special vegetative structures that allow them to reproduce asexually and survive. These perennial structures contain carbohydrates (food reserves, sugars), have numerous buds in which new plants can arise, and include the following:

  • stolons—aboveground, horizontal stems that root at the nodes (e.g., crabgrass, bermudagrass, ground ivy)
  • rhizomes—belowground, thickened stems that grow horizontally in the upper soil layers (e.g., quackgrass, Johnsongrass, wirestem muhly, Canada thistle)
  • tubers—enlarged rhizomes with compressed internodes located at the ends of rhizomes (e.g., yellow nutsedge, Jerusalem artichoke, potato)
  • bulbs—modified leaf tissues for carbohydrate storage that are located at the base of the stem or below the soil line (e.g., wild garlic, onion)
  • budding roots—modified roots that can store carbohydrates and grow both vertically and horizontally (e.g., hemp dogbane, Canada thistle)

Despite these vegetative reproductive structures, many perennials also reproduce by seed. Some depend heavily on reproduction by seed (e.g., dandelion), while for others it is less important (e.g., yellow nutsedge).

Ability to Occupy Disturbed Sites

Weeds are very opportunistic. When conditions are adequate, weed seeds germinate and colonize if left unchecked. When a site is disturbed, weeds are usually the first to emerge. If a weed becomes established first, it has the competitive advantage over crop plants or desirable vegetation.

Problems with Weeds

Weeds are troublesome in many ways. Primarily, they reduce crop yield by competing for:

  • water
  • light
  • soil nutrients
  • space
  • CO2

The following are other problems associated with weeds:

  • reducing crop quality by contaminating the commodity
  • interfering with harvest
  • serving as hosts for crop diseases or providing shelter for insects to overwinter
  • limiting the choice of crop rotation sequences and cultural practices
  • producing chemical substances that can be allergins or toxins to humans, animals, or crop plants (allelopathy)
  • producing thorns and woody stems that cause irritations and abrasions to skin, mouths, or hooves of livestock
  • being unsightly, dominant, aggressive, or unattractive
  • obstructing visibility along roadways, interfering with delivery of public utilities (power lines, telephone wires), obstructing the flow of water in water ways, and creating fire hazards
  • accelerating deterioration of recreational areas, parking lots, buildings, and equipment
  • invading exotic weed species that can displace native species in stabilized natural areas

Weeds Reduce Crop Yield

Weeds reduce crop yield and quality and compete for necessary resources.

Costs of Weeds

Weeds are common on all 485 million acres of U.S. cropland and almost one billion acres of range and pasture. Since weeds are so common, people generally do not understand their economic impact on crop losses and control costs. In 2003, it was estimated that the nonuse of herbicides and the likely substitution of alternatives (i.e., cultivation, hand-weeding) would result in a loss of $13.3 billion in food and fiber production. The total impact of herbicide nonuse would be an income loss of $21 billion, which includes $7.7 billion in increased costs for weed control and $13.3 billion in yield losses. In the early 1990s, the estimated average annual monetary loss caused by weeds, with current control strategies in the 46 crops grown in the United States, was over $4 billion. If herbicides were not used, this loss was estimated to be $20 billion. Losses in field crops accounted for over 80 percent of this total. Other sources estimate that U.S. farmers annually spend over $3.5 billion on chemical weed control and over $2.5 billion for cultural and other methods of control. The total cost of weeds in the United States could approach $15 to $20 billion. Weed control and other input costs (e.g., seed, fertilizer, other pesticides, fuel) vary with the crop. For example, in the mid-1990s, herbicides for soybeans cost about $30 per acre, or about half of the total per-acre purchased input. For corn, the cost was about $32 per acre, or about a quarter of the total per-acre purchased input. Weed control costs for wheat are about $6 per acre, or about 5 percent of the total per-acre purchased inputs. A decade later, these costs are about the same. However, in most situations, herbicide use is still the most economical means to control weeds. The USDA estimates that weed control costs for organic vegetable growers in California can be $1,000 per acre in comparison to $50 per acre that conventional growers spend on herbicides. Several factors help determine the relative costs of herbicides from one crop to another, including the competitive ability of the crop, the weeds present, the contribution of nonchemical control practices, the tillage method, management decisions, the type of crop seed used (e.g., normal versus resistant GMO variety), and the value of the crop. Weeds not only cause losses in crops, but also can affect livestock production if poisonous weeds are present or weeds invade and render the pasture useless.

Benefits of Weeds

Despite the negative impacts of weeds, some plants usually thought of as weeds may actually provide some benefits, such as:

  • stabilizing and adding organic matter to soils
  • providing habitat and feed for wildlife
  • providing nectar for bees
  • offering aesthetic qualities
  • serving as a genetic reservoir for improved crops
  • providing products for human consumption and medicinal use
  • creating employment opportunities

Weeds have a controversial nature. But to the agriculturist, they are plants that need to be managed in an economical and practical way in order to produce food, feed, and fiber for humans and animals. In this context, the negative impacts of weeds indirectly affect all living beings.

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Introduction to Weeds and Herbicides





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