Describe how weeds are categorized by life cycle and how this is correlated with specific control methods.

For intelligent and efficient weed control, some knowledge of the life cycles of weeds is useful. The life cycle of a weed is simply its seasonal pattern of growth and reproduction. For example, an annual life cycle means that a weed goes from seed to seed in one growing season or one year. A perennial life cycle means that a weed regrows season after season. The table below summarizes definitions of a number of life cycles and gives examples of weeds in each category.


Life Cycle Definition Examples
Annual Weed species that completes its life cycle (seed to seed) within one growing season or one calendar year redroot pigweed, kochia
Summer Annual Summer annual weeds are a subcategory of annual weeds. They germinate as temperatures rise in the spring (April to May) through summer, whenever soil moisture is adequate. redroot pigweed, common lambsquarters, green foxtail, Russian thistle,
Winter Annual Winter annual weeds germinate in the fall through early spring (October to March), when soil temperature and moisture are favorable. downy brome shepherd's-purse
Biennial Weed species that completes its life cycle over two growing seasons. wild carrot, common mullein, musk thistle
Perennial Weed that continues to regrow over a few seasons to many seasons. johnsongrass, buckhorn plantain, sagebrush
Creeping Perennial Perennial weeds with vegetative structures (stolons or rhizomes) that permit them to reproduce asexually (without seed). field bindweed, quackgrass, Canada thistle

Control of Annual Weeds in Forages

In general, annual weeds are well adapted to survive and reproduce in annual crops. More specifically, summer-annual weeds are typically a problem in spring-seeded crops, and winter-annual weeds are typically a problem in fall-seeded crops. In conventional forage crop production, the land is cleared of vegetation before a new crop is planted. Preparing an ideal seedbed for a crop will often provide a good seedbed for annual weeds as well. Furthermore, an annual weed with a short life cycle (seed to seed) may be able to complete its life cycle between weed control operations in an annual crop.

Ideally, the best way to control annual weeds in forages is to not allow them to become established in the first place. The use of weed free seed and/or crop rotation are important steps in achieving this goal. Realistically though, even with the use of these techniques, annual weeds may still be a problem.

As mentioned previously, preparing a seed bed for a new crop typically creates somewhat ideal conditions for weed germination as well. One classic non-herbicidal technique to control weeds before establishing a new forage stand takes advantage of this fact. The technique involves preparing a seedbed for planting and then allowing enough time for weed seeds in the soil to germinate and become small seedlings. The newly germinated weed seedlings are then killed by the use of tillage, just prior to planting the forage crop.

In conventional seeding of forage crops, where the land is plowed and where weeds are likely to be a problem, a preplant herbicide may be used. An example of a commonly used preplant herbicide is EPTC. If a grower prefers to avoid the use of herbicides, and is mainly concerned with summer-annual weeds, another option in weed control is to plant in the fall rather than the spring. Similarly, if winter annual weeds are the main problem, a spring planting will probably be preferable. A third option in weed control that also avoids the use of preplant herbicides is to use a companion crop. A companion crop is a fast-growing annual crop that grows quickly enough to out compete weeds. Perhaps the most common companion crops used in forage establishment are the small grains (e.g. oats, wheat).

Controlling annual weeds in established perennial forages may be achieved by careful use of post-emergence herbicides and/or tillage. Use of either technique should be approached with caution since each also has the potential to damage the forage stand. Growers should consult with local weed control experts to determine which specific herbicides are allowed to be used on their particular forage species.

Control of Biennial Weeds in Forages

In general, biennial weeds are less of a problem in annual crops than annual weeds. This is because in most annual crops the land is cultivated at least once a year. One effective weed control operation (e.g. plowing, preplant herbicide) a year should prevent most biennial plants from setting seed. In perennial crops, biennial weeds may become established, especially in older pastures or hay fields where the population of the desired crop plant is beginning to thin out. Controlling biennial weeds may be achieved with careful use of postemergence herbicides and/or tillage. Herbicides will generally be most effective against biennials if applied before a seedstalk is produced, a stage of growth referred to as bolting. As mentioned previously, any use of herbicides or tillage in established stands should be approached with caution because either of these techniques may also damage the forage stand.


Control of Perennial Weeds in Forages

As a general rule, perennial weeds in established forage stands are best controlled by maintaining a healthy, dense, well-managed stand of the desired crop. This makes it very competitive against weed invasion. As a pasture or forage crop stand ages, it tends to naturally thin out, which provides weeds with an opportunity to encroach into the stand. When a thinning stand is the primary cause of weed invasion, the grower must make a decision whether to establish an entirely new stand, rotate to another crop, or simply renovate the existing stand. Each approach has advantages and disadvantages.

Renovating is used to improve growth of desired species and reduce weed problems in established pastures. Renovation typically involves overseeding the pasture with the desired crop by simply broadcasting seed on the surface or using a specialized planter that will plant through the existing crop. The main advantages of renovation are that is may be less expensive (in terms of time and money ) than establishing a new stand, it does not disturb the soil and stimulate germination of dormant weed seed, and it minimizes soil erosion.

Controlling perennial weeds selectively in established pastures or forage crops like alfalfa may be done in several ways and may involve a combination of techniques. With forage crops that are regularly mowed for hay, mowing itself may help control weeds. The control by mowing is due to the reduction in the weed root carbohydrate reserves (a class of substances in the roots that provide the energy for regrowth) and/or by reducing seed production.

Another way to combat perennial weeds is the careful use of selective herbicides. However, before using herbicides the grower should evaluate the cost of herbicide treatment in relation to the potential to increase profits on hay sales and/or animal performance. A significant increase in hay price may be required to compensate for the cost of herbicide application.

In a forage livestock operation the farm/ranch manager may be able to use grazing animals to help control weeds. In a technique known as mob-grazing, animals are placed onto a field at a high density for a period of about 1-2 weeks. Mob grazing, used carefully, can provide weed control under the right conditions. The forage crop itself must be well rooted to avoid animal damage. In mob grazing, weeds are utilized as forage or uprooted by the hoof action of the animals. This method has been used with some success to control weeds in grass pastures and established alfalfa.

© Oregon State University 2008
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