Part 2, Section 1: Pest Management
Pest Management
Weed Management
Economic threshold for weeds
An economic threshold for weeds is the density of a weed population at which control is economically justified because of the potential for yield reduction, quality loss, harvesting difficulties, or other problems that weeds may cause.
Each acre of land has a specific amount of resources for the growth and development of plants. This amount varies from site to site and from year to year with differing environmental conditions. An area will support as much vegetation as possible with available sunlight, water, nutrients, and space. If a weed-free crop is grown, the crop has all of the resources for its own use. If weeds are allowed to grow with the crop, they will use a portion of these resources and may cause losses great enough to justify control measures. Generally, removing an infestation of insects or disease from a crop does not necessarily lead to an infestation by more of these pests. If a species of weed is removed from an area, however, other species may invade unless the crop is sufficiently competitive to prevent it from doing so.
In addition to economic yield loss, other concerns may determine when weed control is justified. For example, eastern black nightshade in soybeans or late-emerging grasses in corn may not reduce yield, but these weeds can clog equipment, causing harvest delays and creating a frustrating situation in which the farmer is prone to accidents. A few velvetleaf plants in corn may not reduce the yield in this year’s crop, but many farmers treat weeds to prevent seed production and subsequent soil infestation. The presence of weeds in an alfalfa crop may not reduce the yield of the first cutting, but their impact on quality and future stand longevity should be considered.
Another problem involves the importance of aesthetics to tenant farmers and their landlords. If the landlord views a few weeds as an indication of poor management, the tenant farmer may be concerned about losing the farm to a "tidier" neighbor as well as about potential yield loss.
Some general assumptions about weed-crop interference can help make the appropriate management decision. Weed-crop competition studies indicate that if weeds are allowed to grow with most field crops under normal environmental conditions for no longer than 4 to 6 weeks after crop emergence and are then removed, and if the crop remains weed-free until harvest, then yield reduction is unlikely. In addition, if weeds are kept out of the field for 4 to 6 weeks after crop emergence, any weeds that later invade will not reduce yield significantly, although they may produce seeds, cause harvesting problems, or reduce crop quality.
Broadleaf and grass weeds will compete at different levels of intensity depending upon the competitiveness of the crop, the tillage system, environmental conditions, and other weeds present. In general, broadleaf weeds are more damaging to a broadleaf crop, while grass weeds are more competitive in a grass crop. For example, velvetleaf is more devastating in soybeans than in corn because it competes for the exact same space and sunlight that soybeans do. It also is harder to control an infestation of broadleaf weeds in a broadleaf crop than it is to control a grass weed. The same theory applies to a grass crop such as corn. Shattercane, which has a growth habit like that of corn, is very difficult to control in corn and competes for the same space and sunlight.
In almost every cropping situation, an economic threshold exists justifying the use of some form of weed control. Crop yield loss information has been determined for certain single weed species growing with corn or soybeans in the midwestern region of the United States. Very little data are available for specific weeds growing in the northeastern United States. Not all weeds have been researched and few data exist for the more common situation -- the effect of infestations of several weed species at various populations. Because the competitive ability of different species can change under various conditions, determining yield loss information for several species growing together is often difficult.
Caution: The values in Table 2.1-7 are averages from several studies conducted in the northcentral region of the Midwest. The values assume that the weeds emerge with the crop and that the crop is planted in 30-inch rows. Time of weed emergence, variations in local environment (soil and weather), and degree of postemergence weed control will affect weed-crop interference. Failure to obtain 100 percent weed control also must be considered. This factor can be rather large if weeds are under stress from drought or cool weather, or if weeds are "off label" with regard to size. Use some judgment in applying these principles, and keep in mind that quality loss, harvesting difficulties, aesthetics, or other problems associated with weeds are not considered in these yield loss estimates.