Weed Management
Most vegetable growers report that weeds are more difficult to manage than insects or diseases. Uncontrolled, weeds reduce yields and quality and harbor insects and diseases which then spread to the crop. For example, European corn borers feed on pigweed as well as many vegetables. Common weed control practices such as herbicide applications and cultivations cost money and, if not done properly, can cause crop and environmental damage.
Herbicides can also affect both beneficial and pest insects. In some cases insect populations increase, and in other cases they decrease or are not affected. The effects can be directly toxic, with herbicides applied during oviposition or early larval development. They can also be indirect as with populations of the egg parasite Trichogramma which were reducted after feeding on insects which had ingested the herbicide alachlor. In other experiments, aphid and thrips populations increased after herbicide use. Green peach aphids preferentially invade weed-free collard patches, probably because there were fewer predators than in more weedy areas.
Integrated pest management of weeds, like insect IPM, focuses on prevention, beginning with identification of weed species and mapping their location and populations in the field. This weed map should show growth stage and size and include distinguishing features such as wet areas and field borders. Localized use of weed control tools may be necessary in areas with high concentrations of perennial weeds such as johnsongrass or bermudagrass. An example of a scouting-based weed management program for pickling cucumbers is presented in Weed species, density and control strategies for cucumber production.
Weed IPM programs differ from insect IPM programs in the way that economic thresholds are used. In insect IPM programs, insecticides are applied when the cost of controlling the insect population is less than the dollar value of the projected crop losses from the insect. This insect population level, called the economic threshold, is higher than the point at which the pest population reduces yield. For some agronomic crops, weed economic thresholds have been determined for use as a basis for making decisions about herbicide application. In vegetables, however, weed economic thresholds are not widely used. First, information is available for only a few vegetable/pest situations. Second, the concept is limited in its usefulness because for most vegetables there are few herbicides registered which can be applied postemergence, i.e. when crop and weeds are growing side-by-side. A more useful concept for vegetables is the critical weed-free period, a concept explained below.
Critical weed-free periods
The critical weed-free period is the minimum length of time during which the crop should be practically weed-free to avoid a yield or quality reduction. As shown in Examples of critical weed-free periods for selected weeds in vegetable crops, the critical weed-free period varies with crop, weed species and environment. The critical weed-free period concept is based on the following observations: At the time of field preparation and planting, the field is virtually free of weeds. Soon after, however, weed seeds brought to the surface by tillage start to germinate.
At some point, crop seedlings and weeds are large enough to compete for light, water and nutrients. Weeds usually win this competition, marking the beginning of the critical weed-free period. Economic losses will occur if weeds are not controlled. For example, California researchers found that direct-seeded bell peppers needed to complete flowering and fruit set without weed competition to reach their full yield potential. The end of the critical weed-free period is generally several weeks later when the crop is large enough to shade late-emerging weed seedlings or is otherwise competitive with weeds. After 8 weeks weed-free, pepper flowering and fruit set were completed, and subsequent weed competition did not influence yields.
The critical weed-free period concept is valuable since season-long absence of weeds from a field may be justifiable aesthetically, but not agronomically. The critical weed-free period concept, does not mean that weeds can be ignored except during the cr itical period, however. The primary objective of an IPM program is to integrate all farm operations. If no provisions have been made to reduce weeds (e.g., mulches or pre-emergence herbicides), weeds may not be controllable by the beginning of the critical period, with or without herbicides. This is because there are few postemergence herbicides registered to control the many grasses and broadleaf weeds found in the southern United States. Even if the crop growth habit allows mechanical cultivation between rows, if no mulch or preplant herbicide is applied, weeds in the row will usually require hand cultivation.
Another consideration is that weeds present after the end of the weed-free period may not reduce yield but can make harvest difficult. For example, mechanically harvesting snap beans is difficult in weedy fields. Workers prefer weed-free fields, especially in cucumbers and other low-growing crops. In a California study, lack of weed control reduced yields 45 percent in direct-seeded cucumbers, making them also hard to harvest. Seeds from late emerging weeds compound next season's problems by increasing weed population levels. Nutsedge, for example, starts sending out next season's reproductive nutlets in late summer. Thus, the concept of a critical weed-free period is valuable, but only as part of a year-round integrated weed management program.
| General Practices to Control Weeds | ||| | Specific Weed Management Practices | ||| | Other Weed Management Practices |
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