Agricultural pest management
Integrated Weed Management
(Reviewed 8/07, updated 12/08)
Effective weed control for dry bean production requires integration of sound cultural practices and herbicide use. It is necessary to carefully plan a program of crop rotation, cultural practices, and herbicide use. The type of program developed depends on bean variety, species of weeds, crop rotation, soil type, and irrigation system used in growing the crop.
and FIELD PLANNING
Record observations on a monitoring form (example form— Determine the dominant weed species present and maintain records of weed species and severity of weed infestations for each field planted to beans. Anticipating certain weeds can aid greatly in designing the optimum weed control program. Avoid fields severely infested with such perennial weeds as bermudagrass, johnsongrass, nutsedge (nutgrass), and field bindweed. Weed control can also be difficult in fields heavily infested with certain annual weeds such as nightshade, groundcherry, and annual morningglory. To obtain a satisfactory crop from a weedy field, a grower must select locally adapted bean varieties and appropriate herbicides, and manage the field with care.
Cultural techniques aimed at suppressing weed growth, and good farming practices in general, are necessary parts of any weed control program.
A preirrigation to germinate weeds followed by a shallow bed tillage before planting will temporarily control many annual weeds and leave 1 to 2 inches of dry soil on the surface to further slow weed seed germination. Vine-type bean varieties are very competitive to weeds and prevent weed germination if allowed to cover and shade the ground before an irrigation is applied. Plant bush-type varieties in narrow rows, 30-inch centers or closer, or in three rows on a 60-inch bed. Vine-type varieties like limas are suited to 30- to 42-inch centers or two rows on 60-inch beds.
If one week or more passes between bed preparation and planting, weed seedlings may begin to emerge. Weeds will need to be treated with a contact herbicide or, before planting, run a blade or rolling cultivator at 1 to 1.5 inches below the soil surface to remove seedlings, giving the bean plants a head start on emerging weeds. This practice conserves more soil moisture than other cultural methods.
Furrow irrigating of beans for germination instead of planting into moisture must be done carefully, otherwise it can cause soil crusting and lead to seed rot. It also causes weeds to germinate simultaneously with the beans. Furrow irrigating for germination is especially risky on heavy clay soils prone to excessive saturation. Sprinkler irrigation is a safer method to use for germination and to avoid problems from excess water.
Herbicides. Combine preplant herbicides with cultural methods to achieve optimum weed control. Weeds that escape control by preplant herbicides may sometimes be effectively controlled by a postplant treatment.
Herbicides registered for use in beans differ in the weeds they control and the conditions under which they are optimally used. Rates may vary, depending on the texture, moisture, and organic matter content of the soil. In addition, different classes of beans vary in their tolerance to different herbicides, so herbicides must be chosen based on the class of beans to be grown. Herbicides can be applied either pre- or postemergence, according to the growth stage of the weeds. Preemergent herbicides are applied and incorporated before weeds have emerged, whereas postemergent herbicides control weeds that have already emerged. Some herbicides are effective as both pre- and postemergents. Herbicide combinations frequently are the most effective way to control multiple weed species.
Most preplant herbicides are applied to the soil surface and mechanically mixed into the soil before the crop is planted; these are called preplant incorporated herbicides and require soil moisture for best performance.
Herbicides may be applied to existing beds and incorporated with a power tiller, or applied broadcast, followed by discing and forming raised beds.
It is important to thoroughly incorporate preplant herbicides 2 to 4 inches deep. When incorporating with a disc or harrow, work the soil to double the depth of incorporation desired (i.e., 4–8 inches). Pull disc and harrows across the field twice at right angles. A power incorporator will incorporate to the depth for which it is set. A ground-driven tiller (rolling cultivator) must be set properly to the correct depth for thorough soil mixing and pulled at the proper speed. Two passes are required with a rolling cultivator for proper soil incorporation.
After bean plants have germinated and reached the third to fourth trifoliate leaf stage, cultivation with sweeps, knives, or rolling cultivators can significantly reduce weed growth that may have survived earlier cultural or chemical control methods. Cultivate with care to avoid damaging bean seedling stems or cutting roots; such injuries predispose seedling to soil diseases and retard crop growth.
Late season weed infestations often hinder harvest operations and increase harvesting and cleaning costs. Weed trash can also lower bean quality, and the presence of certain toxic weeds (nightshade berries) can render the beans unsuitable for canning. In addition, weeds produce a lot of seed that remains in the soil and can infest succeeding crops.
Groundcherry, black nightshade, and hairy nightshade berries severely reduce bean quality during harvesting because they do not dry in windrows. During threshing, berry juices stain the beans; the sticky juices on the beans collect dirt and debris during harvest and warehouse handling that cannot be cleaned off. The sticky seeds of these weeds may also slow down or clog the thresher. In storage, moisture from berries can lower quality. Berries left in the field will infest the soil for the following year. If present, pull and carry these weeds out of the field before harvesting.
Carfentrazone (Shark) may be applied to beans at maturity as a preharvest treatment to burn down broadleaf weeds such as annual morningglory, nightshades, pigweed, and lambsquarters. Make applications when the crop is mature and bean have begun to dry down. Complete coverage is essential for best results.
UC IPM Pest Management Guidelines: Dry
W. M. Canevari, UC Cooperative Extension, San Joaquin County
Acknowledgment for contributions to Weeds:C. A. Frate, UC Cooperative Extension, Tulare County