Crop rotations and cultural practices used to produce each crop influence the weed spectrum in cultivated or tilled fields. Annual weeds that grow and produce seed quickly, or perennials such as yellow nutsedge that are distributed by cultivation and plowing, often predominate in cultivated fields. Also, repeated use of the same or similar herbicides will select for resistant weed species, increase the number of tolerant weed biotypes, and change the population of soil microorganisms that degrade particular herbicides, resulting in shorter soil persistence.
Preventing weed shifts Weeds that survive plowing, cultivation, repeated herbicide treatments, or other routine cultural practices must be eliminated before the tolerant species or biotypes become established. Combine a variety of weed control practices or treatments, rotate practices and herbicides, and spot-treat with a hoe or registered herbicide when the weed first appears. Also, clean your equipment when moving from an infested field.
Weed management in annual cropping systems Successful weed management in annual cropping systems requires a year-round approach using and rotating a combination of weed control practices throughout several years. Developing these strategies requires knowledge of specific weeds that infest your land. Identify and map major weed species and infested patches within each field. With an established point of reference and occasional observations, growers can evaluate weed shifts and adjust crop and weed management strategies before resistant weeds predominate.
Planning weed control options After accurate identification, plan an efficient year-round weed and crop management program by listing previous rotations, possible herbicide residues, and effective control measures for each weed. Consider the strengths and weaknesses of each control method based on your experience, herbicide labels, and local experts including pesticide dealers, consultants, and Extension agents. Examine the information carefully and select a field, soil conditions, weed infestations, and appropriate control methods compatible with the crop you intend to plant.
Field preparation and planting Eliminate perennial weeds before planting, by designing a selective control program in the previous crop or by controlling the weed during a temporary fallow period. Canada thistle, for example, can be controlled with spot treatments of glyphosate (Roundup or similar product) in other crops; S-metolachlor (Dual Magnum or Pennant Magnum) or halosulfuron (Permit or Sandea) reduces yellow nutsedge infestations in other crops. During field preparation, destroy all weedy vegetation and prepare a reasonably smooth surface for uniform herbicide application. Wet soils or delayed applications following the last soil disturbance often result in erratic weed control.
Early and midseason scouting While assessing crop emergence, soil moisture, disease incidence, and other factors, evaluate the performance of the preplant or preemergence weed control treatments. Note field conditions where gravel or a low spot may have caused abnormal weed control or possible crop injury. About midseason, carefully map each field by identifying individual weeds, including patches of perennial weeds. Determine whether additional control measures such as cultivation, application or spot spraying of a postemergence herbicide, or hand-hoeing individual weeds will be necessary to achieve a quality product at harvest yet minimize the chance of allowing a weed shift.
Preharvest scouting and postharvest weed control Verify the location of perennial weed infestations on your field map and note additional weed species since your midseason evaluation. Soon after harvest, destroy existing weeds and crop stubble to reduce unnecessary increases in pest populations. If perennial weeds were present, maintain optimum growing conditions so appropriate herbicides can be applied to actively growing weed foliage for maximum control. Consider factors such as timing of herbicide applications and stage of weed growth, herbicide persistence in the soil, crop rotations, and label restrictions for subsequent crops when selecting a postharvest herbicide for perennial weed control. Following the postharvest treatment, consider planting a rotation crop such as winter grains or other crops that require significantly different cultural practices.
Note Herbicides must be applied at the correct rate and time to selectively control weed growth with minimal chance for injury to the crops. Obtain more consistent results by reading the herbicide label and other information about the proper application and timing of each herbicide. Suggested rates listed in this guide are stated as pounds active ingredient per acre (lb ai/A) or pounds acid equivalent per acre (lb ae/A). For band applications over bulb row, reduce quantity of herbicide applied proportionately to the area within the row actually sprayed.