When properly used, postemergence weed management helps secure crop yields. For example, postemergence herbicides can be applied in any soil type. They are not usually affected by soil moisture, and applications are based on the weeds already present in the field. However, heavy reliance on postemergence herbicides can injure crops, be ineffective, or select for herbicide resistant biotypes. Cultivation at the wrong time also may result in poor weed control as well as result in soil erosion. Proper postemergence weed-control programs require considerable knowledge and management. This article summarizes options and may help you get the most from your postemergence weed management practices.
Timing is critical. Weeds usually emerge throughout the early part of the summer, not just after planting. If postemergence herbicides or cultivation are applied right after planting, late-emerging weeds may still significantly reduce crop yields. Moreover, these late-emerging plants can produce seeds that create problems in the years to come. However, if the cultivation or herbicide application is too late, yield could be compromised by weeds that emerged shortly after planting. Thus, you should time your management practices to control both early and late emerging weeds. Periodic scouting of your crop fields is one of the best policies to secure that management practices are implemented at the precise time.
Control weeds at the correct stage. Plants develop through several growth stages: seed, seedling, vegetative, flowering and maturity. If cultivation or postemergence herbicides are not used during the optimum stage, the results are usually unsatisfactory. As a general rule, weeds are more susceptible at the seedling stage when rapid growth takes place. Spraying large weeds may burn off leaves, but the plant may regrowth. Similarly, cultivation of large weeds may not be very effective. Again, scouting your fields will help you decide when and how you should control weeds.
Minimize crop injury. Herbicides can damage or kill your crops. When excessive herbicide is used, either due to inaccurate application or unfavorable environmental conditions, injury may occur. As a general rule, postemergence herbicides tend to have better crop tolerance than soil-applied products. However, as field crops grow and mature their tolerance to postemergence herbicide changes. For example, barley is very sensitive to several postemergence herbicides such as Puma, Hoelon, and Achieve. To minimize crop injury, these herbicides should be applied prior to crop jointing.
Check the weather before applying herbicides. Temperature greatly influences the effectiveness of several postemergence herbicides. Generally, herbicides should be applied when temperatures range between 65 degrees Fahrenheit and 80 degrees F. Do not apply herbicides such as Puma and Achieve when night time temperatures approach freezing; this can result in reduced weed control due to low plant metabolism. If possible, schedule postemergence herbicide applications for late afternoon to reduce the risk of crop injury due to temperatures above 80 degrees F).
Dry weather can also affect the effectiveness of postemergence applications. Weeds respond to moisture stress by thickening their leaf cuticle and reducing vegetative growth. It is difficult to control these drought-stressed plants because of reduced herbicide absorption and low physiological activity.
Minimize herbicide drift. Wind and air currents can carry suspended herbicide droplets or volatilized herbicides beyond the treated area. This drift of pesticide through the air to areas not targeted for treatment, may injure quite distant crops and is one of the greatest hazards with many herbicides. To minimize herbicide drift, it is important to carefully choose the size of the droplets, the boom height and the nozzle type. For example, you can reduce particle drift by using nozzles that produce coarse spray droplets or by reducing the spay pressure. Also, avoid spraying on windy days and always spray when the wind moves away from any susceptible crop. Reading the “Environmental Hazards” and “Directions for use” sections of the herbicide label can help you minimize the risk of crop injury due to herbicide drift.
Rotate herbicides. In Montana, the shift towards reduced tillage or no-tillage systems has increased our reliance on herbicides. While this approach to farming has benefits in terms of reducing soil erosion and energy use, it increases the potential for selection of herbicide resistant weeds. Every time you use a herbicide, you are imposing a selective pressure on the weed population. The more effective the herbicide and the more consistently it is used, the greater the selection pressure, and the faster the occurrence of resistance. The use of herbicides with different site of action, applied either as tank mixes, premix formulations, or sequential applications, can help in managing resistance.
Develop a multiple-tactic weed management program. The most effective approach to manage weeds and reduce the development of herbicide resistant populations is through the integration of cultural, mechanical and chemical control practices. For example, rotation of crops with different life cycles (e.g., winter annuals such as winter wheat or barley, perennials such as alfalfa, summer annuals such sunflower, proso millet and corn) allow you to select herbicides with different sites of action. Also, diverse rotations involve a diversity of weed management practices that create multiple opportunities to manage weeds with a wide range of tactics. This multiple-tactic approach reduces the rate of weed adaptation and may translate into lower operating costs and greater net returns.
Disclosure. Common chemical and trade names are used in this publication for clarity by the reader. Inclusion of a common chemical or trade name does not imply endorsement of that particular product or brand of herbicide and exclusion
Categories: Postemergence Herbicide, Integrated Weed Management
Date: 04/22/2005