Weed Control Guide for Ohio Field Crops

Management of Soybean Herbicide Programs

A number of broad-spectrum preemergence (PRE) and postemergence (POST) herbicides are available for use in soybeans. In OSU research, almost any type of approach to herbicide management can be used in fields with low weed pressure with little risk of crop yield loss. These approaches include: total PRE, PRE followed by POST, and total POST. However, the biology of some weeds that occur in Ohio soybean fields, in addition to the slow early development of no-till soybeans, can make it difficult to achieve effective weed control with a single application of PRE or POST herbicides. For this reason, a PRE followed by POST program or a two-pass POST program often provides the most consistent control.

Weeds that are especially problematic in a total PRE herbicide program, depending upon their population, include annual grasses, giant ragweed, ALS-resistant common ragweed, marestail, annual morningglory, cocklebur, waterhemp, and most perennial weeds. Another major drawback to PRE herbicides applied at planting is the narrow window of time in which at least 0.5 to 1 inch of rain must occur to move herbicide into the soil. In early May when soybeans are often planted, weeds will typically start to emerge within 8 to 10 days after tillage or an application of glyphosate or paraquat. Rain is needed before these weeds emerge in order for PRE herbicides to be effective. Applying herbicides several weeks or more before planting of no-till soybeans often results in more consistent weed control initially, although herbicide activity may not last as long after planting, compared to application at the time of planting.

Weeds that can be problematic in a one-pass, total POST herbicide program, depending upon their population, include annual grasses, giant ragweed, waterhemp, lambsquarters, marestail, and some perennials (because they may be too small at the time of an early POST application). Most POST soybean herbicides should be applied before weeds exceed 3 to 6 inches in height for consistent control and to avoid crop yield loss. Glyphosate application can be timed for weed heights of 4 to 8 inches, although velvetleaf, lambsquarters and some other weeds are more easily controlled when less than 6 inches tall. Problems with proper timing of a single postemergence application include: 1) giant ragweed grows at approximately twice the rate of most annual weeds, and is likely to be 8 to 12 inches tall when other weeds are 3 to 6 inches tall; 2) wet and/or cold weather can prevent timely application and result in the need to treat many acres within a short period of time once favorable weather returns, and 3) when weed populations continue to emerge after the POST application and are not suppressed by the soybean leaf canopy, a second POST application may be needed.

A planned PRE plus POST approach will provide more consistent weed control than any one-pass approach in many fields, and help solve some of the problems in management of POST herbicides. The most complete PRE plus POST program includes use of a PRE herbicide with activity on key broadleaf weeds that also provides at least some early-season control of grasses, followed by a POST treatment with activity on grass and broadleaf weeds. In many cases, early-spring application of a PRE herbicide with 2,4-D ester in no-tillage fields can reduce or eliminate the need for glyphosate or paraquat, in addition to providing residual weed control. The PRE herbicide can control or reduce the population of some problem broadleaf weeds, such as lambsquarters, waterhemp, and giant ragweed, making it relatively easy to control any later-emerging weeds with the POST treatment. A major advantage of the PRE plus POST approach, compared to total POST, is that the PRE herbicide will often provide enough weed control to prevent major problems if weather delays the POST application. Use of the PRE plus POST approach to allow for a slightly delayed POST application can result in more consistent control of late-emerging weeds such as foxtails, giant ragweed, black nightshade, waterhemp, and perennials.

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