In a survey published by the Minnesota Department of Agriculture, it was reported that 84 percent of soybean acres in 2003 were treated with glyphosate. Although the glyphosate system has simplified weed control for many growers, proper timing of herbicide applications is still important in optimizing soybean yields.

Weeds and crops can co-exist for a period of time before competition for light, water and nutrients leads to crop yield losses. The challenge is to time postemergence (POST) herbicide applications before yield losses occur, while at the same time achieving optimal weed control.

The length of time that weeds can remain with the crop without affecting crop yields is defined as the critical period of weed control. Critical periods of weed control occur early in the season with early-emerging weeds, and later on in the season with late-emerging weeds.

The critical period of weed control early in the season for soybeans generally ranges from two to four weeks after planting, but can be shorter or longer depending on a number of factors. Weed population characteristics (i.e. species, density, emergence patterns), cropping practices (i.e. population, row spacing, tillage), and environmental conditions all influence the critical period of weed control.

Increased weed densities and weeds that emerge early in the season or with the crop shorten the critical period of weed control. A foxtail that emerges with the crop would be much more competitive than a foxtail that emerges four weeks after the crop, for example.

Weed species can also differ greatly in competitiveness. For example, research conducted by the University of Nebraska found that four foxtail plants were equal in competitiveness to one velvetleaf plant.

Cultural practices such as row spacing can also influence the critical period. In another study conducted by the University of Nebraska, the critical period in soybeans in 7.5”, 15”, and 30” rows occurred at the V3, V2, and V1 soybean stages, respectively. Soybeans planted in narrow rows were more competitive with weeds than were soybeans planted in wider rows and yield effects due to weed competition were not seen until later growth stages.

Weed height is a critical factor when making POST herbicide applications. Recommended glyphosate rates vary depending on weed height by species, so be sure to check the label based on your conditions. Weed height affects not only the level of control, but also influences potential yield impacts, with larger weeds being more competitive than smaller weeds. Just because glyphosate may give you a wide window of control for many weed species, doesn’t mean you should necessarily take advantage of it if you want to prevent yield losses due to weed competition.

Many factors affect the critical period of weed control, but probably the most important factor is weed density. If your soybean field looks like a solid green carpet of weeds, you should target a POST application very soon. On the other hand, if you can easily see the crop rows and bare soil between them, you have more time to make a POST application, keeping in mind maximum weed heights at application as stated on the herbicide label.

It is very difficult, if not impossible depending your weed population, to achieve optimal weed control and soybean yield with a weed management program consisting of one POST glyphosate application. Research conducted by the Univeristy of Minnesota in 2004 across six locations confirms this point. Optimal yields were obtained in a glyphosate system by using sequential applications or a preemergence herbicide followed by a POST application. Although not included in this study, a timely cultivation can also be effective in the quest for optimal yields and returns.

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