Even though use of glyphosate in glyphosate tolerant crops has simplified weed management for many growers, paying attention to details, like application timing, can have a significant impact on economic returns.

Researchers at the University of Minnesota evaluated glyphosate programs in both corn and soybeans over the past three years at four to six locations per year. In corn, the best economic returns were achieved when a one-half rate of a preemergence (PRE) herbicide was followed with a postemergence (POST) glyphosate application when corn was in the three- to four-leaf growth stage (V3-V4).

Corn is sensitive to early-season weed competition. When no PRE herbicide was used, a delay of four to seven days after weeds exceeded 4-5 in. in height resulted in a yield loss of 12 bu./acre. Delaying glyphosate application until weeds were 8-9 in. tall resulted in an average yield loss of 27 bu./acre. One-pass glyphosate programs did not maximize yields or economic returns in corn.

The two-pass glyphosate program in corn, where glyphosate was applied at 3-in. weeds and then later at 2- to 4-in. weed regrowth, resulted in optimal yields and economic returns. This program, however, has higher risk since timely applications may not always be possible depending on factors such as the weather and the number of acres you farm.

In contrast to corn, best economic returns were achieved in soybeans when glyphosate was applied to 3-inch weeds and then to 2- to 4-inch weed regrowth. Use of a one-half rate of PRE herbicide followed by POST glyphosate when soybeans were in the two- to three-leaf growth stage (V2-V3) optimized yields but not economic returns. This was due to the higher relative price of the PRE herbicide evaluated compared to glyphosate. Keeping this in mind, use of a PRE herbicide in soybean may enhance control of troublesome weeds, such as common waterhemp and lambsquarters, and is a good tactic to help in resistance management.

When only one application of glyphosate was applied to soybeans in 30-in. rows, yield was reduced more when glyphosate was applied too early (weed heights less than 5 in.) than too late (weed heights of 7 and 9 in.). This was mainly due to competition from broadleaf weeds that emerged after application. Although it was possible to optimize yield and economic returns with one application of glyphosate, this practice is not encouraged due to the high risk involved with getting this timing just right.

urther details about this research can be found by visiting the University of Minnesota Applied Weed Science website at http://appliedweeds.coafes.umn.edu/.