By Stevan Knezevic and O. Adewale Osipitan

Widespread and repeated use of glyphosate in Roundup Ready crops has resulted in 17 weed species becoming resistant to glyphosate in the U.S., confirmed as of 2017. At least six of these species are in Nebraska. These numbers reinforce the need to diversify weed control programs to reduce reliance on glyphosate.

University of Nebraska-Lincoln researchers hypothesized that the use of preemergence (pre) herbicides would delay the first, and possibly only, application of glyphosate; however, they did not know the extent of the delay. So, researchers conducted an experiment at Concord in northeast Nebraska in 2017 to evaluate the influence of preherbicides on the critical time for weed removal with glyphosate in Roundup Ready corn. Three herbicide regimes — no pre and pre-applications of Acuron and of atrazine — were compared.

Acuron contains four active ingredients: atrazine, bicyclopyrone, mesotrione and s-metolachlor. Acuron was applied at 3 quarts per acre, while atrazine was applied at 2 quarts per acre, immediately after planting corn. Weed removal timings with glyphosate were: third (V3), sixth (V6), ninth (V9), 12th (V12) and 15th (V15) corn leaf stages. There were also season-long weed-free and weedy plots. The predominant weed species were common waterhemp, velvetleaf and green foxtail.

CRITICAL PERIOD DELAYED: This corn at V12 received an atrazine treatment. Without a preherbicide, the critical time of weed removal started at V3. With a pre-application, it was delayed to the V5.

The critical time of weed removal with glyphosate was delayed when preherbicides were used. Based on a 5% acceptable yield loss threshold, the critical time for weed removal ranged from 144 to 375 growing degree days (GDD), which corresponded to the V3, V5 and V10 corn growth stages, depending on the herbicide regime (see chart).

Critical time of weed removal in corn based on 5% yield loss with and without preherbicide.

For example, without preherbicide, the critical time of weed removal started at V3 growth stage, while with pre-application of atrazine, the critical time was delayed to the V5 growth stage. Pre-application of Acuron further delayed the critical time of weed removal to the V10 growth stage, which was approximately six days to time of corn canopy closure.

FURTHER DELAY: Corn at V12 received an Acuron treatment. Pre-application of Acuron further delayed the critical time of weed removal to the V10 growth stage, which was about six days to time of corn canopy closure.

Weeds that emerge past the time of canopy closure are typically not competitive enough to impact corn yields. UNL researchers confirmed that preherbicides delayed the need for post application of glyphosate and provided alternative modes of action for fighting glyphosate-resistant weeds in corn.

Osipitan is a postdoctoral researcher, and Knezevic is an Extension weed management specialist at the University of Nebraska-Lincoln. This report comes from UNL CropWatch.