By Stevan Knezevic,

O. Adewale Osipitan

and Jon Scott

Understanding the critical time for weed removal in corn and soybeans would not only ensure effective weed removal and minimize unnecessary weed control inputs but could potentially help minimize the development of herbicide resistance in weeds.

Most studies have shown that the critical time for weed removal in corn and soybeans starts at the early crop stage, as it's well-known that early-emerging weeds are the most competitive against the crop.

This implies that preemergence weed control tactics would help control early-emerging weeds and possibly delay the need for postemergence weed control inputs, including post-application of glyphosate in glyphosate-tolerant corn and soybeans.

In 2017, University of Nebraska-Lincoln researchers conducted a field study at the Haskell Ag Lab at Concord to evaluate the influence of preemergence herbicides on the critical time for weed removal in glyphosate-tolerant (Roundup-Ready) corn and soybeans.

Corn
The three-herbicide program tested in corn was: 1) no pre-application, 2) pre-application of atrazine, and 3) pre-application of Acuron.

Atrazine was used at 2 quarts per acre and was considered a single-active or "basic-blanket treatment." Acuron, which was considered a "loaded treatment" as it contains four active ingredients (atrazine, bicyclopyrone, mesotrione and s-metolachlor), was applied at 3 quarts per acre. The predominant weed species were common waterhemp, velvetleaf and green foxtail.

The critical time for weed removal in corn was significantly delayed by using pre-herbicides. For example, without a pre-herbicide, the critical time for weed removal started at the V3 growth stage, but with a 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 V10 growth stage, which was close to the time of corn canopy closure (Table 1). Weeds that emerge past the time of canopy closure typically are not competitive enough to affect corn yields.

Table 1: Critical time of weed removal in corn based on 5% yield loss with and without pre-herbicide. ¹GDD = growing degree days; SE = standard error in parenthesis; ²DAE = days after corn emergence. Source: UNL CropWatch

This confirms that pre-herbicides delayed the need for early weed removal in corn. Furthermore, the use of products with multiple active ingredients and/or multiple modes of action has proven to be effective in further delaying the need for post-weed control programs and provided alternative modes of action for fighting glyphosate-resistant weeds in glyphosate-tolerant corn.

Soybeans
The three-herbicide program in soybeans tested two pre-herbicides, Authority Assist (12 ounces per acre) and Zidua PRO (6 ounces per acre), against the no pre-application of herbicide.

Authority Assist contains sulfentrazone and imazethapyr, while Zidua PRO contains saflufenacil, imazethapyr and pyroxasulfone. Both pre-herbicides were applied immediately after planting soybeans. The predominant weed species were common waterhemp, green foxtail, velvetleaf and common lambsquarters.

The critical time for weed removal in soybeans was delayed by the pre-herbicides. Without the application of a pre-herbicide, the critical time of weed removal started at 156 growing degree days, which corresponded to V1 soybean growth stage (Table 2).

Meanwhile, pre-application of either Authority Assist or Zidua PRO delayed the critical time of weed removal until 501 to 504 growing degree days, which corresponded to V6 soybean growth stage (Table 2).

Table 2: Critical time of weed removal in soybean based on 5% yield loss with and without pre-herbicide. ¹GDD = growing degree days; SE = standard error in parenthesis; ²DAE = days after corn emergence. Source: UNL CropWatch.

Practical implications

The use of a pre-herbicide protected corn and soybean yield by preventing early-season weed emergence and competition. This also delayed the need for weed removal and post-weed control programs. More importantly, researchers hope the use of pre-herbicides with multiple modes of action would aid in managing glyphosate resistant weeds.

Knezevic is a Nebraska Extension weed management specialist. Osipitan is a post-doctoral researcher at the University of Nebraska-Lincoln. Scotts is a weed science technologist at UNL. This report comes from UNL CropWatch, which is solely responsible for the information provided and is wholly owned by the source. Informa Business Media and all its subsidiaries are not responsible for any of the content contained in this information asset.