Understanding the critical time for weed removal in corn and soybean 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 soybean started at the early crop stage, as it is well known that early emerging weeds are the most competitive against the crop. This implies that pre-emergence weed control tactics would help control early emerging weeds and possibly delay the need for post-emergence (POST) weed control inputs, including post-application of glyphosate in glyphosate-tolerant (GT) corn and soybean. We therefore conducted a field study in 2017 at Concord to evaluate the influence of pre-emergence (PRE) herbicides on the critical time for weed removal in glyphosate-tolerant (Roundup-Ready) corn and soybean.  

Corn

The three herbicide regimes tested in corn were: 1) No PRE, 2) Pre application of atrazine, and 3) PRE application of Acuron^®. Atrazine was used at 2 quarts/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/acre. The predominant weed species were common waterhemp, velvetleaf, and green foxtail.

The critical time for weed removal in corn was significantly delayed by the use of PRE-herbicides (Figure 1). 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 closer are typically not competitive enough to impact corn yields. Thus, we confirmed 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 also provided alternative modes of action for fighting glyphosate-resistant weeds in glyphosate-tolerant corn.

Soybean

The three-herbicide regime in soybean consisted of two PRE herbicides (Authority Assist^® [12 oz/acre] and Zidua PRO^® [6 oz/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 soybean. The predominant weed species were common waterhemp, green foxtail, velvetleaf, and common lambsquarters.

The critical time for weed removal in soybean was delayed by the PRE herbicides (Figure 2). 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 501-504 growing degree days, which corresponded to V6 soybean growth stage (Table 2).

Practical Implications

It is clear that 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, our hope is that the use of PRE herbicides with multiple modes of action would aid in managing glyphosate resistant weeds.