Understanding the critical time of weed removal (CTWR) would not only ensure effective weed removal and minimize unnecessary weed control inputs, but also could help minimize development of herbicide resistance in weeds. Most studies have shown that the critical time of weed removal in soybean starts at very early crop stages (V1 to V3; equivalent to 5-15 days after soybean emergence) as early-emerging weeds are known to be the most competitive with soybean.

It is well known that preemergence (PRE) herbicides control early emerging weeds and delay the need for postemergence (POST) weed control inputs, including POST application of glyphosate in Roundup Ready soybean. Therefore, we conducted a field study in 2017 at Concord to evaluate the influence of PRE herbicides on the critical time of weed removal (CTWR) in glyphosate-tolerant (Roundup Ready) soybean (Figure 1).

We tested how the two PRE herbicide programs (Authority Assist^® [12 oz/acre] and Zidua PRO^® [6 oz/acre]) influenced the CTWR compared to soybeans with no PRE herbicides.

Authority Assist^® is comprised of sulfentrazone and imazethapyr, while Zidua PRO^® contains saflufenacil, imazethapyr, and pyroxasulfone. PRE herbicides were applied immediately after planting soybean. Weed removal timings were done at V1 (1^st trifoliate), V3 (3^rd trifoliate), V6 (6^th trifoliate), R2 (full flowering), and R5 (beginning seed) soybean growth stage, as well as weed free and weedy season long.  Weed species composition included common waterhemp (48%), green foxtail (27%), velvetleaf (13%) and common lambsquarters (12%). The critical time of weed removal was estimated based on a 5% yield loss threshold.

Results

Both PRE herbicides delayed the CTWR (Figure 2). This was expected as the PRE herbicides helped control early-emerging weeds, thereby delaying the need for postemergence weed control with glyphosate. Without the PRE herbicide, the CTWR started at 156 growing degree days after crop emergence, which corresponded to the V1 soybean growth stage (Table 1). Such an early start of weed competition triggers the need for multiple glyphosate applications, which is basically what many farmers had done over the last 20 years. In turn, this led to the development of glyphosate-resistant weeds.

On the other side, PRE application of either Authority Assist or Zidua PRO delayed the CTWR  until 501-504 growing degree days, which corresponded to the V6 soybean growth stage. It is clear that the two PRE herbicide programs tested protected soybeans until the V6 stage, delayed the need for POST application of glyphosate, and provided an alternative mode of action to manage glyphosate resistance.