Preemergence Herbicides Delayed the Critical Time for Weed Removal in Soybean

Field trial comparing timing of soybean weed management
Figure 1. (left) Soybean without preemergence herbicide at V6 stage and (right) soybean with preemergence herbicide at V6 stage.

Preemergence Herbicides Delayed the Critical Time for Weed Removal in Soybean May 16, 2018

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 (1st trifoliate), V3 (3rd trifoliate), V6 (6th 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.   

Critical time of weed removal (CTWR): The time in the crop growth cycle when weeds must be controlled to prevent yield losses. Weeds that emerge before or after this period may not present a threat to crop yields.

Graph of critical time of weed control in soybean
Figure 2: Critical time of weed removal in soybean grown with and without PRE herbicides.

Table 1: Critical time of weed removal in soybean, based on 5% yield loss with and without a preemergence herbicide.
TreatmentGDD (SE)¹DAE²Soybean Stage
No PRE Herbicide 156 (27) 13 1st Trifoliate (V1)
Authority Assist (Sulfentrazone + imazethapyr) 504 (267) 43 6th Trifoliate (V6)
Zidua PRO
(Saflufenacil + imazethapyr + pyroxasulfone)
501 (116) 42 6th Trifoliate (V6)

1GDD, growing degree days; SE, standard error in parenthesis
2DAE, days after soybean emergence