Early-season Weed Control in Roundup Ready 2 Xtend® Soybean

Trial Objective

• Starting weed-free at planting is important for season-long weed control and maximum yield potential.
• Most kochia emerges in early spring with subsequent smaller flushes throughout the growing season, while Palmer amaranth emerges later in the year for an extended period of time.
• When environmental conditions are challenging for weed control prior to planting, failure of burndown control of kochia can occur, which could result in kochia present at planting time.
• The objective of this study was to determine the best strategy for managing kochia that escaped a burndown herbicide treatment in soybean with planting-time and early post-emergence applications.

Research Site Details

• The trial was arranged in a randomized complete block design with three replications.
• Kochia was treated with a blanket application of 4 fl oz of XtendiMax® herbicide with VaporGrip® Technology (approximately 1/5 the labelled application rate) two weeks prior to planting to simulate a lack of control scenario.
• Kochia was approximately 3-6 inches in height at the pre-application timing (PRE).
• Early post-emergence applications (POST) were applied at the V2 growth stage. At this time, kochia was approximately 9-12 inches in height and Palmer amaranth was approximately 4 inches in height.
• Rainfall one day after the POST application (Table 1) resulted in a new flush of Palmer amaranth.
• Some volunteer corn was present in the trial (Figure 5, 7, and 8).

Understanding the Results

• At 21 days after the PRE treatment, kochia and Palmer amaranth control was significantly greater in plots that received a treatment with XtendiMax® herbicide with VaporGrip® Technology + Roundup PowerMAX® herbicide + a residual herbicide (Treatments 2, 3, and 4) compared to XtendiMax® with VaporGrip® Technology + Roundup PowerMAX® herbicide alone, indicating that Tricor® 4F and Valor® SX contributed to better control of kochia (Figure 1).
• Kochia control was significantly greater when using XtendiMax® herbicide with VaporGrip® Technology + Roundup PowerMAX® herbicide compared to paraquat or parquat + metribuzin (Treatments 5 and 6) at 48 days after the PRE treatment (Figure 2).
• The highest level of Palmer amaranth control at 48 days after the PRE treatment resulted from XtendiMax® herbicide with VaporGrip® Technology + Roundup PowerMAX® herbicide + either Valor® SX or Warrant® Herbicide + Tricor® 4F (Treatments 2 and 4) (Figure 2).
• Including Warrant® Herbicide or Warrant® Ultra Herbicide with XtendiMax® herbicide with VaporGrip® Technology + Roundup PowerMAX® herbicide in the POST treatment (Treatments 8 and 9) resulted in a significant increase in Palmer amaranth control 21 days after the POST application (Figure 2).

What Does This Mean for Your Farm?

• Starting clean with an effective burndown herbicide and including a residual herbicide at planting resulted in the best control of kochia and Palmer amaranth.
• Including a residual herbicide in post-emergence applications significantly improved Palmer amaranth control.
• Delaying the herbicide application until the V2 growth stage resulted in applications to large kochia plants and a dense canopy of Palmer amaranth plants, which reduces the effectiveness of post-emergence herbicides.