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Integrating Fall Management Practices with Herbicide Programs to Manage GR Palmer Amaranth

Palmer amaranth is one of if not the most problematic weed in Midsouth row crop systems, and its detriment to crop systems has been magnified by the widespread presence of GR Palmer amaranth. This has been exacerbated by Palmer amaranth’s production of a large amount of seed and thus establishment of a large seedbank in production fields. Thus, researchers have been impelled to investigate how to integrate year-round practices into a management program to combat this weed.

One such study was conducted in Arkansas from fall 2010 through fall 2013, with the objective of elucidating how in-season herbicide programs and post-harvest management practices will impact Palmer population density and seed production over the 3-year period of the study. The results of this research are reported in an article entitled “Integrating Herbicide Programs with Harvest Weed Seed Control and Other Fall Management Practices for the control of Glyphosate-Resistant Palmer Amaranth” which appeared in Weed Science, Vol. 64:540-550 (2016). The premise upon which the research was based is that preventing weed seed inputs into the soil seedbank could prove effective in reducing the impact of weeds on the following crop.

The experiments were conducted on a Sharkey clay at Keiser, Ark. The experimental site had been previously cropped with GR soybean in 2009 and 2010. They determined that this site contained a dense, uniform stand of GR Palmer amaranth at soybean maturity in the fall of 2010.

The researchers evaluated 6 fall management strategies and 3 herbicide treatments as follows.

Fall Management Treatments

No-till, soybean residue spread and retained with no fall tillage

Raised beds reformed immediately following harvest with normal spread of soybean residues

Rye cover crop drill seeded at 80 lb./acre, glyphosate applied for burndown prior to soybean planting

Narrow windrowing of harvest residues, followed by burning

Narrow windrowing of harvest residues without burning

Harvest residue collected and removed from field

Herbicide Treatments/Program*

Glyphosate-only: Glyphosate applied at soybean stage V2 followed by (fb) glyphosate applied at stage V7

Glyphosate + residuals: Valor (flumioxazin) PRE fb glyphosate + Prefix (S-metolachlor + fomesafen) applied at V2 fb glyphosate applied at V7

Glufosinate + residuals: Valor PRE fb Liberty (glufosinate) + Prefix applied at V2 fb Liberty applied at V7

*Glyphosate applied 2-3 weeks prior to planting soybeans to control weeds and rye cover crop, and paraquat applied immediately after planting. Site furrow irrigated as needed.

Results from this study follow.

    Across the 3 years, residue collection and removal, rye cover crop, and windrowing with burning were the most effective fall treatments for reducing Palmer amaranth population density. The residue collection and removal treatment had the lowest weed density, followed by windrowing with burning and rye cover crop, which were equal in their reduction effect. Fall tillage (bedding) was erratic in its effect on weed density.

    The application of glyphosate in combination with PRE herbicides with sites of action different from that of glyphosate resulted in large reductions in Palmer amaranth population density and seed production in all fall management treatments compared to the glyphosate-only treatment, but the reductions were not as great as those following the application of PRE herbicides fb glufosinate. The glufosinate herbicide program resulted in little to no Palmer amaranth plants or seed production across fall management treatments.

    Rye cover crop in the glyphosate-only herbicide treatment resulted in Palmer amaranth population densities and seed production that were significantly below those in the no-till treatment with glyphosate-only herbicide.

    The reduction in Palmer amaranth plants and seeds resulting from the rye cover crop was enhanced by PRE residual herbicides and POST glufosinate.

    These results point to: 1) integrating fall management practices into an effective weed management program that uses PRE residual herbicides fb POST herbicides to reduce Palmer amaranth plant densities and seed production; 2) using a cereal rye cover crop to reduce weed densities; 3) combining herbicides with different sites of action; and 4) using a glufosinate-based weed control program to effectively control GR Palmer amaranth.

It is important to note that utilizing only 4) above will potentially lead to the development of glufosinate-resistant weeds. Thus, it is important that supplemental weed control practices such as the rye cover crop and residue destruction used in this study be considered to supplement herbicide control so that weed seed production is minimized. In the authors’ words “Farmers should broaden and diversify their weed control options by incorporating harvest weed seed control strategies that target Palmer amaranth escapes at crop harvest or integrate a fall-planted cover crop into current production ultimately reduce the soil seedbank”.

Special note: Even small contributions to remedying the problem of GR Palmer amaranth, especially in the reduction of the number of seed that are produced, may be worth the cost and risk in the long run if the useful life of effective current herbicides is to be prolonged.

Composed by Larry G. Heatherly, Oct. 2016,