Managing Glyphosate-Resistant Palmer Amaranth
by Larry Steckel
SITUATION: The problem of glyphosate-resistant Palmer amaranth is not yet extensive in Tennessee, but the threat is critical to conservation-tillage practices there. One field was also suspected to have waterhemp resistance.
DISTRIBUTION: As of 2007, about a dozen fields in three counties have glyphosate-resistant Palmer amaranth. As in other states, some of these populations manifest as small patchy areas in the field. At least one patch is suspected to have originated in a cotton module location. The Tennessee resistant Palmer amaranth is considered to have a low level of resistance, most a 1.5 to 5X level. Plants appear to recover in two ways: some regrow from axillary buds; some plants ‘shut down’ for a few days, then resume growth. Tennessee also is concerned about resistance in giant ragweed (Ambrosia trifida), waterhemp, and horseweed (Conyza canadensis).
IMPACTS: Glyphosate-resistant Palmer amaranth is present in both cotton and soybean. Some producers rotated from cotton to corn and used atrazine applied to 2-inch Palmer amaranth for control. However, it is difficult to apply to 2-inch weeds in large cropping areas with rapidly growing weeds.
- Determination of mechanism of resistance: Tennessee researchers suspect that the mechanism of resistance is absorption/translocation, although more C14 work is needed.
- Management of resistant populations in both cotton and soybean: Liberty Link® systems are one focus for management. Glufosinate (Liberty®, Ignite®) plus flumioxazin (Valor®) post-directed under hoods is a promising treatment.
- Comparative identification of Palmer amaranth and waterhemp: Because one population of waterhemp is suspected to be resistant to glyphosate, one challenge is identification, especially with male plants.
- Continued screening for resistant populations.