WEED SCIENCE Sequential and Co-Application of Glyphosate and Glufosinate in Cotton

Glufosinate controls GR Palmer amaranth (Amaranthus palmeri S. Wats.), but might be less effective than glyphosate on certain weeds. Glyphosate and glufosinate applications in tolerant cotton (Gossypium hirsutum L.) cultivars can broaden the spectrum of control and aid in resistance management. Research investigating sequential application and potential interactions with co-applications of glyphosate and glufosinate is limited. Field research at six locations evaluated Palmer amaranth and annual grass control with glyphosate and glufosinate co-applied or sequentially applied. Herbicides were applied to two-leaf cotton and repeated 14 d later. A GR biotype comprised 10 to 90% of the Palmer amaranth populations. Greater Palmer amaranth control was achieved following sequential applications of glufosinate compared with glyphosate. Co-application of glufosinate plus glyphosate was more effective than glyphosate alone at most locations but never more effective than glufosinate alone. Glyphosate controlled goosegrass (Eleusine indica [L.] Gaertn.) more than glufosinate and a similar response was observed for large crabgrass (Digitaria sanguinalis [L.] Scop.). Glufosinate and glyphosate co-applied were less effective than glyphosate alone on both grasses, but not more effective than glufosinate alone. Glufosinate followed by (fb) glyphosate was the preferred order for sequential application to control Palmer amaranth at most locations. With high percentages of the GR biotype, glufosinate applied sequentially was more effective than any other sequential applications. Glyphosate fb glufosinate and glufosinate fb glyphosate controlled large crabgrass similar to glyphosate applied sequentially and greater than glufosinate applied sequentially. For goosegrass, glyphosate fb glufosinate was more effective than the reverse. Seven days after the second postemergence application, sequential application of glyphosate fb glufosinate controlled goosegrass as well as glyphosate applied sequentially and better than glufosinate applied sequentially.