Winter cover crops influence Amaranthus palmeri establishment

Authors: Webster, Theodore; Scully, Brian; GREY, TIMOTHY - University Of Georgia; CULPEPPER, A - University Of Georgia

Submitted to: Crop Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/20/2013
Publication Date: 6/1/2013
Citation: Webster, T.M., Scully, B.T., Grey, T.L., Culpepper, A.S. 2013. Winter cover crops influence Amaranthus palmeri establishment. Crop Protection Journal. 52:130-135.

Interpretive Summary: Palmer Amaranth is a weed species that has developed herbicide resistance and spread throughout the southeastern U.S. It is a particular problem in cotton production and a number of tactics are under development to mitigate the problem. This research examines the benefit of growing different winter cover crops ahead of the summer cotton crop, with the expected benefit of weed suppression. A greater research effort is needed to devise weed management options for row crops that do not rely exclusively on the diminishing array of herbicide tools. Four winter legumes, including narrow-leaf lupine, crimson clover, Austrian winter pea, and cahaba vetch, were grown in a monoculture and in a mixture with rye, the common winter cover crop for southeast coastal plain. Overall, the rye and legume mixtures reduced Palmer amaranth populations in the following cotton crop by over 60%. In late-June when weed management in cotton is critical, the preceding rye/Austrian pea winter crop had suppressed Palmer amaranth growth by >80% relative to a fallow treatment. Weed suppression from rye mixed with crimson clover, cahaba vetch or narrow-leaf lupine ranged from 64 to 70%. In spite of this suppression from winter crops, the number of Palmer amaranth plants in the cotton rows still required the use of supplemental herbicides to achieve optimal weed management. Where cotton received supplemental herbicide for Palmer amaranth control, the highest yields occurred behind rye, narrow-leaf lupin/rye mixture, and Austrian pea/rye mixture. When crimson clover or cahaba vetch were grown in a mixture with rye their biomass yields were more than doubled as compared to monoculture, while narrow-leaf lupine and Austrian pea mixed with rye produced an increased yield of 49 and 90%, respectively. The rye and narrow-leaf lupine mixture produced the highest experimental yield of nearly 7,000 kg ha-1, which was significantly higher than any other of the combinations.

Technical Abstract: Winter cover crops were evaluated for their effect on Palmer amaranth (PA) suppression in cotton production. Cover crops examined included rye and four winter legumes: narrow-leaf lupine, crimson clover, Austrian winter pea, and cahaba vetch. Each legume was evaluated alone and in a mixture with rye. Cover crop biomass in monoculture was greatest for rye and narrow-leaf lupine (>6,750 kg ha-1), while clover, pea, and vetch were less and ranged from 2,810 to 4,610 kg ha-1. Cover crop biomass was more than doubled when rye was mixed with clover or vetch relative to the legume monoculture. Rye mixtures with lupine and pea increased biomass 49 and 90%, respectively, compared to each legume alone. PA populations in the strip-tilled cotton row ranged from 144 to 404 seedlings m-2; however, densities were lower (46 seedlings m-2) in the non-disturbed areas between cotton rows. PA populations in cotton row middles with rolled vetch or pea were <4 seedlings m-2, while PA populations in clover and lupine were 18 and 29 seedlings m-2, respectively. Rye and legume mixtures suppressed PA populations to <3 seedlings m-2, while rye monocultures had 7.5 seedlings m-2. By late-June, rye and winter pea suppressed PA growth in the row middle >80% relative to the non-cover crop fallow treatment, while suppression from clover, vetch and lupine ranged from 64 to 70%. In spite of this suppression from winter crops, the level of PA plants in the cotton rows prevented yield production where herbicides were not applied. Where PA was controlled with herbicides, the highest yields occurred following rye, with lower yields following lupine/rye mixture and treatments including pea. Management of herbicide resistant weed species requires diverse management tactics. One component of this management could be the use of high-biomass cover crops, which reduced weed establishment in between cotton rows. However, greater research effort is needed to devise weed management options for the crop row that do not rely exclusively on the diminishing array of herbicide tools.