Abstract: Understanding the effects of previous herbicide applications on cover crops is important if cover crops are to be successfully established during the same growing season. Greenhouse research was conducted to evaluate the sensitivity of annual ryegrass (Lolium multiflorum), oat (Avena sativa), crimson clover (Trifolium incarnatum), and medium red clover (Trifolium pratense) to EPTC plus R-29148, pendimethalin, and metolachlor plus CGA-154281 in a loam soil with 2.5% organic matter. Annual ryegrass density was reduced 96% by EPTC at 4.48 kg ai/ha and metolachlor at 2.24 kg ai/ha, whereas oat density was reduced 99% and 35%, respectively, from the two treatments. Pendimethalin at 1.68 kg ai/ha reduced annual ryegrass density 14%, but not oat density. Crimson clover biomass was reduced 33% by EPTC, 45% by metolachlor, and 15% by pendimethalin. All herbicides reduced the density of medium red clover > 25%, but biomass was reduced only by metolachlor. Field trials were conducted to determine whether persistence of these herbicides would affect the seeding time for annual ryegrass and crimson clover establishment. Annual ryegrass was successfully established following application of EPTC at 4.48 kg/ha when seeding was delayed 35 to 40 d after treatment (DAT). Pendimethalin at 1.68 kg/ha injured annual ryegrass > 15%, and annual ryegrass was severely injured by metolachlor at 1.68 kg/ha regardless of seeding time. Successful establishment of crimson clover following all herbicides was possible when seeding timings were delayed. Pendimethalin injured crimson clover less than 13% 15 to 17 DAT, and EPTC injured crimson clover less than 5% 28 to 35 DAT. Crimson clover injury from metolachlor was less than 16% 35 to 40 DAT. Herbicides with longer reported soil persistence generally require more time between herbicide application and cover crop seeding for safe establishment of the cover crop, particularly if the cover crop species is sensitive to the herbicide.
Nomenclature: CGA-154281, 4-(dichloroacetyl)-3,4-dihydro-3-methyl-2H-1,4-benzoxazine; EPTC; metolachlor; pendimethalin; R-29148, 3-(dichloroacetyl)-2,2,5-trimethyloxazolidine; annual ryegrass, Lolium multiflorum L.; crimson clover, Trifolium incarnatum L. #3 TRFIN; oat, Avena sativa L. # AVESA; red clover, Trifolium pratense L. # TRFPR.
Additional index words: Interseeded cover crops, overseeded cover crops.
Abbreviations: DAS, days after seeding; DAT, days after treatment.