Effect of soil-applied herbicides on establishment of cover crop species

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 sens itivity of annual ryegrass (Lolium multiflorum), oat (Avena sativa), crimso n 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 den sity was reduced 99% and 35%, respectively, from the two treatments. Pendim ethalin at 1.68 kg ai/ha reduced annual ryegrass density 14%, but not oat d ensity. 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 we re conducted to determine whether persistence of these herbicides would aff ect 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). Pe ndimethalin at 1.68 kg/ha injured annual ryegrass > 15%, and annual ryegras s was severely injured by metolachlor at 1.68 kg/ha regardless of seeding t ime. Successful establishment of crimson clover following all herbicides wa s possible when seeding timings were delayed. Pendimethalin injured crimson clover less than 13% 15 to 17 DAT, and EPTC injured crimson clover less th an 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 specie s is sensitive to the herbicide.