Cover Crop Establishment Following Commonly Applied Corn and Soybean Herbicides in the Upper Midwest.

Cover Crops are a growing interest for corn and soybean producers in the upper Midwest due to the benefits of reducing soil erosion, providing and scavenging nutrients, and increasing soil organic matter. This study was conducted to determine whether common soil applied herbicides with residual weed control properties applied in the spring during the establishment of corn and soybean crops affect the subsequent establishment of cover crops in the fall. Corn and soybean trials with glyphosate-resistant cultivars were established at Arlington Agricultural Research Station, Arlington, WI on June 2, 2013 and May 28, 2014. Similar trials were also conducted at Throckmorton Purdue Agriculture Center in Lafayette, IN and these trials were established for corn on May 16, 2013 and April 27, 2014 and for soybean May 16, 2013 and May 7, 2014. Corn and soybean trials in Wisconsin had fourteen herbicide treatments, and there were 10 treatments for soybean and 11 for corn in Indiana trials. All treatments were applied at common labeled rates and timings with four replications. Each crop included a control treatment where no residual herbicide applied, however weeds were managed with postemergence (POST) glyphosate for all treatments to remove potential effects from weed competition or residues. Trials conducted in Arlington, WI were harvested for silage near the beginning of September, and cover crops were seeded with a no-till grain drill uniformly across all herbicide treatments. Trials conducted in Lafayette, IN were seeded by hand broadcasting seed at the end of August to simulate aerial seeding and the main crops then were harvested for grain at the end of September. The cover crops included were Tillage Radish® (Raphanus sp;), crimson clover (Trifolium incarnatum), winter rye (Secale cereal), a mixture of 70% oat (Avena sativa) plus 30% peas (Pisum sativum), only at Arlington, WI, and four different annual ryegrass (Lolium multifloram) varieties. The annual ryegrass varieties included ‘Bruiser' and ‘Gulf' in Lafayette, IN and ‘King' and a tetraploid in Arlington, WI. ‘Bruiser,' ‘Gulf,' and ‘King' were all diploids. Nearly two months after seeding, the cover crops were evaluated for herbicide injury. In Arlington, WI percent ground cover was assessed using digital imagery analysis, and accumulated biomass was collected from a 0.25m² quadrat, dried, and weighed. Herbicide injury included the evaluation of plant stunting and loss of plant greenness. In Lafayette, IN injury was assessed by stand counts and visually rating of percent stand reduction.  In 2013, winter rye was the only cover crop without negative growth effects associated from the herbicide treatments applied in the corn or soybean trials (both p-values < 0.05) at both locations.  All other cover crops had significantly reduced biomass (P < 0.05) and percent cover (P < 0.05) for at least one of the residual herbicide treatments applied in the corn and soybean trial. In 2014 at Arlington, WI ‘King' and the tetraploid annual ryegrass were the only cover crops that had growth inhibition because of herbicide treatments applied in the corn or soybean trials (both p-values <0.0001). All other cover crops did not have significantly reduced percent cover (P<0.05) for all of the residual herbicide treatments. At Lafayette, IN none of the cover crops had significant (P<0.05) reduction in stand counts or visual ratings. 

From these results, we suggest several commonly used corn and soybean herbicides have the potential to adversely affect the establishment of many different cover crops, but the severity of damage will be determined by weather, cover crop species, and the specific herbicide combination.  More research will be needed to establish best management practices for farmers interested in the use of cover crops following conventional Midwest corn and soybean production.