Josh Copes, Daniel Stephenson, Donnie Miller and David Blouin

Cover crops provide many benefits. They have the potential to reduce soil erosion, improve soil health, improve soil water availability, reduce weed interference, aid in control of certain pests and diseases, and increase soil biodiversity.

Winter annual weeds are common and troublesome pests for Louisiana grain and soybean producers. Control of species such as annual bluegrass, henbit, shepherd’s-purse, cutleaf evening primrose, sow thistle and others historically require a herbicide “burndown” treatment to remove all vegetation in the spring. Research has shown that burndown applications need to be done four to six weeks prior to spring planting. Unfortunately, many Louisiana producers report inadequate control of winter annual weeds with a single burndown application, leading to a second application to achieve control.

Research in Louisiana has shown that when applied in the fall, a residual herbicide with continued weed control through soil activity provides good to excellent control of many grass and broadleaf winter annual weeds in the spring. From a weed management perspective, excellent winter annual weed control is a positive outcome, but that’s not true from a total management perspective. Excellent weed control during the late fall, winter and early spring can lead to soil erosion and many other negative results. Studies on the effects of combining cover crops and fall-applied herbicides for winter annual weed management were initiated in fall 2016 to investigate cover crop response to fall-applied residual herbicides.

Research was conducted in 2016-2017 at the Northeast Research Station, St. Joseph, and Dean Lee Research and Extension Center, Alexandria, to address the effects of applying fall residual herbicides to cover crops commonly grown in Louisiana. The data presented here are from the Northeast Research Station. Cover crops evaluated were cereal rye, tillage radish, crimson clover and Austrian winter pea. All cover crops were drill-seeded on Nov. 3, 2016. Cover crop seeding rates were 120 pounds per acre for cereal rye, 8 pounds per acre for tillage radish, 20 pounds per acre for crimson clover and 30 pounds per acre for Austrian winter pea. Herbicide treatments evaluated were Zidua at 2 ounces per acre, Valor at 2 ounces per acre, Boundary at 32 ounces per acre and Leadoff at 1.5 ounces per acre with a no-herbicide control. The herbicide treatments were applied two weeks after cover crop emergence on Nov. 21, 2016.

Visual cover crop injury ratings included stand reduction, height reduction and cover crop ground cover. The weeds henbit, annual bluegrass and lesser swinecress were rated for percent control. Cover crop injury and weed control ratings were collected at three, 14, 28 and 57 days after treatment. Cover crops were terminated on Jan. 30, 2017, and cover crops and weeds were counted. Corn was planted March 18, 2017, and yield data were collected at harvest.

The cover crops exhibited varying levels of tolerance to each of the residual herbicides evaluated. Zidua was the only herbicide that caused little injury across all cover crop species evaluated. Valor and Boundary could be used for winter annual weed control in cereal rye and Austrian winter pea, but further research is needed. These herbicides resulted in nearly complete elimination of tillage radish and crimson clover. Austrian winter pea was the only cover crop that was not severely injured by Leadoff. Henbit, annual bluegrass and lesser swinecress control was excellent (99%) for Valor, Boundary and Leadoff treatments. Zidua provided good control of bluegrass (98%) and lesser swinecress (83%), but henbit control was poor (43%). In the no-herbicide control at the last evaluation, there were obvious differences in each cover crop’s competitive ability. Cereal rye and tillage radish were able to compete with the winter annual weeds in this study. The competitive ability of a cover crop combined with fall-applied residual herbicides for weed control could result in terminating just the cover crop four to six weeks prior to planting. Additional research in this area is needed. Corn grain yield was not affected by cover crop treatment or fall residual herbicide treatment.

The use of fall residual herbicides and cover crops has the potential to help Louisiana soybean and grain producers manage winter annual weeds and help reduce soil erosion and other negative effects of bare fields during the winter. Research is ongoing to evaluate fall-applied soil residual herbicide use in cover crops.

Josh Copes is an assistant professor at the Northeast Research Station; Daniel Stephenson is a professor at the Dean Lee Research Station; Donnie Miller is the John B. Baker Professor for Excellence in Weed Science at the Northeast Research Station; and David Blouin is a professor in the Department of Experimental Statistics.

(This article appears in the summer 2019 issue of Louisiana Agriculture.)

Corn grain yield was not affected by cover crop treatment or fall residual herbicide treatment. LSU AgCenter corn specialist Dan Fromme speaks to participants at the second annual field day expo on June 27, 2019, at the Dean Lee Research and Extension Center near Alexandria, Louisiana