Utilizing Fall Applied Herbicides in Cover Crops for Italian Ryegrass Control.

Herbicides are the foundation for weed control in commercial agricultural production systems of the United States.  Reliance on a small number of these herbicides has resulted in extreme selection pressure leading to a number of herbicide-resistant weed species.  Planting cover crops in the fall is one tactic to help mitigate herbicide resistance developing in weeds. In part due to this, integrating cover crops as part of a total weed management system has become more popular in recent years in the State of Tennessee.  Italian ryegrass, primarily a fall germinating weed species, has become more of a troublesome weed in the Mid-Southern United States.  It is very competitive to crops and has developed a biotype in that region that is resistant to a number of herbicides including glyphosate. Growers who utilize cover crops are concerned that Italian ryegrass could become established in their cover crops.  The focus of the research was to determine if fall applied herbicides can be implemented onto a wheat or vetch cover crop to control Italian ryegrass (Lolium multiflorum).

A study to investigate cover crop tolerance to fall applied herbicides and Italian ryegrass control was conducted from 2014-2015 at the Delta Research and Extension Center in Stoneville, Mississippi and at the West Tennessee Research and Education Center in Jackson, Tennessee.  A wheat cover crop was evaluated in Stoneville and wheat and vetch covers were evaluated in Jackson.  Treatments were arranged as a two factor factorial within a randomized complete block design with four replications. The first factor level consisted of cover or no cover and the second factor level was herbicide and consisted of flumioxazin at 89 g ai ha^-1 and pendimethalin at 1123 g ai ha^-1 applied preemergence (PRE) and acetochlor at 1263 g ai ha^-1, flufenacet at 382 g ai ha^-1, metribuzin at 263 g ai ha^-1, pyroxasulfone at 15 g ai ha^-1, and s-metolachlor at 1069 g ai ha^-1 applied at the one- to two-leaf cover crop stage (POST).  Applications to cover and no cover plots were made simultaneously and paraquat was applied immediately after cover planting to insure a clean seed bed.  Cover crop injury and Italian ryegrass control were visually assessed 10, 30, 60, 90, and 150 d after the POST application.  Data were subjected to an analysis of variance with p value = 0.05.  Wheat data was pooled across location.

Injury for hairy vetch and wheat was greatest from flumioxazin (38% and 15%, respectively) 10 DAT.  A interaction of herbicide and cover was detected for Italian ryegrass control in vetch 90 DAT.  Control was greatest from pyroxasulfone with or without (87% and 86%, respectively) a hairy vetch cover.  Control from these treatments declined to 76% and 72%, respectively, at the 150 DAT interval (data not shown).  A wheat cover significantly improved Italian ryegrass control at the 90 and 150 DAT rating intervals.  In the wheat study Italian ryegrass control at 30, 90, and 150 DAT was greatest with pyroxasulfone (79%, 88%, and 87%, respectively) followed by s-metolachlor (76%, 85%, and 74%, respectively), pooled across cover and no cover treatments.  At 90 DAT and 150 DAT rating intervals, cover plots provided 71% and 62% control, respectively, compared to no cover plots providing 63% and 52% control, respectively, when pooled across herbicide treatments.

In summary, early season cover crop injury was reflective of late season cover crop injury (data not shown) and indicates that fall applied herbicide options for a wheat or vetch cover crop can be utilized without causing detrimental impacts to the cover crop.  However, only a limited number of herbicide options are available to provide adequate control of Italian ryegrass.  For maximum control of Italian ryegrass in a hairy vetch or wheat cover crop, pyroxasulfone should be used.