Heavy rains have prevented many fields from receiving proper burn-down treatments and created messes for some Arkansas producers. Many are turning to the herbicide paraquat (the active ingredient in Gramoxone Inteon) and straight applications of glyphosate (the active ingredient in Roundup brand products) just ahead of planting. These treatments fit into this short window, because of the quick burn-down effect of paraquat and the lack of residual or plant-back restrictions with either paraquat or glyphosate.
With over 900,000 acres of wheat scattered around the state, much of it bordering fields that need to be burned down prior to planting, it is no surprise that drift is showing up in some fields.
Although drift from both paraquat and glyphosate can damage wheat in the reproductive stages, the effects on yield and symptomology are quite different. Paraquat drift (figure 1) usually is observed as necrotic spots on leaves that were present at the time the application was made. These spots will show up immediately after application. These spots appear as burned areas, often times encircled by a purple or reddish colored ring. At higher drift rates entire leaves may be affected and can be anything from severely necrotic or have an overall light green, blotchy, or bleached appearance. With the necrotic spot type injury yields will often not be affected, especially if the damage is below the flag-leaf on the affected plants. Paraquat is not translocated readily in the plant and damage usually stays localized. I am not a wheat physiologist, but as long as more than 90% of the flag-leaf is intact the wheat should yield fine.
Conversely, glyphosate drift during the reproductive stages will typically be devastating to wheat yields. Damage may not show up until the wheat begins to head. Glyphosate damage to wheat (figure 2) includes a short flag-leaf, chlorosis of the flag-leaf and collar region of the flag-leaf, and seed-heads that are either malformed or coming out of the side of the leaf sheath. (This is similar to 2,4-D applied late in wheat.) Due to the severe damage often caused to the flag-leaf, yields may be reduced as much as 100 percent.
As with all herbicide applications care should be taken when applying burn-down applications around heading wheat. The damage that these products cause to wheat is similar to drift on rice. I currently have a graduate student, Brad Davis, who is working on comparing glufosinate (Ignite) drift to glyphosate in terms of its effect on sensitive crops. This is in response to an increase in the use of Ignite as a burn-down for resistant weeds and a potential increase in Liberty Link cotton, corn and soon Liberty Link soybeans.