Bumper Crop Basics: Yellow Leaf Mystery Solved

Roundup ¡

Bill Johnson / Farm Progress 5feb01

Soybean leaves can yellow under stress from a variety of sources. During the past few growing seasons, many Missouri growers wanted to know why new leaves in glyphosate-tolerant soybean plants turned yellow after application of Roundup. They saw the effect, to varying degrees -- the leaf surface turns yellow but remains darker green along the veins.

The yellowing occurs because some varieties have less tolerance to glyphosate than others. Glyphosate-tolerant soybeans (such as Roundup Ready and other varieties) contain a gene to give them tolerance to glyphosate herbicides, but the presence of this gene does not mean that glyphosate will not cause crop injury.

When Monsanto developed Roundup Ready technology, it required seed companies to prescreen varieties for tolerance to glyphosate to assure the plants would survive a specific dose of the product. Meeting these requirements was one stipulation for the variety to be sold as Roundup Ready. Still, all varieties are not created equal. Crop response to the chemical will be more evident in some varieties.

Additionally, if the crop is under stress from disease, insect feeding, nematodes, drought, excessive rainfall or injury from other herbicides, it will not be able to metabolize glyphosate as quickly and will show injury symptoms. The 2000 growing season offered challenging conditions for soybeans from the beginning. Extremely dry weather early, followed by abundant moisture, produced all of the above-listed stress factors. In particular, the wet weather experienced in late June caused saturated soils across much of Missouri. Many postemergence herbicide applications were delayed because dry weather reduced weed emergence before the rains. When spray applications were made after the rains, beans were stressed from wet soils and weeds were large.

Therefore, many applicators increased the rate of glyphosate or added a tankmix partner to control large weeds. As a result of adding another stress to the crop, yellow beans were seen in many fields.

Although the presence of crop response to glyphosate may cause concern, keep in mind that it is transitory in nature and usually not present by 10 to 14 days after application unless beans are under additional stresses.

If two to four trifoliates are affected and the plant continues to grow normally, there will not be an effect on yield. We have observed much more severe damage to soybeans with diphenyl ether herbicides (Cobra, Blazer, Reflex) in the past and have not seen a significant effect on yield. Response to glyphosate is minor in comparison.

Bill Johnson is a state extension weed scientist at the University of Missouri-Columbia.