Injury


	Issue 121 Prairie Grains


	Prairie Grains is the official publication of the Minnesota Association of Wheat Growers, North Dakota Grain Growers Association, Montana Grain Growers Association and South Dakota Wheat, Inc.


	Glyphosate Injury to Spring Wheat


	By Dr. Beverly R. Durgan Weed Scientist, University of Minnesota

Minnesota growers have rapidly adopted the use of glyphosate resistance crops. In Minnesota, approximately 95% of the soybean, 60% of corn, 80% of canola and over 50% of sugar beet acres are planted to glyphosate resistant varieties. Glyphosate (sold under numerous formulations including Roundup, Touchdown and others) is a nons-elective post emergence herbicide. The increased use of glyphosate in Minnesota has increased the incidence of off-target movement (herbicide drift) to sensitive crops, especially spring wheat.

Herbicide drift can occur during any herbicide application, depending on the application equipment used and the environment conditions during herbicide application. In most cases the herbicide rate in drift situations is usually somewhere between 1/10th and 1/100th of the applied rate and would in most cases be considered sub-lethal.

However, if the herbicide drift occurs on a field planted to a crop that is very sensitive to the herbicide, crop growth and yield could be significant. Spring wheat is very sensitive to glyphosate drift and that can cause crop injury and yield reductions.

Reports of glyphosate injury on wheat in Minnesota have been increasing; however, limited research has been conducted on the potential for injury and yield loss from glyphosate drift in spring wheat in MN or ND. Research has been conducted in several other wheat producing states and, for the most part, that research can be applied to the situation we are seeing in Minnesota.

Glyphosate drift to wheat during the vegetative growth stages typically stunts normal leaf development and emergence of subsequent leaves. Glyphosate drift on wheat in the vegetative stage can also cause symptoms primarily characterized by bleached or white tissue normally localized in the lower section of upper leaf blades near the collar.

Minor leaf wrinkling, wrapping or cupping is also common on wheat when drift occurs during vegetative growth stages. Slight-to-moderate glyphosate drift injury is normally localized on the youngest, uppermost leaves emerging on the plant. Injury symptoms can be quite variable from plant to plant.

Glyphosate drift to spring wheat in the tillering stage can cause stunting, chlorosis and some dead leaves and tillers; however, prior to jointing this damage does not effect seed head or flag-leaf development. If drift, even in small amounts, occurs after tillering and after jointing, the injury usually remains invisible until the flag leaf emerges. Flag leaves will be stunted and bleached to striped white or yellow chlorosis, often twisted at the base of the flag leaf where it attaches the stem. If the flag leaf is damaged, a damaged head almost always follows. This injury resembles the effect of late applications of 2,4-D as the emerging heads get “caught” in the collar region and can emerge sideways from the sheath.

Spring wheat is especially sensitive to glyphosate drift at jointing when tillering is complete and the first node is visible on the stem, because this is when the seed head is formed. In addition, when wheat is exposed to glyphosate at flowering, a reduction in both the number of spikelets produced per seed head and seed weight has resulted in decreased yield.

A very comprehensive study of the affect of glyphosate drift on wheat was conduct at the Louisiana State University Ag Center – Research and Extension Center. In this study, glyphosate was applied to wheat at first node, boot stage or early flowering at rates to simulate drift representing 1/8th (4 oz./acre), 1/16th (2 oz./acre) and 1/64th (0.5 oz./acre) of the use rate of 32 ounces of Roundup Ultra (one pound active ingredient per acre). Injury from glyphosate appeared within three to five days after application as bleaching of leaf foliage followed by crop shorting by seven to 10 days after application. At 14 days after treatment, wheat injury was 40% to 55% for the 2 oz./acre rate applied at first node and the 4 oz./acre rate applied at all growth stages. Wheat height 28 days after treatment was reduced 47 % when glyphosate was applied at 4 oz./acre at first node and was reduced around 26 % when glyphosate was applied at 2 oz./acre at first node. Wheat height was not reduced for any rate of glyphosate applied at early flowering.

For glyphosate at 4 oz./acre, wheat yield was reduced 72 % applied at first node, 45 % applied at boot stage and 54 % applied at early flowering. At 2 oz./acre, wheat yield was reduced 25 % to 30 % for the three application timings. The visual injury was highly correlated with yield loss results, meaning that if there is a visual injury of 30% then the yield loss will also be approximately 30%.

This research data clearly shows that wheat is very sensitive to glyphosate drift, and that the potential of yield loss is greatest when drift occurs at the tillering to flower stage. Unfortunately, the only way to determine if the injury and resulting yield was due to glyphosate is to have the plant material analyzed by a lab. There are several companies in Minnesota that can do the analysis, however this can be costly.

As was mentioned previously, herbicide drift can occur anytime a herbicide application is be made, depending on the application equipment used and the environmental condition at the time of application. Glyphosate drift is due to particle drift that occurs at the time of application.

In the rush of the weed control season it is important to remember that all herbicides have the potential to drift and cause injury to off target crops. Steps should always be taken to minimize the potential for herbicide drift and especially drift of glyphosate to sensitive crops, such as spring wheat.