Grain Growing Classroom

The Use of Pre-Harvest Glyphosate

By Jochum Wiersma
U of M Small Grains Specialist
wiers002@umn.edu

Questions about the efficacy and benefits of pre-harvest applications of glyphosate resurface each and every year. Last year was no exception.  Following is a short summary of some of the research on the subject. Since the majority of the pre-harvest applications of glyphosate are primarily intended to accelerate crop dry-down, the merits of the weed control provided by pre -harvest applications of glyphosate will not be discussed.

First it should be stated that glyphosate is labeled for suppression and control of perennial weeds prior to harvest in feed barley and wheat.  The label does not explicitly state that the product is intended as a desiccant to aid in dry-down to harvest ripeness. The label states that glyphosate can be applied after the hard dough stage has been reached and when the grain contains 30% moisture or less. Hard dough is reached when the kernel is firm enough so that no moisture can be squeezed from the kernel. The kernel will also have lost all of its green color at this growth stage.  Moisture content can vary from 30% to 40% when wheat reaches hard dough.  Physiological maturity is reached at the end of the hard dough stage.

At physiological maturity, the maximum amount of dry matter has accumulated in the kernels.  The glumes and the peduncle are no longer green and little green coloring remains in the plant.  The loss of green in the peduncle is a very useful marker to aid in the correct timing of preharvest use of glyphosate. The peduncle is the upper internode of the stem that carries the spike. It has a characteristic bend just below the spike. It is this bend that will lose the green color first, signaling that the vascular system no longer communicates and that physiological maturity has been reached.

Canadian research in the late 1980s showed that glyphosate applied to wheat with seed moisture content above 25% slightly improved the dry -down of both the seed and foliage when compared to a standing crop. The authors found that the small differences in the dry-down required 10 to 15 days to become expressed, and warmer or drier conditions tended to make the differences in dry-down between treated and untreated disappear.

The maximum benefit of glyphosate as a desiccant was observed when seed moisture content was above 40%. This was well before the wheat crop reached physiological maturity and consequently the authors reported yield losses and high residue levels of glyphosate in the harvested seed.  When seed moisture content dropped below 40%, the glyphosate applications caused little or no yield losses and little to no differences in kernel weight, test weight, grain protein, or germination.  Bread making quality was largely unaffected by the use of glyphosate, but some of the dough mixing characteristics changed. The authors concluded that the use of glyphosate as a desiccant was limited.

More recent research at North Dakota State University gave similar results.  The NDSU group was primarily interested in the effects of pre-harvest applications of glyphosate on kernel characteristics and milling and baking quality rather than the efficacy of glyphosate as a desiccant or the weed control attained. 

When applied at the soft dough stage (seed moisture content of approx. 50%), glyphosate did hasten crop dry-down, but also reduced kernel weight and test weight. It also reduced protein slightly, varying by variety.  At hard dough (seed moisture content of approx. 35%), glyphosate did not reduce kernel weight, nor test weight. Loaf volume, crumb color, and crumb texture were unaffected as well. Protein also was not affected. Similar to the Canadian results, the dough mixing characteristics were affected by the use of pre-harvest glyphosate at both the soft dough and hard dough growth stage.

So what does this mean? Given the stated label restrictions, do not expect miracles of glyphosate as a desiccant. The impact on dry-down may not be as much as you would expect or hope for even in a cool and wet year like 2004, and the difference in dry-down rates are likely to completely disappear when weather conditions are hot and dry, favoring natural dry -down.

Beverly R. Durgan, University of Minnesota extension weed scientist and associate dean, contributed to this article.

---------------------

What NDSU Says About Glyphosate as a Harvest Aid

Small grain preharvest herbicides can desiccate weeds and crop and provide perennial weed control. Expectations for preharvest weed control may exceed reality. It is difficult to kill or dry down a 3-foot weed in the same manner as a 3-inch weed. Lower portions of the weed may not be affected.

Plant desiccation requires 7 to 10 days or more if wet and cool after treatment. All herbicides labeled for preharvest application are systemic and slow acting, which requires a longer dry down period as compared to contact type herbicides.

The intent of a preharvest treatment should be to facilitate harvest and reduce harvest loss. Preharvest treatments do not decrease yield losses due to weed competition or prevent weed seed production. Herbicide drift from preharvest treatments can cause injury to crops nearby. Consider sensitive crops (sugarbeet, potato, etc.) and other plants (trees, gardens, etc.) in the general vicinity of the field receiving treatment.

Glyphosate is labeled as a harvest aid only in spring wheat and durum - not barley or oats. Glyphosate at 0.5 to 2 pt/A of a 3 lb ae/gal concentrate controls annual grass, broadleaf weeds, and quackgrass and suppresses Canada thistle in hard red spring wheat and durum. Do not apply to barley. Do not apply to wheat grown for seed, as a reduction in germination or vigor may occur.

Glyphosate should be applied after the hard dough stage (30% or less grain moisture) of the wheat and at least 7 days prior to harvest by air or ground in 3 to 10 gpa spray volume. See label for adjuvant use. Always add AMS at 8.5 to 17 lb/100 gallons of water. AMS increases control of annual and perennial weeds and especially control of weeds stressed by dry weather. AMS also eliminates antagonism from ions and carbonates in hard water. Do not use AMS in place of an NIS. Refer to label for addition of other adjuvants. – NDSU Extension Service