The Grain Growing Classroom

Preharvest Glyphosate, Grain Protein, and Falling Numbers

By Jochum Wiersma  and Doug Holen

In Discovery Channel’s “MythBusters,” the duo of Adam Savage and Jamie Hyneman take on myths and urban legends. With engineering and some science, they attempt to discern fact from fiction. Although we at the University of Minnesota never get asked about exploding jawbreakers or killer quicksand, we do get questions from time to time that raise the eyebrows.

Some questions return year after year and get asked so often that we could possibly label them “rural legends.” One of these rural legends includes the use of preharvest glyphosate and the effect it has on grain protein and falling numbers/sprout damage (see related sidebar). As the rural legend goes, both grain protein and FN/sprout damage potential improve when glyphosate is applied a couple days prior to harvest.

It should be noted that the glyphosate formulations labeled  for preharvest weed control in HRSW state that the preharvest interval is seven days. The labels state that glyphosate can be applied at the hard dough stage of wheat and that up to 2% ammonium sulfate by weight can be added.

Both points are of importance as we attempt to debunk this rural myth. First, wheat reaches the hard dough stage at physiological maturity. PM signifies that no additional assimilates/absorbed materials are deposited in the developing kernel. In other words, at PM, the kernel has attained its maximum dry weight.  Therefore, we have to conclude that an application of glyphosate cannot improve grain protein at this stage, because no assimilates can be deposited in the grain kernel at that time. If the grain protein content cannot be altered, then what process/effect can result in an increase in grain protein percentage, as is reportedly the case when applying glyphosate a couple days prior to grain harvest?

Analyzing Grain Protein Content
The NIT analyzer is a useful tool that allows a fast and accurate estimation of the protein content in grain. This apparatus measures the amount of near infrared light of specific wavelengths that is transmitted through a grain sample.  Since (grain) protein consists of a certain amount of N, the amount of absorption can be used to determine the amount of protein in a grain sample.

This technique, however, is not flawless.  First, the moisture content and temperature of samples have to be within a specified range for the calibration curve to be accurate.  In addition, the technique can give false readings if the sample is contaminated with compounds that contain additional N.  This explains why releasing anhydrous ammonia (NH3) in a bin of grain will result in an overestimation of grain protein at the elevator and is illegal.

Like anhydrous ammonia, ammonium sulfate also contains a N.  Thus, we reasoned that an application of glyphosate with ammonium sulfate several days prior to harvest could hypothetically result in an increase of grain protein as determined by NIT analyzersas a result of the presence of ammonium sulfate on the grain sample.

Similar to the MythBusters, we set out to test this hypothesis.

Myth Busting Methods
To test this hypothesis, we designed the following experiment. At two locations (Crookston and Fergus Falls), we applied water, water + ammonium sulfate (AMS), water + labeled rate of Roundup Ultra Max, water + labeled rate of Roundup Ultra Max + ammonium sulfate and an untreated check. The ammonium sulfate was added at a rate of 2% by weight or 1.7 lbs per 10 gallons of spray volume.  These five treatments were applied at physiological maturity (PM), at PM plus 5 days and at PM plus 10 days in a randomized plot design with three replicates. Treatments were applied with a CO2-pressured backpack sprayer equipped with 80015 flat fan nozzles delivering 10 gallons per acre at 35 psi. The experiments were harvested at PM + 12 days, 2 days after the last applications of Roundup Ultra Max were made.

Immediately following grain harvest, grain protein was determined with a Foss Tecator 1290 whole grain analyzer.  To maintain the integrity of the sample, we opted not to clean or dry the grain samples.  Grain protein was again determined at 7, 14, and 28 days after harvest.  Falling numbers were analyzed at 7 and 28 days after harvest.

Results and Discussion
At harvest, grain moisture averaged 12.4% in Fergus Falls and 15.2% in Crookston. The application of Roundup Ultra Max at PM, PM + 5, or PM + 10 did not alter grain moisture content of the harvested grain as determined by the NIT whole grain analyzer in Fergus Falls.  In Crookston, both the main effects of timing and application and the interaction between the timing of the application and application itself had a statistically significant effect on grain moisture (Table 1).  The applications at physiological maturity yielded nearly a whole percentage point drier grain than applications made at PM + 5.  The applications of glyphosate with or without ammonium sulfate yielded the driest grain (Table 3). Spraying straight water increased grain moisture content when compared to the untreated check at PM but not for the applications at PM + 5 or PM + 10.

The observed differences between Crookston and Fergus Falls may be attributed to the weather conditions the week prior to harvest.  Weather in Fergus Falls was hot and dry and may have caused any differences between treatments to dissipate. The weather in Crookston was overcast for several days prior to harvest, allowing possibly any differences to be maintained longer.

Thus, it appears that glyphosate applied at PM can quicken the dry down of the crop and allow for a more timely harvest, if weather conditions are such that dry-down is slowed. This agrees with Canadian research in the late 1980s which 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 a 10 to 15 day to express and warmer or drier conditions tended to make the differences in dry-down disappear.

Neither the application of glyphosate or ammonium sulfate at any of the three application timings resulted in an increase in grain protein in Fergus Falls or Crookston immediately after harvest (Table 2).  This did not change at 7, 14, or 28 days after harvest (data not shown).  The application of glyphosate or ammonium sulfate did not affect falling numbers values either (data not shown).

In summary, we can conclude that preharvest applications of glyphosate with or without ammonium sulfate may hasten dry-down, if weather conditions are such that dry down is slowed. However, a pre-harvest glyphosate application should not be expected to improve grain protein or falling numbers. We therefore conclude that this rural myth is busted. 

Table 1. Grain moisture content of hard red spring wheat on Granite treated with Roundup Ultra Max in Crookston, MN, 2005.

	PM	PM + 5	PM + 10	Mean1
Check	15.4	15.2	15.3	15.5
H2O	15.7	16.5	15.6	16.0
H2O + Glyphosate	13.6	15.7	15.1	14.8
H2O + AMS	15.5	15.4	15.2	15.4
H2O + Glyphosate + AMS	13.4	15.2	14.3	14.3
Mean2	14.7	15.2	15.1	0.93
1The LSD (0.05) to compare the main effect of treatment means is 0.5 2The LSD (0.05) to compare the main effect of timing is 0.4 3The LSD (0.05) to compare the interaction means of timing and treatment is 0.9

Table 2. Grain protein percentage of the hard red spring wheat Knudson treated with Round-Up Ultra Max in Fergus Falls, MN, 2005.

	Timing
	PM	PM + 5	PM + 10	Mean
Check	14.2	14.0	15.0	14.4
H2O	14.4	14.1	14.2	14.2
H2O + Glyphosate	14.2	14.1	14.3	14.2
H2O + AMS	14.5	14.5	14.0	14.3
H2O + Glyphosate + AMS	14.8	14.2	14.6	14.5
Mean	14.4	14.2	14.4	NS

Table 3. Grain protein percentage of the hard red spring wheat Granite treated with Round-Up Ultra Max in Crookston, MN, 2005.

	Timing
	PM	PM + 5	PM + 10	Mean
Check	15.9	15.8	15.8	15.9
H2O	15.3	15.7	16.0	15.7
H2O + Glyphosate	16.5	15.3	16.2	16.0
H2O + AMS	16.2	15.8	15.5	15.8
H2O + Glyphosate + AMS	15.5	15.8	16.2	15.8
Mean	15.9	15.7	15.9	NS