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January
30, 2006
Preharvest Glyphosate, Grain Protein, and Hagberg Falling Numbers
Jochum Wiersma, Assistant Professor and Extension Educator, Crookston and
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Myths And Rural LegendsIn 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 never get asked about exploding jawbreakers or killer quicksand, we do get questions from time to time that raise at least one of our 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 positive effect application of glyphosate has on grain protein and Hagberg Falling Numbers (HFN). As the rural legend goes, both grain protein and HFN improve when glyphosate is applied a couple days prior to harvest. It should be noted that the only glyphosate formulation labeled (Roundup Ultra Max) for preharvest weed control in HRSW states that the preharvest interval is seven days. The label states that glyphosate can be applied at the hard dough stage of wheat. The label also states 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. Physiological maturity signifies that no additional assimilates are deposited in the developing kernel. In other words, at physiological maturity, the kernel has attained its maximum dry weight. Therefore, we have to conclude that an application of glyphosate cannot improve grain protein 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? Grain protein content is commonly determined with a NIT analyzer. This apparatus measures the amount of near infrared light of specific wavelengths that is transmitted through a grain sample. Chemical bonds like C-H2 and N-H will absorb near infrared light of specific wavelengths. Thus, absorption amount of specific wavelengths is directly related to the number of C-H2 and N-H binds in the sample. Since (grain) protein consists of 6.25% N, the amount of absorption can be used to determine the amount of protein in a grain sample. The NIT analyzers are a useful tool that allows a fast and accurate estimation of the protein content in grain. The technique however, is not flawless. First, the moisture content and temperature of samples have to be within a specified range for calibration curves to be accurate. In addition, the technique can give false readings if the sample is contaminated with compounds that contain additional N-H bonds. This explains why releasing anhydrous ammonia (NH3) in a bin of grain will report an overestimation of grain protein at the elevator and is illegal. Like anhydrous ammonia, ammonium sulfate - (NH3)SO4 - also contain N-H bonds. Thus, we reasoned that if an application of glyphosate a couple days prior to harvest could hypothetically result in an increase of grain protein as determined by NIT analyzers basically as a result of the presence of ammonium sulfate on the grain sample. Materials and MethodsTo 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 full factorial design arranged as a randomized complete block design with 3 replicates. Treatments were applied with a CO2-pressured backpack sprayer equipped with 80015 flat fan5 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. Hagberg falling numbers were determined using the Hagberg Falling Number apparatus 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 1290 Foss Tecator 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 were statistically significant (Table 1). The applications at physiological maturity yielded nearly a whole percentage point dryer 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 eighties showing 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 HFN values either (data not shown). In summary, we can conclude that preharvest applications of glyphosate with or without ammonium sulfate did not improve grain protein or Hagberg falling numbers. We therefore conclude that this rural myth is busted.
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