Help, my numbers are falling
Reports of low falling numbers in hard red spring wheat are widespread. The Hagberg Falling Numbers test (HFN) is a measure of the amount of alpha-amylase in the grain. Alpha-amylase is an enzyme that hydrolyzes polysaccharides like starch into simple sugars (glucose and maltose). A low HFN test value indicates that there is a large amount of alpha-amylase present in the seed, resulting in a very rapid breakdown of the starch in the ground wheat sample used in the HFN test, thereby allowing the plunger to fall rapidly through the slurry in the test tube.
The HFN is used as an indirect measure of sprout damage in the harvested grain. Sprouted grain is problematic as a source of seed for next season as the germination and vigor will decline over time in storage. Likewise, milling and baking quality are also affected as the alpha-amylase breaks down the starch in the endosperm and over time the alpha-amylase present will degrade and no longer be reactive when needed during fermentation.
Unfortunately, the HFN is an indirect measure of sprout damage and as such, there are complicating factors that affect the HFN test and yield false positives. First, there are varietal differences for falling numbers. The HRSW breeders in the region have a target of HFN > 400 seconds. In other parts of the world, breeders have lower standards. In the past, varieties have been brought in from other parts of the world that inherently had HFN much lower than the 400-second target.
The temperature during grain fill has an influence on HFN in two distinct ways. Warmer temperatures during grain fill will result in higher HFN values. Conversely, cooler temperatures will yield, on average, lower HFN. A short period of very low nighttime temperatures (<50oF) very late in the grain fill period, however, is enough to yield a disproportionate increase of alpha-amylase in the seed. This phenomenon is referred to as late maturity alpha-amylase (LMA).
In short, a variety that has inherently lower HFN and encounters lower temperature during grain fill and/or the conditions that trigger LMA can have an HFN test below 300 seconds even in the absence of sprouting and the associated degradation of the starch and the functional milling and baking qualities. When elevators use the HFN test to determine sprout damage and the test yields a score less than the lower limit of 300 seconds, it is impossible to tell whether the low test value is a result of sprout damage, presence of late maturity alpha-amylase, or the inherent characteristics of the variety.
At physiological maturity, the grain enters a phase of post-harvest dormancy. This high-temperature dormancy is an evolutionary adaptation to survive the summer heat in the desert and not germinate with the first few drops of moisture on the ground. Once harvested, the grain will slowly lose this post-harvest dormancy, especially when stored cool and dry. Repeated wetting and drying, while standing in the field or laying in the swath, will eventually result in a loss of this dormancy as well. Once the dormancy is broken, the seed is at risk for sprouting.
Again, varieties differ in post-harvest dormancy. The University of Minnesota evaluates these differences in a dedicated trial using a misting chamber and assigns a pre-harvest sprouting (PHS) rating to all HRSW entered in the variety trials. We, unfortunately, do not have a rating for LMA.
With this knowledge at hand, you can test grain yourself for sprout damage without using the HFN test. Select clean, undamaged kernels and count out 2 x 100 seeds from each field, lot, or bin you like to test. Germinate the first 100 seeds now and score the percent germination after five and seven days. You store the second 100 seeds at room temperate for approx. 90 days and germinate them later this fall. If the percent germination decreased over time, the seed had lost its dormancy and had some sprout damage already. If the percent germination stays the same or improves, the lot likely did not have any sprout damage.
If you still have HRSW standing in the field, I encourage you to evaluate whether you can visually already detect sprouting. So-called blind sprout occurs when you see no sprout on the grain, but the dormancy of the seed has already been broken. Often the scutellum will enlarge when this happens, resulting in the embryo being slightly raised up from the seed.
If you suspect sprouting or even blind sprout, store the grain separately as blending of lots with good HFN and poor HFN values is not recommended as it is not a simple linear function. This means that blending a lot with an HFN of 250 seconds with an equal amount of a lot with HFN of 350 seconds will not yield you a blend with an HFN of 300 seconds but something lower and probably closer to 250 than 300 seconds.
Finally, on the question whether pre-harvest glyphosate is worsening the problems with low HFN, the answer is a resounding no. This conclusion is based on some work we did 15 years ago to evaluate whether ammonium sulfate and pre-harvest glyphosate applied at physiological maturity improved grain protein content. Across two locations, two years, and two HRSW varieties, the application of pre-harvest glyphosate did not alter the HFN.