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Low Falling Numbers in Wheat a Concern (08/29/19)

Low falling numbers (FN) is a concern in some areas of North Dakota, particularly in areas where harvest has been delayed by excessive moisture.

Low falling numbers (FN) is a concern in some areas of North Dakota, particularly in areas where harvest has been delayed by excessive moisture. The falling numbers test is used by the grain industry to determine potential or possible sprout damage in cereal grains. This test gives an indication of the level of alpha-amylase (starch degrading) enzyme activity in a sample of grain or flour. The wheat industry typically considers grain with FN values over 300 seconds to be sound, while grain with values below 300 seconds are often discounted. As a result, domestic and international customers set a minimum FN value in their contract specifications.  Low FN number values are possible even if there are no visible signs of germination, yet alpha amylase enzymes levels are elevated. Grain with low FN is discounted because of its reduced value to the baking industry. Flour made from grain with low FN has low water absorption, reduced mixing strength and forms a sticky dough. Bread made from flour with low FN can result in lower loaf volume, crust strength and crumb texture, and creates loaves that may collapse or produce holes in the middle. Durum products made with semolina from grain with low FN has reduced shelf life, are less firm when cooked, can result in greater cooking losses, and may even increase breakage in packages.

There are two primary causes of low FN. The first and most common is due to pre-harvest sprouting. Pre-harvest sprouting (PHS) of kernels in the spike does not occur until they have fully matured and undergone a period of “ripening”, after which repeated rains and prolonged periods of dew can initiate germination. Often the kernel only initiates the first step in the germination process before the kernel then dries and is returned to a dormant state so no visible sprouting can be observed. This first step includes the synthesis of alpha amylase enzymes that degrade the starch in the kernel to produce compounds essential for metabolic activities needed for germination and seedling growth. Varieties differ in their propensity to “sprout” when conditions are favorable for sprouting prior to harvest. Most of the commonly grown spring wheat varieties have relatively good sprout tolerance (see Table 2 in the report of the variety trials conducted by the University of Minnesota for a comprehensive list of preharvest sprout ratings of most varieties of spring wheat grown in the region).

The second potential cause for low FN is called late maturity alpha amylase (LMA). LMA occurs when elevated levels of alpha amylase are produced during kernel development. The literature suggests that LMA can be induced by either a heat or cold shock (most likely type of shock the crop may have experienced this year) during a key kernel developmental stage. I know of at least one report this year where a grower was surprised to learn that his grain had a lower than normal FN even though the grain had been harvested prior to any post-maturity rain events. In my opinion, this is a classic example of LMA. In the literature there are reports of genetic differences in the propensity of varieties to develop LMA, but as far as I know, this has not been documented in spring wheat varieties adapted to North Dakota (i.e. we do not know if some varieties are more susceptible to LMA than others).

I have had several questions about whether a pre-harvest application of glyphosate might affect PHS or influence the FN test, particularly since glyphosate is known to affect the germination of seeds when plants are treated prior to reaching harvest maturity. Though the literature is somewhat limited on this question, it appears that glyphosate when applied for pre-harvest weed control at the labeled time and rate has no effect on FNs, positive or negative.

Discounts for low FN numbers can be significant. Nevertheless, blending grain with low FN with sound grain can be risky because the relationship is not linear but rather exponential, and it is difficult to predict what the FN of the blended grain might be. As a hypothetical example, mixing grain with a FN of 350 sec with grain with a FN of 250 sec will not result in grain with a falling number of 300 sec. Potentially it might be 270 sec, resulting in the entire mixture being subject to discounts.

 

Joel Ransom

Extension Agronomist, Small Grains and Corn

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