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Managing the Risk of Low Falling Numbers in Wheat

Managing the Risk of Low Falling Numbers in Wheat

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Camille Steber, Research Molecular Geneticist, Wheat Genetics, Health and Quality Unit, USDA-ARS, and adjunct faculty, Dept. of Crop and Soil Science, Washington State University Pullman WA
Grain is purchased at a discount when falling numbers are below 300 seconds (sec). This can result in serious financial losses for farmers. This article addresses many commonly asked questions about the Hagberg-Perten Falling Number test, and provides some suggestions for reducing losses due to low falling numbers.
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Grain is purchased at a discount when falling numbers are below 300 seconds (sec). This can result in serious financial losses for farmers. This article addresses many commonly asked questions about the Hagberg-Perten Falling Number test, and provides some suggestions for reducing losses due to low falling numbers.

What is the Falling Number test?

The Hagberg-Perten Falling Number test is used to measure damage to starch in flour (Perten 1964). Low falling numbers result from high levels of the enzyme alpha-amylase (Perten 1964; Kruger and Tipples 1980; Yu et al. 2015). Alpha-amylase catalyzes cleavage of starch chains. This starch damage leads to poor end-use quality of wheat products including bread, noodles, and cakes (Farrand 1964; Batey et al. 1997; Gooding and Davies 1997). For example, Japanese-style sponge cakes show an increasing tendency to fall with increasing levels of alpha-amylase (Figure 1).

Figure 1. Sponge cakes fall with the increasing alpha-amylase content that can come from pre-harvest sprouting. Image reproduced with permission from C. Morris, WWQL, USDA-ARS, Pullman WA.
The falling number test is based on the principle that starch damage from alpha-amylase reduces the ability of wheat flour to “gel.” During the test, a flour/water mixture is heated and stirred, like making gravy. Once the mixture has been stirred for exactly 60 sec, the falling number instrument measures the length of time in seconds needed for the stirrer to fall through the mixture (also see the video What is the Falling Number Test?). With more starch damage, the mixture is thinner, and the stirrer falls faster. The lowest possible falling number is 60 sec (the length of time the gravy is stirred). Grain with a falling number below 300 sec is typically discounted in the Pacific Northwest (PNW). The goal is to keep the falling number of PNW wheat higher in order to compete well in the export market.

What is pre-harvest sprouting?

Pre-harvest sprouting is the initiation of grain germination while still on the mother plant (Lunn et al. 2001). Germinating seeds make alpha-amylase. This alpha-amylase cleaves starch chains into sugars that can be used to fuel seedling growth.

Why don’t all mature seeds sprout right away in the rain?

Some wheat varieties can germinate immediately after the grains mature, while some cannot (Tuttle et al. 2015). Varieties that are unable to germinate immediately after maturity are considered dormant or “asleep.” There are two ways to break the dormancy of such varieties. The first is to store them dry so that they “after-ripen”; this period of dry storage can range from two weeks to one year depending on the variety. The second way is to subject them to cold and wet conditions (wetness alone is not enough). Differences in seed dormancy explain 60–80% of the variation in pre-harvest sprouting susceptibility (DePauw and McCaig 1991).

What kind of weather promotes pre-harvest sprouting?

Not all rainstorms induce pre-harvest sprouting. Cool weather combined with rain increases the likelihood of pre-harvest sprouting. If the temperatures are in the 80s when it rains, then wheat is less likely to sprout than if the temperatures are in the 60s (degrees Fahrenheit). Low falling numbers are also more likely when there are multiple rainy days in a row, as it is more likely that dormancy will be broken and germination started if the wheat stays wet longer. When conditions are cool and wet for an extended period of time, even cultivars that are highly resistant to pre-harvest sprouting will sprout.

How do you spot a sprouted grain?

It takes a lot of rainfall to make a seedling visibly sprout out from a wheat spike (about 3 days of constant rain at 70°F). But you can see visible signs of sprout in as little as 24 hours if you look closely at an individual grain. You can sometimes see a small root protruding from the germ-end of the grain (Figure 2). Such grain can have a very low falling number (under 200 sec). As the sprouted grain dries, the root can shrink back into the grain leaving behind a small crack or crater at the embryo/germ end. Sometimes this cracked end breaks, leaving behind a germ-less grain. A magnifying glass can be useful in spotting these early signs of sprouting.

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