Richard Taylor, Extension Agronomy Specialist; rtaylor@udel.edu
A number of tests have been developed to estimate or measure the quality of grain produced by various types of wheat. Most of us are most familiar with the measure called test weight, which is a measure of the density of wheat or the number of pounds of kernels in a bushel volume. Other than levels of compounds like vomitoxin that are associated with scab infection, most quality problems are associated with pre-harvest sprouting (PHS). Tests for PHS range from visual scoring to enzyme analysis to viscosity analysis.
Pre-harvest sprouting occurs when wheat germinates within the grain head prior to harvest. This most often occurs after the grain has matured and when wet conditions either delay harvest or occur just prior to harvest but after grain maturation. Frequent showers creating within canopy high humidity conditions increase the chance for PHS. Wheat varieties have a variable dormancy period, although most of the wheat grown in our area does not have a very long dormancy as evidenced by the amount of germinated wheat we see in double-cropped soybean fields.
Another quality test associated with PHS that has been getting a lot of attention since the very rainy 2006 harvest season is the falling number (FN) test. The FN was first developed in 1960 and named ‘falling number’ in 1961 by the test developer, Sven Hagber. This test is a method for determining α-amylase activity in sprout-damaged grain. It was accepted as an Official AACC method in 1972 and on an international level in 1982. The method measures the effect of enzymes on wheat quality in flour or meal and is a measure of the time in seconds required for a viscometer stirrer to fall a given distance through hot, aqueous flour gel undergoing liquefaction by hydrolyzing enzymes. To simplify that explanation, let’s say it’s a measure of how fast the amylase enzymes in the wheat grain convert the starch of the grain (ground and put in liquid to make a paste) into sugars that convert the gel into something akin to sugar water. A high FN (>300 seconds) indicates that there was no sprout damage in a wheat grain sample; a number between 200 and 300 indicates some sprouting; and a FN<200 indicates severe sprouting damage.
The occurrence of sprouting signals an increase in the hydrolytic enzyme activity leading to starch and protein breakdown in grain from α- and β-amylase and protease enzymes. This reduces grain yield and quality and can cause the wheat to be downgraded to feed wheat costing the grower money. The reason for the down grade to feed wheat is that pasta and noodles made with low FN flour are fragile, soft, mushy when cooked, and lose more starch to the cooking water, making the water cloudy. Low FN flour also creates processing problems such as uneven extrusion, strand stretching, and irregularities in drying.
A low FN in soft red winter wheat (SRWW) always is detrimental when the flour is used in cakes, sometimes can affect cookie quality, and for pretzels it necessitates changes in the mechanization since pretzels require consistent flour quality. Other wheat products are not affected by a low FN.
Although the amount of dormancy before sprouting varies among wheat types, there hasn’t been much, if any, breeding for improved FN in wheat varieties within a class of wheat. So, how does the grower avoid PHS and low FN wheat grain. Since weather control is still beyond our capabilities—we still can’t even predict weather very well—growers have limited options for protecting themselves from PHS and a low FN. If the grower can dry the grain then harvesting wheat the first time it reaches 20 percent moisture is advisable but if drying is not an option then growers should harvest he first time wheat reaches 15 percent moisture. Growers should also modify the combine cylinder speed and/or concave clearance as harvest conditions change during the day.
Unfortunately all varieties of SRWW will sprout given exposure to the right conditions for a sufficient time. Cool temperatures during the late grain fill period will reduce the dormancy period and signals a need to harvest as early as possible, weather permitting.
Studies have shown that nitrogen fertilization rate either increases or decreases the FN. Fusarium infection does cause a minor decrease in the FN and fungicide treatment can also decrease the FN, although this is cultivar dependent. The largest effect on FN comes from weather conditions and especially rainfall that delays harvest.