As the crop reaches physiological maturity, the seed develops a pre-harvest or primary dormancy. This dormancy is to prevent the seed from germinating as it ripens and dries down in the field. Time and/or a period of cold temperatures, such as during grain storage over the winter months, will break this dormancy.

Unfortunately, not all varieties of wheat have an equal amount of pre-harvest dormancy. The amount of dormancy is partially under genetic control. Conditions during the growing season also influence the amount of dormancy. High temperatures during grain fill tend to decrease the pre-harvest dormancy. Generally, red-seeded wheat varieties have more pre-harvest dormancy than white seeded wheat varieties.

Varieties that have less pre-harvest dormancy are at a higher risk for pre-harvest sprouting. This pre-harvest sprouting results from imbibing or uptake of water of the physiologically mature and ripening wheat kernels. The kernels reach physiological maturity when all the dry-matter has accumulated in the developing seed and the period of ripening starts. At physiological maturity the seed still has a moisture content of 30 – 35%.

Physiological maturity can be recognized by a loss of green color from the developing kernels. This generally coincides with the loss of green color from the peduncle. The peduncle is the last elongated internode that supports the head.

The (repeated) rains will cause the ripening seed to imbibe water. This in itself will result in loss of test weight because of the (repeated) swelling and shrinking of the seed. Heavy morning dews can give the same results, which is why swathing grain increases the odds that grain can take up water and lose pre-harvest dormancy.

Ultimately the kernels will loose their pre-harvest dormancy and may sprout in the field. Pre-harvest sprouting results in: 1) yield losses, 2) decrease in test weight, and 3) activation of the enzyme complex in the seed, in particular α-amylase, which will result in low falling numbers, poor seed vigor, and problems when processing the grain.

Dr. James Anderson, University of Minnesota’s wheat breeder, screened the current HRSW varieties for pre-harvest sprouting tolerance in 2002 and 2003. Below is a picture taken from the pre-harvest sprouting screening as conducted by Dr. Anderson, showing the best and the worst pre-harvest sprouting from left to right (Photo 1). Of the current HRSW varieties Ingot was rated very poor, while Mercury and Dandy were rated poor (Figure 1). All other varieties that were tested were rated acceptable to good. Nonetheless, even the most pre-harvest sprouting tolerant wheat varieties will eventually sprout when wet weather continues for a prolonged period.

Photo 1 Differences in pre-harvest sprouting tolerance among different HRSW varieties (Source: James Anderson, University of Minnesota).

Figure 1 Pre-harvest sprouting rating for 21 HRSW varieties tested in 2002 and 2003 (1=best and 5=worst. Source: James Anderson, University of Minnesota)