As wheat harvest gets started in Washington, recent rainfall and cool temperatures have some growers worried about Mother Nature’s fickle ways with their crop. Rainfall close to harvest can result in preharvest sprouting, which can negatively affect wheat quality. Dr. Camille Steber, USDA-ARS plant geneticist, explains the potential effects of recent rains on this year’s wheat crop and what growers can do to manage this risk.
The Hagberg-Perten Falling Number test is used to measure starch damage due to sprout. Low FN is used as an indicator that grain contains a high level of alpha-amylase, an enzyme that degrades starch leading to poor end-use quality of wheat products (Figure 1). Grain with an FN below 300 seconds is typically discounted in the Pacific Northwest.
Wheat that has been rained on is at risk of low FN. Eastern Washington experienced periods of rainfall July 7 to 12, 2016, just as winter wheat harvest approached. Chances are, buyers will check FN to protect themselves from sprout damage risk. Whether or not your grain is likely to have low FN problems depends on the susceptibility of the cultivar you grew, on the local weather, and the timing of the rain relative to maturity date.
Preharvest sprouting is the initiation of grain germination while still on the mother plant. Germinating seeds degrade starch for use in fueling growth. Lack of seed dormancy explains 60-80% of genetic sprout susceptibility. Dormant grains can’t germinate, and so don’t suffer sprout damage in the rain. Seed dormancy is strongest at maturity, just as the wheat turns from green to yellow. Dormancy is lost gradually over time as the dry, mature grain “after-ripens” (Figure 2). Winter wheat that still had some green color is less likely to have a low FN due to sprouting than wheat that was completely yellow and dry when it rained. Since green grain cannot sprout, spring wheat that was green when it rained should be safe from low FN, as long as there isn’t another ill-timed rain event.
Not all rainstorms induce sprouting. Seed dormancy is broken by cool, rainy conditions. So if the temperatures are in the 80s °F when it rains, the wheat is less likely to sprout than if the temperatures are in the 60s. Low FN is also more likely when there are multiple rainy days in a row, as the wheat stays wet longer.
How do you spot sprouted grain?
It takes a lot of rainfall to make a seedling sprout out from a wheat spike (about 3 days of constant rain at 70 degrees). If you look closely at a mildly sprouted grain, you can sometimes see a small root protruding from the germ-end (Figure 3). Such grain can have a very low FN (under 200 sec). As the sprouted grain dries, the root can shrink back into the grain leaving behind a small crack at the embryo end. Sometimes this cracked end breaks, leaving behind a germ-less grain. So get out your magnifying glass.
Low FN (200-300 sec) can also be caused by late-maturity alpha-amylase (LMA) induced by heat shock or cold shock during grain maturation. LMA causes low FN in grain that appears to be sound. We had some big temperature fluctuations this summer, so there may be some lower FN in wheat that saw no rain. Some LMA-susceptible suspects include SY-Ovation, Bruehl, Jasper, and Alturas.
Plant cultivars with genetic resistance to sprouting and LMA. We can use past preharvest sprouting events to judge which cultivars have more genetic resistance to sprouting. For example, there were major sprouting events in Fairfield, Lamont, Pullman, and other locations in 2013. The FN of all cultivars grown in the WSU Cereal Variety Trial at all locations in 2013 and 2014 can be found on the Project 7599 PNW Falling Number website. One problem is that many of the highly PHS tolerant cultivars such as Mary and Masami are older cultivars that may not compare well to recent cultivars for yield and disease resistance. The falling number versus yield tool on the Project 7599 website can help you take both yield and FN into account when choosing a cultivar. Examples of sprouting-resistant cultivars include Puma, Skiles, Coda, and Bobtail. Sprouting-susceptible cultivars include Bruehl, Xerpha, AP-Legacy, LWW10-1018, and Bruneau. Ongoing research will improve sprouting resistant choices.
Harvest wheat quickly after maturity to reduce the risk of getting rained on. Wait for the rained-on wheat to dry well before harvesting to avoid germination in the truck. Also, avoid harvesting green wheat, as green kernels have higher alpha-amylase (lower FN) than mature grain. Green kernels can be a problem if you combine wheat that is yellow on the hill with green wheat from the draw.
Avoid mixing likely-sprouted grain with likely-unsprouted grain. A little bit of alpha-amylase can cause big FN problems. Mixing equal amounts of FN 200 grain with FN 400 grain will not give you a load at FN 300 sec. Instead, you will end up with something well below 300. If you have one field that was greenish and another field that was fully yellow when it rained, you might make more money if you keep them separated when you sell. The same is true if you planted both a sprouting-resistant and susceptible cultivar in separate fields. Store mildly sprouted grain. Some research suggests that alpha-amylase levels drop during storage – it could be that UV light or heat degrades the enzyme over time. If the FN is moderately low (200-300 sec), it might help to store grain for 2 to 3 months to see if the FN rises. If the FN is very low, storing the grain won’t reverse starch damage that has already occurred. So storing the grain may not greatly improve its value.