Our recent rainy weather combined with saturated soils is going to delay harvest of wheat harvest. In a recent OSU Extension crop newsletter, OSU Extension specialists Pierce Paul and Laura Lindsey outline some of the possible problems this can cause. Late harvest coupled with excessive rainfall mean more time for late-season mold growth, mycotoxin accumulation, test weight reduction, and sprouting, all of which collectively could result in poor overall grain quality.
Test weight (grain weight per unit volume or grain density) is one of the grain quality traits most likely to be affected by harvest delay and wet conditions. Low test weights usually occur if grain is prevented from filling completely or maturing and drying naturally in the field. Rewetting of grain in the field after maturity but prior to harvest is one of the main causes of reduced test weight. Rain and harvest delay may also lead to pre-harvest sprouting in some varieties. Sprouting affects grain quality (test weight). Once moisture is taken up by mature grain, stored reserves (sugars especially) are converted and used up for germination, which leads to reduced test weights. Even before visual signs of sprouting are evident, sugars are converted and grain quality is reduced.
In addition to sprouting, the growth of mold is another problem that may result from rain-related harvest delay. To fungi, mature wheat heads are nothing more than dead plant tissue ready to be colonized. Under warm, wet conditions, saprophytic fungi (and even fungi known to cause diseases such as wheat scab) readily colonize wheat heads, resulting in a dark moldy cast being formed over the heads and straw. The growth of pathogens, usually whitish or pinkish mold, could result in low test weights and poor overall grain quality. In particular, in those fields with head scab, vomitoxin may build-up to higher levels in the grain, leading to further grain quality reduction and dockage. Even in the absence of visual scab symptoms, the fungi that produce vomitoxin may still colonize grain and produce toxins if harvest is delayed.
To minimize grain quality losses, it is best to harvest wheat on the first dry-down. Harvesting at a slightly higher moisture level (18% for example) may also be useful for minimizing quality losses, particularly those associated sprouting and mold growth due to rainfall and harvest delay. However, if grain is harvested at moisture above 15%, it should be dried down below 15% before storage to minimize mold growth and mycotoxins in storage.