At physiological maturity, wheat grain moisture is about 37%, however at this high moisture level, the grain is too soft for combine harvest. Most producers harvest when the grain has dried to between 15 and 13% moisture. Rainfall between maturity and harvest is known to lower grain test weight and because of climatic conditions around harvest time, delayed harvest is a common occurrence in Virginia. Recently published research from North Carolina State University has quantified test weight loss in response to rainfall after wheat grain maturity.
Their study included two soft red winter wheat cultivars, 'Pioneer 26R61' and 'Coker 9704' studied at six site-year combinations across North Carolina in 2001 and 2002. The two treatments evaluated were "timely harvest" when the grain first reached 13.5 % moisture and a "delayed harvest" 8 to 19 days later, depending on conditions at a site.
Figure 1 shows the relationship between the number of precipitation events and the change in test weight between timely and delayed harvests. The more rainfall events, the greater the loss in test weight. On average, a12 day delay resulted in a 6.4% reduction in test weight going from an average of 58.8 lb/bu to 55.0! The number of times the grain experienced wetting and drying cycles was much more important than the total precipitation received. As would be expected, the number of rainfall events increased with the number of days between harvests, so the longer wheat stayed in the field after ripeness the greater the opportunity for more precipitation events and wetting and drying cycles.
Other than hoping nature provides ideal conditions, there are several things managers can do to increase the likelihood of harvesting high quality wheat.
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