We initially anticipated an early wheat harvest this year due to dry weather conditions. However, with cool, wet weather in some areas of the state, wheat harvest may be later than usual. Late harvest coupled with excessive rainfall means more time for late-season mold growth, mycotoxin accumulation, test weight reduction, and sprouting; all of which could result in poor overall grain quality. In 2018, we evaluated wheat harvested on June 29 (at 12% moisture content) and July 8 (at 14% moisture content). Grain moisture increased between June 29 and July 8 due to 0.58 inch of rainfall between the two harvest dates. When wheat harvest was delayed until July 8, yield decreased by 9 bu/acre, test weight decreased by 2.9 bu/acre, and DON level increased by 0.86 ppm. These reductions in yield and test weight and increase in DON are likely attributed to re-wetting of dry grain.
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. When grain is rewetted, the germination process begins, causing photosynthates (i.e., starch) to be “digested”. This leaves small voids inside the grain which decreases test weight. Additionally, grain will swell each time it is rewetted and may not return to its original size as it dries which will also reduce test weight. Thus, the enlarged kernels will take more space but weigh the same, allowing fewer kernels to pack in the measuring container, lowering the test weight.
Rain and harvest delay may also lead to pre-harvest sprouting in some varieties. Sprouting is characterized by the swelling of kernels, splitting of seed coats, and germination of seeds (emergence of roots and shoots) within the wheat heads. Some varieties are more tolerant to sprouting than others, and for a given variety, sprouting may vary from one field to another depending on the duration of warm, wet conditions. 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. Since varieties differ in their ability to take up water, their drying rate, the rate at which sugars are used up, and embryo dormancy (resistance to germination), grain quality reduction will vary from one variety to another.
Another important measure of grain quality that is affected by late-season rainfall and delayed harvest in Falling Number (FN), which provides an indirect measure of the activity of enzymes required for breaking down sugars during sprouting. Remember, once wheat starts to sprout (even before visual signs of germination are seen), enzymes are produced to break down sugars that provide the energy needed for sprouting. FN is reported in seconds. As enzyme activity increases, FN decreases. For instance, FN values of 350 seconds or greater indicate low enzyme activity, and relatedly, good wheat quality (low sprouting). On the other hand, FN values below 200 seconds are usually associated with severe sprouting (high enzyme activity) and low grain quality. For more information on a common test to measure FN, please refer to the article at go.osu.edu/FNtest.
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. This problem is particularly severe on lodged wheat. In general, the growth of blackish saprophytic molds on the surface of the grain usually does not affect the grain. However, 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. While vomitoxin contamination is generally higher in fields with high levels of wheat scab, it is not uncommon to find above 2 ppm vomitoxin in late-harvested fields that have been exposed to excessive moisture. Although head scab was low across the state this year, with limited visual scab symptoms in some fields, the fungus that produces vomitoxin may still colonize grain and produce toxins if harvest is delayed, particularly in fields exposed to frequent rainfall between grain-fill and harvest.
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