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Late-season Rainfall, Late Harvest And Wheat Grain Quality

The month of June was fairly cool which resulted in an extended grain fill period. Combined with low disease levels and low grain contamination with vomitoxin, wheat quality is expected to be good this season. However, growers are now finding it hard to get their crop harvested. It has rained consistently across most of the state over the last two weeks and growers are understandably concerned about grain quality. Indeed, rain and late harvest can certainly reduce test weights, increase fungal colonization of the heads, and cause pre-harvest sprouting.       

What is test weight? Test weight is used to take into account varying densities (weight per given volume) of grain. It is an indicator of grain quality. Generally the higher the test weight the higher yields will be for flour and starch. The standard commodity weight for soft red winter wheat is 60 pounds per bushel at 13.5% moisture. If test weight is below the acceptable range (low test weight), the wheat sale could be “docked.” Depending upon the elevator, dockage for test weight generally does not occur unless the value is below 58 lb/bu. Some elevators will give a premium for test weights 60 and above.

What causes low test weight? Grain density can vary based on weather, production practices, variety, and pests. Low test weights occur if grain is prevented from filling completely and/or maturing and drying naturally in the field. Rewetting of grain in the field prior to harvest can also reduce test weight.  When grain is rewetted, the germination process may initiate 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 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. If possible, for maximum test weight, it is best to harvest wheat on the first dry-down.    

Should I be concerned about sprouting? Rain and harvest delay may lead to pre-harvest sprouting in some varieties in some areas. 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 other, 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.

Why are heads turning black? 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 grain quality. In particular, on scab-affected heads, molds may produce toxins such as deoxynivalenol (DON), leading to further grain quality reduction and dockage. While DON contamination is generally higher in fields with high levels of wheat scab, it is not uncommon to find DON (above 2 ppm) in late-harvested fields that have been exposed to excessive moisture. Even in the absence of visual scab symptoms, the fungi that produce DON may still colonize grain and produce toxin.

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