By Pierce Paul and Laura Lindsey, Ohio State University Extension
This July has been one of the wettest on record for the state of Ohio, and with the extra moisture, comes concerns about diseases in corn and soybeans and harvest and potential quality challenges with wheat.
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.
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 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.
In addition to sprouting, the growth of mold is another problem that may result from rain-related harvest delay. 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. Even in the absence of visual scab symptoms, the fungi that produce vomitoxin may still colonize grain and produce toxins if harvest is delayed.
Not only has rain delayed the wheat harvest of some fields for grain, it has also delayed the baling of straw in several fields that have been harvested. In Ohio, wheat straw is sometimes just as economically important or even more important than grain, as it is used as bedding for livestock, and in some cases, as a feed ingredient. Delayed baling due to excessive rainfall could cause the quality of the straw to deteriorate as a result of mold growth.
To fungi (molds), wheat straw is nothing more than dead plant tissue ready to be colonized. Under warm, wet conditions, saprophytic fungi (molds that feed of dead plant materials) readily colonize wheat stubble, resulting in a dark moldy cast being formed. This problem is particularly severe in lodged fields.
Some of the molds growing on the wet straw in the field, including the fungus that causes head scab (Fusarium graminearum), also produce mycotoxins such as vomitoxin. This could be a concern when using straw for bedding or as a feed ingredient. In general, vomitoxin levels are much higher in straw than they are in the grain, and even when the straw is used as bedding, a small portion of it is consumed.
For instance, depending on the production system, pigs may obtain up to 12% of their total feed intake from straw. Pig are particularly sensitive to vomitoixin — it causes vomiting (hence the name), feed refusal, and low weight gain. A good way to tell if the straw is colonized by the scab fungus it to look at the color of the mold growing on it – a pinkish-white mold would be a good indication that the straw could be contaminated with vomitoxin.
Vomitoxin contamination of straw tends to be highest when the straw is baled from fields with high levels of head scab. In addition, straw from fields that were planted with head scab resistant variates usually contain less vomitoxin than straw from fields plated with susceptible varieties. Thankfully, head scab levels were very low in most fields across the state this year, as a result, initial vomitoxin levels in the straw may be low. However, straw from scab-free fields can still be colonized by the fungus and contaminated with vomitoxin, especially if it goes through repeated wetting and drying cycles before being baled. This would be even more of a problem for those fields that have not yet been harvested, as both the grain and the straw could be contaminated with vomitoxin. Moldy straw could also contain other mycotoxins that are harmful to livestock. So, in addition to examining the straw for mold growth (pinkish color), get a sample tested for mycotoxins before using it for bedding or feed.