By Emerson Nafziger
Professor of Agronomic Extension
University of Illinois

Harvests of corn and soybean are starting very slowly this year, with only 4% of corn and 6% of soybean harvested by Sept. 28. This compares to 56% and 37% harvested a year ago, and 5-year averages of 32% for corn and 25% for soybean. The good news is that crop maturity is reflecting weather conditions, meaning that the crops are not dying early, leaving a lot of unfinished seed-filling. Exceptions to this are noted in the following discussions of some harvest issues for 2008:

  • The few reports I've had on corn yields have ranged from mediocre to very high. This will be a year with wide ranges in yield, sometimes within a fairly small area. While the recovery from the poor start in the spring has been better than expected, there has been enough N loss, ponding, water damage, stand issues, and the like to cause yield losses in fields in some areas.

  • There is a great deal of talk about effects of conditions in recent weeks on "test weight," which is the term that many people use when they really mean kernel weight. As a reminder, test weight is the weight of a volume of grain (one bushel officially is 1.24 cubic feet). While higher kernel weights often mean higher test weight, factors like kernel shape, endosperm density, and seed coat slipperiness affect test weight as well. I recently looked at test weights in our corn hybrid trials from last year and found that test weight had no relationship with yield across hybrids. I also found, to my surprise, that test weight was in some cases negatively correlated with grain moisture. It makes sense that wetter kernels are a little puffy, which lowers test weight. I have not seen an official correction of test weights for grain moisture, but it may be worth keeping in mind if corn delivered at high moisture is docked for low test weight. Taking test weight in dried grain is probably more representative, though there might be rare cases, depending on hardness of endosperm and other factors, where kernels don't shrink in size as they dry, and perhaps even lose test weight as a result of losing the weight of water.

  • Cornfields where winds associated with the remnants of Hurricane Ike flattened the crop on September 14 may well have considerable yield reductions if kernels still had a lot of filling to do and leaf area did not stay green for long after the damage. Unfortunately, this damage was most severe in the southern part of Illinois, where much of the corn was planted late and so was not yet in the final stages of grain fill. Where soils were wet when the wind blew (as was the case here at Urbana), roots pulled out of the soil, resulting in root lodging. In drier soils, the lower stalks may have broken before the roots pulled out, causing stalk lodging. Some lodged -- especially root-lodged -- fields still have a fair amount of green leaf area, so this crop may still be filling grain to the extent that water uptake can still occur and the leaves can intercept light. I expect such fields to yield considerably less than they would have without lodging. It makes a difference whether plants are flat on the ground, as many are with severe stalk lodging, or are only leaned over close to the ground, which is often the case with root lodging. Those close to the ground often have some air movement around leaves and so may be able to take up more water and do more photosynthesis. Grain in such fields will dry down better than where plants are flat on the ground, but it will probably still be slow to dry due to tight husks and limited air movement around ears. Prematurely killed plants also have more sugars in their kernels, which hold water and thereby slow drying.

  • Both fields that have lodged and many that are still standing have developed, or are in danger of developing, kernel and ear rots. The heavy rainfall earlier in September contributed to this, as did insect injury. In some cases, water stood in the husk and encouraged fungal growth, or husks are tight on ears and kept kernels damp. Some have observed that moldy kernels show greater tendency to sprout on the ear. The embryo of the corn kernel has the ability to germinate by only a few weeks after pollination, but it is kept from doing so by a complicated biochemical system. This appears to be the result of some sort of biochemical reaction to injury that breaks the dormancy that normally keeps kernels from sprouting as they start to dry down.

  • One of the big issues this year will be slow drydown, and either the long wait for grain to dry to below 20% or having corn wetter at harvest than it has been in recent years. Unless kernels are infected with fungal diseases or stalks are very weak and likely to lodge, there is little danger that yield will be lost as grain dries in the field. But night temperatures in the 40s and daytime highs in the 60s are not a formula for fast drydown, so many producers will want to start to harvest by the time corn grain moisture reaches the mid- to lower 20s. Drying rates slow as grain becomes drier, and this effect will be even greater when temperatures drop as they normally do in October.

  • One of the painful uncertainties as we wait for corn to dry down in the field is whether stalks are strong enough to stay standing, especially if the wind blows. The wind and rain of mid-September was a good test of this, but as grain-filling comes to an end, sugars might still be moving out of the stalks faster than photosynthesis can replenish them, and stalks can weaken rapidly. This weakening will stop soon after physiological maturity, but it still may pay to go into fields and push on stalks to see if they break easily. All we can really do is to try to harvest fields with weakened stalks as soon as practical and keep our fingers crossed that the wind won't blow too hard before then.

  • While soybean harvest is starting considerably later than corn harvest, it is likely to proceed faster than corn harvest over the next week or so. Yields reported so far have been surprisingly good, after all the concern we've had since early August about dry weather and low temperatures. Even many fields planted in June are dropping their leaves and starting to move toward maturity--more than 50% of the crop in Illinois had dropped leaves by September 28. The obvious lesson is that we don't understand soybeans well enough to predict what they'll do under conditions like we've had this year. We'll keep trying, while also experiencing relief that, at least in some fields, yields are higher than we had thought they would be. I have had a few reports of soybeans dropping to 10% moisture or less before they're harvested. The cooler temperatures should mean fewer problems with very dry seed than we saw last year, and there seem to be fewer problems with green-stem than we have seen in some recent years. But keep looking at the soybean crop as it loses its leaves, and plan to start to harvest as soon as moisture levels drop below 14% or 15%.

    SOURCE: University of Illinois.