Hot & dry: Stress on the corn crop escalates
uestions and concerns about the potential of Indiana's corn crop are increasingly on the minds of growers throughout the state, especially in areas hardest hit by the current drought. Rainfall continues to be hit or miss as isolated storms rumble through the state, dropping decent amounts of rain here and there but missing large areas entirely.
The moderate temperatures enjoyed throughout the state since the end of May delayed the onset of severe drought stress symptoms in many areas, but those temperatures have recently climbed to above-normal levels and, coupled with excessively dry soil conditions, increasingly reveal the severity of the situation in field after field. Areas in the state have already "gone over the brink" to disaster status relative to yield potential.
Eighty-seven percent of the state is currently estimated to have subsoil moisture content that is rated short to very short and is higher for this time of the season than any year since 1988. Estimated crop condition continues to worsen; down to 27% good to excellent as of 24 June 2012 and is lower for this time of the season than any year since 1988.
For some, the effects of the drought began after planting in fields where seedbed moisture was simply inadequate for germination and emergence of the crop. Large areas in these fields are simply devoid of corn or soybean plants. With no rainfall after planting, many farmers elected not to attempt replanting knowing that the second attempt at establishing a crop would not be successful. The lost yield potential in such fields obviously cannot be recovered.
For other fields where initial stand establishment was satisfactory, the severity of the drought stress on crop development has slowly become worse and/or has escalated in recent weeks as the combination of drought and heat stress worsens. The severity of the stress is such in some fields that plants are simply dying. Some growers have already worked with their crop insurance adjusters to assess the yield loss potential of their damaged fields and have basically abandoned the crop for this year.
This early in the season, it is difficult to estimate the effects of the drought on the eventual statewide average grain yield because we cannot accurately forecast the weather for the remainder of the season. Trend yield with "normal" weather for corn in Indiana for 2012 would be 163 bushels per acre (bpa).
There is a moderate linear relationship between crop condition ratings (percent good and excellent) throughout June and grain yield at the end of the season that accounts for about 33% of the variability in historical grain yields. Based on that relationship alone, the predicted statewide average grain yield for corn as of the end of June would be 145 bpa or nearly 11% below the trend yield.
However, if the drought continues and crop condition worsens, this estimate would quickly become outdated. For comparison, statewide grain yield for the drought years 1988 and 1991 were 31% and 27% below trend yield, respectively.
Much of the state's corn crop will enter the critical pollination phase over the next two or three weeks. Continued lack of adequate rainfall during this period will place undue stress on the crop. Forecast excessive heat during the same time period will only amplify the stress; resulting in delayed silk emergence, premature pollen shed, tassels failing to emerge from whorls, potentially inviable silks or pollen, poor synchrony between exposure of silks and availability of pollen, incomplete pollination success, or abortion of newly developing kernels. Yield loss per day during pollination due to severe stress is often estimated to be 5 to 10 percent. Yield loss for any given field could easily approach 100 percent with continued drought/heat stress due to complete failure of the pollination process.
Assessing Drought Effects on Yield
Growers who elect not to abandon drought-stressed fields prior to pollination will be wanting to assess the yield potential of their fields prior to harvest in order to estimate the potential loss of net income and/or to revise their grain marketing strategies for this fall. Assessing yield potential for an individual field cannot be done with any accuracy prior to pollination, but rather depends on assessing kernel set later in the grain filling period. The challenge with assessing yield potential of drought-stressed fields is that of obtaining ear samples that adequately captures the variability of the yield potential throughout a damaged field.
Immediately following pollination
Given the severity of the drought stress in some fields, growers may want to assess the success of pollination itself within the first week after pollen shed by sampling ears and conducting the so-called "ear shake" test. This assessment simply gauges the degree to which pollen successfully fertilized the ovules on the ears via the silks, but does not predict the risk of kernel abortion in the few weeks following pollination or estimate grain yield. However, the test will at least give you an idea whether pollination was an absolute failure or was moderately successful. Unfortunately, there will be fields that appear to be only moderately stressed according to windshield surveys that, in fact, are severely stressed to the point that pollination will be a near or total failure. Better to find out the bad news soon rather than be totally shocked later in the season.
Prior to harvest
Assuming that pollination occurs successfully, then pre-harvest grain yield estimation revolves around kernel abortion and grain filling. Unfortunately, one cannot begin to estimate grain yield for an individual drought-stressed field until the crop is beyond the milk stage of the grain filling period where the risk of kernel abortion tapers off. The risk of kernel abortion is greatest immediately following successful pollination and decreases to nearly zero by about mid-milk stage of grain fill. Once the crop reaches or surpasses the dough stage of grain fill, the "ballpark" grain yield potential can be estimated by documenting effective plant populations and sampling ears throughout the field.
Pray for rain or turn on the irrigation.