Last fall we were concerned that wet fall weather would impact wheat planting and potential wheat yield. Soybean and corn harvests were delayed well past the Hessian Fly free date. This meant that wheat planting was prevented or wheat was planted after the optimum planting date. The Missouri Agricultural Statistics Service estimated that planted wheat acreage in Missouri was less than 450,000 acres. Unfortunately, many of these acres are suboptimal for stand may need to be abandoned.
In the November 3, 2010, issue of the Integrated Pest & Crop Management newsletter I discussed the difficulty of predicting the response of wheat yield to planting date. The complication is that seedling emergence and grainfilling occur in two different years separated by a winter dormant period. The effect of planting date on wheat yield is highly dependent on weather conditions between planting and establishment of dormancy. To maximize yield, wheat plants must accomplish three things during the autumn growth period: develop a root system to resist heaving, store sugars associated with winter hardening, and produce tillers to increase head number.
Autumn weather in 2009 was not kind to wheat plants. Cool weather slowed overall wheat growth and inhibited tillering. Tillers are important because they produce grain heads in the spring. Normal production of tillers triples or quadruples the number of heads. Wheat yield is severely decreased by inadequate tiller development.
Many farmers are discovering that their wheat plants produce few if any tillers last fall. Wheat plants are able tiller in the spring, and early spring application of nitrogen may increase the number of tillers. Wet winter and early spring weather prevented early application of fertilizer. Even if it had been possible to apply fertilizer, it is unlikely that spring tillering could have produce enough tillers to maintain yield potential.
The science of predicting wheat yield from tiller counts is imprecise. As a starting point, it takes about 60 heads per square foot to maximize yield. The formula to estimate yield with fewer than 60 heads is mostly a guess. A conservative approach would be to use a linear relationship. In other words, a field that averages 30 heads per square foot would yield 50% of whatever the yield potential of that field might be. Few things in nature follow a straight line, so this approach might underestimate yield potential of Missouri wheat plants. But, I believe in most instances this calculation will provide at least part of the information needed to determine whether to retain a weak wheat stand or abandon it for another crop.
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REVISED: October 2, 2015