Effect of High Temperatures on Small Grain Development and Yield (7/12/12)
Moreover, during this period soil moisture tended to be adequate for the demands of the developing small grain crop, even though rainfall was below normal. June and July, on the other hand, have been hot and dry. The high temperature has accelerated small grain development and greatly increased water demand by the developing crop. Water stress is now commonly observed which compounds the deleterious effects of the high temperatures. Much of the early planted wheat and barley crops are now approaching maturity. The question of the day is how is high temperature impacting the small grain crops in the state.
Wheat, barley and other small grains are cool season crops. Accordingly, they develop best when temperatures are cool; yield is favored by daily maximum temperatures below 70 degrees. During the vegetative stage, cooler temperatures (in the absence of other stresses) results in more tillering and spikes with more spikelets. During the four to five leaf stage, for every five degrees rise in the maximum daily temperature above 65 degrees, wheat will produce roughly one less spikelet per spike. High temperatures during the early vegetative stage were not an issue for the early planted crop, but for late planted crops, small spikes will predominate and yield will be reduced because of a reduction in the number of kernel the plant will produce. High temperatures hasten development and concomitantly reduce plant height. I think most growers will find their small grain crops at least two or three inches shorter than previous years due to abnormally high temps in June and July. High temperatures at flowering can impact the fertilization process. For crops flowering during the recent hot spell, some reduction in seeds per spike might be seen, either because of pollination failure or kernel abortion shortly after fertilization. This will be particularly pronounced if coupled with moisture stress during this period. Plants have a high water requirement during flowering and early grain fill and this year it is quite common to see bleached spikes and dead plants that simply ran out of water as their diseased roots and vascular system were not able to conduct sufficient water to meet the plant’s needs. These plants and spikes will of course not contribute to the yield of the field.
During grain filling, high temperatures reduce the rate of photosynthesis, thereby reducing the amount of starch available to the developing kernel. Rates of grain fill actually accelerate with increasing temperatures, but the maximum kernel weight is reduced due to a dramatically shortened grain filling period (see accompanying table). This is particularly true if night temperatures are also high. The reduction in kernel weight can be attributed to a reduction in the deposition of starch. So though kernel size is reduced with high temperatures, the amount of protein deposited in the kernel will largely be unaffected, resulting in kernels with a high concentration in protein. One could say that is good news, but in a year like this, there will almost certainly be no premium for high protein, so the value of the higher protein will not begin to make up for the loss of yield. As growers begin harvest, they should expect to see smaller kernels, lower yields and high proteins as a result of the high temperatures we have experienced during these last few weeks. Furthermore, for late planted crops, yields will be substantially reduced because of the added impact of higher temperatures during the vegetative stage on spike size and tiller numbers.
Extension Agronomist for Cereal Crops