The July 9 MN Ag Statistics crop report shows St. Cloud is 2.28 inches of rainfall below average since April 1 and has had 26 percent higher than average degree growing days since May 3. This documents what we can all see as we look across the landscape – the effects of persistently warm and dry weather. Based on research from Wisconsin, the effects of severe drought stress for 7-10 days at various stages of development were as follows (yield decrease percentage – numbers reflect corn stage and per day of stress: 10 leaf, 1-2; 15 leaf, 2-4; tassel/silk, 5-15; blister, 3-6; milk, 3-5; dough, 3-4; dent, 2-3; and maturity, 0. The yield reduction can be greater if the stress extends for longer times. A lot of crop in central Minnesota has endured drought stress for more than 7-10 days at a time and serious damage has occurred on lighter soil where rainfall has been short. The two-week period prior to tassel and silk emergence and for two weeks following pollination are most critical. Some crop is already hurt badly enough so there will not be tassels or silks. This is the fourth consecutive year of drought damaged crops in some neighborhoods. South Dakota Extension colleagues Robert Hall and Edward Twidwell wrote a bulletin on the effects of drought on corn in 2002 describing the normal process for pollination and the effects of drought stress. The pollination period in non-stressed corn generally lasts about eight days. Tassels emerge and begin to shed pollen two days before the first silks emerge. Once the silks emerge, pollination begins and lasts about six days until all the silks have emerged and pollen shedding ends. In contrast, in stressed corn the pollen begins to shed but silk emergence is delayed three or four days. Tassels also stop shedding earlier. So the opportunity for pollination to occur can be reduced from six days under good conditions to one or two days for stressed corn. Pollen shed can stop before all silks have emerged. Partial and blank ears can result. A Nebraska Extension Article in 1995 says that temperatures above 95 degrees with low relative humidity will desiccate exposed silks. Pollen is no longer viable once temperatures reach the mid-90s, especially with low relative humidity. Fortunately, pollen shed usually occurs from early to mid-morning when temperatures are lower. Drought and other kinds of stress can slow the emergence of silks so pollen shed is out of synch with silk emergence. The amount of pollen shed from one plant can be sufficient for 10 plants when tassels and silks develop normally. Determining the success of pollination – If fertilization of the ovule is successful, within one to three days the silk will detach from the developing kernel. Silks will remain attached to unfertilized ovules and be receptive to pollen up to seven days after emergence. Silks turn brown and dry up after pollination.
