Dr. Heather Darby, UVM Extension Agronomist
Very low precipitation over the last few months has caused drought stress in isolated areas of the state. The dry weather can have an impact on corn growth and development. For many farms corn silage harvest will begin in the next few weeks partly due to an above average accumulation of growing degree-days (GDDs) and for some due to severe drought stress. The goal of this article is to describe the impact of drought on corn development and provide suggestions for managing drought-stricken silage.
Impact of drought on corn pollination
Many producers have observed leaf rolling in their corn fields especially in fields where soil compaction is severe. Some of these fields are entering the critical pollination and fertilization period where any type of environmental stress will result in yield loss. About 2 weeks before silk emergence, corn enters the period of grain yield determination. Corn is the most sensitive to drought stress during this period. Continued wilting of the plant at this stage can decrease yield 3 to 4 percent per day. Inadequate plant water can also delay silk elongation and silks that do emerge may become non-receptive to pollen. Obviously this can result in poor pollination. During the silking and pollen shed period, severe stress may reduce yield up to 8 percent per day.
Impact of drought stress on grain-filling
Water stress during grain-filling reduces yield 2.0 to 6.0% with each day of stress.
Abortion of kernels during the first 2 weeks following pollination is common during drought. Kernels can also abort during blister and milk stages if there is severe drought stress. Once kernels have reached the dough stage of development, yield losses will occur mainly from reduced kernel test weight. Drought stress during dough and dent stages can lead to premature black layer formation in the kernels.
Impact of drought stress on corn nitrate levels
Drought conditions can cause nitrates to accumulate in corn plants. Under normal growing conditions, nitrates are quickly converted into plant proteins and other compounds. When plant growth is slowed or stopped, nitrates can accumulate in the plant. Rainfall following an extended dry period may cause an immediate increase in nitrates for 2 to 5 days until the plant can utilize the nitrates.
There are several strategies that can help reduce nitrate levels in drought-stressed plants. First, try to wait 3 to 5 days after an appreciable rain or long cloudy spell before harvesting crops. Since nitrates accumulate in the stalks, consider a higher cut height. Leaving 12-inch stubble in the field can reduce nitrates but would also reduce yields. Ensiling will also help to reduce nitrates by as much as 60 percent. Allow the forage to ferment for 4 weeks to allow for complete fermentation. Any suspect feed should be tested for nitrate levels before feeding. The silage can also be tested at harvest to determine if nitrates are a cause for concern.
Symptoms of acute nitrate poisoning in animals are related to the lack of oxygen in the tissues. These include muscular weakness, accelerated heart rate, difficult or rapid breathing, cyanosis, coma, and even death. Drop in milk production, abortion due to lack of oxygen getting to the fetus, poor performance and feed conversion are seen in chronic cases. The most critical factor influencing possible toxicity is the rate of nitrogen intake, which is affected by forage dry matter intake over a given time period. Feeding practices that regulate dry matter intake can be used to manage high nitrate forages. When stored forages contain more than 1,000 ppm NO3-N, intakes generally must be managed to avoid toxic effects (Table 1).
Lastly, high nitrates can also contribute to elevated levels of deadly silo gas. Silo gas is produced 4-5 days after silo filling. During this period the nitrates are converted to oxides of N. Nitrogen dioxide or NO2 is the most common and is a yellowish orange gas with a bleach-like odor. This gas is heavier than air and can form in the silo and then escape down the unloading chute into the barn, endangering humans and livestock. Exposure to silo gas can cause immediate death or severe lung injury due. To avoid exposure to silo gases, keep the door between the feed room and the barn closed, ventilate the silo by running the blower for at least 20 minutes before entering the silo and learn to recognize the bleach odor and yellow-orange color as signals of silo gas.
For more information on nitrate testing of forage please contact Dr. Heather Darby – (802) 524-6501 or Dr. Sid Bosworth – (802) 656-0478 at the University of Vermont Extension.