In spite of technological advances, weather factors still play a major role in crop production, especially precipitation. The July 5, 2016 U.S. drought monitor shows that more than half of South Dakota is under abnormally dry or moderately to severe drought conditions. The current drought stress in SD is more pronounced in the Northeastern and Western regions. Even though the crop producers with established irrigation systems are usually able to manage crop water needs more effectively, some agronomic considerations may prevent the situation from getting worse for producers under dryland management systems. Consider the following management recommendations during drought conditions.
Weeds under drought conditions tend to show more tolerance to applied herbicides that those growing under optimal conditions because plants may develop a thicker wax layer on the leaf surface to conserve moisture which can obstruct the herbicide absorption. Herbicides applied during highly stressful environmental conditions may cause unwanted injuries to already stressed crops and not affect weeds as intended. Systemic herbicides such as Roundup and SU herbicides are known to have reduced effects during hot and dry conditions.
Weather factors can influence insect populations quite dramatically, both in favor of and against the growing crops. One of the groups of insects to lookout for during hot and dry conditions are grasshoppers. Warmer springs like the one encountered in 2016 can favor early hatch, which can result in grasshopper populations that are denser and mature. During hot and dry conditions grasshoppers may move from less favorable hosts to those that are still green. This can result in serious defoliation depending on the population density. Another important pest of crops during dry conditions are spider mites. Both Banks grass mite and twospotted spider mite populations can increase in response to drought conditions. Other insects such as soybean aphids may also increase during dry conditions, but can have population growth that is limited by excessive heat. The real issue that often occurs is when management of one pest (e.g., grasshoppers or soybean aphids) occurs during dry conditions it can actually lead to outbreaks of spider mites. This is especially true when pyrethroid insecticides are used for management. This chemistry is known to remove natural enemies of spider mites, which allow their populations to rapidly grow.
Nutrient Deficiency Symptoms
Under dry conditions, plants may show moderate to severe nutrient deficiency symptoms even when the soil contains adequate amount of nutrients needed for normal growth and development. These symptoms could be in the whole field or on a small area. One of the major nutrients that could be harder for plants to extract during moisture deficit situations is potassium. When the field shows deficiency symptoms, a good practice would be to sample soil and plant tissues from good and bad portions of the field and conduct nutrient analyses on both sets of samples to confirm the presence (or absence) of these nutrients. Further, if the dry (granular) nitrogen fertilizer is applied late, it may stay on the soil surface for prolonged periods of time and not be readily available to the plant as roots are penetrating deeper in the soil for resources. This will cause plants to show N-deficiency symptoms.
Nitrates in Forages
Nitrates in forage crops when converted to nitrites can cause increased toxicity to livestock. Under drought conditions plants cannot effectively metabolize nitrates into protein and amino acids. These nitrates can build up in plant tissues and cause toxicity when fed to livestock. As grass forages are more likely to be fertilized with nitrogen, it is recommended to test for nitrates when these crops are grown under drought conditions.