What does too much rain mean to your crops?
By Chris Clark, WI-CCA, AgSource
August was the first month I recorded below average precipitation for 2019 in northeastern Wisconsin where I live. With record-breaking precipitation totals being recorded around the state, what is the effect of too much rainfall on crops?
- Crop yield is affected by excessive water just as much as excessive heat or drought. An interdisciplinary study from the University of Illinois looked at the effects of weather on crop yield from 1981-2016. Excessive rainfall reduced US corn yield by as much as 34%, nearly as much as excessive heat which caused a yield loss of 37%. The overall impact of excessive rainfall varies regionally partly because in cooler areas with fewer GDD’s (Corn Growing Degree Days), soils take longer to dry out, especially late in the growing season closer to harvest.
- Delayed planting reduces yield potential. Historically, the University of Wisconsin says 50% of the corn acres are planted by May 7, but this year maturity varies widely across the state with some acres of corn planted as late as July. GDD, which drives corn grain maturity, is 300 units behind the 30-year average and the cool moist weather of October will not help. Frost will be the biggest concern for achieving good yields even if the frost is not early as there is even concern about having enough mature grain to make good silage.
- An ideal soil has 25% air space and 25% water space. But this year has not been ideal. In general, most crops are resilient and can handle ponded or flooded areas for 36-48 hours without problem and some will even survive 3-7 days of excess water stress. But this year we’re seeing more extensive damage because the ground has been wet since last fall in some areas. The water table is often just below the soil surface. And once clay and clay-like soils become saturated, they take a long time to dry out.
- The excess rainfall all year has physically damaged plants. Without oxygen plant roots cannot perform at 100% and shoot development lags because roots need air for water transpiration and nutrient uptake. Overall, excessively wet soils reduce the metabolism of plants decreasing yield and the plants can be easily stressed, have stunted growth, experience root damage or even die. Many crops have thus developed very shallow root systems and are not effectively taking up soil nutrients.
- The excess rainfall also means plants are easily stressed by other environmental factors. In August we actually had a week or more without rain and plants with shallow root systems quickly showed signs of drought stress. A stressed crop is also susceptible to insect and disease pressure and the yield reductions they cause. Soils that stay wet too long start to accumulate mold, fungus and other pathogens and rainfall then spreads the pathogens among plants in the field. Downy mildew and white mold were common but some of this pressure was kept in check by cooler local temperatures, and even the delayed planting shortened the time available for disease pressure to build.
The yield losses can mean excess soil nutrients. Excess soil moisture, late planting, and the anticipated late harvest probably means lower yields, but it could also mean crops are leaving nutrients in the field. Phosphorus and potassium fertilizer applied this year, but not used by the crop, will be retained in the soil. This can be used as available fertilizer for next year’s crop. Test the soil and adjust next year’s recommendation to make use of the excess P and K retained. This is a good opportunity to recover some of the lost revenue from low yields as reduced expense for the next crop. Be sure to use the corn stalk nitrate test to see how the crop used the nitrogen that was available. Use this data with results from other years to adjust the nitrogen fertilizer rates to target good yields.