Our topography and soil types prevent most crop damage during flooding; however, time, temperature and plant growth status are major factors that affect the extent of crop damage after a flood. This may become an issue with a prolonged rainy period. A June or July flood, for example, is often much worse for crop survival than a spring flood. The warmer mid-summer weather increases the rate of damage and death to submerged plants. During spring flooding, temperatures are colder and plants can survive longer under water.
Most forages that encounter flash-flooding along creeks where the water rises and recedes quickly are most likely to survive. They will experience less oxygen depletion than submerged plants. Other factors for survival include water movement and plant height. Standing water is more harmful than moving water. Plants with some leaves protruding from the water are more likely to live.
Alfalfa can withstand submersion for a limited time, depending on its stage of growth. Growing plants can usually withstand submersion for less than three to four days without damage.
The mechanisms for damage include:
- Lack of Oxygen–Alfalfa roots must ‘breath’ just like humans, and respiration is reduced under waterlogging. If severe, lack of oxygen can cause death or lots of damage.
- Temperature–Damage is greater under warm temperatures vs. cold due to increases in respiration rates (in plants and soil microorganisms). Hot temperatures can kill alfalfa within hours, but the crop can survive for days under cold temperatures.
- Death of fine root hairs–Fine root hairs are particularly damaged during waterlogging and must be regenerated later if the plants survive. These are critical for nutrient and water uptake later.
- Micronutrient availability -Under reducing conditions (low oxygen), iron and other micronutrients may become unavailable for plant growth due to excess bicarbonates or other mechanisms including root damage.
- Disease and pests–Since saturated soil conditions favor disease organisms, Phytophora, nematodes, and other organisms can gain the upper hand over a weakened alfalfa plant.
- Weeds–Aggressive cold-favoring, flood-tolerant winter weeds can completely dominate alfalfa stands weakened by flooding.
- Nodules— The Rhizobium nodules are weakened or damaged under flooded conditions, resulting in reduced nitrogen fixation.
After the intense flooding, when fields have drained, inspect roots and crowns for damage caused by anaerobic (without oxygen) conditions in the soil. Dig up roots and examine their health.
Check for disease in crowns or center of roots. Roots that are soft and compress easily when squeezed may be damaged beyond hope. If roots are beginning to release a strong odor, they likely won’t recover. If roots are white, they may.
Assess your stand to evaluate if you will have an adequate stand. Keep an eye on the plants, inspecting for damage and yield throughout the following summer; sometimes disease development is delayed with weakened plants succumbing to damage months later.
Contact your area Extension agronomist for help evaluating potential issues or visit the Extension Dairy Team website for more information.