Key Points:
- Forage and pasture plants are more tolerant of saturated soil conditions in early spring when they are growing slowly.
- Excess water increases the risk of root and crown diseases. Dig up plants to check for disease after the water has subsided.
- Flood water may leave behind silt that can smother a crop, raise ash levels, and/or introduce livestock disease-causing pathogens.
- Reseeding may be required if drowning or disease thin or kill a forage stand or pasture.
Saturated Soils
Roots need both water and oxygen in the soil to thrive. When soils are saturated, the pore spaces in the soil are full of water, and there is no room for air. If the soil remains saturated for too long, plant roots will die. This can create an opportunity for secondary infection of root rot diseases.
Plants are more tolerant of saturated soil conditions when they are dormant or growing slowly (Table 1). As the weather warms and growth rates increase, the roots require more oxygen, so tolerance to saturated soils is less.
| Species | Duration |
| Alfalfa | 1-2 weeks |
| Alsike clover | 2-3 weeks |
| Red clover | 3-4 weeks |
| Bromegrasses | 3+ weeks |
| Meadow fescue | 6+ weeks |
| Meadow foxtail | 6+ weeks |
| Timothy | 6+ weeks |
Flooding
Flooding prevents oxygen from reaching the crown of the plant. Flowing water tends to have more oxygen in it than stagnant water, so plants generally can survive longer if the flood water is moving. In general, perennials have greater flood tolerance than annual crops. Growing plants have much higher oxygen demands than dormant ones, so the time of year will also affect how well a forage stand comes through a flood. For example, dormant alfalfa can survive flooding for up to 7–10 days; if it is actively growing, it will only tolerate submersion for 3–4 days.
Excess water can increase the risk of plant diseases by creating an environment for fungi and bacteria to thrive. Flooding may introduce pathogens to the crop that could make livestock sick. In addition to carrying pathogens, silt deposits left by flood water may smother a forage stand. Alfalfa is more tolerant of silting than white clover. Many perennial species will grow through silt deposits up to 5 cm (2 in.) so long as soil surface crusting is not an issue.
Wait 2–3 weeks after the flood water has receded to assess crop survival. Dig up roots to check whether they are healthy and look specifically for root diseases that may limit future productivity of the stand.
Silt deposits will raise the ash content of a forage, so an analysis will help ensure enough nutrients are supplied in the ration. Consult with a nutritionist to determine the best way to manage elevated ash levels in harvested forage crops.