Extension Communications Extension 4-H Youth Building Ames, Iowa 50011-3630 (515) 294-9915

5/26/04

Contacts:
Stephen Barnhart, Agronomy, (515) 294-7835, sbarnhar@iastate.edu
Jean McGuire, Continuing Education and Communication Services, (515) 294-7033, jmcguire@iastate.edu

Summer Flooding of Hay Fields and Pastures

By Steve Barnhart
Forage Specialist
Iowa State University Extension

Most forage crops perform best when soils have adequate, but not excessive, soil moisture. Standing water, flowing water and waterlogged soils following heavy summer rainstorms or extended periods of higher-than-normal rainfall patterns can all cause management concerns for forage crops.

Alfalfa, clovers and most forage grasses cannot live for very long underwater. Short-term (one to two days) of flowing water is tolerated by most forage plants. Standing, or pooled water, that 'heats" in the sun and 'cooks" the submerged forage plants is more of a concern, and can kill or severely damage most plants within hours.

After the surface water recedes, an extended period of saturated soils continues to be reason for concern. Forage plants (other than perhaps wet site-tolerant reed canary grass) can survive for several weeks in saturated soils, but the lack of oxygen in the root zone will adversely affect their growth. These plants do not take up soil nutrients normally, an increasing part of the root system deteriorates, and legumes cease 'fixing" nitrogen. They appear stunted and yellowish-green in color. If the soils drain quickly, plants begin to recover.

Managing established alfalfa
The only research that I am aware of regarding waterlogged soil and alfalfa management was done in Ohio. In studies there, alfalfa plants, which had reached this degree of yellowing and lack of plant vigor, were more susceptible to additional disease stress and more importantly to stressful cutting practices. Based on this limited information my suggestions would be:

Digging up random plants in several areas of the fields and evaluating the condition of the root systems. Plants with a firm taproot, creamy-white in color, with no evidence of root rot, which have green and visually healthy crowns and crown buds have the greatest likelihood for survival. These plants need a week or more of sunshine and drying soils. Plants with watery, mushy textured taproots, yellowish or tannish in color, those with significant loss of lower taproot to root rot and those with no evidence of active crown buds will be the least likely to survive, even with good growing conditions during the next few weeks.

For the greatest benefit to the plant:

1) Delay harvest for a week to 10 days to allow the plants to regain what vigor and recovery that they can. This management approach will produce a more mature forage crop of lower nutritive value. In addition, take extra care to schedule a five to six week 'fall rest' period for these stands.

2) Potato leafhoppers should be monitored and controlled. Their feeding stress can quickly affect stand regrowth and vigor.

3) I would also caution producers not to move into these fields hastily because they are still quite susceptible to wheel traffic and compaction damage, which will also limit the future productivity of the field.

4) Flooded forage may be silt-covered, which will both add to plant disease potential and detract from the palatability of the harvested hay.

5) Be alert for flood debris carried into hay fields that may damage harvest equipment.

Managing new alfalfa seedings
Newly emerged and developing seedlings are probably more susceptible to flooding and saturated soils than are mature plants. Monitor these fields closely. Regularly reassess stand densities and make necessary replant decisions. There is sometimes a good opportunity for late-summer alfalfa replanting for summer-damaged new seeding fields.

Where a cereal companion crop was used with the new seeding, early removal of the competing companion crop for hay or silage will often improve the vigor and stand density of the new alfalfa seeding.

Managing flooded pastures
Pasture plants are affected much the same as alfalfa when under standing or flowing water, and growing in water-logged soils. Grasses are, however, slightly more tolerant of these conditions than are legumes.

For the greatest benefit to the pasture plants and grazing animals:

1) I would caution producers not to move into these fields hastily because they are still quite susceptible to hoof traffic and compaction damage, which will also limit the future productivity of the field.

2) If possible, allow flood-damaged pastures to recover some vigor before grazing.

3) Flooded forage may be silt-covered, which will both add to plant disease potential and detract from the palatability of the pasture.

4) Clean up debris carried by floodwater into pastures that may be potentially hazardous to grazing livestock or damaging to equipment.

Additional resources to help crop producers make decisions on how to handle hail- and flood-damaged crops can be found on the Web at: http://www.extension.iastate.edu/ag/flooding.html

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