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Managing Flooded Alfalfa

a flooded alfalfa field

As floodwaters rise again in parts of South Dakota, spring planting seems to feel further and further away. One crop that is often overlooked in the hustle and bustle of the approaching planting season is alfalfa.

There are several factors that affect the outcome of flooded alfalfa including: air and water temperature, plant growth stage/age of stand, duration of submersion, pest pressure, and soil drainage. During flooding, plants may experience lack of oxygen, fine root hair death or damage, lack of nutrient availability, weakened root nodules (reducing nitrogen fixation), and exposure to numerous diseases.

One benefit of early flooding is that alfalfa plants are still dormant, making the stand more resilient to flood waters (due to slower physiological process in dormancy) versus late spring flooding when plants are up. However, as temperatures rise and plants make their way out of dormancy flooding may cause more damage.

Typically, dormant alfalfa stands can live about 7-10 days submerged in water. If water is standing stagnant, it is likely more harmful than flowing water, which contains more oxygen. In addition, alfalfa fields may survive up to 2-3 inches of silt deposits, but more than this will typically require re-grading and re-establishment. Well-established stands have a higher probability of survival as compared to newly seeded alfalfa.

What to do After the Water Recedes

  1. Examine root health: Dig up a plant and look over the roots, if they appear very soft and discolored or smell foul, the plants will likely not survive. If roots appear creamy or white with minimal signs of root rot, they may survive but should be monitored throughout the summer for vigor and survival.
  2. If plants will not likely survive: Consider planting a warm season annual for the growing season to make up for lost feed, protect soils, and buy time until a more long-term decision can be made. See the South Dakota NRCS Cover Crop Resource Page for cover crop and annual forage information.
  3. If plants will probably survive: Assess the stand; generally, if a field contains less than 39 stems per square foot, stand replacement or modification should be considered (see stem count method below). In newly seeded stands, we would expect denser stands than in well-established fields. If a stand is mixed with grass or in a more arid climate, lower stand counts are typically acceptable.
    • To estimate stems per square foot, first select four representative areas to count; keep in mind that one plant may have multiple stems. Then, use or create a 2ft2 measuring device at 17”x17” square (1/2” PVC works well) or a 19” diameter ring. Count only stems that fit within the square or ring and would be harvested by the mower (exclude those under 2” tall). Divide your count by 2 to calculate stems per square foot. Use all counts to create an average stem density estimate across the field.

In young stands (1-2 years), over-seeding with other warm season crops (like clover or various grasses) in the spring may be successful but watch for any herbicide issues due to floodwater movement or previous applications. Many farmers may wish to terminate thin-standing, surviving alfalfa and quickly re-seed with alfalfa. However, studies show various rotation interval recommendations for reseeding alfalfa stands, and the safest approach may be to plant annual forages for the season, and attempt to re-plant alfalfa the following spring; this allows time to reduce autotoxicity as well as soil-borne diseases and other pest issues. Some studies show that heavier textured soils can show alfalfa autotoxicity for up to 2 years.

In established stands, replanting in large, dead areas may be tempting if the entire previous alfalfa stand in the location has completely died. However, as previously stated, re-seeding alfalfa or over-seeding into existing stands is not typically highly effective due to autotoxicity and pest problems. A good alternative could be drilling over poor stands with other warm season annuals to create a forage mix for the season. For more general information on how to evaluate alfalfa stands and re-seeding options, see the Assessing Alfalfa Stands article by Dr. Undersander of UW Extension.

When Maintaining Surviving Flooded Stands

  1. Check soil fertility levels and reapply fertilizer if necessary.
  2. If herbicides are necessary, use the lowest labeled rate and avoid those absorbed through the roots or foliar systemics.
  3. Leave extra time before the first cutting; 10% bloom is a good goal. Allowing plants to recover and re-develop roots is an important part of maintaining future yields. Cut above the new re-growth if possible.
  4. Monitor and manage pest populations (plant disease, insects, and weeds) diligently to avoid any further stress in the field than has already occurred.
  5. If plant populations drop below 4-6 plants per square foot by the end of the season, it may be best to plan for a new crop next year.

When floodwaters recede, ensure that alfalfa fields have sufficiently dried before any field operations take place. Should you enter the field while it is saturated, further damage to alfalfa crown roots may take place, causing further yield loss and soil compaction.

There is no simple answer to dealing with flooded stands. By using careful observation and proper management, farmers will most likely be able to salvage some sort of forage crop for the coming year.

Resources and References