Post Flood and Fallow Syndrome in Corn


2016 has been a year filled with heavy rains that has caused many acres to be flooded out, destroying the crop or preventing planting. If another crop was not planted and/or the areas kept clean of plant growth, fallow syndrome may impact crops grown in those areas in 2017, especially corn. Corn in the areas fallowed the previous year often show P (phosphorus) and/or Zn (zinc) deficiency, purpling, light green color and reduced vigor.

During the 1960’s U of MN conducted a studies to evaluate fallow syndrome. Key findings were:

  • Corn after fallow showed retarded growth and lighter green color compared to corn after a cover crop.
  • Even if visual symptoms disappeared latter in the growing season, corn ear moisture percentage was about 5% higher than corn after a cover crop.
  • Corn yield reductions of about 15% occurred following fallow compared to corn after a cover crop.
  • Plant analysis indicated that the lighter green color (indication of possible N deficiency) corn had lower P concentrations than corn planted in the areas of a cover crop. The low P concentrations occurred even with high to very high available soil-P levels.

Studies conducted in Australia in 1987 found an association between a fallow period and decreased mycorrhizal (a particular type of beneficial fungi) colonization for following crop. These fungi are dependent on host plants to complete their life cycle, i.e., no or reduced plant growth the mycorrhizal hyphae and spores are reduced. Mycorrhiza is a beneficial fungus for the corn crop. The mycorrhizal spores germinate and the hyphae invade the small rootlets, growing inside and outside. The fungal hyphae on the exterior of the roots serve as an extension of the roots for water and nutrient absorption. Benefits of this association include:

  • Increased effective root surface for increased absorption and availability of nutrients – especially P.
  • Rootlets function longer.
  • Increased heat and drought tolerance.

Farmers will realize a number of benefits with the use of cover crops:

  • Increase soil organic matter.
  • Improve soil health, quality & productivity.
  • Reduce erosion.
  • Reduce nitrate leaching which will improve water quality.
  • Suppression of winter annual & early season weeds.

Farmers facing post-flood or fallow syndrome should consider:

  • Establishment a cover crop in impacted areas due to wet soils and not being accessible.
  • Choice of cover crops will depend on:
    • Herbicides applied earlier in the season.
    • The effectiveness of the cover crop in alleviating fallow syndrome is related to planting date- earlier is better, but we recognize that in some cases planting delays are unavoidable due to prolonged wet conditions. Even when planted late, any plant and root growth prior to freeze-up will help reduce the fallow effect.
      • Best to have planted by end of August.
      • Small grains are an excellent choice with later plantings
    • Seeding methods
      • Drilled/planted: uniform seeding depth and adequate seed to soil contact.
      • Broadcast: requires higher seeding rate and should be rolled or lightly incorporate for better results. Species with smaller seeds may establish better than larger seed species.
      • Slurry Manure: a mixture of cover crop seed and manure can be applied. The manure nutrients encourage cover crop growth and the cover crop prevents nutrient losses. Be sure to follow manure application rules. MPCA requires incorporation and the planting of a cover crop with most summer (prior to September 1) applications of liquid manure.
      • Aerial/Surface: covers larger areas in late summer.
    • Weeds will support mycorrhiza populations; however, can provide less consistent cover, which can increase erosion risk, and possible increased future weed pressure than cover crops.
    • Use of a high analysis P fertilizer in the starter fertilizer, broadcast applications of P have not been effective at preventing the fallow syndrome symptoms. High soil test P levels will not eliminate the fallow syndrome. In has been suggested that the use of rates of 60-80 lbs. P/acre as starter will help offset the impact of mycorrhizal loss due to flooding and fallow.
    • Soybeans can be impacted but not to the extent of corn.
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