The dry conditions in March and April have given way to extremely wet areas in some parts of Minnesota. Since alfalfa stands got an early start this year there were a few concerns popping up early in the southeastern part of the state on areas of fields yellowing. While there may have been some effects due to the cool weather in April a couple of nutrient could be of concern.
The likely culprit for any deficiencies in early 2012 in the southeast is probably sulfur. Keep an eye on stands prior to the second or third cutting to see if any symptoms remain. Some of the deficiencies may have lessened due to mineralization after rainfall events this spring. Plant analysis can be used to help identify the problem. It is recommended to sample the top 6" from at least 10 plants during early bloom. Current research in Iowa has found that plant concentrations above 0.22 to 0.25% S should be adequate for maximum yield potential. To aid in sampling make sure to same at least 3 areas of the field ranging from good to poor growth to be able to compare tissue results.
If any spots still remain in the field topdressing a sulfate-sulfur fertilizer source such as gypsum, ammonium sulfate, or potassium sulfate can have an impact on yields of future cuttings. Elemental sulfur is a poor choice if a field is deficient since it takes time to become available. Fertilizer guidelines for alfalfa were changed over the winter to include suggested application of sulfur on field areas with 3.0% or less soil organic matter in the top 6-8". In these cases 10-15 lbs of S per acre is suggested annually. You can find more information on the current fertilizer suggestions at:
Fertilizing Alfalfa in Minnesota
J. Sawyer, Iowa State University Extension.