Ayman M. Mostafa
With the rain events we had during the last few weeks, I have received questions about rain impacts on alfalfa hay yield and quality. Alfalfa must be dried or cured for safe storage as hay, and generally we do not have problems reaching this dryness in Arizona. Normally, field and harvesting losses of hay are as high as 20 to 30% due to cutting and curing. Rain can increase these yield losses and reduce quality. The event of rain that occurs between the time forage is cut and baled extends curing time. Consequently, yield and quality are decreased, reducing the value of the crop as an animal feed and marketable commodity. Weather-induced losses can be caused by: 1) increased and prolonged plant respiration that reduces soluble carbohydrates and the overall energy content of forage, 2) leaching of soluble carbohydrates, protein, and certain minerals, 3) leaf shattering and loss, removing the highly digestible and high protein portion of the forage, 4) microbial activity that metabolizes soluble carbohydrates, reduces forage energy content, and possibly produces harmful mycotoxins, and 5) color bleaching.
In a Utah study, artificial rain of 0.8 inches resulted in estimated yield loss of 9.7%, losses of available carbohydrate by 18.8%, 10.2% reduce of crude protein, 19.8% of lipids, and 14.0% of soluble minerals. Hay quality was reduced more by rain damage than by advancement in maturity.
In a study in Michigan to examine the effects of rainfall on field cured alfalfa, dry matter (DM) losses ranged from 4-13% with rainfall intensity that was kept constant at 0.7-inch but spread over periods of 1 to 7 hours, with highest losses occurring when the rain was spread over a longer duration.
Carotene, the precursor for Vitamin A, is sensitive to prolonged field exposure. Vitamin A is the most common vitamin deficiency in beef cows and horses.
You can also see:
Fonnesbeck, P.V., M.M. Garcia De Hernandez, J.M.Kaykay and M.Y.Saiady (1986) Estimating yield and nutrient losses due to rainfall on field-drying alfalfa hay. Animal Feed Science and Technology, 16: 7-15.
Lindquist, K. (2017) How Rain Affects Hay Quality – Frequently Asked Questions. Alberta Agriculture and Forestry. https://www1.agric.gov.ab.ca/$Department/deptdocs.nsf/all/faq14011
Rankin, M. and D. Undersander (2000) Rain Damage to Forage During Hay and Silage Making. University of Wisconsin Extension. https://fyi.uwex.edu/forage/files/2014/01/Raindam.pdf
Rotz, C.A. (1995) Loss Models for Forage Harvest. Transactions of the ASAE. 38: 1621-1631.