Estimating yield and nutrient losses due to rainfall on field-drying alfalfa hay

Alfalfa forage was cut with a swather (3.048 m wide) at two stages of maturity (latevegetative or early-bloom). Drying forage was treated with three levels of simulated rain (0, 5 or 20 mm) applied with a sprinkler irrigation system at 24 or 48 h after cutting. Otherwise, the forage dried under clear skies with only a slight breeze and temperatures ranging from 15 to 34°C. Air-dry hay was baled when it had regained sufficient water from evening dew to prevent leaf loss. Chemical composition of untreated forage or hay taken immediately after swathing, before sprinkling, before baling and before feeding did not significantly differ. Plant cell wall constituents (CW) of undamaged hay increased from 38.1 to 41.6% (P < 0.001) by 1 week advancement in maturity. The 20-mm simulated rain increased CW from 39.4 to 43.6% (P < 0.001) by leaching out soluble constituents. Crude protein content of the hay did not change, but the simulated rain reduced (P < 0.05) available carbohydrate from 27.3 to 24.3% , total lipid from 5.8 to 5.2% and soluble ash from 9.5 to 9.0%. The CW content was used to estimate loss of yield and nutrients from measurements of rainfall on drying hay. Estimated yield loss from soluble nutrients was 9.7%. This consisted of losses of 18.8% available carbohydrate, 10.2% 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.