A close relationship has been established between yields of meadow hay and a meteorological parameter, namely, estimated “actual” transpiration during the period of growth. County data for yield and transpiration were combined, using grass acreages as weighting factors, into regional and national values. After an adjustment had been made for a steady improvement in yield after 1946, the correlation between the actual and calculated yield in England for the years 1939–56 was 0·95. The mean error between the actual yields and the calculated yields was 2·1%, the formula being: Yield (cwt./acre) = 7·05 + 1·56T + 0·27N, where T = estimated actual transpiration (in.) N = number of years after 1946.