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Further Studies on the Availability of Phosphorus in Alfalfa Hay¹

University of California²

Abstract

A method is described for determining the availability of the phosphorous in feedstuffs wherein only one subcutaneous injection of radioactive phosphorus need be made. From the seventh through thirteenth day the specific activity of the plasma inorganic phosphorus and the feces exhibit a linear relationship with time. This observation enables one to estimate the average specific activity of the plasma inorganic phosphorus by drawing blood samples on only the seventh and thirteenth days. The fecal specific activity is determined on a composite sample collected from the eighth through fourteenth day. The ratio of the specific activity of the fecal composite to the mean specific activity of the plasma inorganic phosphorus on the seventh and thirteenth days represents the proportion of the fecal phosphorus which is of metabolic origin.

Using this method it was found that approximately 92 percent of the fecal phosphorus of yearling wethers being fed alfalfa hay was of metabolic origin and 94 percent of the phosphorus consumed was absorbed. The result indicates a high availability of the phosphorus in alfalfa hay.

¹ This study was partially supported by a grant from the United States Atomic Energy Commission.

² Department of Animal Husbandry, Davis.