Radiophosphorus (P32) as Tracer for Measuring Endogenous Phosphorus in Cow’s Feces

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Abstract
Intravenously injected radioactive phosphorus, P32, used as a tracer for phosphorus exchange, permitted estimation of the endogenous phosphorus in the feces of lactating cows. For a steady state, reached after 12 days of two daily injections of P32, and for a period starting three days after the last injection, the ratio of endogenous to total fecal phosphorus could be regarded as equal to the ratio of the mean specific P32 activity in feces to the mean specific activity in the blood plasma of two days earlier. In a cow eating 11 kg air-dry feed and producing 10 kg milk per day, 43% of the fecal phosphorus was endogenous. For another cow, producing the same amount of milk but eating only 3.5 kg of air-dry food daily, 70% of her fecal phosphorus was of endogenous origin. The rate of secretion of endogenous phosphorus into the intestinal contents amounted to 14 gm per day for the cow on the larger food intake and 10 gm per day for the cow on the smaller food intake. Comparisons with two other trials, based on specific activity in milk instead of plasma, support the conclusion that the secretion rate of endogenous phosphorus is positively correlated with the level of food intake or the rate of fecal excretion. Endogenous fecal phosphorus was used to calculate the “true” digestibility of phosphorus. The cow on the higher food intake showed an apparent digestibility of phosphorus of 12%. The “true” digestibility was 50%. The cow on the low food intake excreted even more phosphorus in her feces than she consumed in feed. The “true” phosphorus digestibility, however, amounted to 64%.

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