Phosphorus Metabolism in Lambs Fed Varying Phosphorus Intakes

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Abstract
A growth and metabolism trial was conducted using 24 lambs. Three P levels were fed, namely 0.12, 0.15 and 0.29% P in a complete ration. Phosphorus deficiency, based upon reduced plasma inorganic P levels, was evident after one week in those lambs fed the 2 lower levels of P. Body weight gain and feed intake were adversely affected within 2 weeks. After P deficiency was established (7 weeks), P32 was injected subcutaneously and the metabolism of P and P32 was studied during a 2-week metabolism study. Metabolic fecal P and P balance increased with increasing increments of dietary P. Fecal elimination of the injected P32 was greater for the lambs fed adequate amounts of P. The percentage of endogenous P present in the rumen fluid increased with increasing levels of dietary P. Total endogenous P in the rumen fluid increased markedly with increasing P intake. It is concluded that endogenous P secretion into the rumen is decreased by a deficiency of P probably as a result of decreased plasma inorganic P levels. Equations expressing the P requirement of lambs in terms of total and available P are described.
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