Prediction of the phosphorus content of herbage consumed by grazing cattle

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Abstract
SUMMARY: The determination of feed phosphorus content using oesophageally fistulated cattle is reported in this paper, from an experiment in which salivary phosphorus was labelled with 32P.An intravenous infusion of Na232PO4 to cattle produced an immediate increase in the specific activity of salivary phosphorus, which then fell rapidly to an essentially linear asymptote by 3 h after the infusion. The phosphorus content of consumed feed was calculated from the degree of reduction in salivary specific activity by the feed phosphorus, expressed as the ratio of the specific activities of bolus and saliva phosphorus.A dose of 100 /μCi 32P allowed the accurate prediction of phosphorus content ranging from 0·07 to 0·25% in various feeds, at intervals from 3 to 24 h after the infusion; the predicted and actual phosphorus concentrations were highly correlated (r = 0·95). Similar observations for feeds ranging from 0·14 to 0·25% phosphorus suggested that accurate prediction was also possible 144 h after infusion. Comparison of estimated feed phosphorus content of grazed material with that measured in hand-plucked herbage indicated that this approach is applicable to grazing studies.