Phosphorus metabolism in ruminants. 2. Effects of inorganic phosphorus concentration upon food intake and digestibility

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Abstract
Four sheep, prepared with bilateral re-entrant parotid duct catheters, ruminal fistulas and abomasal catheters, were offered a pelleted high-calcium low-phosphorus diet. By infusing saline or phosphate solutions into the abomasum with or without diversion of the parotid saliva, three treatments were created: low ruminal, low blood; high ruminal, high blood; and low ruminal, high blood inorganic phosphorus concentrations. The organic matter intake of the sheep on the low-blood treatment was 34% less than on either of the high-blood treatments. Although there were no differences between treatments in organic matter digestibility, the digestibility of the neutral-detergent fibre fraction on both low-ruminal treatments was 5% lower than on the high-ruminal treatment. There were no differences between treatments in ruminal fluid volume, but there was a tendency for the retention times of ruminal fluid and caeco-colic digesta to be greater and the rate of flow of fluid from the rumen and daily parotid saliva secretion to be lower when less food was eaten. It is concluded that although phosphorus deficiency may depress microbial digestion, the reduced food intake of the sheep was not due to a depression in microbial digestion and that the primary factor limiting food intake was due to some nondigestive tract disturbance, possibly a disturbance of intracellular metabolism.