Use of sheep maintained by intragastric infusion as a model for studying the endogenous loss of phosphorus in ruminants.

Mature sheep fitted with a rumen cannula and an abomasal catheter were used to evaluate whether sheep maintained by intragastric infusion could be used as a model for studying minimum endogenous phosporus loss in ruminants. In control periods the sheep received 1.29 g phosphorus day-1 by infusion and maintained phosphorus balance. The withdrawal of phosphorus from the infusate led to a reduction in the excretion of phosphorus to 6.4 mg kg liveweight-1 day-1, a level lower than that currently used to estimate maintenance phosphorus requirements. However, salivary phosphorus secretion, based on rumen phosphorus outflow, averaged only about 1.2 g day-1, a level which would be considered insufficient to meet rumen microbial requirements in a normally fed animal. Allowing the sheep access to a small amount of roughage, although contributing little additional phosphorus, greatly increased salivary phosphorus flow and increased their endogenous excretion of phosphorus to levels close to those used to estimate maintenance requirements. These results suggest that this technique may offer an alternative way of measuring minimum endogenous mineral loss to those currently in use.