In vivo measurement of rumen microbial growth using ³⁵S

Abstract

A method for the measurement of the net rate of synthesis of rumen microbes, entailing separate infusions of 35 S-sulphate into the blood and rumen, is described and compared with estimates derived using digesta flow markers. Measurements were made on seven occasions with sheep and on eight occasions with cattle given chopped hay or pelleted hay diets at intervals of 60 or 90 min. Additional measurements were made on two sheep and two cattle given lucerne hay at intervals of 6 or 12 h. Estimates of microbial synthesis using the proposed method for animals fed at intervals of 60 or 90 min were 0-95-1-23 (mean 1-05) that derived using flow markers. For animals fed at intervals of 6 or 12 h the ratio was more variable, 0-91-1-35 (mean 1-17). The agreement between methods tended to be poor in animals with a low rate of irreversible loss of serum sulphate. The proposed method can be accomplished with a low analytical load, and appears to have potential for application in animals fitted with rumen fistulae only, and fed at infrequent intervals.