The seasonal variation in the ruminal microorganisms of grazing sheep

Abstract

Samples of rumen liquor were taken by stomach tube, at approximately monthly intervals for a period of 19 months, from three mature Merino wethers grazing annual pastures in a Mediterranean-type climate in Western Australia. Counts were made, on the rumen liquor samples, of the total numbers of 'free' microorganisms, of protozoa, and of the organism Oscillospira guilliermondi. Qualitative variations in the organisms present were examined by means of wet iodine preparations and the Gram stain. Marked seasonal fluctuations in the numbers of total organisms were found. Two definite levels were evident, a high level associated with the green grazing in winter, with a peak in the spring, and a low level associated with the dry grazing. The lowest mean count obtained was 32 million per cu. mm. in April 1949, and the highest was 88 million per cu. mm. in November 1948. The numbers of protozoa and of Oscillospira were found to follow a similar seasonal cycle, the former showing an extremely high spring peak. Except for minor additions during the period of green grazing, all morphological types were found to be constantly present, but the balance of the many constituent organisms changed at different times of the year. The relation of these various changes in the numbers and balance of the ruminal microorganisms to changes in the chemical composition of the grazing, particularly the protein content, is briefly discussed and the need for further investigation of the influence of dietary changes on these organisms is stressed.