Possible contribution of protozoa to differences in rumen metabolism between cattle breeds

Abstract

Brahman (B) and Hereford (H) cattle were used to study protozoal population density of ruminal fluid (RF) in relation to some aspects of rumen and body metabolism. Steers were fed restricted intakes of lucerne hay (HQ) or low-quality hay (LQ) in each of eight experiments. In the 8 h period following the once-daily feeding of HQ the number of entodiniomorphs declined by 50%, while the number of holotrichs increased by 60% before reducing to pre-feeding levels. There were no significant rises and falls in the numbers of protozoa of steers fed LQ once daily nor in those fed either HQ or LQ at hourly intervals. The numbers of entodiniomorphs, dasytrichs, total protozoa, pH and bacterial content of RF were higher and rumen temperature lower in B than in H. Urinary allantoin excretion used as an index of the total microbial population in the rumen distinguished between diets but not between breeds. The concentrations of propionic, butyric, isobutyric and isovaleric acids in RF and of glucose in plasma were higher in B than in H. The concentrations of all classes of long-chain fatty acids contained in the protozoa and bacteria of RF and of cholesterol and essential fatty acids in plasma were higher in B than in H. The amount of dry matter disappearing from nylon bags in the rumen of steers fed HQ did not differ between breeds, but was higher in B than in H fed LQ. Elevation of pH to peak values in RF with an accompanying disappearance of protozoa and volatile fatty acids during fasting from either HQ or LQ occurred more rapidly in B than in H. The plasma concentrations of non-esterified fatty acids were higher in B than in H during fasting. It is proposed that at the same level of intake of either HQ or LQ: (1) the higher protozoal population density in RF of B contributes to quantitative breed differences in the end products of digestion, (2) that B achieve and maintain a higher body weight because more metabolizable energy and essential nutrients are supplied from the rumen to their body tissues, and (3) after the same period of feed deprivation B have fasted for a longer period of time because the residual feed in their rumens is fermented at a more rapid rate.