A study on consistency of differences between cows in rumen outflow rate of fibrous particles and other substrates and consequences for digestibility and intake of roughages

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Abstract
In 22 cows given complete diets of 500 g straw and 500 g concentrates per kg fresh weight the outflow rate of small fibrous particles was determined on two occasions to investigate the consistency of this measurement and its association with apparent digestibility. The correlation between the outflow rates on the two occasions werer= +0·90 and the correlation with apparent digestibility wasr= −0·80.In a second experiment, the outflow rates of long and small particles were determined together with rumen retention time. There was no difference between outflow rate of long and small particles but rumen retention times were greater with the long particles. The correlation between the two estimates of outflow rate wasr= +0·84. The outflow rate of fish meal was found to be 0·070 while that of short fibrous particles was 0·031 per h. The average liquid outflow rate was 0·146 per h.The three cows with the highest outflow rate and the three with the lowest outflow rate from experiment 2 were selected for experiment 3. While voluntary food intakes of the two groups were similar, the differences in outflow rates between the cows persisted both withad libitumand restricted intakes of both a high and a low roughage diet. The differences were reflected in significant differences between the two groups in apparent digestibility of the diets. The practical implications for breeding and selection are discussed.