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.
Differences in rumen volume can have considerable effects on the digestibility and voluntary intake of ruminants. For instance Mould et al. 1982 showed that cattle in Bangladesh had a much greater rumen volume relative to live weight than reported values of Friesian cattle. They also consume much more straw per unit of live weight than Friesian cattle.Differences between cows in the ability to consume roughage have often been observed. It is however possible that this strategy could be adopted simply by increasing outflow rate and not necessarily by a high rumen volume. Depending on the strategy adopted this would give rise to differences in the digestibility of the roughage consumed.In order to investigate these aspects in more detail, 22 lactating Friesian cows were given doses of Cr-mordanted straw having been ground through a hammer mill with a 2 mm screen. The cows were fed completely mixed diets consisting of 50% ammonia treated straw and 50% concentrate.
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