Voluntary intake, digestion, rate of passage, amount of material in the alimentary tract and behaviour in cows receiving complete diets containing straw and concentrates in different proportions

Abstract

1. Pelleted diets containing concentrates and 0, 20, 40 or 60% chopped straw were fed ad lib. for 5 h daily to four cows. Voluntary intake, digestion and rate of passage of these diets were examined, and also the behaviour of the cows and the amount of material in the rumen before and after feeding.2. Least dry matter (7.5 kg) was consumed when there was no roughage in the diet. When roughage was present, its level had no effect on dry-matter intakes, which were 10.7, 11.3and 10.7kg for 20, 40 and 60% roughage respectively. Digestible energy intakes were greatest with diets containing 20 or 40% roughage.3. The digestibilities of the dry matter of the four diets were 81, 69, 59 and 55% in increasing order of roughage content. The proportion of total digestion which occurred in the rumen decreased as the roughage content of the ration increased. The rate of breakdown of cotton threads in the rumen increased as the roughage content of the diet increased.4. Rates of passage varied greatly between cows and there were no differences between treatments in passage rates through the entire alimentary tract. The diets on which the cows consumed the largest amounts of digestible energy (20 and 40% roughage) passed through the rumen more slowly, and through the hind gut more quickly, than the other diets.5. The times spent daily eating and ruminating both increased as the proportion of roughage in the diet was increased, but in relation to the amount of dry matter eaten, eating times were lowest with diets containing 20 or 40% roughage. Time spent ruminating per kg straw eaten decreased with increasing straw content of the diet. Rumen contraction rate during eating was greatest when dry-matter intake was greatest, but during rumination it was similar with all three levels of roughage.6. Before and after feeding, the amount of digesta and digesta dry matter in the rumen increased as the proportion of roughage in the diet increased. After feeding, there was a highly significant linear relationship between the amount of digesta in the rumen and the digestibility of the diet.7. In a second experiment, similar diets containing 0 or 50% roughage were given to two cows for 5 or 24 h daily. The 24 h intake expressed as a percentage of 5 h intake was 148% for the 0% roughage diet, but only 105% when the diet contained 50% roughage.8. The results of these experiments are interpreted as indicating a declining importance of physical factors in the regulation of the intake by cows of a range of diets of increasing digestibility.