Nutritional status and intake regulation in sheep. VI. Evidence for variation in setting of an intake regulatory mechanism relating to the digesta content of the reticulorumen

Abstract

In each of four experiments, individually penned sheep were fed on a range of diets ad libitum, consuming the feed over 24 hr each day. In preliminary feeding periods each experimental animal became stabilized to a feeding pattern giving low standard deviations between days for the amount of feed consumed at a meal when the animal began and left off feeding at will. The amounts of digesta in the reticulorumen at the end of a specific meal (the first meal after presentation of a new allowance of feed) or before the commencement of the next (second) meal were then measured directly by emptying the reticulorumen or indirectly by estimates based upon a polyethylene glycol (PEG) dilution technique. The amount of digesta present in the reticulorumen at the end of the first meal was significantly greater when animals were fed on chopped lucerne hay (1.8 % nitrogen, 68% digestibility) than on either chopped oaten hay (1.2% nitrogen, 62% digestibility) or chopped wheaten straw (0.8 % nitrogen, 56 % digestibility). The quantity of digesta present in the reticulorumen increased as intake per day, or per meal, increased. For each diet, however, the weight of digesta held at the end of the meal was found to be repeatable between separate observations to within 800 g, which indicated a possible regulatory relationship between 'fill' and the termination of that meal, and the order of precision of the regulation. The intake of chopped wheaten straw was less than that of chopped oaten hay or chopped lucerne hay, both per day and in the second meal, even though the amount of digesta present in the reticulorumen immediately before the second meal was substantially lower for sheep given chopped wheaten straw. Though the amount of digesta in the reticulorumen of sheep given a pelleted, ground lucerne hay-barley mixture was also relatively low before the second meal, intakes were very much greater than when animals were given chopped wheaten straw. When casein was given in the diet or per duodenum, the intakes of oaten chaff (0.9 % nitrogen, 59% digestibility) were increased, and the amount of digesta held in the reticulorumen at the end of the first meal increased. Results support the hypothesis that, while for each diet the intake may be regulated to give a relatively constant level of fill of the reticulorumen at the end of a voluntarily terminated meal, this level of fill is not a constant between diets but is influenced by other, presumably nutritional, factors. One such factor is the protein nutrition of the animal.