The digestion of pasture plants by sheep. II. The digestion of ryegrass at different stages of maturity

Abstract

Ryegrass harvested at various stages of maturity was dried and offered to sheep either ad libitum or at approximately 90% of the ad libitum intake. Feed consumption and the times spent by the sheep in ruminating and eating were measured when the forages were offered ad libitum, and several aspects of digestion were studied when the level of feeding approached 90% of ad libitum intake. The chemical composition of the grasses and the apparent digestibility of the constituents showed the usual changes with maturity. As the grass matured, there was a tendency for: (i) the rate of flow of digesta from the abomasum to increase relative to the flow from the rumen, (ii) the concentration of organic matter in digesta leaving the abomasum to decrease, (iii) the quantity of nitrogen leaving the stomach in digesta per unit intake of dietary nitrogen to increase, and (iv) the sheep to spend more time in ruminating and eating per unit feed intake. No differences attributable to maturity were observed in: (i) the rate of flow of digesta from the rumen, (ii) the retention time of a water-soluble marker in the rumen, (iii) rumen volume, (iv) the distribution of the digestion of organic matter and carbohydrate between the stomach and intestines, and (v) the apparent digestion of nitrogen in the intestines. Although the quantity of nitrogen provided by 100 g of forage organic matter decreased with maturity from 4.3 to 1.2 g, the corresponding change in the quantity of nitrogen, in forms other than ammonia, apparently digested in the intestines was only 2.6 to 1.5 g. It was calculated, after making several assumptions, that the net energy value of the metabolizable energy derived from volatile fatty acids produced in the rumen and from amino acids digested in the intestines was little affected by the stage of maturity. On the other hand, after making assumptions about the energy costs of ruminating and eating, it was calculated that the proportion of the metabolizable energy used in ruminating and eating could increase from 5% with the immature grass to 13% with mature grass. Intraruminal infusion of propionic acid did not increase the sheep's intake of one forage that provided a relatively small quantity of propionic acid during ruminal digestion.