This chapter discusses the development of the concepts of homeostasis and homeorhesis in the regulation of metabolism, regulation of nutrient use during lactation, and the mechanisms of nutrient partitioning during lactation in dairy cows.

This chapter discusses the endocrine regulation of growth and metabolism in ruminants with special emphasis on hormonal control of somatotropic axis (growth hormone, growth hormone receptors, insulin-like growth factor I, insulin-like growth factor binding proteins, insulin-like growth factor receptors and somatomedin).

The aim of this study was to determine whether blood metabolite concentrations in free-ranging indigenous goats are sensitive to expected variations in nutrient supply, and whether they could be used to evaluate different kidding seasons at two locations subject to similar seasonal variations in terms of nutrient supply. Monthly blood samples were taken over a period of one year. At Delftzyl farm, where a winter kidding season (June) was practised, glucose concentrations decreased from February onwards and reached their lowest levels just prior to the kidding season. Plasma glucose concentrations increased sharply after parturition and subsequently decreased until the end of lactation. Glucose concentrations were lower in lactating does than in non-lactating does during the first two months of lactation. In contrast, glucose concentrations during lactation in does at Loskop farm, where kidding took place during spring (October), did not differ from those recorded during the four months following weaning, and neither were there differences between lactating and non-lactating does. Glucose concentrations during lactation at Loskop farm were also higher than at Delftzyl farm. The different responses can be attributed to the fact that lactation at Loskop farm coincided with peak nutrient availability during the summer period of vegetative growth, whereas lactation at Delftzyl farm coincided with low nutrient availability and quality during the winter period of plant dormancy. Plasma urea concentrations were also elevated during the last month of pregnancy and the first two months of lactation at this location, and were higher during lactation than those recorded at the summer kidding site, indicating that body protein reserves may have been catabolized to support gluconeogenesis in these animals. Plasma cholesterol concentrations were higher in lactating goats than in non-lactating goats at Delftzyl farm but not at Loskop farm. Cholesterol concentrations during lactation were also higher at Delftzyl than at Loskop. This suggests that body adipose tissue reserves were catabolized during the winter lactation at Delftzyl farm. These results indicate that lactating does at Delftzyl farm were unable to maintain glucose homeostasis during pregnancy and lactation without significant catabolism of body reserves, and suggests that the winter kidding practised there was inappropriate in relation to the available nutrient supply. It was concluded that the plasma concentrations of all the blood metabolites studied were sensitive to seasonal changes in nutrient supply, and that they could be of use as a management tool in free-ranging farming systems in which conventional methods of nutritional assessment are difficult to apply.

Three experiments were conducted to investigate interactions between acetate and glucose metabolism in sheep fed on roughage-based diets, and to establish whether the clearance rate of an intravenous acetate load would provide a valid index of the dietary acetate:glucogenic precursors ratio. In Expt 1 lambs were fed on a basal diet of wheat straw and supplemented with propionate and protein. Both supplements increased glucose irreversible loss rate (ILR) although not to the same degree. Acetate clearance rates were increased by protein and propionate supplementation and were positively related to glucose ILR irrespective of precursor. In Expt 2 the effects of an increased dietary load of acetate given with or without propionate were investigated. Glucose ILR did not respond to acetate supplementation, but was increased when propionate was fed in addition to acetate. This was reflected in an unchanged ability to clear an intravenous acetate load from the blood when acetate alone was added, but an increased acetate clearance rate when propionate was fed in addition to acetate. In Expt 3 the effects of supplementation with various propionate: acetate ratios were investigated. Acetate clearance was consistently increased by an increased propionate :acetate ratio. These results show that the metabolism of excess acetate is responsive to the dietary supply of glucose precursors, and provide support for the concept that additional glucose precursors are necessary for the efficient utilization of acetate when roughage diets low in protein are fed.Acetate clearance rate : Gluconeogenesis : Ruminant Forage diets typically give rise to a high rumen production rate of acetate relative to that of propionate. This is usually accompanied by a reduced supply of amino acids (mainly as microbial protein), and negligible amounts of glucose and lipids; this results in a high proportion of acetate relative to glucose precursors in the absorbed nutrients. As acetate is the major energy-yielding substrate under these conditions, the efficiency with which it is used will exert a dominant influence on the overall efficiency of feed utilization. As it has been proposed that an insufficient supply of glucose relative to acetate may reduce the efficiency with which acetate is utilized (Preston & Leng, 1987), a need has arisen for some measure of the balance between the availability of acetate and precursors of glucose.A number of workers (Jarrett e t al. 1952;Pugh & Scarisbrick, 1952;Reid, 1958; Jarrett & Filsell, 1960;Egan, 1965) have made estimates of acetate clearance rate and, in general, higher rates of acetate clearance were associated with higher-quality diets. In reviewing the literature, Egan (1965) argued that factors influencing the rate of acetate oxidation or lipogenesis from acetate would include the availability of glucose or glucogenic substrates, and the nitrogen status of the animal. Weston (1966) found a linear relationship between

This chapter discusses the different ruminal microorganisms that are responsible for plant cell wall digestion, isolation of cellulosome binding proteins that are responsible for cell wall adhesion of microorganisms and the catalytic properties of glycoside hydrolases (cellulase, xylanase, glucanase and glycanase) by rumen bacteria, rumen fungi and rumen protozoa in plant cell wall digestion.