Some aspects of constraint to forage consumption by ruminants

Various aspects of forage intake regulation are discussed with the objective of providing a basis on which assessments could be made of (i) the scope for forage intake manipulation, and (ii) priority areas for further research. A simple conceptual model of the regulation is presented which permits the linking of rumen function and energy metabolism. It takes cognizance of upper physiological limits for (i) energy disposal, (ii) the clearance of digesta organic matter from the rumen, and (iii) muscular fatigue, as well as a range of dietary and environmental constraints. The transmission to the brain of signals relating to amount of digesta in the rumen and the ruminant's energy deficit are considered to be important in the intake regulation An alternative conceptual model which recognises the amount of energy in the circulating energy pool, rather than the energy deficit, as the origin of signals relating to energy metabolism, is also discussed. It is considered that over a range of forage qualities neither the rumen digesta load ceiling nor the capacity to use energy limit intake; in this range both the resistance of the forage organic matter to removal from the rumen and the net energy value of the forage act as constraints. A method to calculate forage intake constraint is presented, and theoretical relationships between rumen digesta load, net energy intake, energy deficit and forage intake constraint have been formulated to facilitate interpretation of data obtained in forage intake studies. Forage intake in the reproduction cycle is discussed in the context of an optimum nutritional strategy for ensuring species survival. It is considered that the intake changes at mating and immediately prior to parturition, together with the decrease in rate of nutrient storage in maternal reserve tissues in late pregnancy and the use of these tissue stores in early lactation, are consistent with such a strategy. In this context it is suggested that (i) the relevant reproduction hormones affect intake via modulation of the metabolism of the maternal tissue stores and (ii) this type of regulation and its accompanying production losses need not be necessary in those production systems permitting some control of nutrition.