Metabolic loads to be expected from different genotypes under different systems

Abstract

AbstractThe strategy most widely adopted to improve milk production efficiency is to increase yield per cow. To date, this has been achieved primarily through genetic selection and improved nutrition. Achievement of very high individual yield has had its down-side, especially in terms of reduced reproductive efficiency and there is now quite widespread concern that the high genetic merit cow is at greater risk of metabolic disease than her unimproved counterpart. To quote from the recent Farm Animal Welfare Council Report on Dairy Cow Welfare (FAWC, 1997): ‘High metabolic turnover in cows can be associated with a greater risk of mastitis, lameness, infertility and other production diseases…’. Whilst there can be little doubt that metabolic turn-over is indeed higher in high merit cows, it is not safe to assume that this necessarily equates with more risk; metabolic turn-over is higher in an elephant than in a mouse but risk is certainly not. Metabolic load might be a better term to use. If we think, simplistically, of this being the ‘strain’ on a system it is logical to expect an inverse relationship between metabolic load and health. The extrapolation to high genetic merit cows being at greater risk then presupposes that they experience an increased metabolic load but there has been no rigorous evaluation of whether this is so. In this review we will consider what is meant by metabolic load, examine in qualitative theoretical terms what degree of load might be expected from different commercial systems and present some recently obtained data which addresses directly the question, is metabolic load greater in high genetic merit cows?