Nutrient supply to high-yielding dairy cows

The last decade of the 20th century has seen unprecedented change within the UK dairy industry, with consolidated of production quotas, payment schemes based on milk solids, deregulation of the milk market with the demise of the Milk Marketing Board and more recently a major downward pressure on milk prices. All of these have contributed to further intensification of a large part of the industry. However the largest change has been the introduction of improved genetics, with the accompanying increase in Holsteins (now the preferred breed) at the expense of Friesians. As a consequence individual cow yields have increased dramatically and such cows are becoming an increasing problem in respect of nutritional management, as it relates to optimisation of milk solids content and yield and maintenance of reproductive competence. This paper has attempted to examine some of these issues by first describing the changes in more detail and then from a theoretical consideration of the biochemical principles associated with milk solids production to provide information which may permit better decisions to be made in relation to the feeding of such cows. In this respect the importance of glucose is established as well as the potentially competing processes of energy use for body maintenance or milk fat synthesis. Two experimental studies from this laboratory with high genetic merit cows are used to illustrate how the overall utilisation of energy for lactation and body tissue metabolism may be affected, especially when energy intake in early lactation fails to meet whole body energy demands. Subsequently, the nutrient costs of pregnancy, an important but often forgotten part of the ‘annual’ life cycle of dairy cows, are also considered and it is concluded that increased emphasis needs to be put on the management of cows for lifetime performance rather than simply for lactational output. Finally, whilst recognising that high yielding cows represent a major challenge to all management systems, the paper concludes that existing feeding systems which rely solely on nutritional entities such as metabolisable energy will need to be abandoned in favour of those which recognise specific nutrients, if the full potential of high genetic merit cows is ever to be achieve