Energy Nutrition and Metabolism of the Lactating Dairy Cow

The genetic trend for increasing milk yield in the dairy cow population is impressive evidence of the application of genetic principles in dairy cattle selection. Walton (31) recently predicted that by the year 2000, individual cow records will approach 31,800kg of milk per year and 1,360kg of milk fat. Top individual herd averages will be nearly 16,000kg of milk and 635kg of milk fat. If these predictions are close, the subject of energy nutrition and metabolism will remain a relevant topic for many years. It will be a continuing challenge to design feeding systems and diet formulations that support this high production. An appropriate question is what high genetic merit for milk production means to a nutritionist or physiologist. To me it means the physical capacity and the physiological drive for cows to eat large volumes of energy-dense feeds, to assume negative energy balance through much of the first trimester of lactation, and to partition most of the feed energy plus energy from body stores into milk energy. I will sketch a broad picture of energy nutrition and metabolism in the lactating cow and identify those areas in knowledge and understanding that constrain dairy scientist from assisting cows of superior genetic merit to produce milk more efficiently. I see five areas for potential improvement in use of energy by lactating cows: a) increased production, b) increased energy consumption, c) increased digestive efficiency, d) increased metabolic efficency, and e) changed energy flow to partition more dietary energy to mik and less to body reserves for a longer period during lactation. I will attempt to relate the symposium speakers' presentations to some of these areas.