One of the major limitations in nutritional studies of lactation in dairy cows is the unknown contribution of body tissues to the energy status of the animal. Both fat and protein are thought to be mobilised in early lactation (Bauman and Elliot, 1983) when nutrient intake is insufficient to meet the demands for energy and amino acids. The present experiment investigates the effects of diet on changes in weights of chemically determined fat and crude protein (CP) in the carcass and non-carcass fractions over the first 29 weeks of lactation.

Body composition was measured by serial slaughter of autumn-calving dairy cows offered grass silage (206 g CP and 18.56 MJ GE per kg DM) ad libitum and 3(L), 6(M) or 9(H) kg DM/day of concentrate (206 g CP/kg DM)(Sutton, Aston, Beever and Fisher, 1992). Cows with post-calving live weights between 500 and 700 kg and previous lactation yields between 5000 and 7000 litres were used, and were blocked for parity and expected calving date before allocation across treatments and slaughter times. Six cows were slaughtered at 2 to 4 days post partum (week 0), and two from each treatment (one 2nd, and one 3rd or 4th parity) at 2, 5, 8, 11,14,19, 24 or 29 weeks post partum. After slaughter the right half carcass and all other non-carcass components were frozen and minced before chemical analysis for fat, CP, ash and water. All weights were adjusted by covariance for live weight post partum and parity. Energy content was calculated assuming values of 39.19 and 23.23 MJ/kg for fat and CP respectively.