Changes in body composition of 54, second to fourth parity, autumn-calving Holstein-Friesian dairy cows offered grass silage ad libitum and 3(L), 6(M) or 9(H) kg concentrate dry matter per day were measured by serial slaughter at 0, 2, 5, 8, 11, 14, 19, 24 and 29 weeks post partum.
Concentrate level had a significant effect on the fresh weights of many of the body fractions with the differences generally being greater between L and M than between M and H. Increasing concentrate level generally reduced the extent of weight loss of body fractions in early lactation and enhanced subsequent repletion. Empty body weight decreased to week 8 and then increased steadily over the remaining 21 weeks, but within this pattern different organs were concomitantly increasing and decreasing. Carcass weight and the weights of the internal fat depots showed a decline over the first 8 weeks and a subsequent increase, udder weight declined throughout, weights of various sections of the digestive tract showed an initial increase then remained steady, whilst liver weight increased throughout.
In week 0 the carcass accounted for proportionately 0-61 of the total energy in the body (6278 MJ), of which fat and crude protein (CP) comprised proportionately 0·67 and 0·33, respectively. In early lactation mobilization of fat and CP in the carcass was reduced with increasing level of concentrate. In the non-carcass fraction increasing concentrate level led to a higher weight of CP in the metabolically active organs such as the digestive tract and udder but had little effect on the weight of fat. Nevertheless, there was generally a positive effect of concentrate level on energy content. Total weights of fat, CP and water in the body declined to week 8 then increased over the following 21 weeks. Although weight of CP in the liver increased throughout lactation and weight of fat was elevated in weeks 0 and 2, the energy content of the liver remained fairly constant.
Estimates of the change in net energy (NE) associated with live-weight loss and with live-weight gain showed a slight though non-significant difference between the two, despite evidence of a higher concentration of fat associated with gain than with loss, and CP concentration being the same in both cases. The mean value was 19·3 MJ/kg live-weight change.