Tissue Growth in Young and Mature Cull Holstein Cows Fed a High Energy Diet

Twelve young and 12 mature lean Holstein cows were fed a high energy diet to appetite based on high moisture corn and haylage. In addition to the cows put on feed, six young and six mature cows were slaughtered at the start of the trial to estimate initial body composition. After 68 d on feed, two young cows and two mature cows were slaughtered on a weekly basis until the conclusion of the trial. After slaughter, the left side of each carcass was broken into eight wholesale cuts that were further separated into bone, muscle and depot fat [subcutaneous fat (SF), intermuscular fat (IF)]. Growth patterns of bone, muscle and fat in each meat cut were investigated with the allometric equation (Y = aXb), using carcass weight as the independent variable. There were no maturity differences in bone growth and all the growth coefficients were less than one, indicating that bone was only a small proportion of the carcass gain during the growth period. At constant carcass weight, mature cows had more bone in the hip (P<.01) and plate (P<.05) than young cows. Cow maturity had no effect on muscle growth, although the growth coefficients were higher and closer to one than those recorded for bone. Mature cows had more muscle in the chuck (P<.001) but less in the hip (P<.05) than young cows. Cow maturity had no effect on the growth coefficient for fat, although both the major fat depots (SF and IF) had coefficients greater than unity. Young cows had more total fat in a side and more SF at the same carcass weight. Subcutaneous and intermuscular fat growth for the individual wholesale cuts did show maturity differences, but no clear patterns emerged. It was concluded that the lean cull cows made muscle gains, but the major tissue to be deposited was fat.