Growth and body composition of dairy calves fed only milk replacer at 3 intakes.

Determination of energy requirements for growth depends on measuring the composition of BW gain. Previous studies have shown that the composition of gain can be altered in young dairy calves by the composition of the milk replacer diet. Here, our objective was to determine body composition and the composition of empty body gain in young calves fed increasing amounts of a milk replacer containing adequate CP. Male Holstein calves underwent an adjustment period of 14 d after birth in which they were fed whole-waste milk at 10% of BW. Calves were then stratified by BW and randomly assigned to either an initial slaughter group (n = 11) or to groups fed 1 of 3 milk replacer amounts and slaughtered after 35 d of growth. Calves in all treatments consumed the same milk replacer containing 24.8% CP (DM basis; from all milk proteins) and 18.9% fat, reconstituted to 12.5% solids. Treatments were milk replacer fed at 1.25% of BW (DM basis; n = 6), 1.75% of BW (n = 6), or 2.25% of BW (n = 8), adjusted weekly as calves grew. Calves fed at 1.25% or 1.75% of BW were fed twice daily and those fed 2.25% of BW were fed 3 times daily. No starter was offered. Postslaughter, the bodies of calves were separated into 4 fractions: carcass; total viscera minus digesta; head, hide, feet, and tail;, and blood. The sum of those 4 fractions was empty BW (EBW), which increased linearly as the amount of milk replacer increased. Final heart girth and body length, but not withers height, increased linearly as intake increased. Gain:feed increased linearly with increasing milk replacer. Feeding more milk replacer increased the amounts of lean tissue and fat in the body. The percentages of water and protein in the final body decreased linearly, whereas fat percentage and energy content increased linearly as intake increased. As gain increased, the percentage of protein in gain decreased and the percentage of fat increased, resulting in an increase of energy content of EBW gain. Efficiency of energy use (retained energy:gross energy intake) increased linearly but retained energy:ME available for growth was not different among treatments. Efficiency of protein use increased quadratically as feeding rate increased; there was no further increase at 2.25% of BW. Plasma insulin-like growth factor 1, insulin, and glucose increased linearly, whereas urea-N decreased linearly, as milk replacer intake increased. Our data document changes in body composition that affect estimates of retained energy in the bodies of calves slaughtered at a common age. These data are important for calculations of energy requirements for young calves.