Effects of energy source on growth and body composition of Holstein calves fed only milk replacer or whole milk

Whether lactose or fat is a better fuel for protein synthesis and deposition in young calves has remained controversial. Also, it remains uncertain whether bioactive factors in whole milk might influence growth compared with a milk replacer of similar macronutrient composition. We used Holstein calves (3 wk old, 46.0 kg BW) to determine effects of energy source on growth and body composition over a 35-d experiment. Calves (n = 29) were assigned either to an initial body composition group (n = 11) or to 1 of 3 treatment groups (n = 6 per group). The treatments were (1) a conventional milk replacer (CMR; 22.3% CP, 21.4% fat, and 49.1% lactose); (2) a high-fat milk replacer formulated to be similar to whole milk (HFMR; 25.4% CP, 27.4% fat, 40.1% lactose); or (3) whole milk (25.4% CP, 27.1% fat, 41.9% lactose). Reconstituted milk replacers were fed at a rate of 14% of BW for CMR and 11.65% of BW for HFMR, and whole milk was fed at 11.65% of BW. Initial intakes during wk 1 of CP and ME were similar among diets, but this condition was not maintained for the remainder of the study. Final BW and gain-to-feed ratio were not different among diets, but ADG and empty BW gain were greater for calves fed CMR. Days with fecal score ≥4 (5-point scale) were greater for calves fed HFMR than for those fed whole milk. Liver weight was greater for calves fed CMR than for those fed HFMR or milk. Final body composition did not differ among treatments except that protein percentage was lower for CMR than for HFMR and milk, and higher for HFMR than for milk. Composition of gain behaved similarly to results for whole-body composition. Efficiency of ME use for gain was similar among treatments, whereas efficiency of use of dietary CP for gain was greater for milk than for HFMR. Apparent partial efficiency of use of intake fat was greater for calves fed CMR than for those fed HFMR or milk. Concentrations of IGF-1, insulin, and total protein in plasma did not differ among diets, whereas glucose was higher for calves fed whole milk than those fed HFMR. Plasma urea-N was lower for CMR than for HFMR and milk, and lower for HFMR than for milk. Plasma NEFA were lower for CMR than for HFMR and milk, and higher for HFMR than milk. A greater nutrient supply from a high-lactose milk replacer supported greater gains of lean tissue than higher fat diets. At constant nutrient intakes, whole milk supported more efficient use of dietary CP and shortened the duration of abnormal fecal score occurrences than a milk replacer of similar composition.