Effects of milk replacer composition on growth and development of beef × dairy crossbred calves

The production of crossbred beef × dairy (B × D) calves is increasing; however, evaluation of pre-weaning feeding strategies for B × D calves is limited. We hypothesized that both male and female B × D calves fed added fat and protein through milk replacer would have increased muscle growth, muscle fiber cross-sectional area, average daily gain, BW, and morphometric measurements. We also hypothesized that calves fed the additional fat and protein milk replacer would have upregulation of regulatory genes involved in muscle hypertrophy. SimAngus × Holstein calves (n = 42) were assigned to one of two milk replacers: 30.0% crude protein (CP), 32% crude fat (CF) milk replacer (HPHF, n = 11 males, 11 females), or 22% CP, 20% CF milk replacer (CON, n = 10 males, 10 females) from 0 to 8 wk of age. B × D calves were weighed at birth and weekly thereafter. At week 2 and 8, longissimus muscle biopsies were collected for muscle fiber cross-sectional area (CSA) or gene expression analysis. Ultrasounds were performed at 4 and 8 wk of age to quantify ribeye area (REA), and backfat and rump fat thickness. Morphometric measurements, BW, CSA, and ultrasound data were analyzed with PROC MIXED with animal as the subject and fixed effects of milk replacer, age, sex, and their interactions. Gene expression data were analyzed in R Studio. Calves that consumed the HPHF milk replacer were heavier than calves consuming the CON milk replacer (HPHF: 70.7 ± 0.39 kg; CON: 68.5 ± 0.41 kg; P < 0.01). At 8 wk of age, HPHF calves tended to have 14% larger muscle fiber CSA than CON calves (P = 0.06). No differences due to diet were observed for REA or fat thickness (P ≥ 0.38). Expression of MyoD tended to be 34% greater in CON females than HPHF females at 2 wk (P = 0.06), but at 8 wk, HPHF females tended to express 39% more MyoD than CON females (P = 0.09). Myogenin expression was 3% greater in CON calves than HPHF calves at 2 wk (P = 0.02), and CON females tended to express 52% more IGF-1 than HPHF females (P = 0.07). Feeding a milk replacer with a protein and fat content similar to beef cow milk improves B × D calf growth compared with a conventional milk replacer with less protein and fat. Improvements in early growth may improve B × D carcass quality and quantity, with the potential to increase return to the producer.