189 Effect of milk replacer and grower diet composition on intake, growth and body composition in Holstein-Angus crossbred calves from birth to the end of the growing phase.

Abstract The number of crossbred dairy calves has drastically increased over the past decade, but pre-feedlot nutritional recommendations remain scarce. This study investigated the interaction between pre- and post-weaning diets with different energy substrate inclusions. Male Holstein-Angus calves (n = 68) were assigned at 1 wk of age to 1 of 4 treatments in a 2 × 2 factorial arrangement: fat milk replacer (MR) + control grower diet (FC); fat MR + fat grower diet (FF); lactose MR + fat grower diet (LF); and lactose MR + control grower diet (LC). Fat and lactose MR were 29% and 18% fat, and 38% and 51% lactose (DM), respectively. Mixing rate for fat and lactose MR were 135 and 150 g/l, respectively to balance energy and protein supply. Milk replacer was fed at 15% of arrival body weight (BW) until wk 3 and 18% thereafter until wk 6. Gradual weaning occurred from wk 6 to 10, with grower diets introduced at wk 5 and fed until the end of the growing phase (wk 36). Fat and control grower diets were 7% and 3% crude fat (DM), respectively. At wk 13, calves were transported to a feedlot for the remainder of the experiment. Milk and solid feed intake were recorded daily, and BW was collected weekly. Starting on wk 16, measurements of the ribeye area (REA), back and rump fat thickness, and intramuscular fat (IMF) percentage were taken monthly using ultrasound on the Longissimus dorsi muscle. Analysis of variance was conducted using treatment and calf nested in block as fixed and random effects, respectively. Milk composition had no effect on BW or average daily gain (ADG), though calves fed fat MR had greater milk intake for the first 2 wks (P = 0.01). Calves fed the control grower diet had increased solid feed intake (P = 0.02) and tended to have greater ADG (P = 0.10), leading to a greater BW at wk 36 of age and a larger REA (P = 0.05) compared to calves fed the fat grower diet. Rump fat depots tended to be higher in calves fed the high lactose MR (P = 0.06) and the control grower (P = 0.07), whereas IMF was greater in LC-fed calves compared to FC-fed calves (P = 0.05). Overall, calves fed the fat grower diet had higher feed efficiency (kg DMI / kg BW gain) than calves fed the control grower (P < 0.01). Milk replacer composition had limited effects on growth when fed at isocaloric and isonitrogenous levels. However, growth, body composition and feed efficiency were affected by the type of energy substrate in the grower diet. Whether these differences persist in the finishing phase remains to be addressed.