Effects of dietary carbohydrate:fat ratio on growth and body composition of milk-fed calves

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Abstract
Growth and body composition measurements were made on young calves after 49 days feeding on one of 12 diets having a range of carbohydrate:fat ratios and where protein intake was either adequate or inadequate for maximum growth. Across Diets 1-6, carbohydrate:fat ratio ranged on an energy basis from 2:1 (56% lactose, 12% fat) to 0.4:1 (31% lactose, 32% fat); resulting daily protein and energy intakes were respectively 244 g and 18.7 MJ (designated low energy-high protein regime). Across Diets 7-12 carbohydrate:fat ratio ranged from 1:1 (54% lactose, 24% fat) to 0.4:1 (34% lactose, 39% fat); daily protein and energy intakes were respectively 194 g and 21.8 MJ (designated high energy-low protein regime). Whereas energy intake restricted growth on the low energy-high protein regime, protein intake restricted growth on the high energy-low protein regime. Calves on the high energy-low protein regime had higher gains of body weight (598 v 569 g/day), fat (114 v 55 g/day), and energy (7.0 v 4.8 MJ/day) than those on the low energy-high protein regime; protein and water gains were similar. The efficiency of utilisation of dietary protein for growth was highest for the high energy-low protein regime, but conversely the efficiency of utilisation of energy for growth was highest for the lower energy regime. Diet carbohydrate:fat ratio affected growth and growth efficiency on the low energy-high protein regime only, where gains of body weight, protein, and water increased, and of fat decreased, as diet carbohydrate and fat contents increased and decreased respectively. Carbohydrate:fat ratio did not affect the efficiency of energy retention. Bodies of calves on the higher energy regime contained more fat and less protein and water than those on the lower energy regime. Increasing the dietary carbohydrate:fat ratio resulted in decreasing body fat content at the low energy-high protein regime only.

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