Effects of animal fat and of low ratios of protein to metabolisable energy on the utilisation of dietary energy by medium‐ and fast‐growing strains of poultry

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Abstract
In three experiments each involving 16 groups of 12 birds, diets having normal to low ratios of crude protein to metabolisable energy were fed, for the first 12 weeks of life, to cockerels of a Rhode Island Red X Light Sussex cross and a modern commercial fast‐growing hybrid. Dietary ratios of crude protein to metabolisable energy were lowered by adding either maize starch or animal fat to a diet which was based on cereals and was, by calculation, adequate in all known nutrients. The efficiency with which metabolisable energy was converted to tissue energy was assessed by a carcass analysis technique which also allowed a study of the effect of diet on body composition.In both strains of bird, diets in which the ratios of crude protein to metabolisable energy had been lowered considerably below that corresponding to the 20% protein and 2.9 Mcal. metabolisable energy per kg. recommended by the (U.S.A.) National Research Council, gave rise to a greater metabolisable energy intake and greater loss of this energy, mainly as heat, per unit of live body weight. Deposition of body fat was also greater than in birds fed normal diets. The relative importance of heat loss to the environment and deposition of body fat, for disposal of excess energy ingested through overeating to compensate for inadequate concentrations of amino‐acids in the diet, may be influenced by breed and the proportion of fat in the diet. No significant differences were found among diets in the percentage utilisation of dietary metabolisable energy for tissue energy production and it may be that the differences found between strains arise solely from differences in dietary energy intake. Whatever the diet, the modern fast‐growing type of bird consumed more feed and therefore energy and made about 30% greater live‐weight gain with about 40% greater tissue energy gain over 12 weeks than the Rhode Island Red X Light Sussex type.