Age and Breed Difference in the Effect of Dietary Protein on Carcass Fat Deposition of the Chicken

Abstract

It has been reported that starting chicks and growing meat-type chicks can easily become fatty or lean by simply changing dietary levels of protein, but, in striking contrast, dietary protein levels have little effect on abdominal fat content of laying hens. In this paper, is reported the effect of dietary levels of energy and protein on the body fat deposition of growing chicks of both sexes of two strains at various ages.Both males and females of White Leghorn or meat-type cross-breds were sacrificed at 4, 6, 8, 12, 16 and 20 weeks of age for abdominal fat determination. Abdominal fat was determined because it has highly significant correlation with body fat deposition. Therefore, the latter can be estimated from the former. Except those sacrificed at 4 and 6 weeks of age, 24 males and 24 females were moved from the farm of National Institute of Animal Industry into the chick room kept at constant temperature of 20°C., and divided into 16 groups of 3 chicks each. One of the 4 experimental diets was fed to duplicated lots of males and females for 25 days before the determination of abdominal fat. Two-week-old chicks were reared in an electrically heated battery brooder for 2 weeks in the chick room on the experimental diets and sacrificed at 4 and 6 weeks of age as mentioned above.The combination of 2 levels of crude protein and 2 levels of metabolizable energy (kcal/g air-dry diet) prepared 4 experimental diets. Their contents were determined to be 14.8%-3.00kcal/g, 14.9%-2.52kcal/g, 20.4%-2.97kcal/g and 20.2%-2.47kcal/g, respectively.Effect of dietary energy and protein levels on abdominal fat deposition was analyzed statistically at various ages, and following results are obtained:1) Higher dietary energy level resulted in more deposition of abdominal fat in all stages of growth, except females at 20 weeks of age. Females at 20 weeks of age deposited large amount of abdominal fat even on low energy-high protein diet, presumably related with the sexual maturity of females.2) Higher level of dietary protein depressed fat deposition of White Leghorn chicks of both sexes at 8 weeks or younger and of meat-type chicks of both sexes at 12 weeks or younger. Little difference in abdominal fat deposition was observed between the chicks older than mentioned above, fed either high or low protein diets. It was also observed that the effect of high protein diet on fat deposition disappeared earlier in females than in males. These findings suggest that the depressing effect of dietary protein on fat deposition is related with sex hormones.3) In young chicks, on which the depressing effect of dietary protein on fat deposition was seen, the increase in dietary protein level by 5.5% approximately corresponded to the decrease in dietary metabolizable energy by 0.5kcal/g, in other words, one g of dietary protein apparently corresponded to -9.09 (=0.5/0.055) kcal of metabolizable energy, which closely resembles to -9.44kcal reported previously.4) Body fat deposition is related to actual intake of energy and protein rather than to their dietary content. However, it was revealed that change in feed intake under the experimental condition was so small that it was negligible in the discussion on fat deposition, except the change in feed intake of young White Leghorn chicks at 6 weeks of age. Dietary protein level of 14.5% seemed to be too low for young chicks and feed intake of the low protein diet was less than that of the corresponding high protein diet. This difference in feed intake had influence on fat deposition.