Energy/protein ratio in natural-ingredient diets for weanling rats. II. Energy and protein retention and efficiency of retention

Abstract

SummaryA 3 x 3 factorial experiment was carried out, in which natural ingredient diets formulated at 3 digestible energy levels (DE) (100, 90 and 80% of NRC, 1978 recommendations) and at 3 energy: protein ratios (EPR) (63, 71 and 79 kJ DE/g crude protein (CP)) were fed to 8 weanling male Wistar rats/group. Half of these animals were sacrificed at the end of the 6-week experimental period and their body composition analyzed. Retentions of protein and energy were assessed by difference from a similar group of 8 rats killed and analyzed at the beginning of the experiment. No interaction was found between the 2 factors studied. Main results showed a decrease in intake and retention of both energy and protein, without affecting the efficiency of retention when energy level of diets was reduced by 20%. An increase in the EPR of the diets led to a decrease in protein intake and to an increase in efficiency of protein retention, without affecting the amount retained. No changes in body composition, nor in composition of live weight gains were found as a result of treatments. It is concluded that a 10% reduction in DE level of the diet is of no consequence, and that an EPR of 79 kJ/g leads to more efficient use of the dietary protein without detrimental effects over any other nutritional indicator.The effects of energy and protein concentration in diets for growing animals has been widely studied in different species. In most of these studies the effects of protein or energy level have been analyzed independently and by uSing synthetic diets. The importance of considering both nutrients together, as an EPR to allow a better utilization of them, has been recognized (Clarke et al., 1977; NRC, 1978; McCracken, Eddie and Stevenson, 1980a, 1980b; and others In a previous report (Lopez & Garcia, 1983) weight gain, intake and intake/weight gain ratio for CP and DE were analyzed, when diets formulated with natural ingredients and differing in DE level and EPR were offered to weanling rats. The EPR value ranged within limits considered as normal by NRC (1978) for such diets. The present work reports on the retention and the efficiency of retention of protein and energy in the body of rats in the same experiment. Materials and methodsThe experiment was conducted as a 3 x 3 factorial, the factors being DE level (IOO, 90 and 80% of NRC, 1978 recommendation, which is 17'5 MJ DE/kg dry matter) and EPR (63, 71 and 79 kJ DE/g CP).. The experimental animals were male weanling Wistar rats. Diets, management and experimental layout were described previously (Lopez & Garcia, 1983). To study the initial carcass composition at the beginning of the experimental period (21 days of age), an extra group of 8 rats were sacrificed with diethylether, just after the animals used for the experiment had been distributed into groups. At the end of the 6-week experimental period, 4 rats/group (the ones that were closest to the average final weight of the group) were sacrificed as described above. In both instances, the bodies were prepared for anal...