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The response of the laboratory rat to changes in the caloric density and protein:calorie ratio of its ration McKenzie, Robert Malcolm

Abstract

One hundred and fifteen male weanling rats of the U.B.C. Wistar Strain were used to study the effect of changing energy level and protein:energy ratio of their ration on their growth rate, feed consumption, feed efficiency, body composition and metabolic rate. The rations used in this study were designed to test the hypothesis that the feed consumption of the rat is inversely proportional to the available energy content of its ration. Twenty semi-purified rations were formulated using sucrose, B. C. Herring Meal, hydrogenated vegetable oil, non-nutritive cellulose and vitamin and mineral premixes. The rations were calculated to contain 2000, 3000, 4000, 5000, or 5735 kilocalories of digestible energy per kilogram of dry matter and at each energy level to contain 30, 45, 60 or 75 milligrams of crude protein per kilocalorie of digestible energy. Most other nutrients (vitamins and minerals) were present in essentially constant amounts in each ration when levels were expressed as nutrient content per kilocalorie of apparent digestible energy. Ten of the rats were killed at a body weight of 65 grams for use as initial controls in the calculation of the composition of body weight gain. The remaining 105 animals were divided into twenty-one groups of five each. Each of twenty of the groups was assigned to an experimental ration and one group was fed a stock diet (U. B. C. Ration 10-61) as a secondary control. The animals were individually housed and were supplied with fresh bedding and water every third day. Their body weight and feed consumption were recorded daily. When an animal reached 58 grams body weight it was supplied with the previously assigned ration (prior to this time the animals were fed the above mentioned stock diet) and allowed to grow until it reached 165 grams body weight when it was killed and frozen for future chemical analysis. The experimental data used in the analysis of this study covered the body weight range of 65 to 165 grams body weight only. The apparent digestibility of the rations was determined using the chromic oxide technique. The energy content of the feed and feces was measured by bomb calorimetry and the apparent digestible energy content of the rations calculated. The protein and fat content of feed and feces was determined by Micro-Kjeldahl analysis and extraction with petroleum ether respectively and the apparent digestibility of these two ration components calculated. The proximate composition of the rats was determined on the individual animals with gastrointestinal contents included. The resting metabolic rate of a selected rat in each group was determined when the selected animal reached a body weight of 100, 130 and 165 grams. The resting metabolic rate of each of the initial control rats was determined just prior to the time it was killed. The experimental data indicate that the rat tends to eat to meet an energy requirement. The relationship between feed intake and apparent digestible energy is not in agreement with the initial hypothesis. An attempt to explain this deviation is presented and a general pattern of feed consumption relative to available energy is proposed which is believed to be applicable to all animals. Growth rates increased with increasing protein:energy ratios at fixed energy levels and increasing energy levels at fixed protein energy ratios. Feed efficiency increased with increasing energy level and with increasing protein: energy ratios. It is clearly shown that growth rate and feed efficiency, used alone, have limited usefulness in evaluating the performance of an animal. It is also clearly demonstrated that protein content of the ration when expressed as a percentage of the ration has no bearing on the performance that can be expected of an animal fed that ration. The body composition of the rats followed a pattern which is generally expected. The fat gain increased with increasing energy level at any given protein:energy ratio and increased with decreasing protein: energy ratios at any given energy level. The majority of the change in the fat content was balanced by differences which occurred in the protein and water content of the rats. The ash content was essentially constant in all groups. No apparent trends occurred in the resting metabolic rate measurements when considered on the basis of individual animals. Finally, it is suggested that the design of rations and the expression of feeding standards should be standardized and that the basis of this standard should be energy. In other words, rations and feeding standards should be expressed on a nutrient per available calorie basis rather than on a percentage basis.

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