METHODS OF ENERGY EVALUATION OF FEED INGREDIENTS FOR RABBITS AND THEIR ACCURACY

The influence of the methodology used in the evaluation of feed ingred ients for rabbits on the precision of determination is reviewed. The p rocedures for calculation of the energy values and their standard erro rs are shown and discussed for each method. The direct method used for evaluating some relatively balanced feedstuffs such as alfalfa hay or wheat bran gives consistent results, but over- or underestimation and low repeatability is obtained with most imbalanced ingredients. For t he substitution method, two calculation procedures of the energy value s are compared, showing greater accuracy with the equation in which th e gross energy of the ingredient is determined directly instead of est imated by difference. The basal diet acid the rate/s of substitution m ust be designed to avoid a great nutrient imbalance in all the experim ental diets. The substitution rate is the main factor in the precision of estimation of feedstuff evaluation, being directly related to the accuracy. Thus, the standard error of the ingredient. energy value est imated by difference is 13.4, 6.4 and 2.9 times higher than the standa rd error of diets for 10%, 20% and 40% substitution rates, respectivel y. The number of replicates is of limited importance in comparison wit h the substitution rate; thus, the numbers of rabbits required to esti mate the mean energy value within +/-5% with a 95% confidence are 37 a nd 10 for 20% and 40% substitution rates, respectively. Therefore, the energy value of ingredients that are usually included in diets in hig h proportions (20-30%) can be obtained with good precision by differen ce using only one rate of substitution. Several levels of inclusion (f our or more) must be used when interactions between ingredients are ex pected or very low rates of inclusion are used. In this case, the ener gy value is estimated by extrapolation of the regression between the d igestible energy of diets and the substitution rates and a greater pre cision than by difference is obtained. The multiregression method is r arely used because of the great number of ingredients included in the diets and because of the existence of interactions among them, prevent ing the correct estimation of their energy values.