Effect of body weight on net energy value of feeds for growing pigs

Digestible (DE), metabolizable (ME), and net (NE) energy values of seven diets were measured in castrated male pigs of 45 (Stage 1), 100 (Stage 2), or 150 (Stage 3) kg BW. Diets were prepared from a basal diet supplemented with cornstarch, or sucrose, or a protein mixture (referred to here as protein), or rapeseed oil, or a mixture of fibrous ingredients (referred to as fiber), or rapeseed oil+fiber. Diets were fed at similar levels (x maintenance) at the three stages. Heat production at different feeding levels, as measured by indirect calorimetry, allowed calculation of energy retained by each pig (equal to ME intake minus heat production) and an estimate of fasting heat production of all pigs (360 kcal/kg BW.42). Net energy intake was then calculated for each pig as retained energy plus 360 x BW.42. The amounts of DE digested before the end of the ileum (DEi) and in the hindgut (DEh) were measured. Formulation of diets allowed calculation of energy values of the ingredients added to the basal diet. Digestibility and metabolizability of diets increased significantly from Stages 1 to 3, with higher variations for low-energy diets. The NE:ME ratio (k, %) and dietary NE content were not affected (P > .05) by stage of growth. On average, k was 75%, with higher values for diets containing rapeseed oil or starch (77%) and lower estimates for the fiber diet (72%). As a consequence, k was approximately 90, 82, 80, 72, and 60% for rapeseed oil, cornstarch, sucrose, protein, and fiber, respectively. These values are consistent with the lower k value for ME from DEh (57 vs 78% for DEi). Present data confirm that the hierarchy between feeds is dependent on the energy system (DE vs ME vs NE) and that the NE concept is superior in predicting the "true" energy value. The present results combined with previous ones show that, under practical conditions, the same NE prediction equations based on digestible nutrient contents, or preferably DE or ME contents, can be applied at all stages of growth in pigs. However, attention should be paid to factors such as BW or feeding level that affect digestibility and metabolizability of feeds markedly. The effects are the most important for ingredients.