Growth Performance, Body Composition and Protein and Energy Utilization of Pigs Fed Ad Libitum Diets Formulated According to Digestible Amino Acid Content

Four groups of six growing gilts each were fed ad libitum diets composed of cereals and soyabean (SBM) or rapeseed (RSM) meal and containing two levels of crude protein: high - 18.0% (RSM-H) and 16.9% (SBM-H) or low - 15.6% (RSM-L) and 15.1% (SBM-L). The diets were balanced by supplementation with crystalline amino acids and contained apparent ileal digestible lysine, methionine, threonine and tryptophan in proportions (1.00:0.32:0.57:0.18) according to CVB (1995). Voluntary feed intake, weight gain and slaughter and chemical body composition of animals were assessed. Protein and energy balances from 25 to 70 kg body weight were calculated by the comparative slaughter method. Protein source had a significant effect on voluntary feed intake; it was 0.12 kg/d lower in pigs fed the SBM than RSM-diets. Pigs fed on the SBM-L diet consumed the least amount of feed (2.17 kg). Daily gain (average, $900{\pm}12.59g$) and feed conversion ratio ($2.54{\pm}0.04kg/kg$) were not statistically affected by source (SMB and RSM) and protein level (high and low). In empty body similar amounts (g/kg) of protein (163 g), water (635 g) and ash (28 g) were found. However, pigs fed the RSM-L diet were fatter than those fed the SBM-L diet (188 vs. 161 g/kg). No statistical differences were observed in daily protein deposition, which on average amounted to $142{\pm}11g$, or carcass characteristics. An improvement of crude protein utilization by 6.3 percentage units was found by decreasing the protein concentration in the diets. Heat production in the body was not significantly affected by the treatments.