Effect of high dietary fat content on heat production and lipid and protein deposition in growing immunocastrated male pigs

In immunocastrated (IC) pigs, revaccination (V2) increases lipid deposition (LD) because of increased voluntary feed intake; but little is known on associated effect of diet composition on partitioning of nutrients in IC pigs. Digestibility measurements, N and energy balances in respiration chambers were performed in two subsequent stages in four replicates of two male littermates to determine the changes between 85 (stage 1) and 135 (stage 2) kg live weight due to combined effect of IC, growth and increased feed intake (IC/growth). During stage 1, pigs received a standard low-fat diet (LF diet; 2.5% dry matter (DM) of fat fed at 2.27 MJ metabolizable energy (ME)/kg BW0.60 per day), whereas during stage 2, feed intake was increased to 2.47 MJ ME/kg BW0.60 per day and one littermate was fed LF diet whereas the second received a fat-enriched diet (HF diet; 8.9% DM of fat) to determine the effect of increased dietary fat content on energy utilization in IC pigs. Results from N balance and measurements of gas exchanges were used to calculate respiratory quotient (RQ), heat production (HP), nutrient contribution to fat retention, components of HP, protein deposition (PD) and LD. Nutrients and energy apparent digestibility coefficients, methane losses and N retention (P<0.05) increased with IC/growth. Despite higher ME intake, total HP remained similar (1365 kJ/kg of BW0.60 per day; P=0.47) with IC/growth. Consequently, total retained energy (RE) increased with IC/growth (from 916 to 1078 kJ/kg of BW0.60 per day; P<0.01) with a higher fat retention (625 to 807 kJ/kg BW0.60 per day; P<0.01), originating mainly from carbohydrates associated with a higher lipogenesis (536 to 746 kJ/kg BW0.60 per day; P<0.01) and RQ (1.095 to 1.145; P<0.01). Both PD (from 178 to 217 g/day; P=0.02) and LD (from 227 to 384 g/day; P<0.01) increased due to IC/growth. Feeding HF diet after IC was associated with increased crude fat digestibility (P<0.01) and increased RE as fat (807 to 914 kJ/kg BW0.60 per day; P=0.03), originating mainly from dietary fat (P<0.01) and resulting in increased LD (384 to 435 g/day; P<0.01) and lower RQ (from 1.145 to 1.073; P<0.01). Altogether, present results indicate that increased fatness of IC pigs is a result of increased daily LD caused by higher energy intake and lower basal metabolic rate. In addition, LD is further enhanced by dietary energy enrichment with fat after V2.