The effects of feeding level upon protein and fat deposition in Iberian heavy pigs

Eighteen Iberian barrows of 100 kg initial BW were used in a comparative slaughter experiment to investigate the effects of feeding level (FL; 0.70 and 0.95x ad libitum) on growth, protein- and fat-deposition rates and body composition. They were fed on a diet supplying the optimum level of digestible ideal protein:ME ratio (4.82 g/MJ) and slaughtered at 150 kg BW. The apparent digestibility of DM, OM, total N and GE, and the ME:GE and ME:DE ratios were not affected by FL. The average daily gain increased with the increase in FL (691 and 918 g/day respectively). Neither the gain:feed (g/g DM) nor the gain:ME intake (g/MJ) ratios were significantly affected however. An energy cost of 59.9 kJ ME/g gain was calculated. The rate of whole-body protein deposition was not significantly altered by FL. On average it reached 80 g/day. The pigs fed at the lowest level exhibited lower fat deposition (P <0.01) and total energy retention (P <0.01) than those in the group subjected to only a slight feed restriction. The overall efficiency of utilization of ME for energy gain (ER:ME intake) remained at 0.363 on average, irrespective of the FL imposed. Relative proportions of energy retained as protein and fat were not statistically altered by changes in FL. Mean values for ERP:ER and ERF:ER were 0.100 and 0.903 respectively. No differences between treatments were observed in fat, ash, water or energy content of the whole-body (g/kg) at 150 kg empty BW, the mean values (n =12) being 512±8.5, 27.6±0.63 and 353±6.7 g/kg for fat, ash and water respectively and 22.90±0.31 MJ/kg for energy. Nevertheless, the proportion of protein tended to decline with the increase in FL (111 vs 104; P =0.069). At 150 kg the Iberian pigs contained more fat and energy and less ash and water than at 100 kg. Whilst the percentage of lean tissue decreased significantly (P <0.05) and protein content in the carcass showed a strong tendency to decline in the pigs fed close to ad libitum (104 vs 96; P =0.056), the proportions of fat, ash, water and energy remained constant at 564±9.0, 22.2±0.91, 312±6.6 g/kg, and 24.82±0.32 MJ/kg, respectively. It was concluded that a degree of feed restriction may result in a discernible improvement in carcass quality.