Effects of low protein diets on pigs with a lean genotype 2. Compositional traits measured with computed tomography (CT)

•Lowering dietary protein led to more fat and less muscle in growing pigs.•Partitioning of fat between body depots was unaffected by lowering dietary protein.•Balancing essential amino acids in low protein diets maintained carcass quality.•In vivo CT scanning can accurately predict pig carcass tissue weights/ proportions.•CT-measured muscle density predicts intramuscular fat accurately in live pigs.

The effects on compositional changes across the growing-finishing period (40-115 kg) of feeding pig diets with different protein and amino acid levels were investigated using CT scanning (at 60, 85 and 115 kg live weight). Pigs of a lean commercial genotype were fed a commercial control regime (C), or a low protein regime with either high (LP1) or low (LP2) essential amino acid levels, all balanced for net energy. In vivo CT measurements agreed well with post-slaughter sample joint dissection results for carcass tissue weights/proportions, and CT-measured muscle density predicted intramuscular fat accurately. Pigs on C and LP1 regimes did not differ significantly in composition during growth. However, pigs on the LP2 regime had significantly more fat (in carcass, internal and intra-muscular depots) and less muscle, from 85 kg onwards. Although fat levels differed depending on diet regime (LP2 > others), proportions of fat in different body depots were unaffected.