Glycogen metabolism and ultimate pH of muscle in Merino, first-cross, and second-cross wether lambs as affected by stress before slaughter

Abstract

This experiment compared the metabolism of muscle glycogen and the ultimate pH (pHu) of muscle in Merino and crossbred lambs fed the same diet but subjected to different levels of stress pre-slaughter. Twenty- five Merino, 24 first-cross (Merino dam × Poll Dorset sire), and 23 second-cross (Border Leicester × Merino dam and Poll Dorset sire) wether lambs (6 months old, 30 kg liveweight) were maintained for 8 weeks on a complete pelleted diet based on lupin seed, straw, and barley grain (metabolisable energy, 10.8 MJ/kg; protein, 17.4% in dry matter), with a feed intake of 1.3 kg dry matter /day for each breed. At Week 6, muscle biopsies were taken from 15 lambs of each breed, and at Week 8, 5 animals from each breed were slaughtered at an experimental abattoir 10 min after removal from their pens (low pre-slaughter stress). The remaining lambs, separated into breeds, were transported for 2 h and slaughtered after 24 h lairage at a commercial abattoir. Muscle samples were taken at slaughter, and assayed with the biopsy samples for glycogen concentration (corrected for lactate concentration), myoglobin concentration, and citrate synthase activity. The pHu of muscle was measured 48 h post slaughter.

Compared with commercially slaughtered lambs, the muscle tissues of lambs subjected to low pre-slaughter stress had higher glycogen concentrations post mortem, lower pHu, and no significant loss of glycogen between pen and slaughter. Breed had no effect on glycogen, pH, or the colour of muscle under low-stress slaughter conditions or when the muscle biopsy was taken. In contrast, breed had a significant influence under commercial slaughter conditions, the muscle of Merino lambs having the lowest glycogen concentration post portem, and the highest pHu and loss of glycogen between pen and slaughter. Irrespective of stress, second-cross lambs had higher myoglobin concentrations and citrate synthase activities than the Merino lambs, indicating greater pigmentation of muscle in the second-cross animals. We conclude that higher proportions of Merino genes are associated with a greater sensitivity to stress in lambs destined for slaughter.