Deep litter housed pigs have a faster pH decline compared to conventional housed pigs

The Australian pork industry has focused on developing an eating quality pathway (Channon et al. 2016) to improve the quality and consistency of pork. However, studies have generally focused on conventional housed pigs with an average HCW of 74.9 kg. This study was conducted to identify the effect of deep litter compared to conventional housing systems, over three carcass weights and two sexes of pigs. The hypothesis tested was that housing (H), carcass weight (W) and sex (S), does not affect carcass objective quality.

A total of 384 Large White × Landrace commercial pigs (PrimeGro Genetics™, Corowa, NSW, Australia) selected over eight replicates were used in a 2 × 2 × 3 factorial experiment with the main factors being: (1) housing: conventional partially-slatted birth to bacon housing system (CON) v. deep-litter grow-out system (DL); (2) sex: female (F) or castrated male pigs (IM) (Improvac^®, Zoetis, Rhodes, NSW, Australia); (3) carcass weight specification (Trim 1): Light 60 to 70 kg, 8 to 16 mm P2 (L); Medium 70.1 to 80 kg, 8 to 16 mm P2 (M); Heavy 80.1 to 91 kg, 8 to 16 mm P2 (HV). Pigs of different sex were kept separately on the farms, during transport (~4 h) and in lairage. On arrival at the abattoir a sub-sample per housing type and sex were held in lairage with access to water before slaughter the next day. For each replicate, 48 pigs were selected within housing type, sex, and carcass weight specifications. Carcasses were chilled 24 h at 2°C. Carcass pH and temperature were measured (loin) at 45 min, 90 min, 3, 6 and 24 h post-slaughter using a pH meter (MPI, Topeka, KS, USA). Data were analysed using ANOVA (Genstat 16, VSN International, Hemel Hempstead, UK).

The effect of housing, carcass weight and sex on objective quality measures is reported separately (Lealiifano et al. 2017). DL carcasses were 1 mm fatter compared to CON (SE 0.26, P = 0.002), and were fatter at target carcass weights (L: 10.3 v. 9.8, SE 0.36; M: 11.8 v. 11.1, SE 0.36; HV: 13.5 v. 11.9, SE 0.36 DL v. CON respectively P = 0.023). This could be due to ambient temperature variability in the deep litter housed pigs. DL carcasses had a faster pH decline (Fig. 1a) perhaps indicative of increased glycolytic rate post-slaughter as a consequence of increased pre-slaughter stress or higher glycogen stores. Carcass temperature was unaffected by housing (P > 0.05). Rate of chilling was slower in HV carcasses compared to L; however, there was no difference for 24 h carcass temperature between the weight categories (Fig. 1b). Carcass P2 increased by 1 mm with every increase in 10 kg carcass weight (SE 0.22, P = 0.001). Carcass pH was unaffected by weight differences (P > 0.05). The combination of the increased muscle size and carcass P2 caused greater heat inertia and a slower chilling in the heavier carcass. There was an H × S interaction (P < 0.05) at 45 min post-slaughter only (IM: 6.35 v. 6.53 SE 0.04; F: 6.43 v. 6.47, SE 0.04, P = 0.031 for DL and CON, respectively). The hypothesis was rejected as these results show that H, W and S, significantly influence objective carcass quality but with a large variability between factors.

Fig. 1.  (a) Carcass pH of DL (white) and CON (grey) pigs, and (b) Carcass temperature of L (dots), M (solid black), and HV (grey stripes) pigs from 45 min to 24 h post-slaughter.