Evaluation of genetic and physiological parameters associated with meat tenderness in South African feedlot cattle

Abstract

The objective of this study was to compare prediction of meat tenderness by means of gene technologies (markers) with established physical estimates of meat tenderness. Weaned, young bulls (n = 60) were selected on phenotype from various commercial producers to represent a Brahman (Bos indicus; n = 20), Simmental (continental Bos Taurus; n = 20) and Nguni (Sanga; n = 20) crossbred group. After being raised under intensive feedlot conditions the animals were slaughtered according to normal South African slaughter procedures at an A-age (10 - 12 months) with a fatness class of two or three (lean-medium fatness). At slaughter the carcasses were not electrical stimulated because electrical stimulation influences the processes of meat tenderness, and the emphasis was on the expression of the inherent tenderness characteristics without external post mortem influences. Carcasses were halved, chilled at 4 ˚C within 2 hours post mortem. The M. longissimus thoracis et lumboram (LT and LL) of the right and left sides were removed from the third last rib to the last lumbar vertebra and sub sampled for shear force evaluations, SDS-PAGE, Western-blotting, myofibril fragmentation (MFL), sarcomere length (SL), calpain, calpastatin, total collagen, % collagen solubility and marker analysis. The position of sampling for each test was consistent and the different samples were either frozen immediately at -20 ˚C or -80 ˚C or vacuum packed and aged (2 ± 2 ˚C) for 7 or 14 days post mortem. Two single nucleotide polymorphism (SNP) markers were employed in this study for the bovine CAPN1 gene, which is found or situated on bovine chromosome 29, namely a SNP marker which is situated on exon nine (CAPN1-316) and the other on intron 17 (CAPN1-4751). The inhibitor, calpastatin (CAST) found on chromosome seven was also analysed in this study. Both the CAST markers (CAST and CAST-Brahman) lie in the three prime untranslated regions (3' UTR) of the CAST gene. Genotype data of two- markers were used to determine the two-marker haplotypes. The results of the study showed that differences exist in meat quality of the different crossbreds. Brahman- and Nguni-crosses had lower shear force values (more tender) than that of the Simmentaler-crosses under these specific experimental conditions. The pH decline did not differ significantly (p > 0.001) between the crossbreds. The carcass temperature and temperature decline rate although similar between the Brahman- and the Simmentaler-crosses for three hours and up to eight hours post mortem differed significantly from that of the Nguni-crosses (p < 0.006). The sarcomere lengths were mostly under 1.7 µm, which indicate that shortening (caused by rapid chilling) can not be eliminated. No significant differences were found between the different crossbreds for the calpastatin levels but significant differences were found for the µ-calpain activity and µ-calpain / calpastatin activity ratios. The Brahman-crosses had longer myofibril fragment lengths on average, indicating lower proteolysis / myofibrillar fragmentation compared to the other crossbreds. Myofibrillar protein degradation (titin, nebulin, desmin) and myofibrillar protein formation (30 kDa) during post mortem ageing was examined as a confirmation for the myofibril fragment length results. Significant differences between the crossbreds were found for titin degradation. Nguni-cross animals had significantly (p < 0.033) more titin present than the other crossbreds. Nebulin degradation showed a significant (p < 0.038) breed effect at 7 days post mortem and a significant degradation rate difference for breed types between 1 day and 7 days post mortem (p < 0.03) and 1 day and 14 days post mortem (p < 0.034). Desmin degradation evaluated with SDS-PAGE and Western-blotting indicated that the Simmentaler-cross animals had significantly (p < 0.018; p < 0.024, respectively) lower desmin levels compared to the other crossbreds. For the 30 kDa proteins there were no significant (p > 0.001) differences in data evaluated at 1 day, 7 and 14 days post mortem as well as for the formation rate. The results indicates that Brahman-crosses had the highest frequency for haplotypes that are associated with increased shear force, and thus tougher meat compared to the Simmentaler-crosses that had the highest frequency for haplotyes that are associated with lower shear force, and thus more tender meat, while Nguni-crosses were intermediate. A multiplex marker system incorporating both markers (316 and 4751) and indexes for the markers at CASTand CAPN1 genes were evaluated in this study. Considering the average index for the interactions between the CAST and CAPN1 genes, it can be concluded that the Nguni-cross was overall the breed with the highest potential for inherently tender meat. In general, the animals in this study had the tendency for tougher meat. The genetic markers (CAST and CAPN1) showed no association with Warner-Bratzler shear force (WBSF) (p > 0.05), which suggest that various mechanisms and environmental factors may be involved and give another outcome compared to the genetic make up. Simple correlation coefficients were generated between the different characteristics measured. If the group of animals in this study is indeed a typical representation of South African feedlot finished crossbred animals, the relatively high WBSF values emphasise the challenge to manipulate their intrinsic tenderness potential by making use of various pre- and post-slaughter techniques and procedures.