Enhancing long-chain omega-3 content in Australian lamb using genetics and diet

This thesis examined the effects of breed, sex, supplement and SNP marker on the levels of intramuscular long-chain omega-3 fatty acid content and meat quality traits in five Australian sheep breeds. Five hundred first-cross prime lambs sired by five genetically divergent breeds (Texel, White Suffolk, Dorset and East Friesian) under the same management conditions were utilised. The animals were grazed in a conventional broad acre sheep production system in southern Tasmania on improved and irrigated pastures. Adverse seasonal conditions necessitated the relocation of animals which had not reached slaughter weight to non-drought stressed pastures. A supplementary feeding trial using 38 of the F1 progeny representing the five breeds, two sexes (wethers and ewes), 2 supplementary feeds (canola or lupin) with two levels of supplementation (1% and 2% of body weight) was conducted over a 9-week duration. Overall, females had higher content of intramuscular long-chain omega-3 than males regardless of supplement type. The mean intramuscular eicosapentaenoic acid (EPA) + docosahexaenoic acid (DHA) content of animals slaughtered at the commencement of the trial was only 7.5 mg/100 g. After supplementation, EPA + DHA content increased to 12.5 mg/100 g for canola meal and 14.3 mg/100 g with cracked lupin. Despite this remedial effect, supplementation still did not bring the claimable EPA + DHA content of Australian lamb up to the Food Standards Australia and New Zealand (FSANZ) claimable dietary source level of 30 mg/100 g. The meat quality of 354 of the F1 progeny was also investigated to test the impacts of single nucleotide polymorphic marker, sire breed, dietary supplementation and relocation to non-drought affected pastures on omega-3 fatty acid content and meat quality traits. Age at slaughter had an impact on fat scores and eye muscle shape, with younger animals having leaner subcutaneous fat and smaller eye muscles than older animals of the same carcass weight. The SNP markers tested did not have significant (P>0.05) effects on meat quality traits. The findings showed that as long as animals reach the required target liveweight and fat score before slaughter, rearing lambs on drought affected pasture, relocation or supplementation with canola or lupin meals to attempt to boost long-chain omega-3 content had no negative effect on the meat quality parameters. However, sire breed did have a significant effect on fat score and eye muscle measurement with the East Friesian lambs being leaner and having smaller eye muscle measurements but heavier muscle to bone yield. Fatty acid profiles of the animals revealed an increase in intramuscular fat with time. Long-chain omega-3 fatty acid content closely reflected the quality of pasture on offer with only 7 mg/100 g EPA + DHA in the drought affected animals and this value doubled once animals were relocated to actively growing green pastures. Texel sired lambs had significantly lower (P<0.05) DHA content that other breeds. The use of SNP markers to better understand the genetic variability in long-chain omega-3 fatty acid content and relationships with lipid synthesis and fat metabolism-related genes was also tested. The association between polymorphisms of the fatty acid binding proteins (FABP) and Delta-6 desaturase (FADS2) gene clusters was investigated. There was no significant (P<0.05) association between FADS2 or FABP4 and the intramuscular contents of EPA or DHA. Microsatellite and SNP markers were utilised to genotype 79 sires from diverse Australian locations for the Myostatin gene. It was evident that the East Friesian and Texel breeds shared a common significant Guanine to Adenosine substitution (g+6723G>A) frequency of 0.63, thus suggesting a common phylogenetic origin. As an overall outcome for this thesis, Australian sheep meat producers can better understand the significance of supplementation with canola or lupin, quality of feed, sex, genotype and breed on the long-chain omega-3 content of sheep meat. Future research into the fatty acid profiling of various fodder and pastures commonly used in sheep grazing systems over time is of strong merit, as is the trialling and potential use of a wider range of supplements. The high levels of variation observed and the failure to date to reach claimable source (30 mg/100 g) and ultimately good source (60 mg/100 g) levels are still of concern. These two claimable levels should remain research and production targets, if Australian lamb is to be considered a reliable dietary source of long-chain omega-3 for human consumers.