G.E. Gardner

Genetic parameters for meat quality traits of Australian lamb meat

Genetic parameters were estimated for a range of meat quality traits recorded on Australian lamb meat. Data were collected from Merino and crossbred progeny of Merino, terminal and maternal meat breed sires of the Information Nucleus programme. Lambs born between 2007 and 2010 (n=8968) were slaughtered, these being the progeny of 372 sires and 5309 dams. Meat quality traits were found generally to be of moderate heritability (estimates between 0.15 and 0.30 for measures of meat tenderness, meat colour, polyunsaturated fat content, mineral content and muscle oxidative capacity), with notable exceptions of intramuscular fat (0.48), ultimate pH (0.08) and fresh meat colour a* (0.08) and b* (0.10) values. Genetic correlations between hot carcass weight and the meat quality traits were low. The genetic correlation between intramuscular fat and shear force was high (-0.62). Several measures of meat quality (fresh meat redness, retail meat redness, retail oxy/met value and iron content) appear to have potential for inclusion in meat sheep breeding objectives.

Genetic and environmental effects on the muscle structure response post-mortem

This paper reviewed the mechanisms by which glycolytic rate and pre-rigor stretching of muscle impact on meat quality. If muscle is free to shorten during the rigor process extremes in glycolytic rate can impact negatively on meat quality by inducing either cold or rigor shortening. Factors that contribute to variation in glycolytic rate include the glycogen concentration at slaughter and fibre type of the muscle. Glycolysis is highly sensitive to temperature, which is an important factor in heavy grain fed carcasses. An alternative solution to controlling glycolysis is to stretch the muscle pre-rigor so that it cannot shorten, thus providing an insurance against extremes in processing conditions. Results are presented which show a large reduction in variance (both additive and phenotypic) in tenderness caused by pre-rigor stretching. Whilst this did not impact on the heritability of shear force, it did reduce genotype differences. The implications of these results on the magnitude of genotype effects on tenderness is discussed.

Lamb myofibre characteristics are influenced by sire estimated breeding values and pastoral nutritional system

The objective of this study was to establish the extent to which lamb myofibre characteristics at ~8 months of age were influenced by sire (n = 9) estimated breeding values (EBVs) and pasture-based nutritional systems (low and high quality and availability, LOW and HIGH) from birth to slaughter (n = 56). Immunostaining of myosin heavy chain isoforms in longissimus muscle fibres revealed that as the post-weaning eye muscle depth EBV (PEMD) increased, the percentage of type 2B/X myofibres increased and the percentage of type 2A myofibres decreased. The percentage of type 2B/X myofibres also increased with increasing PEMD in the semimembranosus muscle, but not in the semitendinosus muscle. Post-weaning fat depth (PFAT) EBV was negatively related with the percentage of type 2B/X myofibres in longissimus and semitendinosus muscles and with the relative area of more glycolytic to more oxidative fibre types, although there was an interaction with nutritional system in the semitendinosus muscle, with this ratio increasing with the PFAT EBV among the LOW nutrition lambs. Overall, post-weaning weight (PWWT) EBV had less effect on myofibre characteristics than PEMD and PFAT EBVs. Average cross-sectional area of myofibres was more affected by the LOW than HIGH nutritional system in longissimus muscle (–25%) compared with semitendinosus (–21%) and semimembranosus (–15%) muscles. LOW nutrition resulted in a significantly smaller cross-sectional area of all myofibre types in longissimus muscle and of types 2A, 2AB/X, and 2B/X in semimembranosus and semitendinosus muscles. Overall, the relative area of glycolytic myofibres increased and/or more oxidative myofibre types decreased in LOW compared with HIGH nutritional system lambs in longissimus and semimembranosus, but not in semitendinosus muscle. The results demonstrate significant influences of sire EBVs on myofibre characteristics of lambs, the extent of which depends upon muscle type and nutritional regimen. More specifically, they suggest continued selection for increased muscling and reduced fatness in lambs are associated with shifts in the proportions of myofibre types, with potential for adverse effects on eating quality. They also show that the direction and magnitude of the effects of chronic, moderate postnatal nutritional restriction at pasture on myofibre characteristics vary between muscles with different contractile, metabolic, and/or functional characteristics.

Intramuscular fat in the longissimus muscle is reduced in lambs from sires selected for leanness

Selection for lean growth through Australian Sheep Breeding Values (ASBVs) for post weaning weight (PWWT), eye muscle depth (PEMD) and c-site fat depth (PFAT) raises concerns regarding declining intramuscular fat (IMF) levels. Reducing PFAT decreased IMF by 0.84% for Terminal sired lambs. PEMD decreased IMF by 0.18% across all sire types. Female lambs had higher IMF levels and this was unexplained by total carcass fatness. The negative phenotypic association between measures of muscling (shortloin muscle weight, eye muscle area) and IMF, and positive association between fatness and IMF, was consistent with other literature. Hot carcass weight increased IMF by 2.08% between 12 and 40 kg, reflective of development of IMF as lambs approach maturity. Selection objectives with low PFAT sires will reduce IMF, however the lower impact of PEMD and absence of a PWWT effect, will enable continued selection for lean growth without influencing IMF. Alternatively, the negative impact of PFAT could be off-set by inclusion of an IMF ASBV.

Preliminary estimates of genetic parameters for carcass and meat quality traits in Australian sheep.

Using performance from progeny born in 2007 and 2008 generated by theInformation Nucleus program of the Cooperative Research Centre for Sheep Industry Innovation, preliminary estimates of heritability were obtained for a rangeofnovelcarcassandmeatattributesoflambrelevanttoconsumers,includingcarcasscharacteristics,meatqualityand nutritionalvalueoflamb.Phenotypicandgeneticcorrelationsofliveanimaltraitswithcarcasscompositionandmeatquality traits were also estimated. The data were from progeny located at eight sites, sired by 183 rams from Merino, maternal and terminalmeatbreedsandwererepresentative oftheMerino,BorderLeicester ·Merino,Terminal ·MerinoandTerminal · Border Leicester-Merino production types of the Australian sheep industry. Data were available from 7176 lambs for weaning weight, 6771 lambs for ultrasound scanning and 4110 lambs for slaughter traits. For the novel meat quality traits, generally moderate to high heritability estimates were obtained for meat quality measures of shear force (0.27 aged 1 day, 0.38 aged 5 days), intramuscular fat (0.39), retail meat colour (range of 0.09 to 0.44) and myoglobin content (0.22). The nutritional value traits of omega-3 fatty acids and iron and zinc contents tended to have low to moderate heritabilities (0.11-0.37), although these were based on fewer records. Fresh meat colour traits were of low to moderate heritability (0.06-0.21)whereasmeasuresofmeatpHwereoflowheritability(~0.10).Forthecarcasstraits,estimatesofheritabilitywere moderate to high for the various measures of carcass fat (0.18-0.50), muscle weight (0.22-0.35), meat yield (0.24-0.35), carcassmuscledimensions(0.25-0.34)andboneweight(0.27).Resultsindicatethatformostlambcarcassandmeatquality traits there is sufficient genetic variation for selection to alter successfully these characteristics. Additionally, most genetic correlations of live animal assessments of bodyweight, muscle and subcutaneousfat with the carcass and meat quality traits werefavourable.Appropriatedefinitionofbreedingobjectivesanddesignofselectionindexesshouldbeabletoaccountfor the small unfavourable relationships that exist and achieve the desired outcomes from breeding programs.

The effect of genotype and plane of nutrition on the rate of pH decline in lamb carcasses and the expression of metabolic enzymatic markers

The objective of this study was to establish the effect of sire (n = 9) estimated breeding values (EBVs) and pasture-based nutritional systems (low and high quality and availability) from birth to slaughter on the metabolic characteristics and post-mortem pH changes in muscle of ~8-month-old lambs (n = 56). Nutritional restriction resulted in a general decrease in glycolytic capacity as indicated by the enzymes phosphofructokinase and lactate dehydrogenase (LDH), suggesting greater metabolic efficiency. Alternatively, it also depressed oxidative capacity as indicated by myoglobin and isocitrate dehydrogenase, which aligns with the histological results suggesting a fast glycolytic fibre type characteristic of less mature animals. Increasing PEMD EBV resulted in a marked shift towards a metabolically more glycolytic muscle type as demonstrated by LDH, although this did not correspond strongly with an increase in the proportion of type 2B fibres based on histological assessment of myofibre type using immunostaining of myosin heavy chain isoforms. Myoglobin concentration aligned with this trend, decreasing in all muscles as PEMD EBV increased, suggesting that selection for muscling will lead to a whiter muscle appearance. Rate of pH decline was increased in the low nutrition lambs, and through selection for PEMD EBV, this response aligning with the metabolically more glycolytic muscle type.

Associations of sire estimated breeding values and objective meat quality measurements with sensory scores in Australian lamb

The impact of selecting for lean meat yield using breeding values for increased eye muscle depth (PEMD) and decreased fat depth (PFAT) on the consumer acceptance of lamb meat was evaluated. Consumer sensory scores (tenderness, juiciness, flavour, odour, overall liking) were obtained for the longissimus lumborum (loin) and semimembranosus (topside) muscles of 1471 lambs. On average loin samples were more acceptable for consumers. Sensory scores increased with higher IMF levels, with lower shear force levels, and when animals were younger and less muscular. Increasing PEMD decreased tenderness, overall liking and flavour scores in both muscles, and decreasing PFAT reduced tenderness within the loin samples only. This negative impact of PEMD and PFAT is not solely driven through the phenotypic impact of IMF and shear force on sensory scores. Our results confirm the growing concerns that selecting for lean meat yield would reduce consumer eating quality, and highlight that careful monitoring of selection programmes is needed to maintain lamb eating quality.

Sire and growth path effects on sheep meat production. 2. Meat and eating quality

The meat and eating quality characteristics of 627 crossbred lamb progeny from 20 Poll Dorset sires were studied. The sires were selected on the basis of Australian sheep breeding values (ASBVs) for postweaning growth (PWWT), depth of loin muscle (PEMD) and subcutaneous fat (PFAT). Lambs were either weaned at 20- or 30-kg liveweight and then within each of these groups held at their weaning weight for 55 days or fully fed, giving four treatment groups. Restricted lambs were realimentated subsequent to the period of feed restriction and the lambs within each treatment group were slaughtered when their mean liveweight reached 45 kg. Shear force of the M. longissimus thoracis et lumborum (LL) decreased with temperature at pH 6.0, increased with LL ultimate pH and decreased as sarcomere length increased. Within LL aged for 5 days there was no difference between (P > 0.05) between treatments for shear force. There was a treatment effect on M. semimembranosus (SM) shear force, such that for a sire having an average ASBV PWWT of 9.34, topsides from lambs early weaned and restricted had significantly (P < 0.05) lower values. Further within these same lambs, i.e. early weaned and restricted, as the sire ASBV PWWT increased there was a significant (P < 0.001) reduction in shear force. There was a significant (P < 0.05) increase in sensory tenderness as the sire ASBV PFAT increased and across the range of ASBV PFAT this represented a change in sensory tenderness of four points. Overall liking increased with an increase in the temperature at pH 6.0 (P < 0.05) with a coefficient of 0.43 ± 0.19. There was a significant (P < 0.05) interaction between feeding level and the sire ASBV PFAT, such that for restricted fed lambs overall liking increased as the ASBV PFAT increased (coefficient 2.80 ± 0.89) after adjusting for temperature at pH 6.0, but this is not the case for fully feed lambs (coefficient -0.01 ± 0.89). Subtle interactions between sire ASBVs and growth path suggest that the impact of a period of restriction and refeeding on meat and eating quality will be meditated by the sire genetics.

Longitudinal prevalence, oocyst shedding and molecular characterisation of Cryptosporidium species in sheep across four states in Australia.

The prevalence of Cryptosporidium in sheep in the eastern states of Australia has not been well described, therefore a study of the prevalence, oocyst concentration, species and subtypes of Cryptosporidium were assessed from lamb faecal samples at three sampling periods (weaning, post-weaning and pre-slaughter) from eight farms across South Australia, New South Wales, Victoria and Western Australia. A total of 3412 faecal samples were collected from approximately 1182 lambs across the four states and screened for the presence of Cryptosporidium using a quantitative PCR (qPCR) at the actin locus. Positives were typed at the 18S locus and at a second locus using C. parvum and C. hominis specific qPCR primers. The overall prevalence was 16.9% (95% CI: 15.6-18.1%) and of the 576 positives, 500 were successfully genotyped. In general, the prevalence of Cryptosporidium was higher in WA than the eastern states. Cryptosporidium prevalence peaked at 43.9% and 37.1% at Pingelly (WA2) and Arthur River (WA1), respectively during weaning and at Pingelly (WA2) during pre-slaughter (36.4%). The range of oocyst shedding at weaning overall across all states was 63-7.9×10(6) and the median was 3.2 × 10(4) oocysts g(-1). The following species were identified; C. xiaoi (69%-345/500), C. ubiquitum (17.6%-88/500), C. parvum (9.8%-49/500), C. scrofarum (0.8%-4/500), mixed C. parvum and C. xiaoi (2.4%-12/500), C. andersoni (0.2%-1/500) and sheep genotype 1 (0.2%-1/500). Subtyping of C. parvum and C. ubiquitum isolates identified IIa and IId subtype families within C. parvum (with IId as the dominant subtype) and XIIa within C. ubiquitum. This is the first published description of C. parvum subtypes detected in lambs in Australia.

Sheep genotype, age and muscle type affect the expression of metabolic enzyme markers

The objective of this study was to determine whether genotype, age (4, 8, 14 and 22 months), sex (ewe and wether) and muscle type influence ovine (n = 587) muscle metabolic characteristics. The genotypes represented were Poll Dorsetgrowth × Border Leicester Merino, Poll Dorsetgrowth × Merino, Poll Dorsetmuscling × Merino, Merino × Merino and Border Leicester × Merino. Between 4 and 22 months of age, myoglobin concentration within all muscles and all genotypes doubled, with the bulk of this response occurring between 4 and 8 months of age. Levels in the longissimus thoracis et lumborum (LT) and semimembranosus muscles were double those seen in the semitendinosus (ST) muscle, and Merinos had the lowest myoglobin concentrations of all genotypes. The other aerobic indicator, isocitrate dehydrogenase, had lower activity in the ST compared with the LT, was lower in 22-month-old sheep compared with all other ages, and decreased as selection for leanness increased. Both phosphofructokinase and lactate dehydrogenase activity tended to increase with age, were lower in the ST compared with the LT, and had higher activity in the Border Leicester × Merino sheep. The correlation between the percentage of total myofibre area comprising type 2X myofibres and metabolic markers was far higher for the oxidative indicators isocitrate dehydrogenase and myoglobin, which both decreased as relative area of type 2X fibres increased. However, the strongest correlations were with the relative area of type 2A myofibres, which were consistently high for both oxidative and glycolytic metabolic markers implying positive coregulation with both energy producing pathways.