P. McGilchrist

Beef carcasses with larger eye muscle areas, lower ossification scores and improved nutrition have a lower incidence of dark cutting.

This study evaluated the effect of eye muscle area (EMA), ossification, carcass weight, marbling and rib fat depth on the incidence of dark cutting (pH(u)>5.7) using routinely collected Meat Standards Australia (MSA) data. Data was obtained from 204,072 carcasses at a Western Australian processor between 2002 and 2008. Binomial data of pH(u) compliance was analysed using a logit model in a Bayesian framework. Increasing eye muscle area from 40 to 80 cm², increased pH(u) compliance by around 14% (P<0.001) in carcasses less than 350 kg. As carcass weight increased from 150 kg to 220 kg, compliance increased by 13% (P<0.001) and younger cattle with lower ossification were also 7% more compliant (P<0.001). As rib fat depth increased from 0 to 20mm, pH(u) compliance increased by around 10% (P<0.001) yet marbling had no effect on dark cutting. Increasing musculature and growth combined with good nutrition will minimise dark cutting beef in Australia.

Beef cattle selected for increased muscularity have a reduced muscle response and increased adipose tissue response to adrenaline

The aim of this experiment was to evaluate the impact of selection for greater muscling on the adrenaline responsiveness of muscle, adipose and liver tissue, as reflected by changes in plasma levels of the intermediary metabolites lactate, non-esterified fatty acids (NEFA) and glucose. This study used 18-month-old steers from an Angus herd visually assessed and selected for divergence in muscling for over 15 years. Ten low muscled (Low), 11 high muscled (High) and 3 high muscled heterozygotes for myostatin mutation (High(Het)) steers were challenged with adrenaline doses ranging between 0.2 to 3.0 μg/kg live weight. For each challenge, 16 blood samples were taken between -30 and 130 min relative to adrenaline administration. Plasma was analysed for NEFA, lactate and glucose concentration and area under curve (AUC) over time was calculated to reflect the tissue responses to adrenaline. Sixteen basal plasma samples from each animal were also assayed for growth hormone. Muscle glycogen and lactate concentration were analysed from four muscle biopsies taken from the semimembranosus, semitendinosus and longissimus thoracis et lumborum of each animal at 14, 90 and 150 days on an ad libitum grain-based diet and at slaughter on day 157. In response to the adrenaline challenges, the High steers had 30% lower lactate AUC than the Low steers at challenges greater than 2 μg/kg live weight, indicating lower muscle responsiveness at the highest adrenaline doses. Aligning with this decrease in muscle response in the High animals were the muscle glycogen concentrations which were 6.1% higher in the High steers. These results suggest that selection for muscling could reduce the incidence of dark, firm, dry meat that is caused by low levels of glycogen at slaughter. At all levels of adrenaline challenge, the High steers had at least 30% greater NEFA AUC, indicating that their adipose tissue was more responsive to adrenaline, resulting in greater lipolysis. In agreement with this response, the High steers had a higher plasma growth hormone concentration, which is likely to have contributed to the increased lipolysis evident in these animals in response to adrenaline. This difference in lipolysis may in part explain the reduced fatness of muscular cattle. There was no effect of selection for muscling on liver responsiveness to adrenaline.

Ruminant glycogen metabolism

The biochemistry of glycogen metabolism is well characterised, having been extensively studied in laboratory rodents and humans, and from this stems the bulk of our knowledge regarding the metabolism of glycogen in ruminants. With respect to intermediary metabolism, the key tissues include the liver and muscle. The liver glycogen depot plays a central role in intermediary metabolism, storing and mobilising glycogen during the fed and fasted metabolic states, with these responses modulated during pregnancy, lactation, and exercise. Alternatively, the muscle glycogen depot is particularly important for local energy homeostasis, and is likely to be less important as a key post-prandial sink for blood glucose given the reduced absorption of glucose from the gut in ruminant animals. Yet similar to the liver, this depot is also in a constant state of turnover, with the muscle glycogen concentration at any point in time a reflection of the rates of glycogen synthesis and degradation. Muscle glycogen metabolism attracts particular attention given its importance for post-mortem acidification of muscle tissue, with a shortage at slaughter leading to dark cutting meat. Simplistically the concentration of muscle glycogen at slaughter is a function of two key factors, the on-farm starting levels of glycogen minus the amount depleted during the pre-slaughter phase. On-farm concentrations of muscle glycogen are largely a reflection of metabolisable energy intake driving increased rates of muscle glycogen synthesis. Compared with simple-stomached species the rate of glycogen synthesis within ruminants is relatively low. Yet there also appears to be differences between sheep and cattle when fed diets of similar metabolisable energy, with cattle repleting muscle glycogen more slowly after depletion through exercise. While metabolisable energy intake is the key driver, genetic and age-related factors have also been shown to influence glycogen repletion. The amount of muscle glycogen depleted during the pre-slaughter phase is largely associated with stress and adrenaline release, and several recent studies have characterised the importance of factors such as exercise, age and genetics which modulate this stress response. This paper presents a summary of recent experiments in both cattle and sheep that highlight current developments in the understanding of this trait.

The impact of beef cattle temperament assessed using flight speed on muscle glycogen, muscle lactate and plasma lactate concentrations at slaughter

This study evaluated the effect of animal temperament measured using flight speed (FS) on plasma lactate, muscle glycogen and lactate concentrations at slaughter plus ultimate pH in 648 lot finished cattle of mixed breed and sex. Muscle samples were collected at slaughter from the m. semimembranosus, m. semitendinosus and m. longissimus thoracis (LT) for analysis of glycogen and lactate concentration. Blood was collected after exsanguination and analysed for plasma lactate concentration and ultimate pH of the LT was measured. FS had no effect on muscle glycogen concentration in any muscle or ultimate pH of the LT (P>0.05). As FS increased from 1 to 5m/s, plasma and muscle lactate concentration increased by 54% and 11.4% (P<0.01). The mechanisms through which temperament contributes to variation in glycogen metabolism remain unclear. The risk of dark cutting was not impacted by temperament, indicating that other production and genetic factors have a greater impact on the incidence of dark cutting.

Selection for increased muscling in Angus cattle did not increase the glycolytic potential or negatively impact pH decline, retail colour stability or mineral content

This study determined the impact of selection for greater muscling in Angus cattle on myofibre characteristics, muscle enzymatics, retail colour stability, pH decline and mineral content of the semimembranosus (SM), semitendinosus (ST) and longissimus thoracis (LT). Muscle from 10 low muscled (low) and 11 high muscled (high) steers were analysed. The high steers had myofibres 22% and 24% larger in cross-sectional area in the SM and ST (P<0.05), and 8.6% less type IIX myofibres in the LT than the low steers (P<0.05). The highs had 4.9% lower lactate dehydrogenase activity, 10.2% and 12.3% higher citrate synthase and isocitrate dehydrogenase activity than lows (P<0.05). The highs had 27% more iron in the LT (P<0.05). The results indicate that the oxidative capacity of muscle can be maintained in more muscular cattle with no detrimental effects to mineral content, pH decline or retail colour stability. Myofibre hypertrophy is one mechanism leading to greater muscle mass of these high muscled cattle.

Whole body insulin responsiveness is higher in beef steers selected for increased muscling.

The aim of this experiment was to evaluate the impact of selection for greater muscling on whole body insulin responsiveness in cattle, as reflected by greater uptake of glucose in response to constant insulin infusion and greater glucose disappearance following an intravenous glucose tolerance test. This study used 18-month-old steers from an Angus herd visually assessed and selected for divergence in muscling over 15 years. Eleven high-muscled (High), 10 low-muscled (Low) and 3 high-muscled steers, which were heterozygous for a myostatin polymorphism (HighHet), were infused with insulin using the hyperinsulineamic-euglyceamic clamp technique. Insulin was constantly infused at two levels, 0.6 μIU/kg per min and 6.0 μIU/kg per min. Glucose was concurrently infused to maintain euglyceamia and the steady state glucose infusion rate (SSGIR) indicated insulin responsiveness. An intravenous glucose tolerance test was also administered at 200 mg/kg live weight. Sixteen blood samples were collected from each animal between -30 and 130 min relative to the administration of intravenous glucose, plasma glucose and insulin concentration was determined in order to analyse insulin secretion and glucose disappearance. Insulin-like growth factor-1 (IGF-1) was also measured in basal plasma samples. At the low insulin infusion rate of 0.6 mU/kg per min, the SSGIR was 73% higher for the High muscling genotype animals when compared to the Low (P<0.05). At the high insulin infusion rate of 6.0 mU/kg per min, these differences were proportionately less with the High and the HighHet genotypes having only 27% and 34% higher SSGIR (P<0.05) than the Low-muscled genotype. The High-muscled cattle also had 30% higher plasma IGF-1 concentrations compared to the Low-muscled cattle. There was no effect of muscling genotype on basal insulin or basal glucose concentrations, glucose disappearance or insulin secretion following an intravenous glucose tolerance test. The increased whole body insulin responsiveness in combination with higher IGF-1 concentrations in the High-muscled steers is likely to initiate a greater level of protein synthesis, which may partially explain the increased muscle accretion in these animals.

Divergent breeding values for fatness or residual feed intake in Angus cattle. 6. Dam-line impacts on steer carcass compliance

The present study determined the impact of maternal genetics for estimated breeding values for rib fat (High-Fat, Low-Fat) or residual feed intake (RFI; High-RFI, Low-RFI) on the carcass compliance of Angus steer progeny when reared pre-weaning under High or Low-Nutrition and post-weaning under various finishing system (grazing versus short-term feedlot). The dams were joined to sires of similar genetic background (close to average estimated breeding values) and sires were rotated among all dam genotypes, with herds located at either Struan Research Centre, near Naracoorte in the south-east of South Australia, or Vasse Research Station, in the south-west of Western Australia. The breeding herd was part of the Beef CRC maternal productivity project and cows were managed under either High or Low-Nutrition, achieved by adjustments to stocking rate in rotational grazing systems and supplementary feeding, so as to maintain ~20% difference in cow liveweight. The steer progeny were weaned at ~7 months of age, with individuals from both pre-weaning nutritional treatments being treated the same from then on at each site. Steers from Struan Research Centre in South Australia born in 2008 and 2009 were sold and grown out on pasture on a local commercial property. Steer calves born in 2010 at Vasse remained on the station where they were backgrounded on hay, followed by a short period (111 days) total mixed ration containing 40% grain. In the first year, steers from Struan (n = 58) were slaughtered together at ~2 years of age, and in the second year (n = 85), consigned to six slaughter groups as their ultrasound-scanned subcutaneous P8 (rump) fat reached 7 mm and their liveweight exceeded 550 kg. Steers from Vasse (n = 101) were slaughtered at ~12 months of age, all on the same day. High-Fat-line dams produced steers with carcasses with greater P8 fat than did Low-Fat-line dams at both sites. At Struan, when the 2008-born steers were slaughtered together, more steers from Low-Fat dams failed to meet minimum fat specifications, than steers from High-Fat dams (28% vs 9% respectively). The steers born in 2009 at Struan all met processor fat specifications but steers from the Low-Fat dams took longer to reach the fat threshold, and so had greater carcass weight, but attracted more price penalties because of increased dentition. All steers from Vasse met minimum requirements for fat, with none penalised for dentition. Vasse steers from High- or Low-RFI dams performed in a manner similar to that from High- and Low-Fat dams, respectively, in that the High-RFI group produced fatter carcasses than did the Low-RFI group. Steers reared under low pre-weaning nutrition weighed less at weaning than did those on High-Nutrition, but had higher weight gains after weaning, although insufficient to result in the same carcass weight. The results showed that commercial cattle producers need to be aware of the balance and trade-off among fat breeding value, effect of pre-weaning nutrition and post-weaning growth required to ensure their cattle meet market specifications and to avoid price penalties.

The incidence of dark cutting in southern Australian beef production systems fluctuates between months

Dark cutting is detrimental to meat quality and therefore is the major cause of carcass downgrades under the Meat Standards Australia grading system. This study quantified the variation between months in the incidence of dark cutting, in southern Australia. Four years of Meat Standards Australia grading data, from nine individual beef processors in Western Australia, South Australia, Victoria and Tasmania, was utilised for the analysis. The dataset contained 42162 slaughter groups, of 10 or more grass-fed cattle, which allowed for the percentage of dark cutters per slaughter group to be analysed. The interaction between month, year and state was significant (P < 0.001). The lowest risk of dark cutting for South Australia and Western Australia was in October (1.53% ± 0.75 and 6.96% ± 0.76) and November in Tasmania and Victoria (7.34% ± 0.9 and 5.27% ± 0.81) potentially when feed availability and quality is highest. The incidence of dark cutting was highest for all states during the period from February to June. Lower pasture availability and quality in combination with higher levels of stress due to extreme high or low temperatures during this time could all contribute to the higher incidences. The findings of this study show that procurement and management decisions made by cattle buyers, producers and processors need to change throughout the year to help mitigate the incidence of dark cutting carcasses and reduce financial loss.

Lamb loin tenderness is not associated with plasma indicators of pre-slaughter stress

The purpose of this study was to test if associations exist between plasma indicators of acute and chronic stress and lamb loin Warner Bratzler Shear Force (WBSF). Blood was collected at exsanguination from 2877 lambs from the Meat and Livestock Genetic Research flock with a suite of indicators analysed. Loin (M. longissimus lumborum) WBSF was measured after 5days aging. Plasma indicators of stress did not relate to WBSF, however a positive association was found between WBSF and kill order, indicating that immediate pre-slaughter factors may be causing reduced tenderness in lamb. In addition, selection for decreased fat depth (PFAT) was associated with increased loin WSBF, indicating that genetic selection for increased carcass leanness is negatively affecting lamb loin tenderness.

On farm factors increasing dark cutting in pasture finished beef cattle

The on-farm factors increasing the incidence of dark cutting were studied in 3145 pasture raised cattle consigned in 66 lots. Animal, environmental and farm management factors were recorded and pasture quantity, quality and mycotoxin concentrations were measured. The relative risk of dark cutting decreased by 26% in cattle grazing pastures with magnesium concentrations exceeding 0.24%. There was a 50% increase in relative dark cutting risk of cattle drinking from dams compared to drinking from troughs. Feeding supplements (hay/silage) in the last 7 days prior to slaughter reduced the relative risk of dark cutting by 25%. A high prevalence of mycotoxins was detected in pastures across all farms. In this case pasture ergot alkaloid concentrations above 600PPB increased the relative risk of dark cutting by 45%, while the presence of FumonisinB1 increased risk by 58%. In contrast the presence of 3acetyldeoxynivalenol reduced the relative dark cutting risk by 37%. Sex also affected the incidence of dark cutting, with heifers less likely to cut dark than steers by 47%.