Carcass characteristics and meat quality of dairy-origin steers slaughtered at eight, ten and twelve months of age : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Agricultural Science, Massey University, Palmerston North, New Zealand

Abstract

Approximately 2 million surplus calves are produced annually by the New Zealand dairy industry, which are generally processed at around one week of age as “bobby” calves. Concerns surrounding animal welfare issues, due to the ethics of slaughtering calves at such a young age, pose a threat to the industry’s social licence to farm, consumer acceptability and market access. In addition, the beef sector also faces the challenge of improving production efficiency, whilst minimising environmental impacts. To address these issues, a novel yearling beef production system is proposed, that would utilise dairy-origin calves and implement a slaughter age of between eight and twelve months.

The objective of this study was to compare carcass characteristics and meat quality attributes of Hereford x Friesian-Jersey steers slaughtered at eight, ten and twelve months of age, in order to understand the potential saleable meat yields and type of meat product that could be sourced from yearling dairy-origin cattle. Additionally, the study aimed to determine if postmortem ageing had an effect on meat quality attributes.

Sixty Hereford x Friesian-Jersey weaner steer calves born in spring 2017 were sourced from a commercial calf rearer at three months of age (average 103±1 kg live weight). The calves were managed as a single group; however, they were randomly pre-assigned at eight months of age to one of three slaughter treatments: eight, ten and twelve months of age. Steers were grazed on herb-clover pastures and brassica crops from December 2017 to March 2018, then on ryegrass and white clover pasture until slaughter. Steers were processed in May, July and September 2018 at eight, ten and twelve months of age respectively, at Venison Packers Feilding Ltd.

Growth rates of the three slaughter groups averaged 0.9 kg/day and did not differ between treatments. The final unfasted live weights prior to slaughter at eight, ten and twelve months of age were 252±6 kg, 303±4 kg and 348±5 kg respectively (P<0.001), which corresponded to carcass weights of 119±3 kg, 146±3 kg and 174±3 kg (P<0.001). Steers slaughtered at twelve months of age achieved a greater dressing-out percentage (50.0±0.2%) than steers slaughtered at eight (47.2±0.2%) and ten (47.4±0.2%) months of age (P<0.001). Rib fat depth, P8 fat depth and eye muscle area measured by ultrasound increased at each age (P<0.001). Fat depths at all three ages were below 3 mm, which is the minimum fat depth required for steers to be classified in the ‘P’ fat class designating 3-10 mm of subcutaneous fat, under the current beef carcass classification system. The muscle to bone ratio, intramuscular fat content and yellowness of carcass fat all increased progressively with slaughter age.

Objective meat quality attributes were measured on the M. longissimus lumborum (striploin). The caudal half of each striploin muscle was aged for 21 days in vacuum packaging, while the cranial half was frozen immediately after boning, 24 hours postmortem. Shear force values for unaged samples at eight, ten and twelve months of age were 5.1±0.2 kgF, 5.4±0.2 kgF and 5.5±0.2 kgF respectively. Although there was no difference in shear force values, sarcomere length or drip loss between the three slaughter age treatments (P>0.05), ultimate pH and thaw loss increased with age, while cooking loss declined. Meat colour also became darker and redder as animals became older (P<0.001). Proteolytic ageing for 21 days had a positive effect on shear force values and water-holding capacity at all slaughter ages, though differences were minimal (P<0.001).

Overall, the changes in carcass characteristics with age were small, likely due to there being only four months’ difference in age between treatments. However, the greater saleable meat yields and dressing-out percentages at ten and twelve months of age may be advantageous in terms of processing efficiency and profitability. Differences in objective meat quality measures between eight, ten and twelve months of age were also small, indicating that the yearling beef is very tender and of high eating quality. This suggests that the beef obtained from beef-dairy cross cattle slaughtered between eight and twelve months of age and at live weights of 250-300 kg could be processed together under one category, and that the product could justifiably be targeted at markets which offer a premium.