Fiona Anderson

Increasing lean meat yield % (LMY%) and rapid growth are important profit drivers in the sheep supply chain. To optimise these traits, Australian prime lamb producers use the Carcase Plus Index (CP) for selecting sires. The index originally combined breeding values for weight, eye muscle depth and decreased fat depth, with weightings of 60:20:20 (old CP). Due to perceived gains in leanness in Terminal sires, and concern over reducing intramuscular fat levels in lamb meat, this index was altered to the current weightings of 65:30:5 (new (CP). Given that selection for reduced fat depth results in an increase in LMY%, we hypothesised that the new CP will return less LMY% and therefore reduced carcase value compared to the old CP Lamb carcases (n=1,800) from the Sheep CRC Information Nucleus Flock were collected from 6 research stations over 5 years. Carcases were scanned in 3 sections, fore, saddle and hind, using Computed Tomography (CT) to determine fat, lean and bone weights. Data was analysed using the allometric equation y=axb, fitted in its log-linearised form log y = log a + b.log x. The impact on carcase value was determined for new and old CP indexes in a 23 kg carcase. Within the 76 unit range of Terminal sire CP index values, the old CP Index delivered 2.2, 1.2 and 0.5 percentage units more lean in the fore, saddle and hind sections (P<0.01) compared to the new CP index. Across this same range in CP index values the old CP increased the value of carcase lean by $9.27, compared to $6.49 for the new CR equating to a value difference of $2.78 within a 23 kg carcase. Aligning with our hypothesis there was decreased gain in LMY% and carcase value using the new CP weightings. The cost in LMY% represents a substantial loss in daily profit for processors, especially when the potential improvement to intramuscular fat levels is not currently rewarded.

Carcass lean meat weld percentage (LMY%) is a key profit driver for beef and lamb processors. Currently LMY% is poorly assessed in Australia, as processors rely on carcass weight and a single point measure of fatness to estimate IÆ4Y%. This approach has a limited capacity to predict carcass LMY%, and almost no capacity to describe variation in the distribution of lean between regions of the carcass. In response to this the Australian lamb industry has been assessing and developing a number of different high-throughput technologies to predict LMY% within abattoirs. This activity is being driven at two levels, the first targeting simpler and less expensive devices that will deliver single-site measurements of tissue depth. These devices predict LMY% with less precision (R2 range from 0.2-0.4), and at the whole carcass level only. They include mechanical tissue depth probes, ultrasound, and boning room vision systems that can determine tissue depth at the GR site (11 cm from mid-line over the 12th rib), or muscle and fat depth at the C-site (5 cm from the mid-line over the 12th rib). Secondly we are assessing more expensive whole carcass systems that will enable more accurate determination of LMY% (R2 range from 0.4-0.7) as well as determining lean distribution between different regions of the carcass. These include carcass vision systems, dual energy x-ray absosptiometry, and computer aided tomography scanning (CTscan). In all cases these LMY% prediction devices are trained upon a central ‘gold-standard’ dataset generated using CTscan of carcasses scanned in 3 sections (fore, saddle and hind). This central CTscan dataset has the advantage of generating consistent and repeatable data, not subject to human bias. Thus processors can select an LMY% prediction technology that best optimises their trade-off between cost speed, and precision. This model is now being adapted to the beef industry with obvious constraints being the size and expense of working with beef carcasses.

Increased growth rate and carcass lean meat yield % are key profit drivers for the lamb industry, however redistributing lean tissue to more highly priced parts of the carcase will also increase its value. Faster growing lambs are known to be leaner and less mature at slaughter. Therefore we hypothesised that selection for growth using the Australian Sheep Breeding Value (ASBV) for greater post weaning weight (PWWT) would increase whole carcase lean weight, when animals are compared at the same carcase weight. Lamb carcases (n=1,218) from the Sheep CRC Information Nucleus were scanned in sections (fore, saddle, and hind) using Computed Tomography (CT) to determine fat lean and bone weights. Data was analysed using the log-linearised allometric equation logy = log a + b.logx. Fixed effects were site-year, sex sire type, birth-type rear-type and kill group within site-year, with random terms sire and dam by year. For the same carcass weight PWWT caused no composition differences, except in female lambs which had 3.3% more carcase lean (P<0.01) across the 25 unis PWWT range. Alternatively for the same fat, lean or bone weight these tissues were all proportionately heavier in the saddle region of the high PWWT lambs by 3%, 7%, and 16% across the PWWT range. Aligning with our hypothesis, PWWT was associated with increased total carcase lean, although only in females. Unexpectedly, PWWT caused a redistribution of carcass tissues to the saddle region, particularly for bone and lean, implying an altered conformation in these high growth lambs. Conflicting with the premise of our hypothesis, these effects appear to be independent of maturity as there was no whole body increase in bone weight. Furthermore loin muscle myoglobin concentration in the high PWWT lambs was increased (by 0.03±0.018 mg/g tissue) rather than decreased as would be expected in a less mature animal, In conclusion, PWWT redistributes carcase weight to the saddle region of lambs.

Increasing lean meat yield % and redistribution of lean tissue to more highly priced parts of the carcase will increase its value. Selection for the Australian Sheep Breeding Value (ASBV) for greater post weaning eye muscle depth (PEMD) increased eye muscle area and weight of the eye of the short loin, although had minimal impact on carcase lean meat yield %. We hypothesised that selection using the PEMD-ASBV would increase saddle lean weight, without altering whole carcase lean weight when animals were compared at the same carcase weight Lamb carcases (ie1218) from the Sheep CRC Information Nucleus were scanned in ‘quarters’ (fore, saddle, and hind) using Computed Tomography (CT) to determine fat lean and bone weights. Data was analysed using the allometric equation y=axb, fitted in its log-Iinearised form logy = log a +b.logx. Fixed effects were site-yeast sex, sire type, bulb-type rear-type and kill group within site-year, with random terms sire and dam by year. At a given carcase weight the lean tissue was 4.2% heavier (P<0.01), and fat 8.7% lighter in the whole carcase (P<0.05) across the 7 unit PEMD range. When compared at the same lean weight, the lean tissue in the saddle was 4.9% heavier (P<0.01), and lean in the forequarter was 4.8% lighter (P<0.01) across the PEMD range. Aligning with our hypothesis, there was more lean tissue in the saddle, although unexpectedly this was at the expense of the forequarter only. The mechanistic reason for this redistribution is not clear, and will be investigated with more extensive sampling from tissues across the carcase. In contrast to our hypothesis, PEMD was associated with increased total carcase lean, and reduced fat. The leaner and more muscular composition appears to be independent of maturity as there was not a corresponding increase in bone weight. These impacts on lean weight and distribution to the loin will increase carcass value.