Merinos can produce meat of high quality in terms of tenderness, flavour and overall acceptability.  However, Merinos are more susceptible than other breeds to producing meat with a high post-slaughter pH.

 

Consistent high quality lamb products are required to maintain consumer confidence and consumption. The eating quality of meat is predominantly derived from the tenderness, flavour, odour and juiciness traits and in lamb, flavour is as equally important as tenderness. 

 

 

Figure 1. (Right) A section through the striploin showing dark-cutting meat, one result of high pH meat.

 

High pH meat is a persistent quality defect found in meat from all species of animals. After slaughter, the carcase normally undergoes rigor (stiffening) and the pH of the meat drops (becomes more acid). In some carcases, the muscle tissue pH does not drop enough. This results in meat of high pH is a direct consequence of low muscle glycogen (energy) at slaughter and this can cause the meat to be a noticeably darker colour.

 

Meat with a pH greater than 5.7 has poorer keeping and eating qualities and as the pH increases above 6.0, the colour can be unattractively darker, is less desirable to consumers, is less tender (in the pH range of 5.8–6.2), tends to have off-flavours, spoils more rapidly and requires a longer cooking time.  If high pH meat is vacuum-packed, greening will occur in the bag fairly quickly due to the proliferation of sulphur-producing bacteria.

 

The prevalence of high pH sheep meat in Australia is about 10% in lamb loins purchased at the retail counter as tested in four Australian cities, but data is not available for other cuts.

 

Merinos have produced higher pH meat in experiments when compared to first- and second-cross lambs (sired by Border Leicester and Poll Dorsets).  There were also observed differences between strains of Merinos where two superfine strains exhibited higher pH than the medium and broad wool strains, tested in the loin and the leg.

 

The appearance and colour of the product in retail display determines economic returns to the supermarket and a one-day reduction in shelf-life of a meat product can cost them millions per year with consistent high quality lamb products required to maintain consumer confidence and consumption. Losses at the retail end are inevitability shared back through the production and processing chain resulting in lower prices paid to absorb those costs.

 

Glycogen levels in the muscle can be thought of as available energy. Practices that cause stress such as transport, time off feed, mustering and mob mixing, as well as unfamiliar environments such as saleyards and abattoir lairage, cause the animal to use up this energy. These factors can individually and additively affect the ultimate pH of sheepmeat. Merino lambs are often more susceptible than crossbred lambs to the effects of stress on muscle glycogen concentration. In a comparison of five genotypes, the Merinos had the highest ultimate pH in the knuckle (leg muscle), but pH was similar in the loin. 

 

Merinos do not appear to have inherently low levels of muscle glycogen, but they deplete more muscle glycogen between farm and slaughter. In general, on-farm factors appear to be more important than transport and lairage factors in determining how likely the breed and consignment is to produce high pH meat.

 

Shearing close to slaughter can increase the energy requirements of a lamb for maintenance and cause a negative effect on muscle glycogen concentration, and hence lamb meat eating quality.  If lambs are shorn prior to slaughter this is best done before the finishing period and at least four weeks before slaughter.  Refer to Meat Standards Australia Tips & Tools information kit (to order hard copies telephone Meat Standards Australia on 1800 111 672).

 

There is some evidence that Merinos are more sensitive to stress hormone than crossbreds and therefore are more likely to mobilise glycogen (energy) between farm and slaughter. Therefore Merinos may actually mobilise more glycogen in response to any stressor, emphasising the importance of minimising stress pre-slaughter.

 

Research has indicated that Merinos selected for high fleece weights may pay a penalty in other traits. Selection for increased fleece weight can result in a higher meat pH. High wool production may also be at the expense of fat, which may compromise the ewe’s ability to rejoin and rear lambs. Further work is required to better understand the extent of these penalties.

 

Practices that reduce stress or reduce the animal’s susceptibility to stress, will improve glycogen levels at slaughter and therefore reduce the risks of producing high pH meat.

 

Increasing muscle glycogen at slaughter can be achieved by:

Figure 2. (Right) Merino lambs being finished on-farm.

 

Using sires with genetically superior eye muscle depth (selected on Australian Sheep Breeding Values or ASBVs) improves muscle glycogen in Merino lambs given adequate nutrition.

 

Also, if these high ASBV Merino lambs have their glycogen stores depleted through stress, these animals are more responsive to replenishing glycogen stores when re-fed high energy diets. ­It is therefore recommended to select Merino sires with yearling eye muscle depth (YEMD) ASBVs of greater than zero.

 

The temperament of an animal is likely to have an impact on maternal behaviour (and therefore lamb survival and growth rate) as well as meat quality (and muscle glycogen). Temperament is measured in beef cattle and more recently sheep, using an ‘agitation’ score, which is heritable.  While less agitation in cattle reflects better meat (through more glycogen), results in sheep are not so clear. One experiment showed that Merinos had the highest pH but the lowest agitation score compared to Poll Dorset and Border Leicester crosses. This indicates that this measure may not be a good indicator of stress in Merinos and that outward behaviour as an indicator of internal stress, may differ between breeds. 

 

Figure 3. (Right) Poll Merino rams selected for high yearling eye muscle depth.

 

Understanding and managing the factors that cause stress to Merino lambs pre-slaughter will enable Merino producers to produce a consistent high quality meat, comparable with lambs traditionally bred for meat production. 

 

A better understanding of the measure of stress in Merinos, if it differs to other breeds, may also add value to the management of other aspects of the production system, such as reproduction and lamb survival.

 

 

For further information contact Dr Robyn Warner, Department of Primary Industries, Victoria, on 039742 0477 or robyn.warner@dpi.vic.gov.au.

 

This is another note in a series of Practical Wisdom notes available from the Sheep CRC aimed at improving the quality of Australian sheep meat. Other titles discuss a wide range of innovations and improvements that industry can profitably adopt. These can be found at the Sheep CRC website www.sheepcrc.org.au/pw.

This research was funded jointly by the Sheep CRC and Meat and Livestock Australia.

 

 

 

Plain paper (PDF  183 kb)

Fact sheet paper (PDF 176  kb)