Carcase Weight, Nutrition & Finishing Impact Dark Cutting

Using data collected on 204,071 Meat Standard Australia (MSA) graded carcases at one Western Australia processor from February 2002 till December 2008, Beef CRC researcher Peter McGilchrist and colleagues from Murdoch University investigated a range of animal and pre-slaughter factors that impact the rate of dark cutting.
calendar icon 12 December 2011
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Data recorded for each carcase included weight, fat depth, marbling, eye muscle area and physiological age (ossification). In addition, lot size and finishing system (grain vs. grass) were recorded.

Overall, 8.75 per cent of carcases graded at the processing plant had an ultimate pH greater than 5.7, which tends to result in dark cutting.

From 2002 to 2008, the average carcase eye muscle area and marbling increased while physiological age (ossification) at slaughter decreased, indicating that animals were younger at slaughter.

Carcase rib fat depth averaged between eight and 11 mm and reflected the differences in animal nutrition between years.

Carcase weight and rib fat depth

Increasing carcase weight and carcass rib fat depth were both associated with reduced rates of dark cutting. As carcase weight increased from 150kg to 220kg the predicted proportion of carcases with a pH above 5.7 decreased from around 18 to five per cent.

This indicates there is a very high incidence of dark cutting in very light vealer type cattle. As carcase weight increased beyond 250kg, the proportion of carcases with an ultimate pH higher than 5.7 continued to decrease, but at a slower rate. As rib fat depth increased from zero to 20 mm, the predicted proportion of non-compliant carcasses with an ultimate pH above 5.7 decreased from around 14 to four per cent.

“The relationship between increasing carcase weight and rib fat depth are likely to be associated with better nutrition of heavier and fatter cattle,” Mr McGilchrist said.

“Animals that are heavier and have higher rib fat can be assumed to have received better nutrition in the months leading up to slaughter, allowing for high muscle glycogen concentrations.”

An interesting finding was that carcase marbling as assessed by MSA marbling was not associated with rate of dark cutting, even though carcase rib fat depth and carcass weight were.

Physiological age of carcass

Ossification is measured in increments of 10 from 100 to 590 and is an assessment of physiological age of a carcase. Physiologically older animals have higher ossification scores. Overall, lower ossification scores in cattle at the same carcase weight indicate a more rapid growth rate throughout life.

Generally, ossification score had a significant association with pH compliance of carcases. However in carcases with an ossification score less than 190, the effect was negligable.

In carcases under 350kg, the rate of dark cutting increased from around six to 20 per cent as ossification score increased from 100 to 300, with the biggest increases occuring at ossification scores above 200. In carcases above 350kg, non-compliance increased from around zero to seven per cent as ossification increased from 100 to 300.

Grain versus grass finishing

Season and finishing system (grass versus grain) also had a significant effect on the pH compliance of carcasses. The highest incidence of dark cutting was in spring and summer even though the driest months are in autumn.

“During autumn, around 90 per cent of MSA graded cattle in Western Australia are sourced from feedlots,” Mr McGilchrist explained. “The effect of higher rates of dark cutting in spring and summer is largely due to the reducing amount of metabolisable energy in the pasture in the weeks prior to slaughter."

“Even though the cattle may appear in good condition, low energy levels in the pasture mean there will be low glycogen levels in the muscle, resulting in a higher incidence of dark cutting.”

Overall, grain finished cattle had 5.4 per cent fewer carcases classified as dark cutters based on pH, compared with cattle finished on pasture. Feedlot rations tend to have higher energy content, leading to higher muscle glycogen concentrations.

In addition, lot size was also associated with rate of dark cutting. As lot size increased from 10 to 80, the predicted proportion of carcasses with an ultimate pH above 5.7 decreased from 10 per cent to around seven per cent.

However there was little improvement in pH compliance beyond 80 head. Individuals in large mobs are thought to be more resilient to the stresses involved with the pre-slaughter period than individuals in small mobs.

Nutrition and dark cutting

Heavier cattle with increased fatness had lower rates of dark cutting. This underpins the importance of good nutrition and high muscle glycogen storage prior to mustering, transport and lairage to reduce rates of dark cutting.

December 2011
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