The first step in managing cattle to reduce the incidence of dark cutting beef (DCB) is to maximise the animals' level of muscle glycogen. High blood glycogen enables a beef animal to better protect itself against the effects of stress. The nutrition of the animal determines the level of glycogen in the muscle, which acts as a storage bank of energy. The higher the metabolisable energy intake of the animal through feeding, the higher potential the animal has to store glycogen.
Depletion of muscle glycogen occurs when the animal is exposed to stress. The severity of the decline in muscle glycogen depends on the duration and severity of the stress. This may be likened to a bucket with nutrition replenishing the glycogen store and leaks of glycogen representing stress.
If the animal has been on adequate nutrition—and its starting level of muscle glycogen is high—then a larger amount of muscle glycogen is available for reduction before reaching the critical level of 0.6% muscle glycogen.
If nutrition has been insufficient, and the starting level of muscle glycogen is low, then there is a smaller amount of glycogen available before that critical level of 0.6% is reached. In this situation, the animal is at a higher risk of producing dark cutting beef.
There are two ways a producer can address the issue of glycogen depletion.
Research indicates that cattle from feedlots have higher levels of muscle glycogen (>1.0%) and a correspondingly lower incidence of dark cutting beef than cattle grazing on pasture. This is due mainly to the high energy grain diet of the feedlot.
To ensure grazing cattle have a high level of muscle glycogen, they must graze high quality green pastures for at least two to three weeks before slaughter. If these pastures are not available, then provide a high quality energy supplement during this period.
There are many different factors from the paddock to the slaughter floor, which can cause stress and deplete body glycogen. The severity of these stressors is directly related to the management of the cattle. Livestock handling is a major cause of muscle glycogen depletion.
During normal handling, cattle experience two forms of stress:
By managing the process from paddock to slaughter, we can minimise the level of these stresses experienced by the animal.
Mixing mobs of animals or social regrouping is a major stress for cattle. Cattle form tight social bonds within a group or mob. Mixing mobs, or introducing a small number of animals to a mob close to slaughter time, greatly increases the risk of dark cutting beef.
Social regrouping causes physical and/or psychological stress to an animal. In most instances the increase in activity causes a rapid decline in muscle glycogen. This activity may include fighting to establish social dominance, mounting, mock fighting and chin resting.
Cattle with excitable (poor) temperaments have a higher incidence of DCB, as well as lowered daily weight gains in the feedlot compared with cattle of better (calm) temperament.
Animals with excitable or 'stirry' temperament also have the potential to cause physical and psychological stress to other animals in the mob. These animals should be culled from the herd and treated as a high risk for dark cutting beef.
Cattle in lairage on the farm or at abattoirs, need rest. Disturbances and noise should be minimised.
There is no magic formula for predicting the effect of transport. Neither the distance travelled nor the time in transit can be used to predict the final pH of the carcase.
Transporting stock on rough roads can increase stress as the animals struggle to maintain balance. To minimise DCB, take stock to their final destination via the shortest, smoothest route and avoid travel in severe hot or cold weather conditions.
Fasting cattle show a gradual decline in muscle glycogen levels. Providing some fibrous feed, such as hay, before dispatch can assist in slowing the animal’s rate of digestion.
The sex of an animal and its stage of development affect the incidence of dark cutting meat. This effect can be explained in a number of ways:
In terms of risk, bulls have highest levels of dark cutting, followed by cows, heifers, spayed heifers and finally steers.
Combination implants—androgen and oestrogen to steers—or oestrogen implants to heifers can increase the incidence of dark cutting beef. This is due to their activity in modifying growth curves, rates of gain and nutrient requirements.
To minimise this effect, cattle should be withheld from slaughter until the growth promotant has paid out, as well as ensuring adequate nutrition is provided for any additional growth.
Michael Beer for technical assistance and Meat & Livestock Australia for illustrations.
DAI 244 Dark cutting beef – what is it?