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.
Preventing glycogen loss
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.
- Supply adequate energy via the diet to ensure a good store of glycogen in the animal.
- Reduce the incidence of stress on the animal thereby reducing the amount of glycogen loss.
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:
- psychological stress – restraint, handling or novelty, and
- physical stresses – hunger, thirst, fatigue and injury or temperature extremes.
By managing the process from paddock to slaughter, we can minimise the level of these stresses experienced by the animal.
Managing to reduce Dark Cutting Beef
Avoid social regrouping – or mixing mobs
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.
Minimise noise and disturbance
Cattle in lairage on the farm or at abattoirs, need rest. Disturbances and noise should be minimised.
Safe transport of stock
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:
- The general behaviour of some type of stock, e.g. bulls’ antagonistic behaviour—mounting, fighting and chin resting. Heifers and cows in oestrus—mounting behaviour.
- The nutritional demands of different classes of stock, e.g. pregnancy and lactation in cows and cattle in high growth periods.
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.
A checklist for minimising DCB
- Cull poor temperament cattle;
- Train and educate your cattle. Yard weaning can reduce sickness and increase performance in feedlots;
- Keep the time from mustering to slaughter as short as possible;
- Avoid selling stock during extreme weather or seasonal conditions e.g. hot, humid, cold, wet;
- Do not slaughter heifers in oestrus;
- Do not paddock steers or spayed heifers with unjoined heifers before sale;
- Ensure that cattle grazing pasture are growing at an adequate rate, greater than 0.6 kg/head/day before slaughter. Pastures should provide a minimum herbage mass of 1000 kg of green DM/ha, which is 70% digestible, to achieve this growth rate in stock;
- If pasture quantity and quality is poor, supplementation may be required at least two weeks before dispatch. The supplements must have a high ME (>12 MJ/kg). Examples would be high quality silage, grain or pelleted feed rations. If feeding grain, producers should use rumen modifiers to minimise grain poisoning and introduce grain gradually into the cattle diet;
- Minimise the activity of stock from the start of mustering to slaughter;
- Muster slowly and quietly, and allow stock to rest before transport;
- Do not mix mobs less than two weeks before slaughter;
- Keep noise to a minimum;
- Cattle treated with Hormonal Growth Promotants should be slaughtered only after the payout period for the product has expired;
- Avoid poor handling facilities. These restrict the natural flow and movement of stock. Avoid boggy or poorly drained yards;
- Do not use electric prodders;
- When trucking stock, ensure the truck is of good construction—non-slip flooring, trailer suspension and bruise free panelling. Cattle should never be over loaded or under loaded. Stock mixed in transport from different mobs should be immediately re-drafted at their destination yards. Identify different stock and inform the transporter;
- Do not present sick or injured stock for slaughter.
Acknowledgement – Michael Beer – NSW Agriculture and Meat Standards