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The yellow fat found in grass-fed, finished beef is a natural, healthy byproduct of the animal’s diet. Consumer resistance is based on the perception that yellow fat indicates old animals, and on the historical retail conditioning that fat must be white because the bulk of meat on shelves is produced by grain-fed cattle.
Yellow fat in grass-fed and finished beef starts to develop when cattle consume significant amounts of fresh green forage, which can be rich in beta-carotene and other carotenoids. Most fresh leafy material will contain between 200 and 700mg beta-carotene/kg dry matter (DM), compared to between 1 and 3mg/kg for standard varieties of maize and 4 to 22mg/kg for maize silage.
Beta-carotene is a yellow substance that is converted to vitamin A in the body. When supply exceeds the animal’s vitamin A demand, the surplus is excreted and/or deposited in active developing fat storage deposits. Carotenoids are fat-soluble and are attracted to developing fat cells.
While the yellow pigment can accumulate gradually, the yellow colour becomes most pronounced as the animal matures because the carotenoids accumulate in the fatty tissue over time, particularly in older cattle or cattle on grazing during the finishing period. Grain-fed cattle, instead, produce white fat because their diet lacks these surplus yellow pigments.
Natural fat deposition in cattle
To understand the development of yellow fat in cattle, it is important to first consider how fat is naturally deposited in the different compartments of the body. During the early growth phase, younger animals prioritise protein deposition, which is evident in muscle growth. Skeletal development is also a major focus during this phase.
As cattle reach physiological maturity, at around 65% of their mature weight, the growth pattern changes. Muscle and skeletal development slow, while fat deposition accelerates. For Angus crosses, this occurs at a live weight of around 380kg, although the weight varies among breeds because of differences in mature weights.
In animals older than nine months, fat deposition does not result from the formation of new fat cells. Rather, the existing fat cells enlarge as they become filled with triacylglycerol to their storage capacity. Fat is deposited in a natural sequence: At physiological maturity, the visceral (abdominal cavity) and subcutaneous (beneath the skin) fat compartments develop rapidly, while intramuscular fat (marbling) only begins to accumulate. Marbling is therefore completed only after the visceral and subcutaneous compartments have been filled.
Research indicates that once carotenoids accumulate in adipose tissue, they tend to persist. Although removing cattle from pastures results in a gradual decline, the reduction in fat yellowness is very slow due to the low turnover rate of fat cells.
This is not a concern when cattle are removed from carotenoid-rich environments before substantial fat deposition occurs. During the finishing phase, large amounts of carotenoid-free triacylglycerol are deposited as fat cells fill.
As a result, the relatively small amount of carotenoids present becomes diluted by the accumulating fat tissue to the extent that their yellowing effect is minimal.
Smart management strategies
To manage the yellowness of fat effectively, the following factors should be considered:
- Excess carotenoids are primarily deposited in actively developing adipose tissue.
- The greater the surplus, the more pigment will be deposited.
- When excess pigment is present, fat becomes more yellow with each batch of triacylglycerol that is deposited in the fat cell.
- In finishing cattle, subcutaneous fat deposition markedly begins once animals reach 65% of their mature bodyweight.
With these factors in mind, several practical management strategies can be used to influence the yellowness of subcutaneous fat in finishing cattle:
- Manage the time cattle spend on grazing during the active fat deposition phase by either slaughtering at a lower weight or timely withdrawal from grazing.
- Reduce grazing intake to limit the excess carotenoids available for deposition in fat cells:
- Feeding an energy lick can reduce pasture intake by around 15%.
- Supplementary feeding at 1,25% of bodyweight can reduce grazing intake by around 55%.
- Feeding a concentrate can reduce grazing intake by as much as 75%.
- Alternatively, cattle can be moved to a feedlot and finished on a standard finishing ration.
- Allocate the highest-quality grazing to cattle below 65% of their mature weight. They are highly efficient at converting quality forage into meat. During this growth phase, yellow fat development is of minimal concern.
- Use lower-quality grazing during the finishing phase. Such grazing contains lower carotenoid concentrations while providing a higher ‘scratch factor’, which supports rumen function when concentrates are fed during grazing. This approach can also help reduce the risk of acidosis.
Conclusion
Based on the preceding discussion, the management of yellow fat in cattle depends on limiting excess carotenoid intake during the finishing phase.
The economic advantages of selfproduced roughage can still be utilised, but to a much lesser extent than with animals younger than physiological maturity. If the yellowness of fat is a substantial factor in the market, it will be safer to completely remove cattle from grazing once they reach physiological maturity. Furthermore, producers should avoid applying the 65% mature-weight guideline too rigidly. If observations indicate that fat deposition has begun, management decisions should be made promptly.
In situations where yellow fat in finished cattle must be managed, the general recommendation is to maximise lush grazing during the growth phase before physiological maturity and to avoid it during the finishing phase after physiological maturity. – Sjoerd Ten Cate, technical director, Feedtek
Send an email to the author at sjoerd@feedtek.co.za for more information.
