An earlier article on fat quality addressed lipid oxidation where fat in feed and body tissues is degraded to peroxides by heat and air. The accumulation of peroxides is held in check by antioxidants until conditions overwhelm the antioxidant defense system. The balance then shifts to excessive peroxide formation, which can cause tissue damage and impair animal performance.

Professor Emeritus / Clemson University

This translates to an ongoing need to protect unsaturated fatty acids in feed because they are normal and valuable constituents of many economically priced feed ingredients, including forages, byproducts and fat supplements. Likewise, unsaturated fatty acids are always a normal constituent of body tissues, so their conversion to peroxides is an ongoing threat anytime the animal is exposed to environmental, nutritional or biological stresses.

This article briefly describes the effects of oxidized lipids on animals and how problems can be offset with antioxidants. The risk of lipid peroxides in humans is the subject of much research and opinion. Experts agree, however, that lipid peroxides have the potential to cause damage (“oxidative damage”) to biomolecules, which is thought to contribute to the development and progression of certain diseases, especially cancer and neurodegenerative diseases. Effects of peroxides on health and production of livestock have not been studied as thoroughly.

Questions often arise regarding the fate of peroxides in the body of cows following their consumption in feed. These questions generally focus on the impact of peroxides on the rumen microbial population, the extent to which peroxides are absorbed and if peroxides are metabolized in a way that still contributes useful energy for production. While answers to these questions are lacking in dairy cows, some information is available from studies published in other animal species, particularly swine. In many cases, studies that intentionally fed oxidized fat to swine often reported little to no negative effects on energy or fat digestibility. The lack of effect on energy digestibility is partially explained by the oxidized fat comprising only a small fraction of the total energy consumed. For example, a 2017 study by Vasquez and Jenkins bubbled air through an unsaturated fat blend at 92°C for 24 hours to achieve a high peroxide value (215 milliequivalents per kilogram). When the oxidized fat replaced fresh fat total, undamaged fatty acids only dropped from 45.5 to 41.5 milligrams per gram dry matter despite the high peroxide value. The consumption of harmful peroxides, even without effects on digestibility, still were sufficient to reduce feed intake and lower daily gain.

Adverse effects on feed intake are one of the more consistent negative effects of feeding peroxides. The cause of feed-intake depression is undoubtedly a combination of adverse feed palatability and tissue systemic factors, but little work has been done to delineate the contribution of each. Once external peroxides are absorbed, they can stress body defense mechanisms and potentially cause tissue damage or oxidative stress. The damage is magnified when intake of peroxides is combined with excessive internal peroxides made in body tissues as the result of environmental, nutritional or biological stresses on the animal.


Antioxidants are the defense mechanism preventing peroxidation of feed fat and the uncontrolled spread of lipid peroxides throughout body tissues. In feed, oxidation reactions are one of the major sources of deterioration that occurs during manufacturing, storage, distribution and final preparation of foods. Antioxidants function to control oxidation and are used routinely to preserve food flavor and appearance. The integrity of lipids in high-fat livestock feeds are largely ignored, with the only defense coming from internal plant-based antioxidants such as flavonoids, ascorbic acid, phenols, glucosinolates, anthocyanins, iron and copper, just to name a few. Livestock feeds particularly susceptible to oxidation include free oils (including tallow and liquid fats), high-fat byproducts like some dry distillers grains and bakery by-products, and poorly stored oilseeds that, because of higher moisture, might have higher free-fatty-acid concentrations.

111721 jenkins fg1

Inclusion of antioxidants in animal feed has had variable effects on performance in controlled studies, sometimes showing benefits and at other times no effects. Often when supplemental antioxidants were reported to have no effect on animal performance, there was also no evidence that peroxide values were elevated in the feed or in body tissues. This inconsistency in responses to antioxidants led to the quote shown in Figure 1 that there is faith in the value of antioxidants once the circumstances of their need are better known.

111721 jenkins tb1

Table 1 shows a few examples of published reports demonstrating positive benefits from antioxidants when added to animal feed. Supplemental antioxidants have improved stability of feed fats as well as had positive effects on performance of dairy cows, including improved rumen function, increased feed intake and enhanced milk yield. A recent abstract presented at the 2021 annual meeting of the American Dairy Science Association reported higher milkfat and energy-corrected milk yield when Holstein cows were fed supplemental antioxidants.

In summary, antioxidants play a critical role acting as the defense mechanism to keep peroxide formation in feed and body tissues in check. Natural antioxidants produced by plants and body tissues are not always sufficient to prevent peroxides from reaching harmful levels in livestock. Dairy production benefits from supplemental antioxidants have been reported with the hope that additional studies will refine the conditions and periods of antioxidant critical need.  

The author acknowledges editorial advice and comments for this two-part series provided by Dr. Kevin Harvatine, Professor at the Pennsylvania State University.

References omitted but are available upon request by sending an email to the editor.