Oxidized Fats in Animal Feed: Effects on Pig Growth and Feed Efficiency

Why fats and oils matter in animal nutrition

Lipids are widely incorporated into pig and poultry diets because of their high energy density. Compared with carbohydrates and proteins, fats provide a concentrated energy source that can help formulate diets to meet the requirements of rapidly growing animals.

Beyond supplying energy, dietary lipids contribute essential fatty acids and can influence the absorption of fat-soluble nutrients.

However, the nutritional value of a fat source depends on several characteristics, including its fatty acid composition, digestibility and chemical stability.

One particularly important consideration is oxidative deterioration, which can change the properties of fats before they are consumed.

What happens when dietary fats become oxidized?

Lipid oxidation is a chemical process in which fats react with oxygen, producing a range of compounds that can alter their nutritional properties.

This process may be accelerated by heat, prolonged storage and exposure to oxygen. Oils subjected to repeated heating, such as those used in commercial frying operations, can be particularly vulnerable to deterioration.

As oxidation progresses, primary degradation products can break down into secondary compounds, including aldehydes and other reactive molecules.

These changes are important because oxidation products can interfere with normal physiological and metabolic processes, potentially affecting feed consumption and the efficiency with which animals utilize dietary nutrients.

USDA research identifies important growth losses

The USDA-ARS and University of Minnesota research program examined the nutritional consequences of incorporating fats and oils with different degrees of oxidative deterioration into livestock diets.

Across the research summarized by USDA, animals consuming more extensively oxidized lipid sources generally exhibited poorer productive responses.

The reported reductions reached up to 25% in pigs and 10% in broiler chickens for feed intake and growth-related outcomes under the experimental conditions studied.

These figures illustrate the potential magnitude of the problem, although they should not be interpreted as the expected performance loss whenever recycled or previously heated oils are used.

The response depends on factors such as the type of lipid, oxidation severity, concentration of degradation products and amount of affected fat incorporated into the complete diet.

Up to 25% in pigs and 10% in broilers for reductions in feed intake and growth-related performance under evaluated conditions.

Why some oils are more vulnerable than others

An important contribution of the research is its examination of how fatty acid composition influences oxidation susceptibility.

A related scientific review published in the Journal of Animal Science evaluated evidence from 16 publications and found differences between lipid sources dominated by saturated and unsaturated fatty acids.

Fats containing higher proportions of saturated fatty acids, such as tallow, lard and palm oil, were generally less susceptible to oxidation under the conditions considered.

In contrast, oils rich in unsaturated fatty acids, including soybean, corn and canola oils, were more prone to forming lipid oxidation products.

For these unsaturated lipid sources, greater concentrations of oxidation products were associated with poorer pig growth, feed intake and gain efficiency.

This does not mean that unsaturated oils are inherently unsuitable for animal feed. Rather, it reinforces the importance of appropriate handling, storage and quality evaluation when using oxidation-sensitive lipid sources.

Can fat quality be measured more accurately?

One challenge for feed manufacturers is determining whether a fat source has undergone sufficient oxidative deterioration to affect its nutritional value.

Common laboratory measurements include:

Anisidine value (AnV)
Provides information about certain secondary oxidation products, including aldehydes.

Thiobarbituric acid reactive substances (TBARS)
Estimates selected compounds associated with lipid oxidation, although the test has limitations in complex lipid mixtures.

However, the researchers emphasize that no single conventional measurement fully captures the complexity of lipid oxidation.

Peroxides can form and subsequently degrade as oxidation progresses. Consequently, a relatively low peroxide value does not necessarily establish that an oil has remained free from oxidative deterioration.

The scientific review found that combining peroxide and anisidine values provided useful information for estimating performance responses in pigs consuming oxidized unsaturated fats.

Aldehydes may offer a clearer picture of lipid damage

The researchers also investigated whether individual aldehydes could provide additional information about the relationship between lipid oxidation and pig performance.

Several compounds, including 4-hydroxynonenal, hexanal, pentanal and 2-undecenal, were identified among the oxidation products associated with performance outcomes.

Statistical models incorporating selected aldehydes showed improved predictive ability for some measures of pig performance compared with conventional oxidation indices.

Nevertheless, the improvement was not uniform across all outcomes. The researchers noted that the additional predictive benefit for average daily gain and average daily feed intake was relatively limited.

The practical implication is that multiple complementary quality measurements may be more informative than relying on a single oxidation indicator.

What does this mean for recycled fats and oils?

Recycling fats and oils from food-processing and food-service operations can support more efficient resource use and reduce waste.

These materials may provide economically attractive energy sources for livestock feeds when they meet appropriate quality and safety specifications.

However, their previous exposure to heat and oxygen can influence their chemical composition and degree of oxidation.

Two recycled fat sources with similar total fat content may therefore differ in their actual nutritional value.

For feed mills, this creates a challenge: an ingredient that appears less expensive on a purchase-price basis may become less economical if its quality negatively affects animal performance.

The research supports evaluating alternative lipid sources according to both their analytical quality and expected contribution to the complete diet.

The economic importance of lipid quality

Fat inclusion decisions are often driven by ingredient prices and the energy contribution assigned to different lipid sources.

But a conventional formulation approach may not fully account for the consequences of oxidative deterioration.

If a lower-cost fat source reduces feed intake, growth rate or feed efficiency, the apparent savings in formulation cost may be offset by poorer productive performance.

This becomes particularly relevant in commercial swine production, where even relatively small changes in average daily gain or feed conversion can influence production costs.

Understanding lipid quality can therefore improve decisions about ingredient procurement, feed formulation and the economic value assigned to alternative fat sources.

Practical implications for nutritionists and feed mills

The findings reinforce the importance of incorporating fat-quality evaluation into ingredient selection and quality-control programs.

For feed manufacturers, several considerations are particularly relevant:

Account for lipid source. Fatty acid composition influences susceptibility to oxidative deterioration.

Consider processing and storage history. Repeated heating and prolonged exposure to unfavorable conditions can affect quality.

Assess complete-diet exposure. The concentration of oxidation products in the finished diet matters, not simply their concentration in the fat ingredient.

Evaluate economic value beyond purchase price. Potential changes in animal performance should be considered when comparing lipid sources.

These considerations do not establish universal acceptance limits for oxidized fats. Instead, they provide a basis for more informed quality-control decisions, supported by analytical testing and the intended dietary inclusion level.

Fat quality deserves greater attention in precision nutrition

Precision nutrition is often associated with balancing amino acids, energy, minerals and other nutrients as closely as possible to animal requirements.

However, the USDA research illustrates why ingredient quality must also be part of that precision.

Two diets formulated to contain similar amounts of added fat may not produce equivalent responses if their lipid sources differ substantially in oxidative damage.

Likewise, the energy value assigned to a fat source may not fully reflect the potential biological effects of oxidation products on feed intake and growth.

Integrating lipid-quality measurements into feed evaluation could therefore help nutritionists better understand variation in animal responses and make more informed decisions about alternative energy ingredients.

From cheaper energy to better nutritional value

Fats and oils will continue to play an important role in livestock feed formulation, including opportunities to incorporate suitable recycled resources.

But the research conducted by USDA-ARS and the University of Minnesota highlights an important distinction between the apparent energy value of an ingredient and its practical nutritional contribution.

As the feed industry seeks to improve efficiency, control costs and make greater use of alternative ingredients, evaluating lipid oxidation could become an increasingly valuable part of feed quality management.

The goal is not necessarily to avoid recycled fats, but to ensure that the ingredients selected provide consistent nutritional value without compromising animal performance.

Sources

USDA Agricultural Research Service. 15 Years of Data Could be the Key Ingredient to Animal Feed. USDA Scientific Discoveries.

Kerr, B.J., Hung, Y.-T., Chen, C. & Shurson, G.C. (2026). Aldehydes are key for assessing dietary lipid oxidation and subsequent effects on pig growth performance. Journal of Animal Science, 104, skaf407. https://doi.org/10.1093/jas/skaf407

The Pig Site. (2026, July 8). USDA research links degraded fats in feed to growth losses.