Oxidation of lipids in feeds is a natural phenomenon; it happens whether we take measures to prevent it or not. In reality, what we try to achieve is delaying the process so that when the feed reaches the animal, the oxidation of lipids is minimal. This avoids the drop in feed intake observed when animals are offered rancid feeds. Therefore, measures to control oxidation should be undertaken long before the feed is manufactured, starting from selecting the right raw materials and appropriate antioxidant additives.
How rancidity develops
Oxidation of lipids, also called oxidative rancidity, is a natural reaction between unsaturated fatty acids and free oxygen. Such fatty acids exist in all fats and oils, but certain vegetable oils such as soybean and corn oil are especially rich in them. For example, the ratio between saturated and unsaturated fatty acids in coconut oil is over 11, whereas in corn oil it is only 0.15, indicating the richness of corn oil is unsaturated and thus vulnerable to oxidation. Lipid oxidation is enhanced by exposure to air (open bags), light, high temperatures and certain inorganic minerals such as iron and copper. The reaction is autocatalytic and, once started, oxidized fatty acids continue to form, and peroxides, the end product of oxidation, accumulate.
A drop in feed intake is observed when animals are offered rancid feeds, resulting in reduced weight gain.
Why oxidized feed is undesirable
Oxidized fatty acids, also referred to as free radicals, react not only with other fatty acids but also with amino acids, rendering them unavailable to the animal. Methionine and tryptophan are particularly susceptible to oxidation by free radicals. Methionine and tryptophan are limiting amino acids in diets, and they are often added in the form of crystalline amino acids to supplement natural ingredients. In a controlled study, lipid oxidation caused during flaking reduced bioavailability of methionine and tryptophan in wheat, rye, barley and oats by as much as 26 percent.
Measures to control oxidation should be undertaken long before the feed is manufactured.
Other side-effects of rancidity include the creation of objectionable flavors such as “fishy” or “beany” flavor in oils rich in linoleic and linolenic fatty acids (e.g., soybean, rapeseed and fish oils). This process, called reversion, can take place even under anaerobic conditions, and currently there is no way to prevent it.
It is evident that in nutrient-dense diets enhanced with fats, lipid oxidation can easily become problematic. Increased rancidity in choice white grease (lard), added at 6 percent of the diet, linearly depressed growth rates and feed intake in weaned pigs that were fed complex starter diets. From the same study, it was demonstrated that dietary concentrations of peroxides should not exceed 240 mEq to prevent depression of growth performance. Poultry, and especially young broilers, respond similarly, with older birds being rather more resistant to rancidity, but then, older animals seldom receive feeds rich in oils and fats.
A study demonstrated that dietary concentrations of peroxides should not exceed 240 mEq to prevent depression of growth performance.
Which antioxidants to use
Antioxidants are frequently added to oils to prevent auto-oxidation during storage. Such additives are also added into complete feeds, concentrates and premixes that contain high levels of fats and oils. As mentioned, antioxidants can only delay the process of oxidation by stabilizing reactive fatty acids, but given enough time, fatty acids will eventually react with available oxygen. Common antioxidants include ethoxyquin, butylated hydroxytoluene, propyl gallate citric acid, and vitamins C and E. The last three antioxidants are usually too expensive to be used as antioxidants in feed, yet when they are included in relatively high amounts for other reasons they contribute to the antioxidant status of the feed.
Natural antioxidants are extracts from spices and herbs rich in polyphenols. Such commercial products are derived from spices such as clove, cinnamon, oregano, cumin, garlic and coriander or herbs such as sage, thyme, marjoram, tarragon, peppermint, oregano and rosemary. Several pigments such as carotene, beta-carotene, astaxanthin, zeaxanthin, lutein and canthaxantin also have antioxidant activities.
All incoming lipid sources must be checked for rancidity. It is best to buy lipids that have been enhanced with antioxidants if storage conditions favor rancidity. Finished feeds should be tested regularly for oxidation. In addition, samples should be collected from feeding points (troughs) to assess the oxidation status of feeds as they reach the animal.
Peroxide analysis is a valid index of rancidity, but it does not provide all information required to assess the oxidative status of feed and lipids. Other common tests include analyses for anisidine, malonaldehyde and thiobarbituric acid (TBA). A combination of several tests is usually required to provide a complete picture of oxidation state.
The usual addition of a single, inexpensive and low-dosage antioxidant usually suffices for most cases. This involves production of feed in cold(er) environments and consumption within a few weeks after production. Prolonged storage, shipment and storage in warm(er) climates, and, of course, high levels of unstable lipids, require a more generous approach. To this end, using two or three different antioxidants that complement each other is advisable. Using the highest recommended dosage is always a first thought, but very high levels are not recommended unless the feed is expected to go rancid very quickly. Moderation in supplementation levels is best.
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