Feeds contain lipids, either from natural ingredients (corn contains 4 percent) or added (soybean oil, for example), to increase energy concentration or even to improve manufacturing and handling properties. When feed becomes stale, lipids, which are composed of three fatty acids and a back bone of glycerol, become rancid. 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. Rancid fatty acids will also react with every other nutrient and molecule prone to oxidation, such as vitamins.
It is evident that in nutrient-dense diets enhanced with fats, lipid oxidation can easily become problematic. In a further study, 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.