Although most of the test-weight variation is due to the starch component, it is a general reflection of growing conditions and nutrient content of the grain. The percentage of crude protein of low test-weight grains may, in fact, be higher than that of the normal test weight. This is because in low test-weight grains, the starch content is reduced and the protein is, therefore, at a higher proportion to the other components. Low test-weight grains usually result from early frost conditions, a situation that also encourages the development of mycotoxins. Pig growth rate is usually not affected until test weight is extremely low.
Feed conversions are generally poorer due to the grain's higher fiber content.
Because of potential differences in test weights, volumetric feed mills should be calibrated frequently to assure that the diet contains the correct proportion of feed components. This means that 1 lb. of fat added to a diet will be approximately equivalent to the energy present in 2.25 lb. of corn starch. The addition of fat to a diet thus results in improved feed conversion because the diet contains a more concentrated form of energy. Although the addition of fat to market pig diets improves feed conversion, body-fat deposition is often increased, particularly in barrows during the latter stages of the finisher period. Choice white grease and soybean oil are considered higher-quality fats compared to yellow grease.