Several maternal lines that are being used in the industry today are classified as "high-lean" or "high-producing" (>0.7 lb. fat-free lean per day) animals. These genetic lines should be fed diets that will maximize their lean gain during the first six months of life, but upon entrance into the replacement pool or breeding herd, their body fat content should be emphasized more than during the grower-finisher period. Other maternal lines have an "average" ability to deposit lean (< 0.7 lb. fat-free lean per day). The dietary amino acid concentrations provided during the grower-finisher period should reflect the lean growth potential of the animal. The amino acid requirements of the high-producing replacement gilt are generally similar to those of the high-lean gain grower-finisher pig (Table 4).
Most of the vitamin and mineral requirements for replacement gilts are similar to that of the grower-finisher pig. Replacement gilts, however, should be fed higher dietary levels of vitamin A, vitamin E, calcium, phosphorus, selenium, copper, and zinc than market hogs. This is done to enhance their body reserves of specific nutrients that are needed at higher levels during future reproductive purposes and/or to enhance their immune response (Table 6). Diets fed to replacement gilts should contain the better-quality grains, free of myco-toxins so that the reproductive tissue will not be affected.
Bone development and mineralization should be increased during the prebreeding period. Higher calcium, phosphorus, copper, and zinc levels should be provided than for market animals. Additional calcium and phosphorus can be stored in bone tissue, and these minerals can be later mobilized if fetal and lactation demands exceed that which is provided in the diet. Insufficient dietary levels of these minerals during the prebreeding period results in bones with low mineral contents with structural weakness. These effects may not become evident until late gestation, lactation, weaning, or during later parities. Consequently, locomotive failure or broken bones can occur in older females, particularly those that were not developed properly as replacement gilts.
A higher intake and body retention of selenium and vitamin E fed to replacement gilts will also be beneficial later in the sow's reproductive life. These latter nutrients also help improve the sow's immune-response and prevent subsequent lactation complications such as mastitis, metritis, and agalactia (MMA).