The diets fed to gilts and sows in the tri-state area are generally a vitamin- and mineral- fortified mixture of corn and soybean meal. Nutrients supplied to the pregnant sow are used for maintenance, body-tissue growth, and for the development of fetal and other reproductive tissues. Protein and energy needs of the fetus increase greatly during the last few weeks of pregnancy.
An adequate body-fat content appears essential for the sow to have several successful reproductive cycles. The body-fat content of reproducing animals should be closely monitored. Either excessive body condition or low fat reserves can be detrimental to subsequent reproductive performance. The energy content of the diet and the sow's body-condition score (Figure 7) are the primary factors that determine the quantity of feed that should be provided to the gestating sow (Table 11). High-producing and industry-average genotypes should be bred at a body-condition score of 3.0 (270 to 300 lb. and 240 to 270 lb., respectively).
Older sows, because of their larger size and weight, not only have a higher body maintenance requirement but they often also have a lower body-fat content. Higher productivity during previous reproductive cycles exacerbates the lower body fat content. Because of the wide ranges in productivity and body fat, it is difficult to recommend a specific gestation feed intake for all situations. It is best to use the feeding guidelines in Table 11 and the sow body-condition scores depicted in Figure 7 to determine the quantity of feed to provide for individual sows. A last-rib backfat thickness of 22 to 24 mm (0.90 inch) for parity-one gilts and a backfat thickness of 17 to 19 mm (0.70 inch) when sows reach their fifth parity reflect a body-fat condition that will result in satisfactory reproductive performance.
The quantity of feed to provide gestating animals depends upon the energy content of the diet, sow age, body weight, housing conditions (grouped in pens or individual crates), location of animals such as in complete confinement or in outside lots, and their body condition score as they enter the gestation period. Maternal genotypes that are leaner may need higher feed intakes to sustain an adequate body condition. Attaining an average pig birth weight of 3.0 to 3.2 lb. is a good indicator that the gestation sow feeding program is adequate. There is, however, a tendency in the United States, particularly with parity-one gilts, for them to become too fat by the end of their first gestation period. This condition may have detrimental effects during farrowing, on lactation feed intake, and on subsequent rebreeding performance.
With the higher energy needs of larger litters during late pregnancy, the sow begins to use her body fat reserves during the last few weeks of pregnancy even though she is gaining body weight. Although one may simply increase the sow's feed intake by 1 to 3 lb. during the latter portion of pregnancy (depending upon body fat scores), care should be taken not to allow the sows to become too fat.
There are several methods of feeding gestating sows (once or twice daily, every third day), but feeding once daily in individual stalls or crates is the most popular and will result in reproductive performance that is comparable to feeding twice daily. Feeding sows "every third day" can be satisfactory in outside lots during the warmer seasons and with mature sows, but during colder weather and with first-parity gilts, the results are often less than desirable. Managerial observation of sow health is critical, regardless of feeding method.
Incorporating fat in the gestating sow's diet during the last 10 to 14 days of pregnancy has frequently resulted in improved pig surviva-bility during the first week of life, particularly with litters containing pigs of light birth weights. Supplemental fat provided during the latter portion of gestation has resulted in an increased milk fat content during the subsequent lactation.
Dietary protein (amino acids) concentration is important for the pregnant animal, more so for parity-one gilts who are also developing body muscle along with the fetal and mammary tissue. Older sows in good body condition need less dietary protein (amino acids) for body muscle formation than parity-one gilts.
A 14 to 15% protein diet (i.e., 0.75% lysine) fed to parity-one gilts allows for adequate muscle and fetal development and will be of benefit during the subsequent lactation. A dietary protein concentration of 12 to 13% provided to older high-producing maternal-lean-genotype lines during pregnancy will then be desirable as it will result in an increase in the sow's body-fat content. It is advisable to feed pregnant sows a higher protein diet during the last portion of gestation or no later than when they are transferred to the farrowing crate. The lactation diet can be fed during this period.
Because gestation diets are normally fed once daily in a limited quantity and there are differences in the rate of absorption and utilization of synthetic amino acids (e.g., lysine) compared with the amino acids within the grains or protein source, the use of synthetic lysine in gestation diets is discouraged. Amino acids and starch (glucose) from grains are absorbed more slowly than synthetic lysine, and some of the more rapidly absorbed synthetic lysine is not effectively utilized, whereas lysine from intact protein is more gradually absorbed and better utilized.
Feet and leg problems may be associated with inadequate dietary minerals, particularly calcium and phosphorus, but other minerals and vitamins are also essential for normal development of skeletal and hoof tissue. During the latter portion of gestation, the reproductive demands on the sow for transferring minerals to fetal bone tissue is high. If the sow does not receive adequate dietary calcium and phosphorus, demineralization of her skeleton will occur. The vertebrae and ribs are the bones most vulnerable to the demineralization process if dietary calcium and phosphorus levels are inadequate. This will result in weakened vertebrae. As fetal litter weight increases, displacement of the vertebrae may occur with the spinal cord becoming pinched (Downer Sow Syndrome).
Inadequate biotin has been associated with poor hoof development and cracking of hoof tissue. Because hoof development occurs over a long time, the vitamin should be fortified during the entire weaner-grower period for replacement gilts.
Grains in the tri-state area are generally low in selenium and possibly other trace minerals, particularly copper. Vitamin E is low in high-moisture grains, or grains that are stored for an extended period. Because the grains are inadequate in both nutrients, additional selenium and vitamin E must be fortified in the sow's diet. There is a close relationship between selenium and vitamin E, and both nutrients have also been effective in increasing the immunological capability of the sow. The newborn pig is born with low body reserves of both selenium and vitamin E. When the sow is deficient in vitamin E and selenium, an injection of iron in the newborn pig may cause its death (iron toxicosis). Consequently, gestation diets should be fortified with vitamin E and fortified to the approved concentration of selenium to enhance colostrum and fetal tissue reserves. The dietary levels of other minerals and vitamins in Table 10 are ade-quate to meet the reproductive needs of gestating sows. The organic form of selenium has been found to be more effective than sodium selenite for reproducing animals.