Introduction
Milk by-products have been intensively used in weanling pig diets because of their beneficial role from both their protein and lactose components (Mahan and Newton, 1993). When a lactose and corn gluten meal based diet was fed, weanling pigs grew and utilized the diet as well as pig fed the diet containing dried whey (Mahan, 1993). Subsequent trials demonstrated that when the lactose and milk protein components were added to diets separately the greatest response occurred to the added carbohydrate component. These results suggest that the carbohydrate lactose contributed greatly to the improved growth responses to postweaning pigs and has been underestimated as to its value as an energy source.
Fat is also an energy source but provides energy from lipid products. The incorporation of fat into the diets of the grower pigs has shown that it clearly enhances diet acceptability and improves feed efficiency. Increased performance responses during the postweaning period have, however, not always produced performance improvements as with the grower pig. In addition much of the early weanling pig research conducted with added fat used diets that were comprised largely of corn-soy bean meal mixtures. These diets did not contain the ingredients that are commonly used in many modern weanling pig diets and most did not contain a highly digestible carbohydrate source (Cera et al., 1988; Tokach et al., 1995). The initial postweaning responses may also have been due to the shortened villi (Cera et al., 1988) or lowered intestinal lipase activity (Cera et al., 1990) that normally occurs in the small intestine during the initial weeks postweaning.
Consequently, the use of plasma proteins and milk by-products, both of which have now been shown to be beneficial to weanling pigs has shown improved performance responses. As a result of these and other dietary ingredients the pig can now be weaned earlier. These dietary ingredients have allowed the young pig to effectively utilize the nutrients in these special feed ingredients.
Although the weaning pig reportedly can utilize dietary fat efficiency, the interaction effect of dietary carbohydrate and fat has not been studied with the early weaned pig. Therefore, our study evaluated the effects of different levels of a digestible fat source (soybean oil) and a simple carbohydrate (lactose) on the postweaning performance of young weaned pigs.