There is compelling evidence that the responses noted when whey is fed relate almost entirely to the lactose content of the whey and not to its protein contribution. Lactose is a large part of whey products and also very digestible for piglets. Research has indicated that lactose in the diet for the first few days after weaning is essential for success in feeding pigs weaned at less than 3 -4 weeks of age. The inclusion of lactose containing ingredients assists the transition at weaning from sow’s milk to a dry diet (O’Doherty et al., 2004a). The addition of high quality lactose sources has demonstrated linear improvements in pig performance as levels of lactose increase in the diet. The weaned pig responds with greater weight gains when between 34.5 % and 45 % simple carbohydrates are provided during the initial weeks post-weaning (Mahan and Newton, 1993). Most complex carbohydrates such as starch present in grains are utilised less efficiently than lactose by the young pig during the early post-weaning period. Work by our group has shown that lactose is not only important during the first week post-weaning but results in improved pig growth responses during the entire starting period (Pierce et al., 2007). Apart from its importance as a very digestible energy source for the young pig (Jin et al., 1998), lactose also acts as a specific substrate for lactobacilli (Pierce et al., 2005).
Dried skim milk
The main use of skim milk is as a protein supplement in the diets of simple stomached animals and diets fed to pigs immediately post-weaning have routinely contained 10 to 25 % dried skim milk (Tokach et al., 2003). The improvements in performance of early-weaned pigs fed diets containing milk products apparently are due to the ability of the young pig to utilise the carbohydrate and protein fractions from milk more effectively than those components from plant feed ingredients. There are two problems with skim milk however. Firstly, skim milk is an expensive protein source relative to vegetable protein such as soya bean meal and secondly, the casein fraction of skim milk has been found to reduce feed intake (Dritz et al., 1994). Dritz et al. (1994) found no benefit to having skim milk in the diet, when the diet contains adequate quantities of plasma and lactose.
Skim milk generally contains around 350g/kg protein, the quality of which varies according to the manufacturing process i.e roller-dried skim milk is subjected to a higher drying temperature than the spray dried product and has a lower digestibility and biological value. The poorer clotting and increased rate of stomach emptying of severely heated milk reduces the time available for proteolysis in the stomach.
The interaction between lactose and soya bean meal
Certain components of feeds, such as soya bean meal, have been implicated in causing intestinal mucosal damage in piglets. When fed in creep feeds, it has been suggested that soya protein may act as a primer for hypersensitivity reactions after weaning that, in turn, predispose pigs to post weaning diarrhoea (Miller et al., 1984).
Changing from skimmed milk powder to soyabean and maize has been shown to increase the severity of diarrhoea and appearance of enterotoxigenic E. coli in 3-week-old pigs. O’Doherty, et al., (2004) showed that high dietary concentrations of lactose allows for increased soya bean meal inclusion (> 200 g/kg) in weanling pig diets without affecting performance or health. Pierce et al., (2004) found that increasing the inclusion level of lactofeed from 100 g/kg to 300 g/kg, when soya bean meal comprised the principle protein source, improved food efficiency and increased daily gain and daily food intake compared with diets where dried skim milk comprised the principle protein source. The inclusion of increasing levels of lactofeed (86% whey permeate, 14% soya bean meal) was also found to improve nitrogen digestibility. The use of soya bean meal in combination with high concentrations of lactose could offer an alternative protein source to animal proteins in starter diets for pigs, thereby reducing the cost of starter diets.
In conclusion, lactose is an essential nutrient in the diet of the postweaning pig. For pigs weaned at less than 6 weeks of age, optimal performance can only be achieved by feeding feeds that contain significant quantities of lactose.
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