Starch carbohydrates are an excellent energy source for pigs. However, in the early period after weaning, carbohydrate sources must be highly digestible to serve as a readily available source of energy for weanling pigs, particularly to counteract the low feed intake after weaning.
Lactose is the carbohydrate component derived from milk and provides an easily digestible source of energy for pigs. The addition of lactose in nursery diets improves growth rate of weanling pigs, particularly in the early period after weaning (Tokach et al., 1995; Grinstead et al., 2000). There is a benefit of providing 5 to 30% dietary lactose during the entire nursery period (Mahan et al., 2004; Cromwell et al., 2008), but it is recommended to direct the use of lactose to initial nursery diets and for a short period of time because the magnitude of response dramatically decreases, and lactose products are expensive. Typically, initial nursery diets contain around 18% lactose (Mahan et al., 2004).
Common lactose sources are crystalline lactose (100% lactose), whey permeate (80% lactose), and dried whey (72% lactose). Whey products also provide a source of protein to the diet. When replacing one lactose source by another in the diet, care must be taken to evaluate both lactose and amino acids levels. The key is to know the lactose concentration and replace on an equal lactose basis, and then replace the amino acids with an appropriate high-quality protein source.
Whey products are derived from milk curdling during production of milk products like cheese and yoghurt (Grinstead et al., 2000). Whey products used in nursery diets are preferentially derived from cheese production (‘sweet whey’) rather than yoghurt production (‘acid whey’). The use of edible-grade whey are also preferred over feed-grade whey products (Nessmith et al., 1997).
Quality of whey products is affected by excessive heat during processing, which reduces lactose and amino acid contents and results in whey with a brownish color. Over-heating also reduces digestibility of lactose and amino acids, which leads to scouring in pigs. Spray-drying is the preferred method to prevent over-heating because of the fast evaporation at lower temperatures compared to roller drying (Grinstead et al., 2000). The levels of minerals also vary during processing and above 11% mineral content leads to scouring in pigs (Nessmith et al., 1997). Indicators of high-quality whey products include white or yellowish color, absence of black specs, and mineral content below 8.5% (Nessmith et al., 1997).
From a feed milling standpoint, the addition of lactose products in the diet influences feed processing. For pelleted diets, high levels of lactose can increase friction during the pelleting process. In this case, the addition of fat in nursery diets with high levels of lactose is often used to enhance lubrication of the pellet die and prevent heat damage. For meal diets, high levels of lactose can increase bridging and reduce flowability in bins and feeders.
Sucrose is a simple carbohydrate extracted from sugar cane or sugar beet. Sucrose provides an easily digestible source of energy for pigs from glucose and fructose. Young pigs have limited ability to utilize sucrose, but sucrase activity rapidly increases after weaning.
Sucrose can be considered as a replacement for lactose in nursery diets, particularly in diets with no animal-based products. The addition of 5 to 10% sucrose in initial nursery diets improves growth performance of weanling pigs as effectively as lactose (Mavromichalis et al., 2001). Products based on sucrose and other simple sugars can also partially replace lactose without an impact on growth performance (Naranjo et al., 2010; Guo et al., 2015).
Dextrose is a form of glucose typically extracted from corn that provides an easily digestible source of energy for pigs. Similar to sucrose, the use of dextrose or products based on dextrose and other simple sugars can partially replace lactose in initial nursery diets without an impact on growth performance (Turlington et al., 1989; Mahan and Newton, 1993; Bergstrom et al., 2007).