Lactose in piglet formulas, a necessity

By Ioannis Mavromichalis, Ariston Nutrition SL, Spain

The beneficial effects of lactose on growth performance during the post-weaning period are well documented. The most common sources of lactose used in these early studies were skim milk (50% lactose) and dried whey (70% lactose). Of course, today, other dairy products that may be less expensive yet equally effective in supplying lactose include crystalline lactose, deproteinised whey, whey permeate, milk chocolate product, cheese powder, and whey protein concentrate.

It has long been known that pigs younger than 8-10 weeks of age cannot utilise starch very effectively, although starch digestive enzymes can be rapidly induced by dietary substrate. Nevertheless, weaning stress accompanied by low feed intake prolong physiological maturation of the digestive system, and for this reason, simple sugars (such as lactose) and cooked cereals are generally more digestible than raw starch. It is, thus, widely accepted that piglets benefit from readily digestible carbohydrates until their digestive system is fully capable of utilising raw starch. Diets based on cereals and vegetable protein sources can support greater growth performance when supplemented with milk products such as dried whey and skim milk. Inclusion of 10-20% dried whey in simple diets (maize, soybean meal, and oat groats) has consistently improved growth performance by at least 15% in pigs weaned at three weeks of age. Other dairy products are equally effective in enhancing post weaning growth performance, although they are usually more expensive. Initially, the effectiveness of dairy products was attributed to increased palatability and protein digestibility. However, it has been demonstrated since then that lactose and not the protein fraction of whey is responsible for improved feed intake and weight gain during the first weeks post-weaning. In these trials, lactose supplementation restored growth performance in diets devoid of dried whey, whereas lactalbumin supplementation failed to improve performance (Table 1).

It should be remarked that supplementation with a highly digestible source of protein or crystalline amino acids was essential for lactose to elicit its beneficial effects in whey-free diets. In the same studies (Mahan, 1992), it was demonstrated that after the first two weeks post-weaning, piglets responded more to protein supplementation, an indication of a rapidly maturing digestive system and increased need for highly digestible protein.

Lactose specifications
Several studies have investigated the level of dietary lactose concentration that supports maximal growth performance in weaned pigs. These data suggest the following:

1. high concentrations of lactose are needed immediately post- weaning,
2. lactose concentrations can be reduced rapidly afterwards, and
3. piglets heavier than 10-12 kg in body weight do not benefit greatly from supplemental lactose.

Although responses to lactose have been variable, due to basal diet composition, genetics, environment, and health conditions, most experiments report similar lactose requirements for nursery pigs. Therefore, practical recommendations for dietary lactose specifications are presented in Table 2.

Immunoglobulins
Addition of in-feed immunoglobulins (from egg antibodies or animal plasma) may lower lactose requirements because it promotes feed intake post-weaning. For example, pigs fed diets with 6.75% plasma protein required about 15% added lactose for maximal growth performance during the first week post weaning. In contrast, when plasma protein was replaced with extruded soy protein concentrate, performance peaked between 30 and 45% dietary lactose. It appears that when feed intake is high, lower levels of dietary lactose are needed, perhaps because feed intake is closely related to digestive system maturation.

Lactose equivalents
Earlier studies indicated that although weaned pigs prefer lactose over starch, they can also use other forms of simple sugars. Monosaccharides (for example, glucose, fructose, and maltose) and oligosaccharides (for example, sucrose and maltodextrins) can be as effective as lactose in supporting growth performance. The term “lactose equivalent” has been proposed as more appropriate to express dietary specifications for simple sugars, as piglets do not have a requirement for lactose per se. Lactose equivalent values for several sources of simple sugars that can replace lactose are in Table 3. Several concerns arise, however, with the replacement of lactose by other simple sugars in pelleted diets. For example, sucrose may increase pellet hardness as it “burns” easily in the Maillard reaction during pelleting. Also, dextrose and other reducing sugars may readily destroy amino acids during pelleting. For these reasons, lower pelleting temperatures or even cold pelleting are recommended for diets rich in reducing sugars.

Simple sugars also differ considerably in sweetness, and, therefore, changes in diet palatability should be taken into consideration when using lactose alternatives. Pigs prefer simple sugars in the following order:

sucrose>fructose>maltose=lactose> glucose>galactose.

Excessive concentrations of simple sugars are also known to predispose pigs to secretory diarrhoea. For instance, by changing the amount of sugars (sucrose and corn syrup solids), dietary osmolality was manipulated between 250 and 700 mOsm/kg. Absorption of water and carbohydrates from the small intestine in piglets increased, and thus chances for diarrhoea were minimised, as osmolality increased, at similar dietary electrolyte balance. Maltodextrin was shown to increase water absorption, whereas glucose increased water secretion in the gastrointestinal tract, indicating that sugar oligomers are more effective than monomers in preventing secretory diarrhoea.