It is always rewarding studying older research reports, and indeed in trials conducted in the early 50s, researchers had already identified that piglets could thrive not only on lactose, but also on any other simple sugar. Today, the term "lactose equivalent" is used with increasing frequency to reflect the fact that lactose and other sugars are interchangeable. Such sugars are examined here, because each one has its own unique characteristics.
Sucrose (table sugar)
This is the most readily available replacement for lactose, but quite often, the most expensive. Sucrose is a disaccharide composed of fructose and glucose, and it is well digested and absorbed by young pigs. Fructose is actually absorbed by a gut transportation system quite disparate from that used for glucose and thus, it is believed it might provide for the possibility of enhanced energy intake. On the other hand, sucrose is rather poorly digested by piglets that are less than 7 to 10 days old, because they lack specific enzymes responsible for the digestion and metabolism of sucrose. Therefore, feeding high levels of sucrose (for example, more than 5 percent) to neonatal pigs invariably leads to diarrhea and even death from toxicosis. Thus, sucrose is best avoided in milk-replacer formulations destined for orphan neonatal pigs.
Nevertheless, as pigs rapidly start to produce enzymes essential for sucrose digestion and metabolism, they are capable of producing sufficient quantities to fully digest high levels of sucrose by the second week of age. Indeed, studies at Kansas State University have clearly demonstrated that up to 20 percent sucrose may be used in piglet diets without any adverse effect. This assumes precaution has been taken to account for the highly hygroscopic effect of sucrose that can upset the gastrointestinal osmotic balance and lead to secretory diarrhea or appearance of soft feces (both without health or performance consequences, but rather unpleasant!)
The most common form of dextrose is glucose monohydrate, which in essence is pure glucose with a molecule of water. Consequently, dextrose contains less energy (90 percent) than lactose. Based on research from Purdue University, weaned pigs appear to utilize lactose, glucose and dextrose all with the same efficacy. Anhydrous dextrose is equivalent to pure glucose, but it tends to be more expensive than glucose monohydrate. Both products are highly hygroscopic and agglomerate rapidly during storage (but don't cause as much gastrointestinal "leakage" as sucrose when used at levels less than 10 percent of the complete formula).
This is one of the two co-products of the sucrose industry and the one with relatively higher nutritive value. Molasses is frequently used in piglet diets to enhance palatability (sweet taste and aroma) and quite often to facilitate the pelleting process. To this end, 1-2 percent molasses is usually deemed sufficient. Molasses can be of cane or beet origin but the nutritive value is similar as long as quality remains constant. Molasses has about 75 percent dry matter, and it has 46-49 percent sugars in the form of sucrose and its hydrolysis products, glucose and fructose. Quite often diets based on molasses have a laxative effect in young pigs due to high levels of potassium and other salts. In several studies, however, molasses replaced lactose readily up to 20 percent of the diet without any negative effects on health or performance in young pigs. It should be also noted that the sucrose in molasses can be as toxic as crystalline sucrose for neonatal pigs. Finally, molasses-based diets require low temperatures (less than 60 C) during pelleting because they are prone to form Maillard reaction products that render dietary lysine unavailable to the animal. Pelleting diets with high levels of molasses is another intricate process that can be facilitated with proper diet formulation to ensure sufficient absorptive capacity.
Raw starch is poorly utilized by weaned pigs (and thus, it cannot replace lactose), but enzymatically- or acid-hydrolyzed starch products can be quite efficacious in replacing lactose. These products are derived from the industry of nutritive sweeteners and include dextrins, maltodextrins, corn syrups and even free glucose. Depending on cost and availability, these products can be effectively utilized in starter diets and even in milk replacers. They are characterized in terms of Dextrose Equivalents (DE), which reflect their concentration in free glucose (e.g., dextrose anhydrous = 100 DE).
The most highly prized starch derivative is high-fructose corn syrup, which is basically corn syrup, but with about double the concentration of fructose; it is the main component found in many soft drinks. According to evidence from North Carolina State University, high-fructose corn syrup is an excellent alternative for lactose as it can be utilized readily as an energy source by young pigs. High-fructose corn syrup in contrast to sucrose, which also contains fructose, is not toxic to neonatal pigs.
Alternative sugars should not replace lactose on a weight-to-weight basis (although this is the most common practice!). Instead, a specification called "lactose equivalents" (LacEq) should be entered in any ingredient matrix to reflect the actual concentration of simple sugars (Figure 1). For example, table sugar contains about 98 percent sucrose (thus, 98 percent LacEq), whereas anhydrous dextrose contains about 90 percent glucose (thus, 90 percent LacEq). Nevertheless, this only becomes of commercial significance in diets already very low in lactose, as is the case in Europe.
Concerns using alternative sugars
Apart from being a nutrient for pigs, lactose acts also as a prebiotic for the gut microflora, promoting a healthy balance. It is certainly logical to assume this role diminishes in diets that contain a plethora of antimicrobial agents and/or zinc oxide and copper sulfate, but perhaps a small part of lactose should be retained in diets devoid of these antimicrobial agents.
In addition, when switching from one source of sugars to the other, it is prudent to ensure that overall diet palatability is not greatly affected, because not all sugars have the same index of sweetness (Figure 2). This becomes especially important in low-cost diets, which often use ingredients of second quality that impair an off-taste that requires strong sweeteners to cover it.
Replacement of lactose with other simple sugars may also affect pellet quality. For example, replacing lactose with 5 percent sucrose or 10 percent fructose increases pellet friability and softness. Also, sugars such as lactose and glucose (in molasses) readily react with amino acids during pelleting in the Maillard reaction. Therefore, lower pelleting temperatures and even cold pelleting is often suggested when diets contain high amounts of reducing sugars.
Lactose and dietary immunoglobulins
Adding in-feed immunoglobulins (from egg antibodies or animal plasma) may lower lactose requirements because this component promotes feed intake post-weaning. It should be reminded that lactose requirement studies were conducted before the advent of animal plasma. In one preliminary study, pigs fed diets with 6.75 percent plasma protein required about 15 percent added lactose for maximal growth performance during the first week post-weaning. In contrast, when plasma protein was replaced with extruded soy protein concentrate (an excellent protein source that can fully replace the protein function of plasma, but totally devoid of immunoglobulins), performance peaked between 30-45 percent dietary lactose. It appears that when feed intake is already high, lower levels of dietary lactose are needed, perhaps because feed intake is closely related to digestive system maturation.