Young pigs do not eat enough to grow to their full potential during the first few weeks post-weaning, because they are faced with a myriad of overwhelming challenges under most commercial practices. This review provides an update of current technology in feeding young pigs in the face of such challenges.
By Dr Ioannis Mavromichalis, Ariston Nutrition, Madrid, Spain
Under most commercial conditions today, pigs struggle to adapt to post-weaning diets at this age and thus, they do not weigh more than 9 to 10 kg at best. At 50 days of age, pigs in the study reached 32 kg with an astonishing overall growth rate of over 700 g/day.
Under commercial conditions, pigs receiving dry diets rarely weigh more than 20 kg at the same age. Today, it is clear that three-week old pigs can easily reach 10 kg body weight under non-limiting conditions, but in most commercial operations pigs rarely exceed 7 kg at the same age.
Piglets with no experience of creep feeding are required at weaning not only to recognise an unusual diet as nourishment but also to decide when and how much to eat. At the same time, they must distinguish thirst from hunger and learn to satisfy these needs via separate media, because water is no longer part of their feed, as it was in the sow’s milk. These problems coupled with limited digestive capacity curtail growth potential in young pigs.
Post-weaning anorexia is frequently associated with damage to gastrointestinal epithelium from allergenic reactions to dietary antigens, which invariably leads to nutritional diarrhoea and nutrient malabsorption. When starving pigs start eating (usually between the second and fifth day post-weaning), as they associate dry feed with nourishment, the already challenged digestive system is rapidly overloaded with partially digested feed that may promote microbial proliferation and pathogenic diarrhoea, most frequently from collibacilossis. Thus, abnormal feed intake patterns may lead to digestive upsets post-weaning under practical conditions.
The major impact of post-weaning anorexia is growth depression, because feed intake is the main determinant of growth during early stages. For every 0.1 kg extra feed consumed per day during the first week post-weaning, body weight increases about 1.5 kg at the end of the fourth week post-weaning, indicating a considerable carry-over effect. High feed intake during the first week post-weaning also promotes growth and health during the entire growing-finishing period. Pigs that maintain or lose weight during the first week post-weaning require ten extra days to reach market weight compared with pigs that gain about 250 g/day during the same period. For practical purposes, optimal feed intake for piglets under non-limiting conditions may be predicted with reasonable accuracy by the equation by Dr Whittemore:
It is accepted that maintenance of pre-weaning growth rate during the first week post-weaning indicates minimal nutritional challenge. An average of 200-250 g/day is often used as a benchmark for pre-weaning growth rate, unless pigs consume sufficient quantities of a creep diet in a prolonged lactation period. Feed intake of 200 g/day during the first week post-weaning should be considered satisfactory.
Differences among published estimates of nutrient requirements are a reflection of the differences in production methods, ingredients, and research and statistical methods used in original research reports that formed the basis of reviews. Because these requirement estimates do not include margins of safety, they are regarded as base levels in establishing practical dietary specifications under commercial conditions.
Thus, most nutritionists add margins of safety to allow for flexibility in bag label declarations, nutrient variability in ingredients, formulation method (total or digestible nutrient concentrations), nutrient losses during thermal processing and storage, and variable animal performance. Typical margins of safety range from 5 to 30 (Table 1).
Pigs are able to maintain constant energy intake by adjusting daily feed intake, but the ability to fully compensate for decreasing dietary energy concentration is related to pig size. Thus, pigs with high genetic merit growing below their potential for lean gain (because of limited voluntary feed intake) are not able to properly adjust energy intake when presented with low energy diets. Thus, dietary energy concentrations below 15.5 MJ ME/kg are expected to reduce energy intake in pigs less than 15 kg, whereas, a minimum of 14 MJ ME/kg is suggested for pigs over 15 kg, as each MJ ME below this threshold is likely to reduce daily energy intake by at least 1.5 MJ. Recommended dietary energy concentrations are in Table 2 and should be used as a guideline only, because ingredient selection and availability of added fat determines upper levels in dietary energy density.
Among the ten essential amino acids, the most likely to become limiting in low-protein nursery diets are lysine, methionine, threonine, tryptophan, isoleucine, and valine (Mavromichalis et al., 1998). In contrast, histidine, phenylalanine, leucine, and arginine are not likely to become problematic in most practical diets. Glutamine supplementation in early nursery diets may also be warranted on grounds of gut health, but no specific requirement has been determined for this amino acid.
Maintenance requirements are 36 mg true ileal digestible lysine daily per kilogramme metabolic body weight. For protein accretion, 120 mg true ileal digestible lysine are needed per gram protein. On a total lysine basis, maintenance and protein deposition requirements are 44 mg/kg BW0.75 and 146 mg/g protein, respectively, assuming an average true ileal digestibility of lysine of 82% for most practical nursery diets.
Diet formulation using the ideal protein profile requires first the establishment of a lysine specification, which is available through dose-titration studies. Then, the dietary specifications for the essential amino acids are calculated based on lysine specification and the ideal protein profile (Table 3).
There is a current interest in the reduction of dietary protein in nursery diets, and this is deemed beneficial for both animal performance and protection of the environment. Pigs fed low-protein diets (less than 21%) have been shown to be less prone to Escherichia coli infection that leads to post-weaning diarrhoea. Low-protein diets have been also shown to reduce nitrogen excretion by 30-50%, depending on original specifications, ingredient selection, and magnitude of reduction in dietary protein concentration. In general, for each percentage unit reduction in dietary protein concentration, nitrogen excretion can be reduced approximately by 8%.
When diets based on cereals and plant protein sources are supplemented with milk products such as dried whey and skim milk, growth performance of young pigs is markedly improved. 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 around three weeks of age.
Several studies investigating minimum dietary lactose concentrations in weaned pigs suggest that dietary lactose concentrations can be rapidly lowered after the initial two weeks post-weaning, whereas in pigs over 12-15 kg bodyweight there are no real benefits from feeding lactose. Dietary lactose specifications (based on research and field experience) presented in Table 4 have yielded consistent results in commercial applications.
Earlier studies indicated that although weaned pigs prefer lactose over starch, they can also use other forms of simple sugars. Sucrose, dextrose, maltodextrins, and molasses can be as effective as lactose and thus, the term ‘lactose equivalent’ is more appropriate in expressing dietary specifications for simple sugars, as piglets do not have a requirement for lactose per se.
Most nutritionists would consider vitamin concentrations recommended by scientific review committees as too conservative and of limited practical application. It is not uncommon for most vitamins to be included at levels that are up to ten times higher than those recommended. From the fat-soluble vitamins, vitamin E is often included at levels up to 250 IU/kg because of perceived health benefits, whereas recent research has clearly demonstrated that NRC (1998) levels are far too inadequate.
Most water-soluble vitamins are added at considerably elevated concentrations based on earlier reports with claims on enhanced growth performance, but not reproduced in later. Most interestingly, vitamins like vitamin C and carnitine are often suggested for nursery diets despite variable results from research reports. Lately, a combined effort from several research institutions in the USA indicated that supplementation of nursery diets with vitamin levels that exceed recommended concentrations did not improve growth performance in young pigs. Clearly, there is considerable confusion regarding the appropriate levels of vitamins in nursery diets and the fact that stability during storage and feed manufacturing is not standard across all vitamins does not add clarity.
Calcium is the least expensive nutrient in a pig diet because its main source, calcium carbonate, can be provided inexpensively from limestone. Thus, many low-cost formulas tend to be over-fortified with calcium, with some containing over 1% Ca. Calcium carbonate, however, has such a strong buffering activity that can significantly lower the acidity of gastrointestinal contents and, thus, interfere with protein digestion. It is strongly recommended, therefore, that dietary calcium specifications do not exceed actual requirements and the calcium to phosphorus ratio be no more than 1.2:1.
Recent research revealed that post-weaned pigs clearly benefit from additional sodium and chlorine in diets based on animal products that meet or exceed established requirements for sodium and chlorine. Sodium was partially responsible for the positive response, but the greater response was attributed to chlorine. It is, thus, recommended that starter diets for pigs less than 10 kg contain 0.4-0.5% Na and at least as much Cl.
The balance of dietary cations and anions is in close relationship with pig performance and affects metabolism quite differently than individual. Dietary electrolyte balance, also known as cation-anion difference (CAD), is calculated using only monovalent ions:
Trace minerals are added in most nursery diets in the form of a premix. Iron, zinc, copper, manganese, iodine, and selenium are the major trace minerals used. Cobalt is also included although pigs do not require supplemental cobalt. Upper inclusion limits for most of these trace minerals are regulated by law, especially in the European Union, whereas selenium is regulated in most countries worldwide. Research is limited on the appropriate levels of trace minerals in nursery diets and thus, most practical diets contain at least twice as much as recommended by scientific review committees, with the exception of selenium.