The immediate post-weaning period is one of the most stressful phases in a pig's life. The piglets are exposed to environmental, social and psychological stressors, among which:

• abrupt separation from their mother
• mixing with other litters
• new environment
• switching from highly-digestible (liquid) milk to a less-digestible plant-based dry diet containing complex protein and carbohydrates, including various antinutritional factors

All these elements affect animal health and performance. When approaching health, welfare or performance problems in nursery pigs, it is essential to focus not just on infectious agents such as Escherichia coli or Streptococcus suis. Optimization of non-infectious factors may directly improve the health and performance of the pigs, control infections with pathogens, and/or prevent that the latter result in clinical disease. This is particularly important as antimicrobial usage should be reduced to decrease the risk for antimicrobial resistance. The following factors should be considered to optimize the health, welfare, and performance of nursery pigs:

• management and biosecurity
• hygiene
• environmental control
• nutrition and drinking water

Management and biosecurity

All-in, all-out (AIAO) production is an important factor in the control of infectious disease since it can interrupt or decrease the cycle of pathogen transmissions from older to younger pigs. It allows the producer to tailor environmental conditions to a uniform population of pigs and to clean the facilities between groups of pigs.

Mixing or sorting pigs is a source of stress to the animals and it increases the probability of pathogen transmission. Therefore, it should be minimized. An AIAO system in which the same pigs are moved as a group through the different production stages is to be preferred compared to one where pigs are regrouped during transfer from one unit to another.

Runt pigs should be euthanized or moved forward with their group to avoid affecting the next batch.

Proper stocking densities should be maintained, as crowding may lead to increased transmission of pathogens and stress reactions, making the pigs more susceptible to infectious diseases.

Early weaning (< 3 weeks) can reduce transmission of some pathogens from the sow to the offspring, but it is not allowed to be applied systematically in the EU. In addition, studies have shown that early weaning is associated with increased antimicrobial consumption post-weaning. Management strategies like weaning at a later age (e.g. 28-33 instead of 21 days) ameliorate the stress of the piglets and reduce effects of weaning. Weaning weight is further determined by birth weight and colostrum intake, both having long-term effects on the piglet's growth. High variability in pig weight at weaning is related to a high within-litter variation of piglet birth weight. An increase in litter size in hyperprolific sows was shown to be associated with a decrease in the mean piglet birth weight and an increase in the within-litter variability of birth weight.

Parity segregation has been used in large production systems as a means to control several diseases in the breeding herd. Gilts and their offspring are kept separated from the sows until they reach their second gestation. By that time, they are expected to have acquired the desired immune status and to pose no risk of increasing the infection level in the herd.

Keeping age groups separate and working in a well-defined sequence is recommended, i.e. first the youngest animals, followed by the older age groups, and thereafter the quarantine and sickbay and finally the cadaver storage.

Materials and equipment are critical in the transmission of a large number of pathogens. Syringes and needles might also serve as transmitters of disease, e.g., for porcine reproductive and respiratory syndrome virus and porcine circovirus type 2.

Hygiene

Implementing a correct procedure of cleaning, disinfection, and empty period in the compartments/pens will reduce the infection pressure. A lack of cleaning after batches can be seen as a risk factor for several pathogens. Hygiene testing i.e. testing of bacterial contamination of surfaces, or adenosine triphosphate (ATP) analyses of surfaces can be used to check whether the cleaning and disinfection were performed properly.

Environmental control

Housing and/or environmental changes that optimize the climate of the pigs' environment are important in the control of infections, as they allow piglets to contain the progress of the pathogen with their own immune system.

Special attention should be paid to several essential factors:

• temperature set points
• fan staging
• air inlet and curtain settings
• sensor placement
• heater capacity
• drafts
• building maintenance

A warm, dry, draft-free environment should be the target, avoiding temperature fluctuations and sudden causes of environmental stress. The ambient temperature for recently weaned pigs should be approximately 28°C. Making environmental changes for improving the climate in inappropriate or old barns however might entail extensive remodeling, and therefore, they may be difficult and expensive to implement. Worn slats are more difficult to clean and disinfect, and constitute a risk for enteric pathogens.

Nutrition

Properly formulated diets, i.e. low protein, good quality ingredients, milk products or feed additives (including pre- and probiotics, essential oils, and acidifiers), can enhance intestinal health and thus help with preventing post-weaning infections. Investment in the development of in-feed additives to reduce the incidence of post-weaning diarrhea has been extensive in the last decades.

Acidifiers are frequently used because of their ability to create a favorable intestinal environment for beneficial microbes, resulting in increased nutrient digestibility, higher growth, and reduced diarrhea. They can be used as organic or inorganic salts, salts of acids, or blends and are successful additives to control E coli-related diseases.

Recently weaned piglets have more problems lowering the pH in the stomach, which is needed because the stomach is a buffer against pathogens. If the pH remains too high for too long, it increases the risk for disease. Try to minimize nutrients that have a buffering capacity, such as proteins or minerals.

Infections produce free radicals, which are toxic substances that can damage the intestinal tissue. It is important to have sufficient antioxidant activity. Therefore, the feed should contain sufficient vitamins, like A, E, and C, and selenium as they have antioxidant activity. Omega 3 fatty acids in weaning diets have been shown to be associated with fewer E. coli infections and more feed intake. Damaging substances like lectins should be avoided.

Prebiotics in diets fed to pigs have been related to increased fermentability. The subsequent synthesis of short-chain fatty acids results in lowered intestinal pH and reduced protein fermentation in the intestinal tract. The addition of probiotics (e.g. Bacillus, lactic acid-producing bacteria, and yeast) to pig diets may improve gut health by modifying the microbiota, which may help control pathogens and improve health status. However, further research is needed to assess and improve the efficacy of pre- and probiotics.

Zinc oxide has been shown to be effective in controlling post-weaning E. coli infections, although the mechanisms of action are not completely clear. However, its use is banned in the EU (June 2022) because of environmental concerns and because it has been associated with selection for zinc resistance, other heavy metals like copper, and also methicillin resistance in S. aureus.

Apart from the feed composition, also other factors like particle size and feed intake play a role. Fine grinding increases feed interaction with pig's digestive enzymes, improves nutrients absorption, and enhances pig performance. However, grinding too fine is expensive, may cause stomach ulcers in pigs, and the time and energy inputs may outweigh any improvement in feed efficiency.

Providing more light during the first days after weaning may increase feed intake. However, increasing light intensity or photoperiod may increase the risk for aggression or biting, resulting in skin injuries.

Feed intake post-weaning is also dependent on feed intake prior to weaning. Therefore, strategies that increase feed intake of piglets prior to weaning are generally beneficial to alleviate problems post-weaning. More research is needed in the area of nutrition management, including effects of co-mingling systems for piglets, group lactations, or systems for supplementing milk products to piglets.

There should also be a sufficient number of feeding places, and they should easily be accessible by all piglets in the pen. The same applies to the drinking nipples. The flow of the nipples should be sufficient (0.5-1.0 liter per minute) and should also be checked regularly. Quality drinking water is also important. On many pig farms, the quality of the drinking water is not optimal, with deviations in chemical composition and/or microbial contamination.

Finally, feed is the most important cost factor in pig production. Optimal feeding goes beyond merely providing the nutrients to optimize the animal's growth. Feeding plays a role in gastrointestinal diseases, but it also affects body condition, lameness and immunity.

Proper nutrition may increase resilience and prevent disease. While optimal performance has economic and ecological benefits and should be the main goal on farms with optimal health conditions, most farms need to balance trade-offs between feeding for optimal performance and minimizing the risk of health problems. This is particularly the case for recently weaned piglets. In farms with good health status, feeding for optimal performance may be the logical strategic choice. Under more challenging health conditions, safer diets may be formulated with the trade-off of resulting (temporarily) in suboptimal performance.