The first two months of a piglet’s life is decisive for the performance during the rest of its existence. During that time it has to develop a digestive system which initially is made to digest milk to a gut that can deal with grain based nutrients. Understanding this transformation process and providing the right food supports the development of a healthy pig.
By Dr Alejandro Ramirez, DVM, MPH, Diplomate ACVPM, Iowa State University, College of Veterinary Medicine, USA
Pigs are monogastric animals, which mean they have one stomach (compared to four in cattle). They are omnivores, indicating they are anatomically designed to handle food from both animal and plant origin. Their digestive system is very similar to that of humans. Digestion is defined as the breaking down of large molecules into smaller components. In animals the digestive process involves both mechanical (chewing, stomach and intestinal contractions) and chemical (acids, enzymes, etc.) steps each with specific purposes in breaking down feed into substances which can be used by the body (water, protein, fats, carbohydrates, etc.). Although the primary function of the digestive system is to assimilate nutrients for energy and growth, it is also a critical part of the immune system. The digestive and immune functions of the pig’s intestines can be greatly affected by nutrition and management. The focus of this article is to emphasise the small intestine changes occurring in the piglet from birth through weaning in relation to both nutrition and animal health.
The digestive process
A very brief and generalised description of the digestive process of monogatrics is as follows: The digestive process starts with the feed entering the mouth. It then travels to the stomach where the gastric acids and saliva start breaking down the feed which then travels to the small intestine. The small intestine is then divided into three general sections starting with the duodenum, followed by the jejunum, and finally the ileum. The main functions of the small intestine are to further digest feed and absorbed necessary nutrients. The digestive process of the small intestine utilises enzymes and other compounds produced by intestinal cells, as well as those produced by other organs such as the pancreas and liver.
The small intestinal content is very liquid which helps ensure proper mixing of content for a more effective digestion process. The intestinal content then travels to the large intestine where fermentation of fibres (primarily via good bacteria) occurs especially in the cecum (called appendix in humans). It is here in the large intestine where the majority of the water is absorbed along with other nutrients. As ingesta travels from the small intestine to the rectum, it becomes less liquid and more formed. Diarrhoea is defined as an increased in frequency and/or volume of manure. In pigs, it is difficult to identify an increased frequency of bowel movements so we almost always are more focused on identifying loose stools (increased volume due to increased liquid content of manure).
The small intestine of a newborn pig is about 2 to 4 m long and will grow to 16 to 21 m by the time they reach full maturity. In the first 10 days of life, pig’s intestines are said to increase 80% in length and 30% in diameter thus doubling their absorptive capabilities. Intestines are not a hollow tube but rather have cells forming finger-like projections called villi. These villi can increase the surface of the intestines by 30 fold. Cells are produced at the base of the villi (crypt) and migrate to the top as they mature and are sloughed off at the tips. Through the maturation process, cells gain the ability to produce digestive enzymes and later increase their absorptive capabilities through microvilli (small villi on the surface of the cell). In the pig’s intestine, intestinal epithelial cells are replaced every three to four days. The villi and microvilli together are reported to amplify the effective surface area of the intestines by 420 to 1,200 fold! Intestinal insults from pathogens (bacteria, viruses), toxins, or any other irritating products (including some feedstuffs), can damage intestinal cells causing significant losses in digestive and absorptive capabilities of the intestine.
The intestine also serves as a major defense mechanism to help protect pigs. Pigs are immunologically competent (have all necessary parts) at birth (actually since day 70 of gestation) but their immune system is not fully mature and efficient at this time. One of the main functions of the intestinal cells is to protect pigs from pathogens (bad bugs) and compounds (toxins, foreign proteins, etc.) that enter the digestive tract. This is done primarily by having intestinal cells tightly joined (tight junctions) to each other forming a ‘skin’ layer throughout the digestive tract. These tight junctions make sure no large molecules enter the body while the cells use their absorptive capabilities to allow nutrients in. When pigs are born, these tight junctions are quite loose for the first few days of life allowing antibodies, cells, and other immunological compounds in colostrum to be absorbed whole. This is a critical part of the piglet’s immune system which will be discussed later.
Other defense mechanisms
Besides tight junctions, the gastrointestinal system has many other mechanisms that help protect the pig from pathogens. Gastic acids lower the pH of intestinal content significantly. This low pH kills many pathogens. Gut motility also plays an important role. Keeping intestinal content moving minimises the time pathogens and toxins have to interact with intestinal cells. Chilling of pigs decreases gut motility which makes them more susceptible to enteric diseases. Bile acids (compounds produced by the liver to help digestion) also have strong antibacterial properties. Pigs also produce secretory antibodies (secretory IgM and IgA) which help bind to the surface of pathogens and/or toxins thereby preventing them from adhering or being absorbed by intestinal cells.
Pigs are not able to obtain antibodies from their mothers while in utero due to the placental characteristics and therefore need passive antibodies through colostrum. Researchers have estimated that piglets need about 250 ml of colostrum to survive. This amount is not only for the antibodies (IgG) and other immunological cells and compounds but also for its nutritional value (especially energy sources) found in colostrum.
As mentioned previously, piglets are born with loose intestinal tight junctions to allow for the absorption of IgG and other immune components in colostrum. Although many publications emphasise the importance of allowing piglets to obtain colostrum within the first 24-36 hours after birth before gut closure occurs, getting colostrum within the first six hours of life is critical as absorptive capabilities dramatically decrease thereafter.
Colostrum also plays an important role in eliciting dramatic changes in intestinal growth, structure and function of newborn pigs during the first six hours of suckling. This is highly related to the amount of colostrum ingested and can result in approximately 100- fold increase in absorptive area in the intestines. It should be the goal of all farrowing house personnel to maximise piglet immunity and intestinal function by maximising the opportunity for piglets to have access to good amounts of high quality colostrum as soon as possible after birth. This requires not only that the mothers produce the colostrum, but that the right husbandry skills are used to enable this process as the future success of piglets, including the post-weaning period, is impacted by what happens early on in the farrowing house. Colostrum contains large concentrations of IgG, while regular milk contains mostly IgA type anti-bodies. IgA antibodies are not absorbed by the gut but rather stay in the intestinal lumen acting against pathogens and their toxins.
The fat content of colostrum is also critical as it is a major energy source for newborn pigs. Pigs need this energy for survival and growth including keeping themselves warm (thermoregulation) as well as for their immune system. The production of antibodies does require significant energy consumption. Minimising pathogen exposure throughout the pig’s entire life (birth to market) minimises energy and maximises pig growth.
Weaning and post-weaning
The weaning and early post-weaning periods are probably some of the most challenging periods for the pig’s intestinal system. Not only is the pig making a dramatic change in food source (moving from a milk diet to a grain one), the intestines also undergo significant changes. At this same time intestinal cells are undergoing changes in enzyme productions which are needed to properly digest these new feedstuffs. Immunologically the intestines are being exposed to many different new proteins. These different proteins can trigger immune reactions throughout the intestines. It is important for the pig’s intestine to not over react immunologically otherwise inflammation will occur resulting in negative impacts in the digestive and absorptive capabilities of the gut. A good example of this is seen when large amounts of soybean protein are introduced into a pigs diet in the first few weeks post weaning. The piglet’s digestive system does not have the necessary enzymes to break down the soybean protein and therefore an allergic reaction can occur (soybean meal hypersensitivity). There are some genetic variations in the age and degree of piglet sensitivity to different proteins. For the most part, foreign proteins are what trigger immune responses in animals.
Weaning also causes a change in piglet intestinal villi height as well as disrupting gut integrity causing a temporary increase in gut permeability. Approximately 50% of the villi height is lost in the first five days post weaning. It will take approximately two weeks for intestinal villi to return to their previous height. Remember that villi height is associated with intestinal digestive and absorptive capabilities. Low feed intake and starvation have also been shown to significantly affect villi height. The inclusion of zinc oxide, plasma proteins, and dried whey has been shown to minimise the severity of intestinal villi losses in the immediate post weaning period.
Stress from weaning causes a decrease in gut pH which then causes an increase in gut permeability (affect tight junctions). Gut permeability appears to be highest immediately after pig transport and continues for two to three hours after pigs start resting. Several researchers have suggested that this increase in gut permeability is a major factor contributing to piglet enteric problems associated with shipping.
Maturation of the gut
It is really the first two months of life (farrowing house and early weaning period) that are nutritionally the most challenging. After this time, the intestinal tract continues to grow and increase its surface. The gastrointestinal tract of a pig in utero is sterile.
Through the birthing process as well as within the first few days of life, the piglet is exposed to many different microorganisms (some good, some bad) primarily from sow faeces. These microorganisms will become part of the animal’s gut flora and contribute to the digestive process.
As pigs mature, they can digest more complex feedstuffs. These more complex feedstuffs many times also rely on the fermentation capabilities of the gut flora in the large intestine. The composition of this intestinal microflora is then dependent on, type of diet (especially in regards to fibre), environmental exposure, and antimicrobial usage. The ultimate goal is to minimise the proliferation of pathogens in the intestines while maintaining the correct mixture of ‘nonpathogenic’ organisms to maximise digestive effectiveness. Any chronic irritation to the intestinal system will result in thickening of the digestive tract, decreasing its absorptive capabilities.