Many breakthroughs in swine nutrition research lead to improved digestion, and at the same time, eliminate or suppress the many bacterial and viral pathogens that cause enteric infections in pigs.
Increasing nutrient digestibility and maximizing feed intake are common ways promoting efficient gain, while limiting the impact of swine enteric disease in nursery and growing pigs. The results of these efforts at Kansas State University and elsewhere reflect the need for further research to unlock some of the remaining mysteries surrounding pig enteric health challenges.
After digestion in the stomach, further digestion and a large portion of the absorption of fat, protein, carbohydrates, vitamins and minerals occur in the small intestine. In the large intestine, there is very little direct nutrient absorption except water and electrolytes.
However, there is a large bacterial population that ferments undigested carbohydrate, fat, and protein into short chain fatty acids. Some of these fatty acids are then absorbed and utilized as an energy source. Undigested carbohydrate consisting of plant cell wall remnants, bacterial particles and other unabsorbed nutrients are then voided as fecal matter. This undigested carbohydrate is commonly termed as non-starch polysaccharides or “dietary fiber.”
Consultants commonly recommend the addition of dietary fiber to swine diets to aid in controlling swine enteric disease. This includes using oat products in nursery diets, and wheat bran, soy hulls or dried distillers grains to control porcine proliferative enteritis (ileitis) and colitis.
It appears that there are two primary drivers of these recommendations. The first is European research from the 1960s and 1970s evaluating low-crude protein and high-insoluble fiber diets for the prevention of death loss associated with postweaning Escherichia colibacillosis (E. coli).
Unfortunately, these diets were only effective after feeding a high enough proportion of the low-crude protein and high-fiber diet to severely restrict growth rate. Intermediate levels of crude protein and fiber were not successful in preventing E. coli-associated death loss.
Still, this research is the common basis for adding fiber to the diet and lowering nutrient content.
Since the studies were conducted, there have been many major developments in diet formulation for nursery pigs. These include the use of highly digestible lactose sources and animal proteins and the management of the negative effects of the immune response to soybean protein antigens.
Additionally, the development of segregated batch pig production and implementation of improved hygiene procedures have led to better nursery pig performance and disease control.
Recommendations to add fiber to pig diets to control enteric disease are also based on widely documented benefits of including dietary fiber in human diets. These benefits include the role fiber plays in altering the microbial population to help stimulate growth of colon cells and lymphoid tissue.
However, the average adult human intake of dietary fiber is 15 to 20 g./day, compared to a recommended intake of 30 to 35 g./day.
In contrast, a finishing pig eating a corn-soy diet is consuming approximately 250 g./day of dietary fiber. Simple corn-soybean meal-based diets should contain more than adequate amounts of fiber to stimulate growth of the enteric cells. Thus, documentation from human literature should be applied to the pig with caution.
Nonetheless, addition of fiber to swine diets continues to be a widely promoted practice to control swine enteric disease.
Fiber is defined as the plant cell wall components that are left over after enzymes break down starch, fat and protein. Thus, there is a direct, negative correlation between dietary fiber content and energy value. This correlation indicates that as fiber content increases, the ability of the pig to utilize the energy of the feedstuff decreases (Figure 1). Fiber also produces negative effects on amino acid and fat digestibility.
Recent research suggests that increasing the digestibility of the diet by reducing the amount of soluble fiber also reduces the proliferation of infectious E. coli organisms. These fiber sources increase the viscosity of the digestive system, seemingly providing a microenvironment in the small intestine to allow for the proliferation of the E. coli.
Spirochete colitis associated with Brachyspira hyodysentariae causing swine dysentery, and Brachyspira pilosicoli causing porcine colonic spirochetosis, are known to trigger diarrhea in growing pigs. Environmental management and the use of antimicrobials have been the primary means of disease control.
But research efforts suggest dietary changes can greatly influence clinical disease associated with these spirochete (slender, undulating bacteria) organisms.
Swine dysentery challenge studies completed in Australia, utilizing highly digestible cooked rice and animal protein diets, have demonstrated a significant reduction in colonization and clinical disease. Highly digestible feedstuffs minimize readily fermentable material entering the colon.
Conversely, pigs consuming diets with higher levels of non-starch polysaccharides (hemi-cellulose, pectins, lignins) and resistant starches showed greater clinical signs of swine dysentery. These diets resulted in a greater amount of fermentable material entering the large intestine. The spiral colons of pigs on the highly digestible diets also tended to have a reduced hindgut (back part of the alimentary canal which extends from the mouth to the anus) organ mass and lumen contents. Finally, the large intestine and fecal material contained less water in the pigs fed highly digestible diets.
In a colonic spirochetosis challenge study, which evaluates the influence of diet on clinical disease, similar findings were demonstrated. This study compared the effect of feeding diets with varying degrees of digestibility (corn-animal protein, corn-soybean, and wheat-barley-soybean diets) to weaned pigs challenged with porcine colonic spirochetosis. The corn-soybean diet was designed to cause a delayed-type hypersensitivity reaction in the small intestine, leading to temporarily poor absorption and an increase in fermentable material into the colon. The impact on hindgut pH and volatile fatty acid production was similar to the previous studies.
The clinical disease and extent of severity of the lesions were reduced in pigs fed the more highly digestible, corn-animal protein diets. While there were no significant differences in growth rate, this experiment illustrates that dietary composition definitely has an effect on factors in the microenvironment of the colon that are conducive to disease promotion.
These evaluations indicate that the volume of rapidly fermentable material entering the hindgut leads to elevated volatile fatty acid production and a lower pH, which seems to promote spirochete growth.
These findings may indicate that other factors leading to incomplete, small intestinal digestion likely enhance the susceptibility to spirochete-associated colitis. Predisposing factors such as bacterial or viral enteritis, stress, dietary transitions or high-fiber diets may lead to an increased level of fermentable material entering the hindgut, which enhances the pig's susceptibility to spirochete colitis. It is not known if the altered hindgut environment has a direct effect on these two spirochete pathogens.
Feed has long been implicated as a possible source of salmonella contamination. Therefore, heat treatment such as pelleting has been advocated for reducing feed-related salmonella infection. Reduction in salmonella infection has been associated with feeding pelleted rations in poultry even though salmonella was not detected in raw ingredients.
However, swine survey information points to pelleted feeds as posing a significant risk for salmonella. It is not known if these results are due to delivery of contaminated feed or an effect of the pellet diet on factors that promote salmonella growth in the intestine. Current research at Kansas State University seems to indicate that in corn-based diets, there is little impact of pellet or grain particle size on salmonella shedding.
Recent research has indicated that a diet with high carbohydrate digestibility decreases strongyloid nematode parasite establishment, size and female ability to reproduce. In contrast, a diet high in non-starch polysaccharides provides favorable conditions for parasite establishment and sustainability.
This work is further supported by an epidemiological study indicating that in herds positive for ascarid roundworms and trichinae eggs, much higher levels of insoluble, non-starch polysaccharides and total, non-starch polysaccharides were being fed.
The source and percentage of soybean protein in diets for early weaned pigs have been controversial subjects among swine nutritionists. The controversy is due to the implication that soy protein causes a temporary hypersensitivity in the early weaned pig.
Briefly, the hypersensitivity response occurs three to four days after exposure to adequate soy proteins. The temporary hypersensitivity results in digestive abnormalities, including disorders in digestive movement and inflammatory responses in the intestinal mucosa or mucous membrane. Villi (fingerlike projections of the intestine) are sloughed or cast off from the small intestinal mucosa; absorption abilities are reduced and susceptibility to enteric toxins and bacterial infections are increased.
Although the exact mechanisms are not known, these changes are thought to be the result of antigen-antibody complexes that initiate the pig's own immune system to produce cytokines (cells that produce immunity) and complement (proteins that destroy bacteria). The cytokines and complement are thought to directly cause the damage to the intestinal mucosa.
Most importantly, these changes result in reduced growth performance. Recovery occurs after seven to 10 days when oral immune tolerance begins to develop and, eventually, the intestinal mucosa returns to normal with little evidence of long-term damage.
One approach used successfully to reduce the negative effects of the temporary digestive problems is to carefully select high-quality, digestible protein and carbohydrate sources while increasing the amount of soybean meal in each subsequent diet. Exposing young pigs to increasing levels of soybean meal in each diet will allow them to overcome the hypersensitivity to soy protein more quickly, without causing a long-term reduction in performance.
That early exposure permits inclusion of soybean meal at higher levels in future diets without reducing growth performance. This approach has consistently proven more economical than delaying exposure to soybean meal.
A large epidemiological study, designed to assess the risk of several factors related to nursery pig enteric disease, indicated that restricting feed intake was the most important risk factor linked to increased amounts of enteric disease in the nursery (Figure 2).
This data showed that as feed intake increased during the first week postweaning, growth rates improved and diarrhea cases declined.
As shown in Figure 2, farms for which average feed consumption was less than 0.33 lb./day in the first seven days after weaning had 33.6 times greater risk of diarrhea than those farms in which feed consumption was 0.44-0.55 lb./day in the first seven days after weaning. Also, note that farms having an intermediate level of feed consumption (0.33-0.44 lb./day) had 18.6 times greater risk for diarrhea than those consuming the higher amount of feed. Thus, farms that practiced restriction of feed intake in the first week after weaning were more likely to have enteric disease problems and slow nursery growth rates.
The scientific evidence suggests adding fiber or restricting feed intake are contrary to a significant body of science describing the biology and epidemiology of controlling enteric disease.
However, maximizing feed intake does not mean that feeders should be left wide open with excessive amounts of feed in the feed pan. We continually observe decreased growth rate as a result of improper feeder adjustment.
In an attempt to stimulate feeding behavior, large amounts of the first diet are placed in the feeding pan. Although the intention is correct, the outcome is negative. Pigs “sort” the diet, which causes a buildup of fines in the feeding pan. These fines then lodge in the feed agitator mechanism, making it difficult for new feed to flow from the feeder. This problem is remedied by decreasing the amount of feed flow in the pan to stimulate development of feeding behavior.
Approximately 25% to 50% of the bottom of the feeding pan should be visible in the first few days after weaning. As the pigs become more accustomed to the location of the feed and adjust feeding behavior, the amount of feed in the feeding pan should be decreased rapidly to 25% or less coverage. Also, feed agitators need to be tested frequently to ensure that the buildup of fines does not prevent them from working freely.
Use good veterinary diagnostics to determine if a primary pathogen is present. Some 92.9% of farms from one study with a clinical diagnosis of “non-specific colitis” actually had infections with known, enteric pathogens after proper diagnostic investigation was performed.
Evaluate environmental cleaning practices. Poor sanitation practices have been shown to be a significant risk factor for in-creased enteric disease.
Evaluate processing techniques; grain particle size should be in the 600 to 800 micron range, which reduces the amount of undigested material entering the large intestine.
Change to cereal grains with higher starch content and less non-starch polysaccharides. This change includes corn, milo or wheat from barley, oat or by-products.
Change protein source from canola to soybean meal.
Limit use of higher fiber containing feedstuffs such as soy hulls and wheat middlings.