What nutritional factors cause scours in weaned pigs, and how can they be prevented?

When we think about weaning pigs from the sow, the change in diet is profound–from at least 16 meals a day of a warm liquid containing 30% protein, 35% fat and 25% lactose, to a low-fat, low-lactose, lower-protein, high-carbohydrate dry creep feed. This nutritional change is involuntarily added to the pig’s sociological, environmental and immunological stresses at weaning.
Nutritional stress is easily the most important aspect of the post-weaning check, and the pig’s failure to eat is the critical component of this nutritional stress. The change imposed on the weaned pig by removing the dam may deter the pig from eating, but peristalsis continues. Thus after perhaps only 3 hours the stomach, duodenum and the forward part of the small intestine are virtually empty. After 6 hours, the situation is even worse: the pig(s) is now feeling cold and confused and maybe even miserable.
Eventually hunger and the example of penmates drives the pig to eat, probably overeat. The stomach cannot cope with more than 30 ml of solid feed before it fills completely, and the extra quantity eaten by the hungry pig forces food out of the stomach prematurely and into the sensitive absorptive lining of the anterior small intestine. The food is thus insufficiently prepared for digestion in the small intestine. In addition, bacteria and their toxins entering the small intestine may attack the surface of the villi, not having been exposed long enough to the hostile acid conditions of the stomach to be rendered harmless. The villi then become truncated and the absorptive capacity of the gut falls dramatically. The food, still being pushed through in overload, forms a digestive blockage in which the hostile bacteria breed. The gut lining responds by exuding fluid to remove the congestion, and scouring (diarrhea) results.

Nutritional solutions for this problem can be as follows:
1. Choice of carbohydrates: the stomach provides enzymes to precondition proteins and carbohydrates in particular. Sources of these nutrients should be chosen so that they are as digestible as possible in the intestinal villi. Because feed intake may be limited in early-weaned pigs (5-7 kg), a highly digestible carbohydrate source is advantageous, both to stimulate feed intake and due to t he relatively high net energy value. The high lactase enzyme levels at birth and high digestibility of lactose make crystalline lactose or one of several lactose sources (dried whey, deproteinized whey, whey permeate, etc.) an excellent carbohydrate source for young pigs. As long as the diet contains a basal level of lactose, several other carbohydrate sources can be used for the remainder of the diet with acceptable performance.
2. Choice of proteins: The source and level of soy protein in diets for early-weaned pigs has been a controversial subject among swine nutritionists. Some nutritionists believe soybean meal should not be included in the first diet after weaning to prevent an allergic reaction to the unprocessed soy protein. These nutritionists typically will use a further refined soy protein such as soy protein concentrate, isolated soy protein or extruded soy protein concentrate to replace the soybean meal portion of the diet. If a refined soy protein is used in the diet, several research trials have demonstrated an advantage to the moist extruded soy products compared to soy products that have not been moist extruded.
Other nutritionists take a different approach. They believe that exposing the young pig 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 pig performance.
3. Acidification: the stomach also provides the initial acid conditions that attenuate acid-susceptible bacteria, which comprise many of the most serious enteric pathogens. Solid creep feed and the post-weaning feed therefore needs to be acidified so as to assist the milk protein fractions, which are largely self-acidifying. The weaned pig struggles to maintain sufficient gastric acid up to about 15-20 kg body weight; if pH exceeds 4.2-4.5, protein predigestion is affected. While the early research data on acidified pre-and peri-weaning feeds were variable; the evidence now seems to be that such feeds are indeed cost-effective. Even so, while most creep feeds are acidified, too many post-weaner diets are not. One of the reasons for conflicting early evidence on acidification could have been the addition of acid-binding ingredients that largely neutralize the addition of organic acids.
4. The stomach also has a role to play in polysaccharide predigestion. Again, as with protein, cereals and starch-providing ingredients may need processing to assist digestion. When using a cereal grain as a main carbohydrate source (corn, sorghum, wheat, barley, or oat products), grinding the ingredients finely (<700 microns) is important to improve digestibility.
5. Choice of fats: pre-processed in the duodenum, both added fats and fats found in food raw material also need to be under strict control until the weaned pig’s digestive tract can cope. There may be difficulties in reconciling cost and ease of manufacture with nutritional theory, and the variability in European and North American post-weaning diets may often be due to a choice of fats based on cost. The most preferable fats are of the short-chain PUFA (polyunsaturated fatty acid) variety with small globule sizes.
6. Reduction in natural digestive enzymes: a temporary fall in naturally produced enzymes reduces digestive competence and further worsens digestive problems in the small intestine. Inclusion of synthetic enzymes in addition to an immediate post-weaner feed may be cost-effective, and some commercial diets already contain them.
7. Antigenicity: As discussed in number 2 above, regarding the use of soybean meal in post-weaning diets, there is evidence to suggest that certain raw materials that are suitable and economically desirable for pigs over 20-25 kg can set up a form of allergic reaction in the gut wall of pigs at the creep and weaned pig stage, particularly between 3 and 10 kg. The absorptive cells on the surface of the villi apparently fail to distinguish between such antigenic feed ingredients and hostile bacterial toxins. This antigenicity phenomenon seems to be aggravated by poor weaning conditions (less than favorable housing/environmental conditions for the weaned pig).
You also asked how could you prevent post-weaning diarrhea, without using antibiotics. I’m not sure you can completely prevent this without antibiotic usage, however, growth promotant levels of zinc (zinc oxide at 2,500 to 3,000 ppm) are almost routinely used in the diet of the weaned pig, for 14 days. When zinc oxide is used as a growth promotant, high levels of copper sulfate (250 ppm) should not be used in the diet. Recent research has shown that there is an additive effect when high levels of zinc are added to the post-weaning diet (2 weeks) in combination with an antibiotic.