Stomach pH in mature animals is controlled by secretion of hydrochloric acid from gastric mucosa. Hydrochloric acid is a powerful inorganic acid required to initiate protein digestion. It does so by activating the precursor enzyme pepsinogen to its active form, pepsin, the major gastric protease. Mature animals have a relatively low gastric pH (2 to 3, very acidic), which is required for the digestion of plant-derived proteins. An acidic environment also results in the elimination of most pathogens that enter the digestive system through feed and water. Thus, low pH is a prerequisite for efficient feed digestion and good gut health.

Neonatal pigs have a rather high gastric pH (5 to 6) facilitated by the strong buffering capacity of colostrum. This might appear contradictory to the above, but there is a reason. A more tolerant gastric pH allows passage of ingested environmental bacteria (not all of them are pathogens, after all) from the stomach to the small and large intestines for the establishment of normal gastrointestinal microflora. This is considered essential and even beneficial for the animal’s long-term health. Usually, the predominant beneficial bacteria in the stomach are lacto- and bifido-bacteria, whereas in the intestines there is a mix of bacteria. Nevertheless, after the first few hours of suckling, gastric pH drops to about 4 to remain there until weaning, and in most cases, during the first three to four weeks post-weaning. Afterwards, gastric pH drops gradually until it reaches mature levels (2 to 3).

The benefits of early high pH

A moderate gastric pH of about 4 in suckling pigs is favorable for the activation of chymosin (rennin), the enzyme responsible for milk-clotting in the stomach. Without the action of chymosin, milk would pass quickly and largely undigested in the small intestine where it would serve as substrate to pathogens. Although pepsin, also a protease, can also clot milk, albeit at reduced efficacy than chymosin, the latter (chymosin) has weaker proteolytic activity. This might again appear counterproductive, but it is beneficial as it protects the very important milk immunoglobulins from digestion.


Acidity and alkalinity are a natural phenomenon encountered in everyday life, affecting all living organisms. | Alain Lacroix,

Having a moderate gastric pH also benefits proliferation of lactobacteria while excluding other pathogenic organisms. A healthy population of lactobacteria produces copious quantities of lactic acid that stabilize gastric pH. However, this results in the depression of hydrochloric acid. In other words, in the presence of lactic acid, there is no incentive for the secretion of another acid. This is one of the reasons why many post-weaning diets are fortified with lactic acid, among many other organic acids. Thus, in brief, it is apparent that suckling piglets exhibit very limited capacity for secretion of hydrochloric acid, which is not strongly stimulated by sow’s milk. This is erroneously considered as a negative aspect, but in contrast it is important for the survival of piglets receiving sows’ milk.

Weaning upsets pH balance

At weaning, usually too early for piglets to consume enough creep feed, gastric pH remains relatively high. A lower pH is required for the efficient digestion of plant- and animal-derived proteins (other than milk) found in most post-weaning diets. This is because the activity of pepsin peaks at very low pH levels about 2 to 3.5. Some resistant proteins (mostly of plant origin) are digested only at the lower optimum pH, whereas no significant digestion of any protein source occurs above pH 4. Provision of dairy products (mimicking the sow’s milk diet pre-weaning) in post-weaning diets has been shown to reduce gastric pH in weaned pigs, aiding digestion.


Protein indigestion in the young pig not only reduces efficiency of feed utilization but it also gives rise to intestinal microflora populations that thrive on protein.

Protein indigestion in the young pig not only reduces efficiency of feed utilization but it also gives rise to intestinal microflora populations that thrive on protein, especially to opportunistic pathogens such as Escherichia coli. This situation usually ends with the development of diarrheas, and occasionally death, unless pigs are treated with antimicrobial agents (antibiotics, zinc oxide, copper sulfate, organic acids, phytogenics, etc.) that control intestinal pathogens. Thus, in many cases, enhancing protein digestion eliminates the need for expensive medications and prevents the outbreak of costly diseases. In most commercial farms, however, feed digestibility is ignored when diarrhea problems are encountered.

How to prevent protein indigestion

To prevent protein indigestion in newly weaned pigs, several measures are proposed.

First, low-protein diets composed of highly-digestible protein sources are strongly recommended for the first few days post-weaning. To this end, ingredients like refined vegetable proteins (from soya, wheat, peas, etc.), animal-derived ingredients (blood products, egg protein, fish meal) or dairy products are of exceptional value in diet formulation.

Second, if ad libitum feeding (the normal method of feeding weaned pigs) results in overloading of the immature digestive system, then restricted feeding may be employed for a limited period of time (usually, two to five days post-weaning is enough). Prolonging this period only results in reduced performance.

Third, addition of lactic acid or any other acids in post-weaning diets has been shown repeatedly to improve animal performance, mostly due to their antimicrobial properties and less due to any lowering of gastric pH. Here, the best acid to use is less important than the amount of the acid used. However, after establishing the correct amount of organic acids to use, picking the correct blend to match the protein profile of the diet remains crucial. Original research on organic acids calls for levels up to 20 kg per metric ton of feed, but 10 kg are more reasonable. This is because some, but not all, organic acids can be corrosive and even depress feed intake at such high levels. For cost and marketing considerations, however, modern practice calls for 1 to 2 kg of acids per metric ton. At this level, there is virtually no possibility of having any effect at gut level, unless of course, the feed or the animal environment is extremely contaminated with pathogens. In addition, protein digestion is unlikely to be affected by such insignificant additions of organic acids.

Summing up

In brief, gastric pH is correctly set by natural development at moderate levels during the suckling period. At weaning, however, dietary measures must be taken to ensure the feed’s protein and organic acid profile match so that gastric pH is reduced rapidly to levels that ensure maximal protein digestion and gut health.