Digestive Characteristics of Pigs and Nutrient Absorption Explained
I. Digestive Characteristics of Pigs
Pigs are monogastric omnivores; in the wild, they are constantly searching for and consuming various foods. In modern farming production systems, pigs can be domesticated to eat at set meals; or when feed is continuously supplied for free-choice feeding, pigs will also eat frequently during the day. Although pigs generally eat during the day, under certain circumstances such as hot weather, they may also shift to eating at night. There is currently no evidence that the time of daily feeding affects the efficiency of nutrient utilization in pigs.
The teeth in the pig's oral cavity chew feed rather thoroughly, and the length of chewing time is related to the softness of the feed and the age of the animal. Generally, coarse and hard feed requires longer chewing time, and as age increases, chewing time shortens accordingly. In production, pig feed should be appropriately ground to reduce the energy expenditure of chewing, while also helping enzymatic digestion in the stomach and intestines. The crude fiber in pig diets is mainly digested through microbial fermentation in the large intestine and cecum, and the digestive capacity is relatively weak.
II. Absorption of Nutrients in Pigs
(1) There are three ways for the absorption mechanism of digestible nutrients in pigs
1. Pinocytotic absorption: Pinocytotic absorption is when a cell extends pseudopodia or the membrane at the point of contact with a substance invaginates, thereby enclosing these substances into the cell; substances absorbed in this way can be in molecular form, or in the form of clumps or aggregates; the absorption of immunoglobulins from colostrum by newborn mammals is pinocytotic absorption, which is of great significance for newborn animals to acquire antibodies.
2. Passive absorption: Passive absorption is through several forms such as filtration, osmosis, simple diffusion, and facilitated diffusion (requiring carriers), absorbing digested nutrients into the blood and lymphatic system; this form of absorption does not require the body to expend energy; the absorption of some low-molecular-weight substances, such as simple polypeptides, various ions, electrolytes, and water, is passive absorption.
3. Active absorption: Active absorption is the opposite of passive absorption; it must consume energy by the body and is a form of substance transport against the electrochemical gradient accomplished by relying on "mercury proteins" in the cell wall; this form of absorption is the main way higher animals absorb nutrients.
(2) Utilization rate of amino acids in feed ingredients
In most natural ingredients, amino acids cannot be completely digested, absorbed, and utilized. Therefore, an important point in properly formulating diets is to understand the degree of absorption and utilization of amino acids in commonly used ingredients by pigs. Two terms need to be explained here: digestibility and utilization rate. Digestibility refers to the amount of amino acids from feed absorbed in the intestinal tract; utilization rate refers to the proportion of amino acids in feed actually used for growth. Although the concepts are simple, actually measuring these two values is very complex.
Most nutritionists tend to adopt digestibility values as a reasonable indicator of utilization rate.