Basic Overview of Protein in Pig Nutrition

(1) Carbohydrates

Carbohydrates are the most widely available and the largest proportion of nutrients in the diet, and are the main energy source for pigs. In cereal grain feeds, there is very little soluble grass sugar and disaccharide, mainly starch, so it is the main energy source for pigs. Starch is broken down by amylase in the digestive tract into glucose, which is then absorbed into the blood as blood sugar. It provides energy through biological oxidation in the body. The digestibility of soluble sugar and starch by livestock is 95%-100%. For piglets before 2-3 weeks of age, due to insufficient pancreatic amylase and intestinal disaccharidase secreted in the digestive tract, piglets fed large amounts of starch feed grow poorly. Before 7 days of age, piglets fed glucose and lactose can utilize them effectively. After 7-10 days of age, they can utilize fructose and sucrose, but young pigs after feeding generally suffer from severe diarrhea, weight loss and high mortality. Crude fiber, after treatment with a certain concentration of acid and alkali, the remaining residue is called crude fiber. This determination method is incorrect. Generally, crude fiber in feed contains cellulose, hemicellulose and lignin, and their composition ratio is unstable. In typical feed crude fiber, the contents of cellulose, hemicellulose and lignin are 50%-80%, 20% and 10%-50% respectively. Cellulose and hemicellulose are polysaccharides. Cellulose is a glucose polymer, and hemicellulose is composed of hybrid sugars (glucose, fructose, mannose, galactose, arabinose, xylose, rhamnose, uronic acid). Lignin is a group of complex and difficult-to-digest substances - incomplete polymers of phenylpropane derivatives. These substances are the main structural components of plant cell walls. In the Van Soest analysis method, neutral detergent fiber (NDF) estimates the total plant cell wall components mainly composed of cellulose, hemicellulose and lignin. Acid detergent fiber (ADF) includes cellulose and lignin. The difference between NDF and ADF is the hemicellulose in the feed. The pig small intestine has no enzymes to digest crude fiber and cannot digest cellulose, hemicellulose and hemicellulose, but crude fiber enters the large intestine and undergoes microbial fermentation. The main end product of its digestion is volatile fatty acids, which supply about 5%-28% of the maintenance energy requirement. The digestibility of crude fiber is generally 30%-39%. Its digestibility is affected by fiber source, degree of lignification, content in the diet and degree of processing, so it varies greatly. The utilization of fiber is affected by the physical and chemical composition of the diet, the nutritional level of the diet, and the age of the animal. The digestibility of crude fiber varies from 0 to 97%. There is still no conclusion on the fiber level in diets for growing-finishing pigs. It is generally believed that for growing pigs around 20 kg body weight, the dietary crude fiber level is 6%. Some also believe that the level of neutral detergent fiber with low lignin should be less than or equal to 15%. Some experts believe that the crude fiber level in sow diets is 10%-12%. If the crude fiber level in the diet is too high, it reduces the digestibility of feed organic matter and energy. Increasing crude fiber in the diet by 1% reduces total energy digestibility by about 3.5%. It has also been reported to affect protein digestibility. In diets in the late finishing period, a higher level of crude fiber can be used to limit feed intake, reduce body fat deposition, and improve carcass quality. For every 1 percentage point increase in crude fiber content in the diet, backfat thickness is reduced by about 0.5 mm.

(2) Lipids

Lipids include fats and oils. Feeds generally contain about 5% fat. Fat has high heat energy, 2.25 times the caloric value of carbohydrates. Pigs deposit large amounts of fat in the body, mainly through body tissue synthesis of fatty acids. The main raw material for synthesizing fatty acids is acetyl-CoA, which mainly comes from glucose, the decomposition product of carbohydrates in feed. Fat and certain amino acids can also produce acetyl-CoA. Triglycerides are synthesized from acetyl-CoA. However, pigs cannot synthesize certain fatty acids, which must be supplied by the diet or formed through specific precursors in the body. Fatty acids that have important protective effects on normal body function and health are called essential fatty acids (EFA). Essential fatty acids include linoleic acid (octadecadienoic acid), linolenic acid (octadecatrienoic acid) and arachidonic acid (eicosatetraenoic acid). Linoleic acid must be supplied in the diet. The latter essential fatty acids can be supplied directly by the diet, or synthesized through internal molecular conversion in the body by supplying sufficient linoleic acid. The mechanism of essential fatty acids in the body is still unclear. The symptoms of essential fatty acid deficiency are skin damage, keratinized scales, fragile capillaries, decreased immunity, and stunted growth. Young and rapidly growing animals are more sensitive. Pigs can obtain the required essential fatty acids from feed. Common feeds are relatively rich in essential fatty acids, so deficiency generally does not occur. NRC stipulates that the linoleic acid requirement for pigs accounts for 0.1% of the diet. ARC stipulates that for pigs of 30 kg and 30-90 kg body weight, the essential fatty acid requirements are 3% and 1.5% of dietary digestible energy, respectively. It has been reported that for pigs from weaning at 5 weeks to 90 or 100 kg body weight, the linoleic acid requirement for maximum growth and optimal feed utilization is 0.2% of total energy or 0.1% of the diet.

Protein and Amino Acids Protein is an important component of cells. It is involved in most life-critical chemical reactions in animal metabolism and plays an important role in life processes. Protein is the structural component of the body, the main raw material for tissue renewal and repair, and also a component of enzymes, hormones, antibodies and immune substances. Protein can be converted in the body to supply energy or to sugar and fat. Protein is a general term for a large class of nutrients composed of amino acids. The animal's need for protein is essentially a need for amino acids. The physiological functions of proteins in organs, body fluids and other tissues are completely consistent. The main differences are caused by differences in the amino acids that make up the protein, their quantity and the way they are combined. Animal protein is different from plant protein, but the protein needed by animals comes from plant protein. About 30%-70% of the protein needed by animals is supplied by protein in cereal grain feeds, and the rest must be met by plant oilseed meal feeds. Protein in feed generally refers to crude protein, that is, the nitrogen content in feed multiplied by 6.25 is crude protein. Its definition is mainly based on the assumption that 100 g of protein contains 16 g of nitrogen.

(3) Essential Amino Acids and Non-Essential Amino Acids

There are about more than 20 kinds of amino acids that make up animal body proteins. Some are synthesized in the animal body, while others cannot be synthesized and must be supplied by feed. Therefore, amino acids are divided into essential amino acids and non-essential amino acids. Amino acids that cannot be synthesized in the body or whose synthesized amount cannot meet the physiological needs of pigs and must be supplied by feed are called essential amino acids. Research has shown that growing pigs require 10 essential amino acids (lysine, methionine, tryptophan, histidine, isoleucine, leucine, phenylalanine, valine, threonine and arginine). Growing pigs can synthesize 60%-75% of the body's requirement and arginine. Adult pigs can synthesize enough arginine for their needs. For pigs, 50% of the methionine requirement can be replaced by cystine, and 30% of the phenylalanine requirement can be replaced by glutamic acid. Therefore, cystine and phenylalanine are called semi-essential amino acids. However, note that cystine and phenylalanine cannot be converted into methionine and glutamic acid. Non-essential amino acids are not unnecessary in pig nutrition. Rather, they are amino acids that can be synthesized in the pig body from other amino acids or nitrogen sources to meet the body's needs. It can be seen that the ability to provide sufficient essential amino acids and non-protein nitrogen to synthesize non-essential amino acids in the feed determines the appropriate level of feed protein. Therefore, the actual protein requirement of pigs is the pig's requirement for essential amino acids and nitrogen sources for synthesizing non-essential amino acids.

The nutritional value of feed protein mainly depends on the composition and content of essential amino acids in the feed. The closer the content of essential amino acids and the ratio of each amino acid in the feed are to the pig's essential amino acid requirements, the higher the nutritional value of the feed protein. The quality of feed protein from different feed sources varies. If one or some amino acids in feed protein are insufficient, it will limit the utilization of other amino acids. This amino acid is called a limiting amino acid. In a certain feed or a certain diet, the amino acid whose ratio to the pig's required amino acid is the smallest is the first limiting amino acid, the next slightly larger one is the second limiting amino acid, and so on. Common limiting amino acids in pig feed include lysine, methionine, tryptophan, threonine and isoleucine. The first limiting amino acid in pig diets is often lysine. In the diet, the combined use of several protein feeds can exert a protein complementary effect and improve feed protein utilization or the biological value of protein. Adding synthetic amino acids can improve the biological value of feed protein. In diets based on plant protein sources, the amino acid generally likely to be deficient is lysine, so lysine is often added to pig diets.