Threonine was isolated and identified by W.C. Rose in 1935 from fibrin hydrolysate, and has now been shown to be the last essential amino acid to be discovered, it is the second or third limiting amino acid in livestock and poultry, and it has extremely important physiological roles in animals. Such as promoting growth, improving immune function, etc.; Balance the amino acids in the diet, so that the amino acid ratio is closer to the ideal protein, so as to reduce the requirements of livestock and poultry for the protein content in the feed. Lack of threonine can lead to symptoms such as reduced feed intake, stunted growth, reduced feed utilization, and suppressed immune function. In recent years, lysine and methionine synthetics have been widely used in feed, and threonine has gradually become a limiting factor affecting animal performance.

1. Physical and chemical properties of threonine

The chemical name of threonine is α-amino-β-hydroxybutyric acid, which is named threonine because its structure is similar to threonate, and it is the last essential amino acid discovered. Its molecular formula is C4H9NO3, the structural formula is CH3-CH(OH)-CH(NH2)-COOH, and the relative molecular weight is 119.18. From the structural formula, it can be seen that there are 2 asymmetric carbon atoms and 4 isomers in the threonine molecule, among which D-threonine cannot be absorbed and utilized by animals. Naturally occurring L-threonine is colorless or yellowish crystal, odorless, slightly sweet, soluble in water, solubility at 20 °C is 9 g/100 mL, insoluble in organic solvents, melting point is 253~257 °C; D-threonine is orthorhombic, is a colorless or white crystalline powder, soluble in water, insoluble in organic solvents, easily destroyed by alkali, melting point 229 °C~230 °C. Threonine is the only substance that is not deamino- and transamino-chemical, but catalyzed by threonine dehydrase and threonine dehydrogenase, as well as threonine aldolase. In addition to balancing amino acids in the body, threonine can also be used as a one-carbon unit source, involved in the biosynthesis of purines and pyrimidines, as well as the biosynthesis of S-adenosylmethionine, and can be used as a donor for the methylation of various compounds.

2. The role of threonine in pig production
2.1 Balance amino acids, promote protein synthesis in the body, and reduce dietary protein levels

An important role of threonine is to balance amino acids and promote protein synthesis. Different levels and ratios of amino acids in the diet can affect nitrogen utilization in animals. The addition of threonine can balance the overall amino acid level, and maintaining an appropriate ratio of threonine to lysine has a particularly obvious effect on production performance, and Feng Jie and Xu Zirong (2003) showed that the ratio of lysine to threonine is best when the ratio of lysine to threonine is 0.72 [1]. Zimmerman (1987) argues that the addition of threonine, which is higher than required for normal growth (l2% higher), favors maximum deposition of lean pork meat. The simultaneous use of lysine and threonine can reduce the protein level of the diet without affecting the performance of the animal, conserve protein resources, and help prevent diarrhea in piglets [2]. Zhang Hong et al. showed that the protein level of the growing pigs decreased from 16% to 13%, and rapeseed meal was used to completely replace fishmeal and threonine, which could achieve the corresponding level of growth performance of conventional protein and reduce the cost of feeding. Threonine can be converted into non-essential amino acids such as serine and glycine through the tricarboxylic acid cycle in the body, and threonine can be converted into butyryl-CoA and succinyl-CoA allogeneic sugars to maintain the balance of blood sugar in the body.

2.2 Effect

on feed intake An important role of threonine is its effect on feed intake in livestock and poultry. Cole et al. reported that feed intake gradually increased and peaked with the increase of threonine levels before animals achieved optimal performance, and that feed intake fell to the limit zone when animals reached optimal performance. Wu Xilin et al.'s study on the effect of threonine on feed intake of piglets showed that threonine had a certain regulatory effect on feed intake, and that the feed intake and daily gain of piglets increased with the increase of threonine levels, but began to decrease again when the level of threonine increased [3]. Rossell et al.'s study of weaned piglets showed that when threonine exceeded the maximum requirement, feed intake and daily gain continued to decrease as threonine levels increased. Too little or too much threonine can reduce feed intake and daily weight gain, which may be related to the fact that threonine supplementation affects the palatability and amino acid balance of the diet.

2.3 Improve immune function

in the body's immune system. Antibodies and immunoglobulins are proteins. Threonine deficiency inhibits the production of immunoglobulins and T and B lymphocytes, thereby affecting immune function. Hu reported that the important role of threonine in the immune system may be related to the fact that threonine can promote antibody synthesis. Threonine plays a leading role in humoral immunity in pregnant sows, with threonine added to sorghum-containing diets. It can prevent the reduction of IgG content in the plasma of sows. The addition of threonine and lysine to the diets of test animals increases thymic weight, and enhances skin rejection of xenografts and antibody titers to sheep erythrocytes[4]. Hou Yongqing's study showed that different levels of methionine affected the proportion of thymus to body weight, while different levels of threonine affected the proportion of spleen to body weight, and the two amino acids significantly affected the content of IgG in the blood and half of the hemolysis value, indicating that these two amino acids were related to immune function [5]. Zheng et al. reported that increasing the dietary threonine level was helpful to rapidly increase the serum globulin and IgG levels in growing pigs (P<0.05), but did not affect the final content [6]. Serum anti-bovine serum albumin antibody levels increased with increased dietary threonine levels.

2.4 Improve carcass quality

The addition of threonine to animal diets has a significant effect on fat metabolism in the body. Westermeier's addition of threonine to animal diets led to a decrease in serum triglyceride and low-density lipoprotein (LDL) concentrations, suggesting that the effect of threonine on fat metabolism may be due to the reduction of body fat due to the promotion of lipolysis. Different dietary ratios of threonine to lysine had no significant effect on the visceral organs of growing and fattening pigs [7]. Wu Xilin reported that when the dietary threonine level was 0.68%, the anti-fatty liver effect was stronger. It was found that dietary threonine supplementation significantly increased lean meat rate, carcass muscle and eye muscle area, and water and protein content in longissimus dorsi muscle (P<0.05), and with the increase of dietary threonine content, the fat content of backfat thickness, carcass fat, and longissimus dorsi muscle at the 10th rib showed a linear downward trend (P<0.05) [8]. Feng Jie and Xu Zirong showed that the different ratios of threonine and lysine have a certain impact on the production performance and carcass quality of growing pigs, and on the basis of ensuring the need for lysine, the ratio of the two can promote growth and improve carcass composition when the ratio of the two is 0.72.

3. The requirement
of porcine threonine The demand for threonine in pigs is affected by various factors such as pig breed, type, growth stage, gender, threonine metabolism in the body and amino acid balance of feed. Feeding standards also vary from country to country. NRC (1998) recommended that the requirements for 20~50 kg and 50~80 kg weight growth pigs were 0.61% and 0.51%, respectively. ARC (1981) recommended 0.82% and 0.63%, respectively. Xu Zhenying (1992) summarized the requirements of the main limiting amino acids from the 50s to the 80s, and pointed out that the ideal ratio of 10~20 kg of porcine was 0.47±0.1, and the ideal ratio of threonine in growing porcine was 0.5±0.01.

4. Deficiency and excess
of threonine The overall manifestations of threonine deficiency in animals are decreased feed intake, stunted growth, decreased feed utilization, fatty liver disease, and immunosuppression. At the same time, it also causes changes in a series of biochemical indicators: increased hepatic glycogen concentration, decreased plasma free limiting amino acid concentration, decreased serum, lipoprotein, and free fatty acid concentration, and increased plasma and brain threonine concentrations [9]. Kalenyuk et al. fed piglets a diet with deficient or excessive lysine and threonine, resulting in reduced feed intake, decreased protein and energy deposition, and decreased protein biosynthesis. Exacerbates the elimination of amino acids and nitrogens in stool and urine. Causes the concentration of amino acids in the blood to decrease. Serum urea nitrogen (SUN) is elevated, and activity of amino acid oxidase and urea synthase in the liver is increased. Accelerates the degradation of proteins. Wu Xilin comprehensively reported that feeding feed without threonine caused a series of growth and morphological changes. Weight gain is sluggish, liver weight gain is significant, and gastrocnemius muscle weight is reduced. Atrophy of the pancreas, submandibular glands, thyroid gland, and spleen. In addition, severe threonine deficiency caused an increase in glucose-6-phosphate dehydrogenase activity, a significant decrease in phosphorylase and carboxylinase activities, and no significant changes in the activities of glycine synthase, arginase, histidine and acid phosphatase.

5. Problems
that should be paid attention to when adding threonine
5.1 Do not overdose
Excessive threonine can not play a beneficial role in the growth performance and carcass composition of pigs, and will also lead to the relative deficiency of other amino acids due to the excess of threonine, so that the utilization rate of other amino acids is reduced, resulting in unfavorable results such as increased feed costs. Excessive threonine should not occur especially during gestation in sows, because excess threonine can increase the concentration of glycine in the cerebral cortex, which can affect the balance of neurotransmitters in the brain and adversely affect the brain development of the fetus.

5.2 Pay attention to the level

of dietary protein and lysine Dietary protein level and dietary lysine level are important factors affecting the need for threonine in animals. However, adding threonine to a high-protein diet is difficult to achieve the desired effect.

5.3 Prevent the abuse
of threonine
Not all animal diets must be added to threonine, when using feed raw materials with high threonine content, if threonine can meet the requirements of animal growth or production, there is no need to add threonine in the diet, generally speaking, threonine needs to be added in corn, peanut type, sorghum and barley type diets.

5.4 Pay attention to the antagonism

between amino acids Because there is a synergistic or antagonistic relationship between threonine and other amino acids. Therefore, the content of other amino acids also affects the amount of threonine required. In particular, the interaction between threonine and lysine, methionine, glycine, and tryptophan. For example, different levels of methionine do not affect plasma urea nitrogen and serum total protein levels, but do affect serum free methionine and threonine levels. When dietary methionine levels are high. Serum levels of free methionine and threonine are also high. The amount of threonine required is largely related to the amount of lysine required. When there is an excess of lysine or methionine in the diet. The demand for threonine increases. At least 30% of the threonine in the diet is converted into glycine, accounting for 10%~50% of the entire glycine synthesis in the body.

In addition, vitamins, hormones, and fat types can all affect the utilization of threonine in the body, and it is important to consider the addition of threonine in order to formulate an efficient and economical diet.