Lecture 4.4: Discussion on the main causes of fatty liver formation in laying hens

Fatty liver has always been the biggest headache for laying hen nutrition workers and breeding owners. It seems simple but there is nothing we can do about it. Fatty liver not only reduces the production performance of laying hens in the middle and later stage of laying, but also is the main factor of death Amoy. Regardless of the nutritional status of feed, laying hens generally have different degrees of fatty liver symptoms in the middle and later stages of egg production, which is in sharp contrast to growing chickens and mammals. The reason is related to the unique mechanism of protein metabolism in laying hens.

Ammonia, the tail product of protein metabolism in animals, is toxic to the body and must be transformed and excreted as soon as possible. In order to remove ammonia from the body, mammals are excreted through the synthesis of urea (which does not consume methyl), while birds mainly synthesize uric acid through the liver and then excreted through the kidneys. The process of synthesizing uric acid in the liver is very complex. Each molecule of uric acid requires two molecules of active methyl and can bind to four molecules of ammonia (one of which is contained in glycine). The average nitrogen content of protein is 16%, which is equivalent to 19.4% of ammonia. based on the average bioavailability of protein, feed with 1kg crude protein of 16% is metabolized in the body to produce about 3.1 g of ammonia (1000g*16%*10%*19.4%=3.1g)-all of which are synthesized and excreted by uric acid, requiring the consumption of 1.368 g active methyl (3.1g/2*15/17=1.368g). If active methyl is provided by betaine (1 molecule of betaine can provide 3 molecules of active methyl and 1 molecule of ammonia is also produced through metabolism), 4.27 g pure betaine hydrochloride (1.368g/3*117.15/15*120%=4.27g) is required. If active methyl is provided by choline chloride (1 molecule of choline chloride can provide 3 molecules of active methyl, while 1 molecule of ammonia is also produced through metabolism), 5.09 g of pure choline chloride (1.368g/3*139.63/15*120%=5.09g) or equivalent to 10.18 g of commercial choline chloride of 50% is required.

Methyl is necessary for the synthesis of methionine, choline, carnitine, creatine, phospholipid, epinephrine, RNA and DNA. Folic acid and vitamin B12 are coenzymes related to methyl metabolism. Stable methyl supply is needed for both growing and adult animals. It is generally believed that methyl can not be synthesized in animals. ), which needs to be supplied from food. Substances that can provide active methyl groups in feed include methionine, tryptophan, histidine, serine and glycine, as well as choline and betaine.

The above five amino acid molecules such as methionine contain an active methyl, which can be used as a donor of active methyl when the body needs or protein is metabolized as energy. These amino acids provide active methyl groups for uric acid synthesis. For each molecule of uric acid synthesized, two molecules of amino acids provide active methyl and bind to four molecules of ammonia-two of which also produce two molecules of ammonia. So in fact, only two molecules of ammonia can be combined with two molecules of ammonia-that is, the synthesis of uric acid from active methyl provided by these amino acids can only eliminate one additional molecule of ammonia. In the general layer feed, the total amount of the above amino acids is about 15%, and its main function is as a raw material for protein synthesis and as an active methyl donor when needed by the body.

Betaine, also known as trimethylglycine (molecular formula C5H11NO2, molecular weight is 117.15), is a quaternary amine alkaloid. Betaine exists widely in animals and plants, and the content of beet molasses is the highest. Betaine can be chemically synthesized. Betaine used as a feed additive includes compound betaine with a content of 30% and 40% and betaine hydrochloride with a content of 98.5% (the effective content calculated according to the chemical formula is 75%, and the actual content is usually only about 65%). Betaine has a wide range of physiological functions: participating in protein and lipid metabolism, regulating osmotic pressure, relieving stress, promoting food intake and so on. A large number of experiments show that betaine as a new type of feed additive has a broad application prospect.

As a methyl donor, betaine can promote protein synthesis. It was found that the content of betaine in the liver of rats fed with betaine increased and the activity of homocysteine-S-methyltransferase (BHMT) increased significantly with the increase of betaine content. This enzyme is the key enzyme of methionine methyl metabolism of essential amino acid in liver and kidney. BHMT can catalyze the synthesis of methionine from betaine and homocysteine. (more than 90% of the active methyl needed for metabolism in the body is provided by methionine-homocysteine is also produced, which is then converted to methionine by betaine. Betaine can promote fat metabolism and inhibit fat deposition, thus improving lean percentage and carcass quality. Betaine synthesizes a large amount of carnitine through the liver, enhances the transport of fatty acids, promotes the β-oxidation of fatty acids in tissue and cell mitochondria, and provides sufficient energy for cell metabolism, thus reducing fat deposition.

Betaine can cushion the sudden change of osmotic pressure and maintain the balance of cellular osmotic pressure. When tissue osmotic pressure changes, betaine can be absorbed by cells, prevent cell water loss and salt entry, improve cellular Na-K pump function, maintain normal osmotic pressure of tissue cells, and regulate the balance of cellular osmotic pressure.

The addition of betaine to layer feed can increase the laying rate of laying hens, reduce the ratio of feed to egg, prevent fatty liver, reduce broken eggs and abnormal eggs, improve egg quality and promote the synthesis of concentrated protein and yolk. Reduce the content of cholesterol in egg yolk.

Choline, also known as trimethylethanolamine, is the raw material for the synthesis of acetylcholine and lecithin. It has the effect of anti-fatty liver in animals. As a methyl donor, it can also reduce the consumption of methionine and tryptophan. The choline used in the feed is choline chloride, its molecular formula is C5H14CINO, its molecular weight is 139.63, and the commercial dosage forms are 50% powder, 60% powder and 70% water agent, while 50% powder is widely used in our country, and its appearance varies with different carriers.

In terms of methyl content, 98.5% betaine hydrochloride is equivalent to 2.35 kg50% choline chloride and 3.75 kg99% methionine. Methionine can transfer methyl directly, and the methyl transfer titer is the highest; betaine must first transfer methyl to homocysteine to synthesize methionine and indirectly provide methyl, and the methyl transfer titer is only 50% of methionine. Choline needs to be oxidized to betaine before it can provide methyl-the oxidation process takes place in mitochondria, and the oxidation process requires the participation of VB12 and VB2, and is inhibited by Ni, Co, and Fe ions, so the conversion rate is not high, only 3-4%.

Thus it can be seen that as an active methyl donor, betaine is the most suitable and economical.

Discussion on the causes of fatty liver in laying hens: fatty liver is common in laying hens, especially in the later stage of laying. The reason is related to the unique metabolic activity of laying hens. Different components (yolk, egg white, eggshell) in eggs are formed at different times of the day (which is different from the balanced production of livestock and poultry during the growing and lactation periods). Therefore, protein synthesis and metabolism in vivo is also a discontinuous process, so the need for protein is also uneven at different times of the day-large in the morning and small in the afternoon and evening. Most of the protein digested and absorbed in the afternoon and evening is metabolized as energy, that is, about 30% of the total daily feed intake contains protein that is metabolized as energy and produces ammonia. To sum up, the feed with 16% 1kg crude protein will produce 8.69 g ammonia after metabolism in laying hens (1000% 70% 16% 10% 19.4% 1000 30% 16% 19.4% 70% 2.17 6.52 8.69).

Ammonia is toxic to the body, and it is urgent to eliminate ammonia, which must be transformed and excreted as soon as possible. About 30% of them can be metabolized by methionine and other amino acids containing active methyl groups (the total amount is about 15%). The remaining 70% of 8.69 g ammonia requires 7.67 g active methyl (8.69g*15/17=7.67g), which is equivalent to 19.97 g 98.5% betaine hydrochloride (7.67g/3*117.15/15=19.97g) or 47.6 g 50% choline chloride (7.67g/3*139.63/15*2=47.6g).

To sum up, laying hens have a large demand for betaine and choline, and the general feed can not fully meet this need. In order to ensure that the body is protected from ammonia, laying hens give priority to synthesize uric acid to remove ammonia as soon as possible. In this way, all the active methyl groups in the body are bound to be consumed. Betaine and choline are closely related to fat transport and fatty acid oxidation and catabolism. The lack of betaine and choline leads to the disorder of fat metabolism in the body and the formation of fatty liver.

Betaine is the raw material for the synthesis of carnitine, and carnitine is the carrier of fatty acid transmembrane transport in vivo, which can promote the β-oxidation of fatty acids in tissue and cell mitochondria and provide sufficient energy for cell metabolism. Due to the lack of betaine, carnitine synthesis is blocked, which will lead to the inhibition of fatty acid catabolism, and the increase of fatty acid concentration will further promote fat synthesis.

Choline is the raw material for the synthesis of lecithin, and lecithin is the carrier of fat transport in the body. Choline deficiency will affect the transport of fat, resulting in fat retention in the liver and the formation of fatty liver.

The corn-soybean meal type layer feed contains about 1g choline (equivalent to 2.68kg 50% choline chloride) and also contains some betaine (there is no such data), while the laying hen feed generally only adds 1kg hand t 50% choline chloride, which is far from meeting the active methyl needs of laying hens. Of course, the body may also synthesize part of serine through glucose metabolism and transamination (this is just a personal guess), and serine can be converted into glycine, both of which can provide active methyl for the body. however, because the demand is really too large, the speed of synthesis in vivo is far from enough to meet the needs of metabolism. This may be because serine and glycine are also essential amino acids in birds (these two amino acids are non-essential amino acids in mammals). The data in this article are quoted only for illustrative purposes, and some of them are just personal guesses. The point of view is only a management opinion, welcome to discuss and criticize and correct! My observation: hot summer season, easy to become fatty liver-read teacher Xie's article, a lot of gains, thank you! Ask Mr. Xie another question: which is the best, the most economical and what's the difference between choline and betaine in pig feed? thank you. Teacher Xie has been emphasizing betaine, but I still don't think choline is as good as choline. As far as I know, many big manufacturers just add betaine to the fish. -- is the principle of fatty liver the same for broilers and laying hens?-- Betaine is more efficient than choline and is the most cost-effective as a methyl donor. I think that as a special function of choline itself, the content in general feed is enough. As for what to use, it is more about habit. The principle is the same, except that it is not as serious as that of laying hens. Pigs need much less active methyl than birds, and there is no need to synthesize uric acid. I think methyl is enough in general pig feed (serine, glycine, histidine can all be used as methyl donors, pigs can synthesize serine and glycine), so It may be more appropriate to add choline chloride to pig feed. However, betaine can also be converted into choline, and what is added is determined according to habit. Of course, betaine does not damage the multi-dimension of the premix. As Mr. Wang said, laying hens have more fatty liver in summer. Why is that, Mr. Xie? What is the mechanism? How to avoid or alleviate it? Look forward to your advice! -- that makes sense, thank you for sharing!-- according to my experience, fatty liver with bean cake ingredients seems to be more likely to occur, suspected to be related to Aspergillus flavus.