Oxalic acid and oxalate are an anti-nutritional factor widely found in plant feeds. They can significantly reduce the utilization of mineral elements in animals and cause damage to many organs and cause poisoning. Therefore, in the production practice, attention should be paid to preventing the harm of oxalate to animals.
Source and physicochemical properties of oxalate
Oxalic acid is also known as oxalic acid. It is mostly found in plants in the form of oxalate. Oxalates are widely found in the plant kingdom. A survey of the distribution of oxalate in the plant kingdom showed that among the 93 genera, except for 11 species that did not contain oxalate, most of the other species contained large amounts of oxalate. Common oxalate-rich forage plants and wild plants are as follows:
Feed beet
Fresh stems and leaves contain a lot of oxalate, especially in the leaves. The early harvesting period of the early luxuriant harvest is high. Its oxalate content can reach 0.3% to 0.9%.
Pasture and wild plants
Such as sheep's hooves, sorrel, sorrel, blue scorpion, salt grass, purslane, foxtail and so on. The oxalate content (fresh weight) in these plants can reach 0.5% to 1.5%.
Leafy vegetable feed
Such as spinach, leeks, leeks, kale and so on. It has been reported in China that the use of raw spinach to feed the bred chicken has caused calcium deficiency in the breeder (the eggshell is thinner, the breakage rate is increased, and some breeders produce soft shell eggs), and the hatching rate of the eggs is reduced. Feeding pigs and poultry for spinach can also cause slower growth and reduced calcium reserves.
straw
The hay made from rice straw and rice after harvesting contains a large amount of oxalate. In India, there has been a balance of calcium imbalance in the body caused by cattle eating straw.
Sesame cake
The sesame shell contains oxalate, so the outer shell of the sesame cake must be completely removed.
Water lotus
The water lily is an aquatic plant of the family Araceae, which has been widely used as a feed for pigs in the southern provinces of China. Some pig farms have been poisoned by feeding water lotus, showing neurological symptoms such as chewing, nervousness and convulsions. The cause of poisoning is still inconclusive. Some people believe that it is caused by a large amount of oxalate in the water lily.
Oxalic acid in plants is present in the form of the free acid, but is generally present in plant cells in the form of soluble potassium salts, sodium salts and insoluble calcium oxalate crystals. Oxalate can be present in the whole plant, with the highest content in the leaves of plants, followed by flowers, fruits and seeds, with the least content in the stems. The oxalate content in plants also varies with the growth stage and the cultivation environment of the plants. It has been found that the oxalate content in sugar beet leaves can be reduced with the seasons; the oxalate content of sorrel has a day and night change.
In addition to plants, oxalates can also be derived from microorganisms in nature. It has been reported that fungi of the genus Aspergillus, Penicillium, Pythium, Sclerotinia, Chrysosporium, Pseudomonas, Rhizoctonia, and Fungi can secrete oxalic acid. Many pathogenic fungi, such as Sclerotinia, attack plants by secreting oxalic acid toxins.
The harmful effects of oxalate and its mechanism
When oxalate is ingested by animals, it can form insoluble oxalate precipitates with divalent and trivalent metal ions such as calcium, zinc, magnesium, copper and iron in the digestive tract and excrete with feces, thus making these mineral elements The utilization rate is reduced. For example, the addition of 1% oxalic acid to a small horse diet containing 0.45% calcium can significantly reduce calcium utilization.
Oxalate has a strong stimulating effect on the mucosa, so when the oxalate is ingested in a large amount, it can stimulate the gastrointestinal mucosa, causing diarrhea and even gastroenteritis. Oxalate can be degraded by rumen microbes of ruminants and metabolized to carbonates and bicarbonates, so ruminants are more tolerant to oxalates than non-ruminants. Ruminants only ingest a large amount of oxalate in a short period of time (such as when starved), because a part of the oxalate can not be converted into the abdomen and absorbed by the animal before causing poisoning. When ruminants take a small amount of oxalate for a long time, the rumen microorganisms can gradually adapt, and the rumen's ability to break down oxalate is improved. Therefore, the newly introduced animals are more sensitive to oxalate than the local animals. When the animals first graze to the oxalate-rich grassland, they do not adapt to the ingestion of oxalate. The tolerance of oxalate. However, if the ruminant continues to ingest a slightly higher level of oxalate, the pH of the rumen content changes (this may be related to the decomposition of oxalate into bicarbonate and carbonate compared to the normal pH). And digestive disorders of cellulose (oxalate can reduce rumen microbes), may also cause rumen dysfunction.
After oxalic acid is absorbed into the blood in a large amount, it can combine with blood calcium to form calcium oxalate precipitate, leading to hypocalcemia, which seriously disturbs the metabolism of calcium in the body, and increases the excitability of the neuromuscular (expressed as muscle tremor, convulsion, etc.) and heart. The function is reduced and the blood clotting time is prolonged. Under the influence of chronic chronic hypocalcemia, it can lead to hyperparathyroidism, increased bone decalcification, and fibrous osteodystrophy.
Oxalic acid combines with calcium and magnesium in the body to form insoluble oxalate crystals that can be deposited in organs. Rats were given an intravenous injection of 0.3 mmol/kg of oxalic acid, and 14C autoradiography showed that tiny oxalate crystals such as glomeruli, liver and heart were observed within 1 hour. These oxalate crystals can cause damage to organs. Oxalate also crystallizes in blood vessels and penetrates into the blood vessel wall, causing blood vessel necrosis and bleeding. Oxalate crystals can sometimes form in brain tissue, causing dysfunction of the central nervous system.
The kidney is almost the only way to discharge oxalate in the body, and more than 90% of the oxalate ingested is excreted by the urine. When oxalate crystals are excreted through the kidneys, they can cause tubular obstruction, degeneration and necrosis, causing renal dysfunction. Oxalate crystallisation has a stimulating effect on the bladder wall. It is believed that the occurrence of certain bladder tumors may be related to this. Increased urinary oxalate excretion can also increase the incidence of urethra stones. The results of cell culture with oxalic acid showed that oxalic acid had inhibitory effects on various enzyme system activities such as lactate dehydrogenase, pyruvate dehydrogenase, succinate dehydrogenase and microsomal enzyme. After the activity of these enzymes is inhibited, it can interfere with the body's sugar metabolism.
Different types of animals have different sensitivities to oxalate in feed. Equine animals are more sensitive to oxalate, and a relatively small dose can cause poisoning or even death. However, it has also been pointed out that in the use of minerals, oxalate has a greater impact on ruminants than on Equine because oxalate can reduce microbes in the rumen and make calcium and oxalic acid in the feed in the rumen. The combination becomes insoluble calcium oxalate, which causes the loss of calcium in the ruminant's feces to be higher than that of the Equine. Because of the strong metabolism of calcium in the body and the high demand for calcium, laying hens have a great influence on oxalate.
Regarding the amount of poisoning and lethal dose of oxalate, it is generally difficult to determine. Using oxalic acid as a result of an animal acute toxicity test, the dog's oral MLD (minimum lethal dose) was 1000 mg/kg. Horses are given 200 grams of sodium oxalate daily for 8 days to cause poisoning; horse oral sodium oxalate 450 grams can be fatal.
Clinical symptoms and pathological changes of oxalate poisoning
Oxalate poisoning in animals is mainly caused by the large amount of oxalate-rich plant-based feed or the large amount of oxalate-containing pasture or weeds in a short period of time during grazing, especially when animals are hungry. It is more susceptible to poisoning than newly introduced animals. In addition, some moldy feeds can also cause animal poisoning due to the presence of oxalates produced by fungi.
Clinical symptoms
Animals can develop symptoms of poisoning 2 to 6 hours after eating large amounts of oxalate-containing plants. Mainly manifested as loss of appetite, vomiting, abdominal pain, diarrhea, ruminant ruminal motility reduction and mild rumen amine gas. Sick animals are uneasy, frequently standing up and lying down, muscle weakness, abnormal gait, increased heart rate, muscle tremors and convulsions. Frequently want to urinate, and occasionally discharge brown-red urine. It is difficult to breathe quickly, and the nose flows out of the foamy liquid with blood. In the end, it’s awkward, lying down, and even coma. Acutely poisoned animals can die from 9 hours to 11 hours after poisoning. Chronic poisoning often manifests as depression, muscle weakness, growth retardation, and chronic gastroenteritis. Horses and pigs present with fibrous bone dystrophy. The laying of laying hens is reduced, producing thin shell eggs and soft shell eggs.
Pathological changes
Diffuse gastrointestinal mucosal bleeding, mesenteric lymphadenopathy, abdominal and pleural effusion, pulmonary congestion, bronchial and bronchioles filled with bloody foam, kidney enlargement, yellow streaks visible in the renal cortex, especially at the junction of cortex and medulla obvious. Microscopic examination showed oxalate crystal deposition in the renal tubules, renal pelvis, and ureter. This crystallization was also seen in the vascular wall of the rumen.
Allowable amount of oxalate in feed
At present, China has not yet established the allowable amount of oxalate in feed. There are few reports abroad. According to the Indian national standard, the edible sesame cake obtained by the solvent extraction method and the screw pressing method has a maximum oxalic acid content of 0.5%.
Measures to prevent oxalate hazards include the following:
Control the amount of feed
When feeding oxalate-rich feed and forage, the amount of feed should not be too much. It should be fed with other feed and forage. Equine animals are sensitive to oxalate, and the amount of feed must be strictly controlled. The amount of ruminant fed can be gradually increased to increase its tolerance to oxalate. At the same time, care should be taken to prevent animals from eating wild plants rich in oxalate, especially in areas where animals are hungry and cannot graze in areas where oxalate-rich plants are grown.
When adding oxalate-rich feed and forage, supplementing with calcium (such as calcium hydrogen phosphate, calcium carbonate, etc.) can reduce the body's absorption of oxalate and alleviate the symptoms caused by oxalate damage. Usually 1 mg of calcium can be combined with 2.25 mg of oxalic acid, so for every 100 mg of oxalate, 50 mg to 75 mg can be added. Further, elements such as zinc, magnesium, iron, and copper are also appropriately added.
Removal of oxalate from forage. Since the oxalate in plants is mostly water-soluble potassium and sodium salts, the green mash can be removed by soaking in water or scalding with hot water to remove most of the oxalate. The oxalate-containing straw can be soaked in water or immersed in light calcium carbonate or calcium hydroxide.
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