Abstract: Cadmium is a toxic heavy metal element that is widely distributed in nature but its content is very low. The United Nations Food and Agriculture Organization (FAO) and the World Health Organization (WHO) have classified cadmium as a food contaminant after aflatoxin and arsenic. When cadmium enters the body, it will cause great toxicity to the body. At present, the extraction method of cadmium in feed uses the extraction method (GB/T 13082-1991), and the sample processing time is short. The article reviews the sources of cadmium in feed, the harm to livestock and poultry and its determination methods. Cadmium is located in the fifth cycle of the Periodic Table of the Elements, and Group IIB, a non-ferrous metal element, was discovered in 1878. Cadmium is one of the most toxic heavy metals in the environment. As early as 1972, the United Nations Food and Agriculture Organization (FAO) and the World Health Organization (WHO) listed cadmium as a food contaminant next to aflatoxin and arsenic. Cadmium is a heavy metal element that is harmful to the body and highly toxic. Long-term consumption of cadmium-contaminated feed by animals can cause chronic cadmium poisoning, resulting in decreased production performance. Eating a small amount of cadmium can have an adverse effect on animal performance, and cadmium is also likely to remain in animal products (Ren Jiping, 2003). Due to the intensification of environmental pollution and the extensive use of trace element additives, the phenomenon of excessive cadmium in feeds has become increasingly prominent. Cadmium not only harms the animal body, but also passes through the feed into the food chain and then transmits it to the human body, endangering human health. Therefore, it is endangered by animal nutritionists. Emphasis (Yu Yanhu, 2001). 1. Source of cadmium in Cadmium is widely distributed in nature, but its content is very low. According to reports, the content of cadmium in the earth's crust is 0.15~0.20 mg/kg; the mass concentration in seawater is about 0.11μg/kg; the water in rivers and lakes is 1~10μg/kg, up to 130μg/kg; cadmium in air The content is 0.002~0.005μg/m3, and the cadmium content in the soil exceeds 1 mg/kg. Usually, the cadmium content in plants is very low, up to 1 mg/kg. Different from plant-based feed, aquatic organisms have strong enrichment ability for cadmium. For example, algae can enrich 11~20 times of cadmium, and fish can enrich 10,000-100,000 times of cadmium. The content of cadmium in fishmeal is higher than that of plant-based feed. The average cadmium content of fishmeal is as high as 1.2 mg/kg, while the cadmium content of fishmeal in polluted seas can be as high as 25 mg/kg. China's feed hygiene standards (GB 13078-2001) require that the content of cadmium in fishmeal of feed ingredients should not exceed 2 mg/kg. Since cadmium exists mainly in the form of sulphur-cadmium ore in nature and often coexists with minerals such as zinc, lead, copper and manganese, there is a large amount of cadmium polluting environment in the refining process of these metals. Raw materials of feed grade mineral element additives with poor processing quality, such as zinc sulphate, are often also high in cadmium content. Mineral feed additives with high cadmium content, especially zinc sulfate and zinc oxide, are one of the main sources of cadmium in animal feed. In addition, attention should also be paid to the content of cadmium in raw materials such as calcium hydrogen phosphate and bacitracin zinc. Although the content of cadmium in the carrier such as zeolite powder is not high, but the amount is large, it is also necessary to pay attention. Yuan Hui et al. (1997) investigated the content of cadmium in feed in some areas of Hunan Province. The results of measurement and statistical analysis showed that the cadmium content of most types of feed exceeded the national feed hygiene standards. The results also showed that the higher the industrial level In the urban area, the cadmium content of the feed samples is significantly higher than that of the general city, which indicates that the industrial three wastes have a great impact on the feed. Xing Chenghua et al. (2007) conducted a sample survey of feeds produced by a number of feed manufacturers in Jinhua, and determined the content of lead and cadmium in feed by atomic absorption spectrophotometer. The results showed that the content of lead and cadmium in the tested feedstock samples completely met the national standard, and the pass rate was 100%. The lead and cadmium contents were in the range of 0.005~0.157 and 0.06~0.104 mg/kg, respectively. The highest lead content was Corn, 0.157 mg/kg, the lowest is 0.005 mg/kg for broad bean; the highest cadmium content is 0.104 mg/kg for bone meal; the lowest is pea and broad bean for 0.006 mg/kg. The lead and cadmium content in Jinhua City's feed meets the national feed hygiene standards and can safely feed livestock. 2. Toxicity of cadmium to livestock and poultry Excessive levels of cadmium in the feed will have a series of toxic effects on the kidneys and bones of the livestock and poultry body. Cadmium enters the body through the digestive tract, respiratory tract or skin. However, the main part of cadmium absorbed in the animal is in the duodenum. Although it is absorbed in the jejunum and ileum, the absorption rate of the duodenum is jejunum and ileum. 2 times. After entering the body, it can be applied to all organs and systems of the whole body, but mainly distributed in the liver and kidney. Conto and Petit and Brownd and Shurben also obtained similar test results. Distribution of cadmium in animal organs: muscle <liver < kidney; small distribution of ovary, uterus, gastrointestinal system, heart, testis, pancreas, bones and lungs. Cadmium enters the body and affects calcium and phosphorus metabolism, resulting in decalcification of bones and fractures. After cadmium poisoning in pigs, the spleen is swollen, gastric mucosal ulcer and necrotic enteritis, the heart is obviously hypertrophied, myocardial fibers and hepatocytes and renal tubular epithelial cells are swollen, alveolar emphysema, congestion and hemorrhage, kidney is yellowish, and renal pelvis is pale ( Han Xinyan et al., 2005). Cadmium can cause anemia in animals, promote endothelial cell death, significantly reduce the number of endothelial cells, form a stripping cell layer, cadmium can cause damage to renal proximal tubular epithelial cells, leading to proteinuria, diabetes, and hypercalciuria, which in turn leads to calcium imbalance. Osteoporosis occurs; cadmium easily binds to hemoglobin, inhibits bone marrow hematopoiesis and causes anemia, and can also cause spermatogenic epithelial degeneration and affect reproductive function (Yuhu, 2001). Studies have shown that trace amounts of cadmium reduce the performance and antioxidant function of laying hens, and as time goes on, the effect of this decline will become more and more obvious (Wu Wenping, 2006; Sun Tao et al., 2012). 3. Detection method and improvement of cadmium The detection methods of cadmium include colorimetric method (Ba Shuhong, 1999), flame atomic absorption spectrometry (Zhang Ying and Peng Zhu, 1999), graphite furnace atomic absorption method (Feng Shangcai, 2004), and inductively coupled plasma optical emission spectrometry ( ICP-AES) (Chen Weizhen et al., 2008) and inductively coupled plasma optical emission spectroscopy-mass spectrometry (ICP-MS) (Wang Peilong et al., 2007). At present, the method used in the determination of cadmium in feed (GB/T 13082-1991) adopts the extraction method. Although this method has high stability, scientificity and accuracy, the sample processing time is long. Fu Zuolong et al. (2009) used hydride generation-atomic fluorescence spectrometry (HG-AFS) method to digest samples with hydrochloric acid, nitric acid and perchloric acid, and cobalt ions as cadmium fluorescence signal enhancer to determine feed and feed additives. Cadmium has a good linear relationship at 0~2.0μg/L. The regression equation y=1 317.8x+78.203 (r=0.999 5), the relative standard deviation (RSD) of the method is 1.6% (n=11, 2μg/L). The detection limit was 0.08μg/L, and the recoveries were 78.3%~106.7%. The method for determining cadmium in feed and feed additives has simple and rapid operation, high sensitivity and good reproducibility, and is suitable for detection and analysis of cadmium in feed and feed additives. Yuan Shurong (2009) improved the national standard GB/T 13082-1991 "Method for determination of cadmium in feed", selected the best digestion temperature, shortened digestion time and improved work efficiency, and the recovery rate reached 97.4%~102.4%. Wu Lamu Gu Lahong (2011) used the dry ash method to verify the determination method of cadmium in feed. The linear relationship of the method was good in the range of 0.05~1.00μg/mL, and the recovery rate was 94.8%. ~100.3%, precision is 0.6%~4.6%, the method is accurate, reliable, simple and fast, with certain scientific and stability. Wang et al. (2006) used a microwave dissolution method to dissolve samples with HNO3-H2O2, and then used diammonium hydrogen phosphate as a base modifier to determine cadmium in feed by graphite furnace atomic absorption spectrometry and pretreatment of several different samples. The method was compared. The recovery of this method was 91.0%~106.0%, and the RSD was 2.5%~6.3%. 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