Cadmium is a toxic heavy metal element that is naturally present in the environment but in very low concentrations. The United Nations Food and Agriculture Organization (FAO) and the World Health Organization (WHO) have classified cadmium as one of the major food contaminants, following aflatoxin and arsenic. Once ingested, cadmium can cause serious harm to the body. Currently, the extraction method (GB/T 13082-1991) is commonly used for determining cadmium levels in feed, although it involves a lengthy sample preparation process. This article provides an overview of the sources of cadmium in feed, its harmful effects on livestock and poultry, and various analytical methods for detection.
Cadmium is found in the fifth period of the periodic table, in group IIB, and is a non-ferrous metal. It was first discovered in 1878. Cadmium is considered one of the most dangerous heavy metals due to its high toxicity. Since 1972, FAO and WHO have listed it as a food contaminant, ranking it alongside aflatoxins and arsenic. Long-term exposure to cadmium through contaminated feed can lead to chronic poisoning in animals, resulting in reduced productivity and performance. Even small amounts of cadmium can negatively affect animal health, and it may accumulate in animal products, posing risks to human health. Therefore, cadmium contamination in feed has become a growing concern among animal nutritionists.
The sources of cadmium in feed are diverse. It is naturally present in the earth's crust at low levels, with concentrations ranging from 0.15 to 0.20 mg/kg. In seawater, the concentration is about 0.11 μg/kg, while in rivers and lakes, it varies between 1 to 130 μg/kg. In the air, cadmium content is typically around 0.002 to 0.005 μg/m³. Soils with cadmium levels above 1 mg/kg are common. Plants generally have low cadmium content, up to 1 mg/kg, but aquatic organisms such as algae and fish have a strong ability to accumulate it. For example, algae can concentrate cadmium up to 11–20 times, while fish can accumulate it up to 10,000–100,000 times. Fishmeal, therefore, often contains higher levels of cadmium compared to plant-based feeds, with an average of 1.2 mg/kg and even reaching 25 mg/kg in polluted areas.
In China, the national feed hygiene standard (GB 13078-2001) limits cadmium content in fishmeal to no more than 2 mg/kg. Cadmium often coexists with other minerals like zinc, lead, and copper in nature, and during the refining process, it can pollute the environment. Poorly processed mineral additives, such as zinc sulfate, may contain high levels of cadmium. These additives, especially zinc sulfate and zinc oxide, are significant contributors to cadmium contamination in animal feed. Additionally, raw materials like calcium hydrogen phosphate and bacitracin zinc should also be monitored for cadmium content. Although zeolite powder has low cadmium levels, its large usage makes it important to consider.
Studies have shown that cadmium levels in feed vary by region and industrial activity. For instance, Yuan Hui et al. (1997) found that many feed samples in Hunan Province exceeded national standards. Similarly, Xing Chenghua et al. (2007) analyzed feed samples from Jinhua City and found that lead and cadmium levels were within safe limits, meeting national standards. These findings highlight the importance of continuous monitoring to ensure feed safety and protect both animal and human health.
Cadmium toxicity in livestock and poultry primarily affects the kidneys and bones. It is mainly absorbed in the duodenum and distributed in the liver and kidneys. Chronic exposure can lead to bone decalcification, fractures, and kidney damage. It also interferes with calcium and phosphorus metabolism, causing anemia, renal dysfunction, and reproductive issues. Studies show that even low levels of cadmium can reduce egg production and antioxidant capacity in laying hens over time.
To detect cadmium, several methods are available, including colorimetric analysis, flame atomic absorption spectrometry, graphite furnace atomic absorption, and inductively coupled plasma mass spectrometry. The GB/T 13082-1991 method, though reliable, is time-consuming. Improved techniques, such as hydride generation-atomic fluorescence spectrometry (HG-AFS), offer faster and more accurate results. Other methods, like microwave digestion and dry ashing, have also been optimized to improve efficiency and accuracy, making them suitable for routine analysis in feed laboratories.
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