Effects of five trace elements on the quality of eggs

Egg quality includes external quality and internal quality such as egg weight, eggshell quality, eggshell color, egg shape index, egg protein height, yolk color, yolk membrane strength, chemical composition, functional properties, blood spot and meat spot, flavor and so on. The egg quality that producers and consumers pay most attention to is egg weight, eggshell quality, yolk color and functional properties.

1. Effects of Zinc and Manganese on Egg quality

There are few reports on the use of zinc alone in laying hens. The study of Wedekind (1990 ~ 1992) shows that zinc amino acid has medium stability, good solubility under physiological conditions, high biological potency, and can improve eggshell quality and immunity. When NaCl of 2g/L was added to the drinking water of 71-week-old laying hens, the eggshell quality decreased. When 2g/LNaCl was added to the drinking water, zinc methionine or zinc sulfate of 500mg/kg could significantly reduce the eggshell defect and improve the eggshell strength. Most studies have shown that zinc and manganese are effective only when added to layer diets at the same time (Holder,1978;Hunntley,1978). Tahatapob (1985) made an in-depth study on the relationship between manganese and zinc, and concluded that manganese and zinc in 75mg/kg made the eggshell have a higher ratio of calcium to nitrogen and increased uronic acid content, thus improving the quality of eggshell. The addition of organic zinc and manganese to high calcium diet has high bioavailability and can significantly improve the quality of eggshell. Feeding at night can ensure the continuous release of calcium from digestive tract, thus improving the quality of eggshell. In hot seasons, night feeding can prevent the decline of eggshell quality (Lowe,1996;Hecher,1996). However, it was also reported that layers without molting at 80 weeks and once molting at 110 weeks were fed with enough manganese and zinc to reach safe levels to improve eggshell quality (Daleet.al,1998). The hatching rate of eggs laid by zinc deficient hens was low, the development of chicken embryos was blocked, and the mortality rate was high.

Manganese has different effects on the quality of eggshell and yolk. Yan Yuming et al. (2000) conducted a study on 25-38-week-old and 49-57-week-old Beijing red hens. The results showed that there was no significant difference in growth performance, eggshell quality and egg shape index among groups. The content of manganese in eggshell increased negatively, and the content of manganese in eggshell in organic manganese group was lower than that in inorganic manganese group. The content of manganese in egg yolk increased positively, and the content of manganese in methionine manganese group was the highest, which indicated that the retention of manganese in eggshell and yolk showed opposite results, and organic manganese was beneficial to the retention of manganese in egg yolk. When manganese supply is insufficient, egg production and hatching rate decrease, eggshells become thinner and eggs with scars are laid (Hao Zhengli, 1991; Tian Yunbo, 1996). It is generally believed that the addition of 55~75mg/kg manganese to the feed can significantly improve the eggshell quality, once reduced to 7mg/kg, no matter the content of calcium and phosphorus in the feed, it will cause the eggshell quality to decline. The effect of high manganese on eggshell quality is controversial. Liu Jianshu (1992) and Fari (1999) thought that high manganese had no effect on eggshell quality. Other researchers believe that excessive manganese in feed will affect the absorption and metabolism of calcium and adversely affect the formation of eggshell. When manganese in 180mg/kg is added to the feed, the rate of broken eggs and soft shell eggs will increase.

two。 Effect of iodine on the quality of eggs

The content of iodine in eggs is mainly deposited in egg yolk, which is directly related to the content of iodine in feed. Therefore, through strict drug control, iodized eggs can be used for the treatment and prevention of goiter. From the point of view of hatching, the maximum allowable amount of iodine in layer feed is 50mg/kg. Due to chronic iodine deficiency in the diet of laying hens, the ovarian function was limited and the luteinizing function of pituitary was affected. The hatching rate of eggs produced by laying hens with severe iodine deficiency is extremely low, and the weight of embryos decreases, even if there are hatched chicks, they are also weak and accompanied by goiter. However, feeding hens with iodine of more than 300 to 1000 times the appropriate dose temporarily stopped laying and decreased the hatchability of eggs (Xu Zirong, 1991).

3. Effect of Chromium on Egg quality

Chromium is a component of glucose tolerance factors, which participates in the physiological function of insulin and plays an important role in glucose and lipid metabolism. The results showed that chromium could significantly increase the laying rate of laying hens and significantly decrease the level of yolk cholesterol and increase Haugh units. According to the study of Lin Xianglin (1997-1999), the action mechanism of chromium is to increase the activity of insulin, promote the deposition of lipids in the body and reduce the lipids in circulation, thus reducing the content of cholesterol in plasma and egg yolk. The effect of adding organic chromium was more obvious than that of inorganic chromium, and 0.8mg/kg was the best level. The results are consistent with the results of Devegowgr (1997). Jenson (1978) reported that the addition of chromium to 5mg/kg could significantly increase the Haugh unit of eggs and improve the quality of eggs, while the yolk cholesterol content decreased significantly when chromium was added to the diet of laying hens (Li Haifeng et al., 1999). However, there were different reports that supplementation of chromium picolinate to laying hens could not confirm the changes of egg Haugh unit and yolk cholesterol content (Page,etal 1991, 1992; Yin Jingdong, 2000). The reasons for the above differences may be related to the breed, age, chromium source and the content of substances related to absorption and metabolism in the diet. Anderson et al. (1989) after adding 25mg/kg and 100mg/kgCrCl3 to the turkey diet, the egg yolk chromium concentration increased linearly and had little effect on the protein chromium concentration. The egg yolk chromium concentration was 10 times and 23 times higher than that of protein in wet weight and dry weight, respectively. Dietary chromium level plays an important role in maintaining the normal physiological status of laying hen protein, the mechanism may be: as a protein structure, participate in protein connection; chromium is necessary for ovomucin synthesis; promote cation (possibly magnesium) transport into egg protein.

4. Effect of selenium on the quality of eggs

Selenium is not only an indispensable component of enzymes that protect somatic cell membrane, but also a necessity for the effective utilization of dietary protein, carbohydrates and fats. Selenium can convert methionine into cystine in poultry. Dietary selenium deficiency can aggravate cystine deficiency. Selenium deficiency will lead to a decrease in egg production and egg weight. Selenium is also associated with fatty liver syndrome. Dietary selenium can reduce liver bleeding caused by fatty liver. Selenium and VE work together to maintain the body's resistance to disease and stress. Eggs are the best indicator of selenium content. The diagnosis of selenium deficiency should analyze the selenium content of eggs rather than feed. The content of protein selenium is about 0.20mg/kg, the amount of selenium is very small, 2mg/kg will cause the decrease of egg production. The price of selenium is low, and it is easy to recover the cost. Adding selenium to the diet of laying hens is beneficial to improve the nutritional composition of eggs. But selenium is a highly toxic element. Mashan (1973-1996) has done a lot of research on the nutrition and toxicity of selenium. He believes that as long as the amount of selenium is increased, there is no selenium deficiency. After summarizing a large number of experimental studies in the 1970s and 1980s, NRC (1998) recommended that the amount of selenium in the diet of laying hens should be 0.10~0.15mg/kg, and the addition of organic selenium was easier to deposit (Cantoretal.,1999) in eggs than that of inorganic selenium. Ullrey (1992) pointed out that considering the pollution of selenium to the environment, it is suggested that the level of selenium should be reduced to 0.1mg/kg.

5. Summary

To sum up, trace elements play an important role in regulating the quality of eggs, which can affect not only the sensory quality, but also the nutritional quality of eggs. According to the current research results, the metabolic relationship of trace elements in animal bodies is complex, there are synergistic or antagonistic effects, some metabolic mechanisms have not been clearly studied, and, most of the studies at home and abroad are on the effects of one or two trace elements on the quality of eggs. Therefore, in the preparation of laying hens, scientific and reasonable nutritional allocation of trace elements can effectively improve the egg quality of laying hens, meet the market demand, and improve the nutritional value and economic benefits of eggs. Based on the metabolism of the above trace elements and the NRC standard, we think that the dietary contents of manganese 60mg/kg, zinc 50mg/kg, iodine 0.35mg/kg, chromium 0.65mg/kg and selenium 0.12mg/kg are beneficial to the production performance and egg quality of laying hens.