How 5 Trace Elements Affect Egg Quality

Egg quality includes external qualities such as egg weight, eggshell quality, eggshell color, and egg shape index, as well as internal qualities such as albumen height, yolk color, yolk membrane strength, chemical composition, functional properties, blood spots and meat spots, and flavor. The egg qualities of greatest concern to producers and consumers are egg weight, eggshell quality, yolk color, and functional properties.

1. Effects of zinc and manganese on egg quality

There are few reports on studies of zinc used alone in laying hen feeding. Research by Wedekind (1990, 1992) showed that zinc amino acid has moderate stability, good solubility under physiological conditions, and high biological value, and can improve eggshell quality and immunity. Adding 2 g/L NaCl to the drinking water of 71-week-old laying hens reduced eggshell quality; when 2 g/L NaCl was added to the drinking water while 500 mg/kg zinc methionine or zinc sulfate was added to the feed ration, eggshell defects were significantly reduced and eggshell strength was improved. Most studies, however, show that zinc is effective only when added to the laying hen diet together with manganese (Holder, 1978; Hunntley, 1978). Tahatapob (1985) conducted an in-depth study of the relationship between manganese and zinc and concluded that 75 mg/kg of manganese and zinc gave the eggshell a higher calcium-to-nitrogen ratio and increased uronic acid content, thereby improving eggshell quality. Adding organic zinc and manganese to high-calcium feed results in high bioavailability and can significantly improve eggshell quality; feeding at night can ensure continuous release of calcium in the digestive tract, thereby improving eggshell quality. In hot seasons, feeding at night can prevent a decline in eggshell quality (Lowe, 1996; Hecher, 1996). There are also contrary reports: feeding sufficient manganese and zinc at safe levels to 80-week-old laying hens that had not molted and to 110-week-old laying hens that had molted once was found to improve eggshell quality (Dale et al., 1998). Eggs laid by zinc-deficient hens have low hatchability, hindered embryo development, and high mortality.

Manganese affects eggshell and yolk quality to varying degrees. Gu Yuming et al. (2000) studied Beijing Red hens at 25-38 weeks and 49-57 weeks of age and showed that different manganese source addition levels produced no significant differences among groups in growth performance, eggshell quality, or egg shape index (P > 0.05). Eggshell manganese content showed negative growth, and the eggshell manganese content of the organic manganese group was lower than that of the inorganic manganese group. Yolk manganese content showed positive growth, with the manganese methionine group being the highest, indicating that manganese retention in the eggshell and yolk shows opposite results, and that organic manganese favors manganese retention in the yolk. Insufficient manganese supply reduces egg production and hatchability, thins the eggshell, and produces scarred eggs (Hao Zhengli, 1991; Tian Yunbo, 1996). It is generally believed that adding 55-75 mg/kg manganese to the feed can significantly improve eggshell quality, and that once it drops to 7 mg/kg, regardless of the calcium and phosphorus content in the feed, eggshell quality will decline. The effect of high manganese on eggshell quality is highly controversial. Liu Jianshu (1992) and Fari (1999) believe that high manganese has no effect on eggshell quality. Other researchers believe that excessively high manganese in the feed affects calcium absorption and metabolism and has an adverse effect on eggshell formation; when 180 mg/kg manganese is added to the feed, the rates of broken eggs and soft-shelled eggs increase.

2. Effects of iodine on egg quality

The iodine content in eggs is mainly deposited in the yolk, which is directly related to the iodine content in the feed. Therefore, through strict drug control, iodine-treated eggs can be used for the treatment and prevention of goiter. From the perspective of incubation, the maximum allowable amount of iodine in laying hen feed is 50 mg/kg. Chronic iodine deficiency in the laying hen diet restricts ovarian function and affects the luteinizing function of the pituitary gland. Eggs laid by laying hens with severe iodine deficiency have extremely low hatchability, and embryo weight decreases; even if chicks hatch, they are very weak and have enlarged thyroid glands. However, feeding hens iodine at 300-1000 times the appropriate dose causes temporary cessation of laying and reduces egg hatchability (Xu Zirong, 1991).

3. Effects of chromium on egg quality

Chromium is a component of glucose tolerance factor, participates in the physiological function of insulin, and plays an important role in glucose and lipid metabolism in the body. Studies show that chromium can significantly increase the laying rate of laying hens and significantly reduce yolk cholesterol levels and increase Haugh units. Lin Xianglin (1997, 1999) concluded that the mechanism of chromium action is by increasing insulin activity, promoting lipid deposition in the body, and reducing circulating lipids, thereby lowering cholesterol content in plasma and yolk. Adding organic chromium produced more obvious effects than adding inorganic chromium, and the optimal addition level was 0.8 mg/kg. This result is consistent with the research results of Devegowgr (1997). Jenson (1978) reported that adding 5 mg/kg chromium can significantly increase the Haugh unit of eggs and improve egg quality, while adding chromium at a concentration of 0.6-0.7 mwg to the laying hen diet significantly reduced yolk cholesterol content (Li Haifeng et al., 1999). There are also different reports: supplementing laying hens with chromium picolinate failed to confirm changes in egg Haugh units and yolk cholesterol content (Page, et al. 1991; Achee et al., 1992; Yin Jingdong, 2000). The reasons for the above differences may be related to the chicken breed, age, chromium source, and the content of substances related to its absorption and metabolism in the diet. After Anderson et al. (1989) added 25 mg/kg and 100 mg/kg CrCl3 to the diet of breeding turkeys, yolk chromium concentration increased linearly, with little effect on albumen chromium concentration; yolk chromium concentration was 10 times and 23 times that of albumen on a wet weight and dry weight basis, respectively. Dietary chromium level plays an important role in maintaining the normal physiological status of laying hen albumen. The mechanism may be: as a protein structure, it participates in protein bonding; chromium is necessary for ovomucin synthesis; and it promotes the transport of cations (possibly magnesium) into egg albumen.

4. Effects of selenium on egg quality

Selenium is an indispensable component of enzymes that protect somatic cell membranes and is also essential for the effective utilization of dietary protein, carbohydrates, and fat. Selenium enables methionine in poultry to be converted into cystine; dietary selenium deficiency can worsen cystine deficiency, and selenium deficiency can cause a decline in egg production and reduced egg weight. Selenium is also related to fatty liver syndrome. Adding selenium to the diet can reduce liver bleeding caused by fatty liver. Selenium works together with VE to maintain the body's resistance to disease and stress. Eggs are the best indicator of selenium content. To diagnose selenium deficiency, the selenium content of eggs should be analyzed rather than that of the feed. Albumen selenium content is about 0.20 mg/kg; the amount of selenium used is very small, and 2 mg/kg will cause a decline in egg production. Selenium is low in price and easy to recoup in cost; supplementing selenium in the laying hen diet helps improve the nutritional composition of eggs. But selenium is a highly toxic element. Mashan (1973-1996) conducted extensive research on selenium nutrition and toxicity and believed that as long as the amount of selenium is increased, there is no selenium deficiency. After summarizing a large amount of experimental research from the 1970s-1980s, NRC (1998) recommended that the selenium level in laying hen diets be 0.10-0.15 mg/kg, and that compared with adding inorganic selenium, adding organic selenium makes selenium more easily deposited in eggs (Cantor et al., 1999). Ullrey (1992) pointed out that, considering the environmental pollution problem of selenium, it is recommended to reduce the selenium addition level to 0.1 mg/kg.

5. Summary

In summary, trace elements have important nutritional regulatory effects on egg quality; they can not only affect its sensory quality but also affect the nutritional quality of eggs. Based on current research results, the metabolic relationships of trace elements in the animal body are intricate, with synergistic or antagonistic effects; some metabolic mechanisms have not yet been clearly studied, and most studies at home and abroad have been conducted on the effects of one or two trace elements on egg quality. Therefore, when formulating laying hen diets, only through scientific and rational trace element nutrition allocation can the egg quality of laying hens be effectively improved, market demand be met, and the nutritional value and economic benefits of eggs be enhanced. Integrating the above metabolic roles of various trace elements with the NRC standards, we believe that during the laying period of laying hens, dietary levels of each trace element of manganese 60 mg/kg, zinc 50 mg/kg, iodine 0.35 mg/kg, chromium 0.65 mg/kg, and selenium 0.12 mg/kg are relatively favorable for the production performance of laying hens and egg quality.