Efficacy evaluation of novel organic iron complexes in laying hens: effects on laying performance, egg iron content and blood biochemical parameters

Iron (Fe) is an indispensable element for organisms that plays crucial roles in several fundamental metabolic processes, including erythropoiesis, oxygen transport, DNA synthesis, energy metabolism, mitochondrial electron transport, immune-protection, and cognitive function (Abbaspour et al., 2014; Zhou et al., 2018; Drygalski and Adamson, 2013). Iron should be maintained at an appropriate level which is required to satisfy the metabolic needs and specialized functions for animals (Wang and Pantopoulos, 2013), while Fe deficiency causes anaemia, abnormal development, and severe deficiency could lead to death (Sarlak et al., 2021; Baker and Greer, 2010). Traditionally, hens are fed with a form of inorganic Fe salts in the diets, such as Fe sulfate (FeSO4) (Bess et al., 2012). However, the inorganic Fe sources have several issues, like low bioavailability, high oxidation, and excretion, which can pollute the environment (Ma et al., 2014). As a result, chelates or complexes of Fe with amino acid or protein, organic Fe compounds has been developed as substitute for inorganic Fe. Meanwhile, it was reported that organic trace elements have higher bioavailability compared with inorganic microelements (Xie et al., 2019). The relative bioavailability of organic mineral elements was determined by the chelation strength and greater chelation strength indicates higher biological value and beneficial effects to laying hens (Zhang et al., 2016). Studies are mainly focused on ferric glycine (Fe-Gly). Compared with the same dosage of FeSO4, dietary supplementation of 60 ppm from Fe-Gly improved egg weight and increased Fe in egg albumen and yolk (Xie et al., 2019). However, a new organic form of Fe active compounds, Fe amino acid complexes (1:1 complex of Fe lysine and Fe glutamic acid, Fe-Lys-Glu), in laying hens has not been studied. The purpose of this study was to investigate the effects of dietary Fe-Lys-Glu level on laying performance, egg Fe content, and blood biochemical parameters in laying hens.

A total of 1260 18-week-old Beijing White laying hens with similar body weights were randomly allocated into 7 groups with 12 replicates (15 birds/rep). After 2-week adaptation to the environment and to the basal corn-soybean meal diet (formulated nutrient requirements based on NRC, 1994), hens were fed with 0 (negative control, NC), 15, 30, 45, 60, and 75 ppm Fe as Fe-Lys-Glu or 45 ppm Fe as FeSO4 (positive control, PC) for 24 weeks. Analyzed Fe content on basal diet was 75.1 ppm. Every three birds were placed in a cage (0.50m length×0.45m width ×0.40m height) with a 16L:8D photoperiod with 15 to 20 lux light intensity during experimental period. Feed and water were provided ad libitum. The housing temperature and relative humidity were maintained at 24°C±3°C and 50% to 60%, respectively.

Egg numbers and total egg weight of each replicate were recorded daily, and the feed consumption was weighed once a week. Based on the collected data, the laying rate, average daily egg mass, average daily feed intake, and feed conversion ratio were calculated.

For the Fe content in egg yolk at the end of 12 and 24 wk, 6 eggs per replicate were randomly selected to measure the Fe concentrations in egg yolk using AAS according to the method described by Revy et al. (2004). The results were expressed as mg/kg fresh weight. At the end of the trial, one bird was chosen in random from each replicate (n=12). After fasting for 12 h, blood samples from the wing vein were collected to determine blood indexes as red blood cells (RBC), hemoglobin (HGB), and hematocrit (HCT) by a Sysmex microcell counter CL- 180 automated hematology analyzer (Sysmex Company, Lincolnshire, IL, USA). Part of the blood was centrifuged at 3500 r/min for 10 min at 4°C to obtain samples for serum Fe content determination measured by AAS (Revy et al., 2004).

All data were analyzed with general linear model procedure of SPSS software (SPSS 21.0, SPSS Inc., Chicago, IL, USA) and multiple comparisons between the groups were performed by least significant difference test. Besides, the first six groups of hens fed with 0, 15, 30, 45, 60, and 75 ppm Fe from Fe-Lys-Glu in diet of data were analyzed by linear and quadratic regression analysis. P < 0.05 was considered statistically significant.

The results are summarized in Table 1 and the regression analysis of sensitive indexes are presented in Table 2. 

The present study found a remarkable increase in LR of layers fed with 15 to 75 ppm Fe and ADEW when fed 30 to 75 ppm Fe as Fe-Lys-Glu compared with diet without adding exogenous Fe. Additionally, FCR of hens fed 45, 60, 75 ppm Fe from Fe-Lys-Glu was decreased markedly in comparison with NC group. It is because Fe organic source elevates the absorption of trace mineral elements in hens by reducing the ability to bind with nutrients or antinutritional factors, thereby improving the laying performance (Kwiecień et al., 2015). However, when the dosage of Fe-Lys-Glu reached 75 ppm Fe, the laying performance showed a downward trend compared to 60 ppm. This suggests that optimal dosage of Fe-Lys- Glu should not exceeding 60 ppm. Additionally, even though the addition level of Fe-Lys-Glu was lower than that of FeSO4, it still showed better laying performance. It indicates that Fe-Lys-Glu has greater bioavailability than inorganic ferrous. Through the quadratic regression analysis of LR and ADEW, we can see that when the supplemental dose of Fe-Lys-Glu reached 48.6 and 49.5 ppm Fe, respectively, the laying performance of laying hens reached the best.

Several studies reported that Fe contents in egg yolk were affected by different sources of Fe and experiment period (Sarlak et al., 2021; Bess et al., 2012). They found that adding Feamino acids (Fe-AA) to diets distinctly increased the enrichment of Fe in egg yolk, compared with adding FeSO4. We found that additive level of Fe-Lys-Glu and experimental period effectively affected the deposition of Fe in egg yolk, which was consistent with previous studies. 

Iron is a necessary element for RBC production and synthesis of HGB (Trivedi and Bardi, 2021). It was reported that adding Fe-Gly in piglet diets increased serum Fe, RBC count, HGB content, and HCT compared with negative control and FeSO4 groups (Li et al., 2018). Similar results were found in pregnant sows (Egeli et al., 1998). The results may demonstrate that organic Fe has higher biological value than inorganic Fe as our study presented an upward linear tendency in Fe content of serum with increasing dosage of Fe-Lys-Glu. The RBC count and HGB content were significantly increased in hens fed 30 to 75 mg Fe/kg from supplemented Fe-Lys-Glu diet.

In conclusion, the present study demonstrated that Fe-Lys-Glu was an effective Fe source that could substitute for FeSO4 in laying hen diets. Dietary Fe-Lys-Glu supplementation is beneficial to improve the laying performance, yolk Fe content and blood biochemical parameters of laying hens in varying degrees. Based on the quadratic model analyses, supplemental 45 ppm Fe via Fe- Lys-Glu could provide the optimum laying performance (LR and ADEW). It is recommended that the dosage of Fe-Lys- Glu should not be higher than 60 mg Fe/kg.