Essential oils are aromatic, volatile liquids obtained from plant material through steam distillation and named after the plant from which they are derived [1] . They are volatile and highly concentrated in active molecules having diverse properties including antiparasitic, antifungal, antibacterial, anti-inflammatory and antioxidant activities [2] . In animal production, these properties are utilized to limit the loss of energy in favor of growth and reproduction [3] .

Curcuma longa (turmeric) is a perennial medicinal plant that belongs to the ginger (Zingiberaceae) family and is a major source of prominent polyphenol curcumin [4] . Kamari et al. [5] and Tozo et al. [6] reported that Curcumin may act as an anti-infectious, anticarcinogenic, antifungal, anti-inflammatory, antiparasitic, antiviral, antioxidant, and antimutagenic agent [7] [8] . Diferuloylmethane is the main phenolic compound of turmeric rhizome powder that has an antioxidant effect [9] [10] . It inhibits lipid peroxidation, scavenges the superoxide anion and hydroxyl radicals [11] , and enhances the activities of detoxifying enzymes, such as glutathione-S-transferase and Superoxide dimustase [12] . It also increased the level of antioxidatives enzymes [13] . In addition, Curcumin is a phytoestrogen [14] and can, therefore, interact with the endocrine system, affect the hypothalamo-hypophysial-ovarian axis [15] . Inano et al. [16] and Thakur et al. [17] reported that this interaction of Curcumin with the endocrine system may be increased respectively the levels of LH and Estrogen in mousse irradiate. Moctar et al. [18] indicated that supplementation of laying hen diet with Curcuma longa root at the rate of 3% increased hatchability traits and eggs weight.

It is widely known that during the later stages of egg incubation, chick embryos may experience stress caused by excessive heat production [19] . The development of the chick embryo is associated with the accumulation of highly polyunsaturated fatty acids within the lipids of several embryonic tissues [20] [21] . This makes embryonic tissues highly susceptible to lipid peroxidation and free radicals through hatching period [22] . Tissue specific features in the susceptibility to lipid peroxidation were found with the brain being the most susceptible to lipid peroxidation at day 25 and in day-old poults of Turkey [22] and at day 15 of incubation period of chicken eggs [20] . Therefore, the integrated antioxidant systems in the chicken tissues are responsible for protection of polyunsaturated fatty acids, protein and DNA from damaging effect of free radicals and toxic products of their metabolism [22] . In such condition, oxidative stress may be a problem during the last days of prenatal and 1^st days of postnatal chick life. This leads to decrease hatchability and increase mortality post hatch and consequently economic impact on the poultry industry [23] . These necessitate the development of effective antioxidant capacities in the tissues to prevent lipid peroxidation. The antioxidant system of the embryo and newly hatched chick is based on antioxidant enzymes such as superoxide dismutase, glutathione reoxidase and catalase [24] , ascorbic acid [25] , reduced glutathione [26] and carotenoids [27] .

Several studies have been carried out on the use of Curcuma longa rhizomes powder in the diets of broiler chickens to improve growth [28] [29] and reproductive parameters [30] - [32] . These studies showed some positive effects of Curcuma longa root powder on reproductive performances but, the effects of Curcuma longa rhizomes essential oil which is highly concentrated in active molecules, show limited studies on animal reproduction, especially in birds. The objective of this study is to investigate the possible effects of Curcuma longa rhizomes essential oil on some reproductive parameters in laying Japanese quails and study the effect on egg yolk oxidative stability during storage.

The improved performance of the birds’ tread with 225 μl/kg b.w of Curcuma longa roots essential oil in the present study agrees with the finding of Suvanated et al. (2003) [48] and Durrani et al. [49] who reported that broiler chicks fed dietary turmeric powder had a higher final body weight, and body weight gain. However, Tchoffo et al. [3] reported that ginger roots essential oil did not have effect on the growth characteristics in female quail. The variation could be as a result of the nature of essential oil. The current study has also shown that beyond 225 μl/kg b.w, there was no additional benefit from consumption of curcuma longa roots essential oil. In fact, it appears that this is the tolerable level for the birds as it could be seen that administration of female quail up to 225 μl/kg b.w enhanced the egg yolk MDA. The improved performance in the current study could be attributed to the antioxidant activities of curcumin which could have stimulated feed intake and protein synthesis by the enzymatic activities in the birds [10] . This trial has revealed that inclusion of 225 μl/kg b.w of turmeric essential oil is sufficient to improve the performance of bird. This result is an indication that turmeric at this level enhanced efficient use of nutrients in the birds. Superoxide dismutase is one of the antioxidant enzymes, which are capable to minimize oxidative stress in the organelles [50] . The higher level of SOD, GPx and CAT activities in the yolk eggs in the present study is in agreement with the observation of Tchoffo et al. [51] who reported that ginger essential oil increased activity of SOD, GPx and decreased MDA level in yolk eggs in female quail. The same results were observed by Zainali et al. [52] who reported that turmeric powder enhanced the antioxidant status of heat-stressed broilers by improving the activity of glutathione peroxidase and superoxide dismutase and reducing the concentration of MDA. The high-performance indices recorded in the bird’s treated group may be explained by the inhibition of the deleterious effect of lipid peroxidation brought about by the decreased free radical generation as reflected by low yolk MDA and high SOD, GPx and CAT activities. Altan et al. [53] [54] reported that the elevated level of SOD, GPx and CAT may be an indication of a protective mechanism against oxidative stress and lipid peroxidation.

The degree of lipid peroxidation is usually a biomarker of ROS-mediated damage [55] and the concentration of MDA is often used as indicators of lipid peroxidation [56] . MDA is a product of peroxidation of unsaturated fatty acids in phospholipids and is responsible for cell membrane damage [32] . MDA of the birds received curcuma longa essential oil was lower than those of the birds in the control group. This may be due to antioxidant compounds (Curcumin) present in curcuma longa essential oil transferred to eggs and deposited into yolk. This antioxidant scavenges superoxide anion and hydroxyl radicals. Masuda et al. [57] stated that, an antioxidant mechanism of curcumin in polyunsaturated lipids was proposed, which consisted of an oxidative coupling reaction at the 3-position of the curcumin with the lipid and a subsequent intramolecular Diels-Alder reaction. [58] suggested that dietary turmeric lower lipid peroxidation by enhancing the activities of antioxidant enzymes (superoxide dismutase, catalase and glutathione peroxidase).

Egg and yolk weights increased in quails exposed to the curcuma longa rhizome essential oil with reference to the control. These results are in agreement to those reported by Tchoffo et al. [51] with ginger essential oil in laying Japanese quail. The increase of egg and yolk weight in treated quails is related to the phenolic compounds because of their antioxidant property would reduce the peroxidation of cells or nutrients involved in the production of eggs and subsequently promote its good formation [3] . A negative and significant (𝜌 = −98; p < 0.05) correlation was found between the level of MDA and egg weights, while the positive and significant (𝜌 = +0.92; p < 0.05) correlation was found between the egg and the yolk weights. Based on these results, the level of MDA decreases while the egg weight increases and the egg yolk weight increases as the egg weight increases. The increase in egg component provides sufficient nutrients and centrally placed yolk to support embryonic growth and development [3] .

Our study revealed that the curcuma longa rhizomes essential oil at dose of 150 µl/kg b.w caused significant (p < 0.05) decrease of embryonic mortality compared to those of control group. The low level of embryonic mortality recorded in the birds in the group who received 150 µl/kg b.w may be explained by the inhibition of the deleterious effect of lipid peroxidation brought about by the decreased free radical generation as reflected by low egg yolk MDA and high SOD, GPx and CAT activities.

The present study showed a significant increase in estradiol with the increase of curcuma longa rhizomes essential oil dose. The increased estradiol level may explain by the presence of curcumin which is a phytoestrogen [14] [15] compound content in the essential oil of curcuma longa and can, therefore, interact with the endocrine system, and may stimulate the synthesis of estradiol by acting on the hypothalamic-pituitary-gonad axis. Estrogens are steroid hormones which, together with other hormones, control the ovulatory cycle in the female animal [35] . This estrogen acts in a feedback mechanism, influencing the production of follicle stimulating hormones FSH and LH from the pituitary gland [59] . It is known that the FSH in turn promotes the development of the immature ovarian follicles, which increases the production of estrogen from the ovary. This is readily done if normal quantities of exogenous estrogen are administered, thus prevent ovulation by stimulating the release of the gonadotropin releasing factor from hypothalamus, that is exogenous hormones exert positive feedback on the hypothalamus in a manner similar to that by the naturally occurring hormones.

Our finding shows significant increase level of LH and FSH in female Japanese quails which received the essential oil compared to the control. The similar observation was reported by Mengjie et al. [32] in the Hy-Line brown hens exposed to heat stress and treated with curcuma roots powder. We suggest that the increase level of LH and FSH in the present study were caused by the action of curcumin which has a strong phyto-estrogenic properties [31] and can, therefore, interact with the endocrine system [15] .

In the present study, it was observed that: fertility, total hatchability and the hatchability of fertile eggs rates increased with essential oil dose. These results are comparable to those found by Ali et al. [60] who reported that individually added of 0.5% to 1% curcuma to chicken diets increased fertility and hatchability. Tchoffo et al. [3] reported the same result in the female Japanese quail who received orally 100 µl/kg b.w of essential oil of ginger. In fact, the Curcuma longa rhizome essential oil contains a number of antioxidants compounds such as tetrahydro-curcuminoids [10] ; curcumin, dimethoxy-curcumin, and bisdemethoxy-curcumin [61] . These properties increased oxidative metabolism, especially in the final few days before hatching, as a normal result of embryonic growth. It is reported that over increment lipid peroxidation may lead to tissue damage [23] , whereas diets with antioxidants properties as ginger may protect the embryo and therefore increase hatchability and chick’s survival rate. A negative and significant correlation was recorded between the MDA and: the embryonic mortality (𝜌 = −0.92; p < 0.05); the hatchability of fertile eggs (𝜌 = −0.75; p < 0.05) and the fertility (𝜌 = −0.95; p < 0.05).

The results of this study suggest that the inclusion of Curcuma longa rhizome essential oil in Japanese quail diet can improve the reproductive performances and oxidative yolk parameters in female quails. The effects on the quail production performance seemed to be dose dependent and Curcuma longa rhizome essential oil at the highest tested level (225 µl/kg.b.w) was the most effective treatment. Based on these findings, Curcuma longa rhizome essential oil can be considered a promising feed additive for managing reproductive processes in female poultry at level of 225 µl/kg.b.w. However, further studies are needed to conclude on the effect of Curcuma longa rhizome essential oil on egg quality and the quality of the poultry products as well as its mechanism of action.

Conceptualization, H.V.N.; methodology, H.V.N., H.T., and F.N.; software, H.V.N., H.T. and F.N.; validation, H.V.N., H.T., and F.N.; formal analysis, H.V.N., H.T., and F.N.; investigation and resources, H.V.N., H.T., S.N.D., D.A.K., S.T.N., and F.N.; data curation, H.V.N., H.T., S.N.D., D.A.K., S.T.N., and F.N.; writing original draft preparation, Z.M. and Z.N.; writing—review and editing, H.V.N., H.T., S.N.D., D.A.K., S.T.N., and F.N. All authors have read and agreed to the published version of the manuscript.

Experimental protocols used in this study were approved by the ethical committee of the Department of Animal Science of the University of Dschang (ECDAS-UDs 23/02/2015/UDs/FASA/DSAES) and was in conformity with the internationally accepted standard ethical guidelines for laboratory animal use and care as described in the European Community guidelines; EEC Directive 86/609/EEC, of the 24th November 1986 [39] .

Qualified researchers can obtain the data from the corresponding author.

The authors declare that no financial help was received.