This study was carried out to monitor the fertilizing ability of rabbit semen frozen in the presence of melatonin as antioxidant. Semen from 10 mature Egyptian Baladi red bucks was pooled, extended with Tris-glucose-citric extender (1:1 v/v) supplemented by three concentrations of melatonin (10^-9, 10^-6 and 10^-3 M) in addition to control, and finally cryopreserved at -196°C. After thawing, sperm kinetics, antioxidant capacity and fertility traits were evaluated. The results showed that total and progressive motility percentages were higher significantly in samples cryopreserved by melatonin (10^-6 M) as compared to control group. Curvilinear and average path sperm velocities were significantly higher in melatonin groups of 10^-9 and 10^-6 M than in the control group. Supplementation of melatonin at 10^-9 M and 10^-6 M groups decreased (P < 0.05) malondialdehyde activity when compared to the control one. In contrast, the total antioxidant capacity and catalase activity were significantly (P < 0.05) elevated at concentration of 10^-6 Mmelatonin as compared to the control group. In addition, transcript abundance of NFE2L2 and SOD1 genes increased in groups supplemented with melatonin compared to the control group.

The fertility trial indicated that pregnancy rate, and the total and live born increased significantly (P < 0.05) in rabbit does inseminated with semen of 10^-6 M melatonin group in compared to the control samples.
Conclusively, these results concluded that using melatonin with 10^-6 M level in rabbit freezing extenders could be recommended to improve semen quality and fertilizing ability of buck sperm post-thawing.