Amani  Firoozabad, esmaiil, Hassanpour, Ali, ganj khanloo, amir, Dolatabadi, Abdollah. “Comparison of serum beta-carotene and vitamin A levels in sheep of Khalkhal region in open and closed breeding system.” Veterinary Clinical Pathology The Quarterly Scientific Journal, vol. 14, no. 54, 2020, pp. 185-193. 10.30495/jvcp.2020.515131.1261