Abstract
Background and Aim: Improving reproductive efficiency is a major objective in sheep production because increased lamb output directly enhances flock productivity and economic returns. Ovariocytotoxic serum (OCS), an organ-specific hyperimmune preparation produced against ovarian tissue antigens, has been proposed as a biological alternative to conventional hormonal reproductive management. However, its effects on innate humoral immunity and reproductive performance in sheep remain insufficiently characterized. This study evaluated the effects of OCS administration on hematological and humoral immune parameters, estrus and insemination dynamics, and reproductive outcomes in Kazakh Merino ewes under field conditions. Materials and Methods: A controlled, non-randomized field study was conducted during the breeding season using 200 clinically healthy Kazakh Merino ewes allocated into an OCS-treated group (n = 100) and an untreated control group (n = 100). Treated ewes received two subcutaneous OCS injections before the planned insemination campaign. Hematological indices and serum lysozyme, bactericidal, and complement activities were evaluated in a laboratory subset of 20 ewes per group at five sampling points. Estrus detection, artificial insemination, lambing performance, twinning rate, lamb output, and lamb survival were recorded. Reproductive outcomes were analyzed using Fisher's exact test or chi-square test, and effect estimates were expressed as relative risk (RR), odds ratio, and 95% confidence interval (CI). Results: OCS administration was associated with descriptive increases in humoral innate immune parameters. Mean lysozyme activity increased from 29.1% to 37.3%, bactericidal activity from 49.4% to 58.8%, and complement activity from 19.8% to 21.6% around estrus. By day 25 of the insemination campaign, 93.0% of OCS-treated ewes had exhibited estrus and been inseminated compared with 46.0% of controls (RR = 2.02; 95% CI: 1.62-2.52; p < 0.0001), reducing the insemination period from 21 to 14 days. Twin lambings were significantly more frequent in the OCS group (32.0% vs. 14.0%; RR = 2.29; 95% CI: 1.30-4.02; p = 0.004), increasing lamb output from 104 to 129 lambs per 100 ewes. Although lambing rate was numerically higher following OCS treatment (97.0% vs. 90.0%), the difference was not statistically significant (p = 0.082). Conclusion: OCS administration was associated with enhanced humoral innate immune responses, accelerated estrus and insemination dynamics, increased twinning rate, and greater lamb production in Kazakh Merino ewes. These findings support the potential of OCS as a hormone-sparing biological approach for improving reproductive efficiency under field conditions. Nevertheless, randomized placebo-controlled multicenter studies with comprehensive endocrine, immunological, and long-term safety evaluations are required before routine application in sheep breeding programs can be recommended.