Superovulatory response and embryo production following a vitamin–mineral rumen bolus in Holstein donor cows

Abstract Background Micronutrients involved in antioxidant defense may influence ovarian function and embryo competence, but evidence regarding combined vitamin–mineral rumen boluses administered before superovulation is limited. This randomized exploratory trial evaluated whether a single rumen bolus containing vitamin A, beta-carotene, vitamin E, zinc, and selenium, administered 21 days before superovulation, altered circulating micronutrient concentrations, ovarian response, and embryo outcomes. Twenty-two clinically healthy primiparous Holstein donor cows were allocated to bolus ( n = 11) or untreated control ( n = 11) groups across two experimental periods. All donors underwent the same synchronization, superovulation, artificial insemination, and embryo-recovery protocol. Blood was collected at baseline and on the day of embryo recovery, and embryo morphology was assessed by an evaluator unaware of group allocation. Results The bolus and control groups yielded 115 and 86 recovered structures, respectively, of which 103 and 65 were transferable. After adjustment for experimental period, however, neither total recovered structures (incidence rate ratio [IRR], 1.34; 95% confidence interval [CI], 0.91–1.92; P = 0.211; false-discovery-rate-adjusted P [q] = 0.685) nor transferable embryos (IRR, 1.56; 95% CI, 0.97–2.45; P = 0.127; q = 0.685) differed significantly between groups. The adjusted difference in donor-level transferable-embryo proportion was 12.7 percentage points (95% CI, − 5.2 to 32.3; P = 0.294; q = 0.685). Follicle, ovulation, corpus luteum, and non-transferable embryo counts also showed no statistically supported differences. No measured blood analyte showed a differential change between groups; all 95% CIs for fold-change ratios included 1 ( P ≥ 0.394; q = 0.853). Sensitivity analyses did not alter these interpretations. Conclusions Under the conditions of this exploratory study, bolus administration did not result in statistically supported improvements in circulating micronutrient status, ovarian response, or embryo outcomes. Although point estimates for total and transferable embryo counts favored the bolus group, the small sample and wide confidence intervals preclude firm conclusions about benefit or lack of benefit. Routine use solely to enhance superovulation outcomes is therefore not supported by these data. Larger, adequately powered trials incorporating dietary micronutrient characterization, verified bolus-release kinetics, and control of semen-source effects are warranted.