Long-term micronutrient and amino acid supplementation increases embryo recovery per flush and ovarian responsiveness in donor mares.

Abstract: Nutritional strategies may modulate ovarian function and embryo recovery in donor mares, but field evidence for long-term multinutrient supplementation remains limited. Objective: To determine whether long-term micronutrient and amino acid supplementation increases embryo recovery per flush and ovarian responsiveness in donor mares. Methods: A total of 115 uterine flushes were performed under a crossover design across two consecutive breeding seasons in 12 multiparous, non-lactating crossbred donor mares (4-12 years). Mares received a basal diet or the same diet plus 40 g/day of a multinutrient supplement for 150 days; treatments were reversed after a 6-month washout. Follicular development was monitored by transrectal ultrasonography, and ovulation was induced when preovulatory criteria were met. Fixed-time artificial insemination with frozen semen was performed 24 h after ovulation induction. Ovulation was assessed 48 h after induction; if ovulation had not occurred, mares were rechecked every 12 h until ovulation was detected. Day 0 was defined as the day ovulation was first detected for scheduling embryo recovery. Results: Of the 115 flushes (supplemented: 60; control: 55), embryo recovery rate was higher with supplementation (49/60, 81.7%) than in controls (36/55, 65.4%), risk difference +16.2 percentage points (95% CI 0.3-32.1; P = 0.048). Mare-level follicle counts increased during supplementation for multiple small follicles (≤15 mm; 115.3 ± 10.3 vs 112.7 ± 11.0; P = 0.003) and follicles ≥30 mm (24.7 ± 3.1 vs 23.1 ± 2.9; P = 0.012). Conclusions: Long-term multinutrient supplementation increased embryo recovery per flush and mare-level ovarian follicular activity in donor mares; embryo morphology-based quality grade distributions were similar between the control and supplemented phases.