Abstract
This study aimed to evaluate the influence of seasonal variations on foal estrus breeding success in mares in Iraq by analyzing pregnancy rates and preovulatory follicular diameters across four distinct seasons over a calendar year. The investigation was conducted at the Veterinary Clinic of the College of Veterinary Medicine, University of Diyala, Iraq, and involved forty-seven lactating mares aged between 5 and 14 years. Follicular dynamics, cyclicity patterns, and preovulatory follicle sizes were monitored via transrectal palpation followed by ultrasonographic examinations using a portable ultrasound scanner equipped with a 5.0–8.5 MHz multi-frequency linear transducer, performed by a single operator from day 7 until day 13 postpartum. All mares were naturally bred during the foal heat on days 9, 11, and 13 postpartum, and pregnancy diagnosis was confirmed between days 15 and 40 after the final breeding session via the visualization of the embryonic vesicle or early embryo. The results demonstrated that the mean preovulatory follicular diameters peaked significantly during autumn (37.9±2.17 mm) and summer (37.6±2.06 mm) compared to winter (34.1±1.59 mm) and spring (33.6±1.84 mm) (P≤0.05), with the least significant difference value established at 3.081. Concurrently, seasonal shifts exerted a highly significant effect on breeding outcomes (P=0.0087), yielding an overall pregnancy rate of 57.45 %. The highest pregnancy success was recorded during summer (66.67 %) and spring (65.00 %), followed by autumn (50.00 %), whereas the winter season exhibited a severe reproductive decline, characterized by the lowest pregnancy rate (37.50 %) and a corresponding peak in non-pregnancy outcomes (62.50 %). The study concludes that regional seasonal climatic conditions act as primary drivers of equine fertility, heavily favoring warmer and photo-stimulated periods. These findings underscore the necessity of integrating strategic seasonal planning and management into local equine breeding programs to optimize reproductive performance and minimize the economic losses associated with failed breeding attempts in unfavorable seasons.