Effect of Age on the Date of First Ovulation in Mares Subjected to an Artificial Lighting Regimen During Seasonal Anestrus

Previous studies have shown that artificial lighting can reduce the duration of anestrus in the horse. Here, we investigated whether age can affect the date of first ovulation in mares subjected to an artificial lighting regimen beginning before the winter solstice. Mares (n = 224) of different breeds aged 1 to 27 years were divided into five groups by age: a) 1–5 years (n = 71), b) 6–10 years (n = 38), c) 11–15 years (n = 44), d) 16–20 years (n = 30), and e) 21–27 years (n = 16). 25 mares apparently failed to respond to treatment and were excluded from the analysis. Light treatment began on a mean starting date of November 27. Ovulation was determined through transrectal ultrasonography at regular intervals. The age of the mare significantly influenced the date of first ovulation, with mares aged 1 to 5 years ovulating significantly earlier than all other age groups (P < 0.01). The interval from the start of treatment to ovulation in this group was shorter than for all other groups. A significantly larger proportion of mares in the youngest group (25.7%) ovulated before February 1 compared to all other age groups combined (6.9%). The proportion of mares that failed to respond to treatment was significantly larger in mares older than 15 years (P < 0.01). These results show that under winter artificial lighting, first ovulation occurs earlier in young mares, likely due to a lessened entrainment to the natural photoperiod.