Artificial photoperiod in pregnant mares and its effect on pregnancy length and postpartum reproductive performance

The aim of the present study was to evaluate the effect of prepartum artifi cial photoperiod on pregnancy length and reproductive performance in mares. The study included 145 Thoroughbred mares with age between 4 and 23 years. The mares were submitted to a 15 hour light/9 hour dark photoperiod/day (1 0W/m(2)) during different lengths of time before fooling, constituting the following groups: <30: less than 30 days of artificial photoperiod; 30-45: from 30 to 45 days of artificial photoperiod; 46-60: from 46 to 60 days of artificial photoperiod; >60: more than 60 days of artificial photoperiod. I n the moment of delivery, mares were classified according to body condition score (crescent range from 1 to 5) and, when necessary, submitted to surgi cal repair of the vulva within 30 minutes. Mating occurred when a follicle larger than 40 mm was detected, associated with decrease of uterine edema, Twin pregnancies were reduced manually. The number of days in which mares w ere submitted to artificial photoperiod influenced significantly the anoest rus rate postpartum (p=0.033), as well as the interval between parturition and the first ovulation (p=0.019). Mares submitted to a larger artificial p hotoperiod showed anoestrus postpartum rate significantly smaller than that observed in mares submitted to artificial photoperiod of less than 30 days (>60 - 0%; <30 - 16.7%). Parturition-ovulation interval was significantly shorter in mares submitted to artificial photoperiod for more than 30 days (p<0.001). Body condition was influenced by photoperiod. Treatment groups s ubmitted to artificial photoperiod for more than 45 days presented better b ody condition than that submitted for less than 30 days. The body condition influenced anoestrus postpartum rate (p=0.07) and parturition-ovulation in terval (p=0.073). Comparing only mares with body condition score 4 (n=81), a reduction in parturition-ovulation interval in mares submitted to a longe r artificial photoperiod was seen (p=0.072). Pregnancy length was not influ enced (p=0.077) by the length of photoperiod. There was no influence either on pregnancy rate in foal heat (p>0.1), or on embryo death rate (p>0.1), o r on number of cycles per pregnancy rate (p=0.798). It was concluded that t he use of a 15-hour light/9 hour dark photoperiod/day (10W/m(2)) during at least 60 days before delivery decreases significantly the incidence of anoe strus postpartum. The use of this photoperiod for more than 30 days decreas es parturition-ovulation interval. There is no influence either on pregnanc y rate during foal heat, or on embryo death rate, or on number of cycles pe r pregnancy.