Utilização de diferentes períodos de fotoestimulação em éguas acíclicas para o controle da sazonalidade reprodutiva

The mares have a period of reproductive activity during the summer and in winter, poor follicular activity (anestrous). In order to anticipate the reproductive phase, this study aimed to evaluate the effect of different periods of photic stimulation on the occurrence of first ovulation of the breeding season in mares in seasonal anestrous. The experiment was conducted during the breeding seasons of 2009 and 2010, from 1 July to 1 August, were used 45 mares crossbred in anestrous between four and 12 years in Sao Simao - SP, Brazil. In the absence of the corpus luteum and follicles larger than 20 mm, mares were randomly separated into the following groups: G1 - control, without stimulation with artificial light (G1, n=15), G2 - stimulation with artificial light for 35 days (G2; n=15) and G3 - stimulation with artificial light for 60 days (G3, n=15). We used the Fisher exact test for analysis of ovulation rate to the first 60 days of the experiment and ANOVA followed by Tukey test to analyze the average number of days to first ovulation, all statistics were considered significant when P<0.05. The Fisher test was used to compare the mare proportion that ovulated until the first 60 days of experiment between the study groups. The Bonferroni method was used to adjust the  level for multiple comparisons. Survival curves and Cox proportional hazards model were used to compare the ovulation rate between the study groups (estimating the expression of hazard ratio. Statistical significance was defined as P<0.05 or P<0.02 for the Fisher test with multiple comparisons. The proportion of mares that ovulated until 60 days after the beginning of the treatment was significantly larger (84.6%) in the group treated with artificial light by 60 days, when compared to the control group (15.4%, P=0.001). There was statistical difference between the control group and the group treated with artificial light by 35 days (61.5%, P=0.041), however was not statistical difference between the groups treated by 35 and 60 days (P=0.378). The average ovulation time in the group treated with artificial light by 60 days was 53 days, while mares in the group treated with artificial light by 35 or mares in the control group ovulated at the 56 and 125 days, respectively. It is concluded that the reduction of treatment with artificial light for 35 days was not as effective as the conventional protocol of 60 days.