ABSTRACT The objective of this study was to evaluate the effect of environment on follicular diameter and repeatability of the size of preovulatory follicles of mares. Temperature, rainfall, and photoperiod were measured, and their effects were evaluated on follicular dynamics. Data were studied by analysis of variance of follicular size and environment traits. During the five years, 7% double ovulations and 159 anovulatory follicles were recorded. Mean preovulatory follicle diameters of left and right ovaries were 39.3±3.8 and 39.2±3.5 mm respectively. There was no effect of evaluation year on follicular diameter. Reproduction season affected the right preovulatory follicle diameter. There was no effect of temperature and rainfall on follicular diameter. Repeatability values of the preovulatory follicular diameter were low. In the subtropical climate studied, temperature and rainfall caused no effect on follicular dynamics, and photoperiod had only a small effect on dominant preovulatory follicular diameter. Repeatability of follicular diameter was low, so this characteristic should be evaluated daily to predict ovulation and the optimal time to inseminate mares.

RESUMO Avaliação do tamanho dos testículos é muito importante na seleção de garanhões, uma vez que é um determinante indireto do seu potencial reprodutivo. O objetivo deste estudo foi avaliar as características de biometria testiculares, calcular o volume testicular e analisar a concentração sérica de testosterona de garanhões. Durante seis meses, foram medidos os testículos de garanhões Pôneis da raça Brasileiro adultos, criados no Norte do Estado do Rio de Janeiro, Brasil. O comprimento a largura e a altura dos testículo (TL, TW e TH) foram mensurados. A partir dessas medidas, calculou-se o volume testicular total (TTV). Os dados foram avaliados por análise de variância e foram calculadas as correlações entre as características. A média geral das características de biometria testicular e volume testicular foram: TL 5,8 ± 0,6 cm; TW 3,8 ± 0,5 cm; TH 4,7 ± 0,4 cm e TTV 111.2 ± 27,7. A média de concentração sérica de testosterona foi de 0,89 ± 0,52 ng/dl. Altas correlações foram observadas entre as características de biometria testicular e volume testicular. Baixas a médias correlações entre as características testiculares com produção espermática e com concentração de testosterona foram observadas. O tamanho testicular pode ser uma importante característica que auxilia na predição do potencial reprodutivo do garanhão pôneis da raça Brasileira. Sugere-se que não há variações na concentração sérica de testosterona nos pôneis da raça Brasileira que poderia afetar em sua fertilidade.

SUMMARY Evaluation of the testis size is very important in stallions’ selection, since it is an indirect determinant of their reproductive potential. The aim of this study was to evaluate the biometric testicular traits, calculate the volume testicular and analyze the serum testosterone concentration of the stallion. During six months were measured the tests of adults Brazilian Ponies stallions from northern of Rio de Janeiro State, Brazil. The length, the width and the height of the testicles (TL, TW and TH) were measured. From these measurements, the total testicular volume (TTV) was calculated. The serum testosterone concentration was also evaluated. The data were studied by analysis of variance and were calculated the correlations among the traits. The average of the testis biometry were TL 5.8 ± 0.6 cm; TW 3.8 ± 0.5 cm; TH 4.7 ± 0.4 cm and 111.2 ± 27.7 for TTV. Mean of serum testosterone concentration was 0.89 ± 0.52 ng/dl. High correlations between biometry testicular traits with testicular volume were observed and low to moderate correlations were observed for testicular traits, testosterone and sperm output. The testis size could be an important trait that assists to the prediction of stallion's reproductive potential of the Brazilian ponies stallions. It is suggested that there would be no fluctuation in serum testosterone concentration in Brazilian ponies stallions that coult afect on its fertility.