The aim of this study was to evaluate the apparent digestibility coefficient of a diet with Angleton hay and concentrate based on rice by-products, and its impact on parameters of follicular dynamics. Seven Colombian criollo mares were fed Angleton hay ad libitum (T1) and Angleton hay plus concentrate containing rice by-products (T2) for periods of 75 days. Samples of the two diets were taken for bromatological analysis, faecal samples for the determination of the apparent digestibility coefficient and blood samples for the determination of metabolic and hormonal analytes. Additionally, follicular monitoring was performed through transrectal ultrasonography. The concentrate made with rice by-products improved the apparent digestibility coefficient of the fibrous fraction of the diet and influenced the daily growth of the preovulatory follicle diameter, doubling the levels of FSH and LH.