The most used reproduction method in beef cattle in Brazil is natural breeding, which corresponds to 84% of calves born. Breeders adopt the bull:cow ratio of 1:25, which results in a sub utilization of bulls, making natural breeding antieconomic and underestimating the reproductive ability of competent bulls. The bull:cow ratio is determined by several factors, such as climate and nutrition, but the most determining factor is the fertility of the bull, estimated with more precision through breeding soundness examination of bulls. The aim of this study was to develop a simulation model as an aid to choose the best bull:cow ratio according to the combination of the many factors that determine this ratio. It is a conceptual, empiric, static, and determinist model which, through the processing of the input data, simulates the best BCR. Developed on the Vensim PLE 6.1 software, the model describes variables related to bulls and cows. Two equations were generated to predict the mating potential of males. In the same way, according to the reproductive status of females, equations were generated to predict the cyclicity rate of the cows. The sum of these equations originated the one that determines BCR according to the reproductive status of females. This model might serve as a tool to support decisions as to the best BCR and could be used with several combinations of models´ characteristics.

Objective: Selenium (Se), as the form of selenite, is commonly supplemented in poultry diet, which has low bioavailability and high toxicity. Here, we compared the effects of the supplemen¬tation of the diet with Se nanoparticles (nano-Se) on the growth, sexual behavior, and reproduc¬tive performance (gonad size, sperm quality traits, and plasma testosterone levels for males and egg production for females) of Japanese quail (Coturnix coturnix japonica). Materials and Methods: Quail chicks (n = 300) aging 14 days were divided into three groups: Group 1 (basal diet and Se at 0.2 mg/kg ration), Group 2 (basal diet and nano-Se at 0.2 mg/kg ration), and Group 3 (basal diet and nano-Se at 0.1 mg/kg ration). Several parameters relating to body weight and egg were measured. Sexual behaviors of the birds were observed by continuous visual scanning. The sperm viability, sperm morphology, and concentration of spermatozoa were determined after staining and microscopic examination. The plasma testosterone levels were determined by indirect enzyme immunoassay assay. The Se concentrations in the testicular, ovar¬ian, and ration samples were measured by flame emission atomic absorption spectrophotometer. Results: The type or concentration of nano-Se administration had no impact on body weight, feed efficiency, egg production, or egg weight. However, the total feed intake throughout the experi¬ment was reduced in Group 2 at 0.2 mg/kg. Nano-Se supplementation significantly increased the sexual behavior. In general, the deposition of Se in the testicular and ovarian tissues increased with increasing supplement concentration. At the same supplement concentration, the tissue deposition of nano-Se was more enhanced than that of inorganic Se. Nano-Se supplementation improved the testicular functions by enhancing plasma testosterone level and sperm quality traits (sperm count, motility, and viability). This improvement was found more prominent with the lower supplement concentration (when comparing 0.1 vs. 0.2 mg/kg diet). Conclusion: It is concluded that the use of nano-Se (at 0.1 mg/kg) in the ration of Japanese quail improves several reproductive performance parameters. [J Adv Vet Anim Res 2020; 7(4.000): 597-607]