Viana, C.H.C. et al. 1999. Relationships between the characteristics weaning-to-estrus interval, estrus duration and moment of ovulation by ultrasonography in sows. Brazilian Journal of Veterinary Research and Animal Science. 36, 4 (Jan. 1999), 212–218. DOI:https://doi.org/10.1590/S1413-95961999000400008.