Ultrasonographic evaluation of the pre-pubertal development of the reproductive tract in beef heifers

To study the development of the reproductive tract in heifers, the ovaries, uterus, cervix and vagina were examined by transrectal ultrasonography every 2 weeks, from 2 to 60 weeks after birth. First ovulation occurred at 63.7±1.1 weeks of age. Ovarian dimensions increased rapidly from 2 to 14 weeks of age, and increased again after 34 weeks of age (P<0.05). The size of the largest ovarian follicles increased from 8 to 14 weeks of age, from 38 to 42 weeks of age, and finally from 52 to 60 weeks of age (P<0.05). The number of follicles ≥3 mm in diameter tended to increase from 6 to 14 weeks of age (P<0.10) and increased significantly from 6 to 60 weeks of age (P<0.05). Mean numerical pixel values of the ovarian images decreased from 4 to 26 weeks of age, and then rose to 44 weeks of age (P<0.05). Diameter of the uterine body, cervix and vagina increased from 2 to 20–24 weeks of age, and again after 32 weeks of age (P<0.05). Mean numerical pixel values for the uterus and vagina decreased initially (uterus: 4–8 weeks and vagina: 6–22 weeks of age) and then increased (uterus: 14–42 weeks and vagina: 22–32 weeks of age; P<0.05). Pixel heterogeneity showed a consistent peak at 20–22 weeks of age for the uterus, cervix and vagina (P<0.05). In summary, in the heifer calf, the marked growth of the reproductive tract in the first few months of age, and prior to first ovulation, reflects phases of increased ovarian follicle (≥3 mm in diameter) numbers and size. Ultrasonographic image analysis revealed patterns of numerical pixel values and heterogeneity that may be useful in determining important stages of growth and differentiation of the reproductive system.