Estrus and luteal function in suckled beef cows that were anestrous when treated with an intravaginal device containing progesterone with or without a subsequent injection of estradiol benzoate.

The objectives in this study were to determine whether treatment with progesterone (P4) via an intravaginal device would induce estrus and development of corpora lutea (CL) with typical life spans and whether treatment with estradiol benzoate (EB) following device removal would enhance the responses. At treatment initiation (d 0), suckled beef cows (n = 362) that were anestrous received one of the following: 1) an intravaginal device containing P4 for 7 d plus an injection of 1 mg of EB 24 to 30 h after device removal (P4+EB), 2) an intravaginal device containing P4 for 7 d (P4), 3) a sham device for 7 d plus an injection of 1 mg of EB 24 to 30 h after device removal (EB), or 4) a sham device for 7 d (control). Progesterone treatment increased the proportion of cows that formed CL with a typical lifespan (P < .001) and exhibited behavioral estrus (P < .05). Treatment with EB in combination with P4 increased (P < .001) the proportion of cows that exhibited estrus. Treatment with P4 alone had no effect, but the combination of P4 and EB increased (P < .01) the proportion of cows that formed short-lived or typical lifespan CL by the end of the experiment compared with untreated cows. In summary, treatment with P4 increased the incidence of estrus and the proportion of cows that formed CL with a typical lifespan. Treatment with P4 resulted in resumption of luteal function in suckled beef cows that were anestrous, with most cows developing CL with a typical lifespan, whereas EB enhanced the expression of estrus.