Extending the duration of treatment with progesterone and equine chorionic gonadotropin improves fertility in suckled beef cows with low body condition score subjected to timed artificial insemination

The objective of this study was to evaluate the effect of an extended progesterone treatment on follicular development and fertility in postpartum, suckled beef cows subjected to timed artificial insemination (TAI). In experiment 1, cows (n = 24) with body condition score (BCS) ≥4.5 received either a 2-g progesterone intravaginal device on Day −23 or a 0.558-g progesterone intravaginal device on Day −9. Then, all cows received 2 mg of estradiol benzoate on Day −9; removal of the device, 1-mg estradiol cypionate, and PGF2α on Day −2; and TAI on Day 0. Metabolic status was assessed between Days −9 and −2. Ovarian structures and plasma progesterone were determined weekly from Day −23 to −9, daily from Day −9 to 0, and weekly until Day 28. In experiment 2, cows (n = 302) with BCS ≥4.5 received identical treatment to cows in experiment 1, but on Day −2, cows received 400 IU of two different commercial preparations of equine chorionic gonadotropin (eCG). Ovarian structures were determined on Days −23 and −9 on a subset of cows (n = 40). Pregnancy was determined 39 days after TAI. In experiment 3, multiparous cows (n = 244) with BCS