Comparison of two methods of progesterone application in a fixed time artificial insemination protocol in beef cows
Two experiments were conducted at two commercial farms to compare pregnancy rates of nulliparous (n = 178) and primiparous (n = 130; Farm 1), and multiparous (n = 161; Farm 2) Aberdeen Angus cows, when 200 mg s-c of injectable oil-based progesterone (MAD-4) or a 558 mg progesterone releasing intravaginal device (DIV) were used in a fixed time artificial insemination protocol based on estradiol benzoate (EB), prostaglandin (PG), equine chorionic gonadotropin (eCG) and gonadotropin-releasing hormone (GnRH). Primiparous and multiparous cows were weaned 30 d before treatment (60–90 d postpartum). The ovarian status was determined by rectal palpation and the animals classified as cycling or in superficial anoestrus or deep anoestrus. The P4 source affected (P<0.0001) the pregnancy rate of primiparous and multiparous cows, but not that of nulliparous cows (Farm 1: Nulliparous: 51% and 42%; Primiparous: 42% and 10%, DIV and MAD-4, respectively, with a treatment x category interaction (P = 0.0056). Farm 2: DIV: 51% vs. MAD4: 9%, P=0.0001). Independently of the hormonal treatment, the ovarian status influenced pregnancy rates (P<0.0001), cycling cows showed the highest value and cows in deep anoestrus the lowest. No significant treatment x ovarian status interaction was found. It is concluded that replacing the DIV by MAD-4 decreased pregnancy rate of primiparous and multiparous cows, but not that of nulliparous ones.