Effect of postpartum nutrition on the onset of ovarian activity in beef cows

Abstract

Scope and Method of Study: Effects of nutrition on insulin-like growth factor-I (IGF-I) and insulin in plasma and dominant follicles (DF), and IGF binding proteins in DF were evaluated in anovulatory primiparous Angus x Hereford cows after calving. Cows were assigned to one of two postpartum nutritional treatments: maintain (M), 2.27 kg of a 40% CP supplement per day and ad libitum hay; or gain (G), ad libitum access to a 50 % concentrate diet and hay. Ovarian follicles were aspirated by ultrasonography guided needle at 72+-2d (n= 12) and at 56+-9 (n= 28d) after calving. The effect of treatment of postpartum anestrous beef cows with gonadotropin releasing hormone (GnRH) or estradiol on onset of first estrus and luteal activity was evaluated. Thirty-four cows were assigned to one of three treatments: GnRH, estradiol cypionate, or control. Estrous behavior and ovarian luteal activity were evaluated. Maximum size of DF was influenced by nutritional treatment. Concentrations of IGF-I and insulin in FF and plasma were greater for H than M. Concentrations of insulin in FF and plasma were greater for G than M cows in Exp. 1 and Exp 2. Concentration of IGFBP-4 and -5 were greater in G than M cows. Changes in FF IGF binding proteins may have a role in dietary induced changes in postpartum follicular growth. Percentage of cows detected in estrus within 6 d after treatment was greater for estradiol than GnRH or control cows, and was similar for GnRH and control cows.

Findings and Conclusions: Body condition score at calving did not influence the effect of treatment on estrus and luteal activity. Treatment of anestrous beef cows with GnRH initiated short luteal activity without estrus, indicating that GnRH caused an ovulatory surge of LH. Treatment with estradiol increased the incidence of estrus without altering luteal activity. Our results indicate that concentrations of IGF-I and insulin in FF are influenced by nutritional intake and may be related to follicular function. Follicles did not ovulate in response to estradiol and an ovulatory surge of LH was not induced at this stage postpartum in anestrous beef cows.