ABSTRACT Nutrient intake, body energy reserves, and suckling are major regulators of reproductive performance of beef cows. Inadequate body energy reserves at parturition increase the interval to first estrus and ovulation, and postpartum nutrient intake can influence the duration of the interval in cows with thin to moderate body condition score. Suckling can increase the postpartum anestrous interval in thin cows, but has little effect on mature cows with adequate body energy reserves. The purpose of this review is to evaluate signals by which nutrient intake and body energy reserves may regulate ovarian function in postpartum beef cows. Nutritional restriction causes decreased secretion of GnRH and LH, reduces follicular growth, and decreases concentrations of estradiol in plasma. In addition to direct and indirect effects of decreased energy intake on the hypothalamus and pituitary, nutrition may influence ovarian function. Metabolic signals that communicate the adequacy of body energy reserves and nutrient intake may stimulate changes several weeks before ovulation occurs, have a permissive role, be regulated by binding proteins or receptors, or interact with stimulatory or inhibitory factors produced by adipose tissue. Metabolic signals may also have autocrine and/or paracrine effects. Adequate body energy stores and sufficient plasma concentrations of metabolic signals are prerequisites for ovulation in postpartum cows. Complex interactions between hormones, metabolic compounds, and other factors control follicular maturation, estrus, and ovulation in postpartum beef cows.
Implications
The interval from calving to conception greatly influences profitability of beef production. Inadequate body fat stores at calving and reduced postpartum nutrient intake increase the interval from calving until ovulation. Suckling suppresses ovulation during the early postpartum period in cows with moderate body fat stores, and the suppression is longer in thin cows. Restricted suckling or early weaning of calves can be used to improve reproductive efficiency in very thin cows. Insulin, insulin-like growth factor-I, and leptin may be metabolic signals or permissive cues; however, other interactive factors must be involved. Determination of metabolic signals by which body energy stores and nutrient intake regulate the interval from calving to first ovulation will allow development of management strategies to increase pregnancy rates in beef cows.
Key Words: Cow, Insulin-Like Growth Factor, Ovulation, Postpartum Period, Reproduction
© 2003, by the American Society of Animal Science. All rights reserved.
J. Anim. Sci. 2003. 81(E. Suppl. 2):E48-E59
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