Relationship between plasma progesterone concentration and number of conceptuses and their growth in superovulated cattle

Elevated concentrations of circulating progesterone (P₄) in the immediate post-ovulation period are associated with advancement of conceptus elongation in cattle. Superovulated (SOV) cattle have not only elevated plasma P₄ concentrations but also multiple embryos in the uterus because of the formation of multiple corpora lutea. We examined the relationship between plasma P₄ concentration and uterine glucose level in the immediate post-ovulation period and the presence and growth of multiple conceptuses in SOV cattle. SOV cattle were artificially inseminated with frozen-thawed semen at standing estrus (day 0), and the conceptuses were recovered by nonsurgical flushing of the uterus on day 13. In the SOV cattle, there were quadratic relationships between plasma P₄ concentration on days 4, 5 and 7 and conceptus length and between number of conceptuses in the uterus and conceptus length. These results suggest that conceptus growth in SOV cattle is regulated by both systemic P₄ level and number of conceptuses and that there are ranges of plasma P₄ concentrations and numbers of conceptuses in the uterus that are suitable for conceptus growth and development. Plasma P₄ concentrations on days 5 and 7, but not the numbers of conceptuses, were quadratically correlated with uterine glucose levels on day 13 in SOV cattle. In addition, conceptus length was positively correlated with uterine glucose level in SOV cattle. Accordingly, regardless of the number of conceptuses in the uterus, the plasma P₄ concentration was well correlated with the regulation of conceptus growth via changes in uterine glucose levels in SOV cattle.