Double vision

Abstract

Twinning in Holstein dairy cows has increased over time concurrent with increased milk production. More than 95% of twins in Holsteins arise due to double ovulations resulting in dizygotic twins. It is now clear that low progesterone during growth of a preovulatory follicle increases the incidence of double ovulation. Increased hepatic metabolism of progesterone as a result of the increased feed intake associated with high milk production provides a physiological mechanism for decreased progesterone levels in high-producing dairy cows resulting in an increase in di zygotic twinning. Twinning will likely continue to increase with milk production over time, so strategies to effectively manage twinning in dairy cows need to be identified and implemented. Cows carrying unilateral twins had greater pregnancy loss compared to cows carrying bilateral twins, and bilateral twins had increased survival and body weight at birth, a longer gestation length, and less dystocia than unilateral twins. Selective embryo reduction by manual rupture of the amnion followed by progesterone treatment did not increase the risk of pregnancy loss for cows with unilateral twins but increased the risk of loss for cows with bilateral twins. Hormonal synchronization protocols that increase progesterone during development of the preovulatory follicle increase conception rate, decrease pregnancy loss, and decrease double ovulation rate thereby decreasing twinning in high-producing dairy cows. Twinning can be decreased in high-producing dairy cows using a two-pronged approach. First, the incidence of double ovulation and dizygotic twinning can be decreased in high-producing Holstein cows by manipulating ovarian function to increase progesterone during growth of the preovulatory follicle before timed AI. Second, cows identified with bilateral twins using transrectal ultrasonography early during gestation should be allowed to continue gestation with extra assistance provided at calving, whereas selective reduction can be attempted for cows diagnosed with unilateral twins.