The effect of strain of Holstein-Friesian cow on size of ovarian structures, periovulatory circulating steroid concentrations, and embryo quality following superovulation

Abstract

When managed under grass-based systems of production, the NZ strain of Holstein-Friesian cow has superior reproductive performance compared to the NA strain despite having similar SCM yields. This study compared the ontogeny of early pregnancy events in NZ and NA cows. Ten NZ and 10 NA cows were submitted to a superovulation protocol on three occasions. Blood samples were collected daily from every cow from day -3 to +7 relative to a synchronised oestrus during each superovulation protocol. Pre-ovulatory oestradiol concentrations, follicle diameter, post-ovulatory progesterone concentrations, CL diameter, and circulating insulin-like growth factor-I concentrations did not differ between the two strains. Uteri were non-surgically flushed 7 d post AI, embryos were isolated and graded. The proportion of transferable embryos recovered was higher (P<0.01) in the NZ cows compared with the NA cows. A greater (P=0.01) proportion of the recovered structures were at the blastocyst stage in the NZ cows. Peak SCM yield and BCS at the time of peak SCM yield were not different between strains. However during the experimental period the NA cows maintained significantly higher daily SCM yields, whereas the NZ cows replenished significantly greater levels of BCS. The results indicate that differences in periovulatory steroid concentrations and size of ovarian structures do not explain the differences in embryo quality between the two strains. However, strain differences in nutrient partitioning from the time of peak SCM yield through late lactation may provide the key signals responsible for superior embryo quality in NZ cows.