The objective of this study was to determine whether administration of 1500 IU of hCG on Day 29±1 after artificial insemination (AI) would reduce pregnancy loss by inducing accessory corpora lutea and increasing serum progesterone concentrations in dairy cattle.

Three hundred and seventy lactating Holstein dairy cows from a farm with 1200 lactating cows were randomly assigned to a treatment and a control group. The animals of the treatment group (n=185) received hCG 1500 IU (IM), while in the control group they received the control group (n=185) received no treatment. Ovarian structures, serum progesterone levels and pregnancy status were determined on days 29±1 and 42, respectively.

The Progesterone concentration on day 42 was higher (P=0.005) for cows in the hCG group (14.9±0.7 mg/ml) compared to the control group (12.1±0.7ng/ml). On day 42, the number of accessory CLs  for cows in the hCG group (1.6±0.04) were higher (p˂0.0001) than cows in the control group (1.2±0.04). A Positive correlation was detected between the circulating progesterone and CL volume in the hCG group (r=0.47, p=0.001). Pregnancy losses did not differ between days 29±1 and 42 both in the control (7.6%) and in the hCG group (5.9%).

It was concluded that the administration of hCG (1500 IU) on day 29±1 of pregnancy in lactating dairy cows increased the number of corpora lutea and serum progesterone concentration but did not have any effect on pregnancy loss rate or pregnancy losses.