Silent ovulation, based on walking activity and milk progesterone concentrations, in Holstein cows housed in a free-stall barn.

The objectives of the current study were to determine the incidence of silent ovulation (based on walking activity and milk progesterone profiles), identify risk factors for silent ovulation, and investigate its impact on reproductive performance in high-yielding dairy cows in free-stall housing. Overall, 277 lactations in 161 Holstein Friesian cows from a commercial dairy herd in northern Japan were studied. Walking activity (measured with pedometers) >80% above the mean for the preceding 2 d was defined as estrus, whereas day of ovulation was estimated using milk progesterone concentrations. Ovulation not preceded by increased walking activity was considered silent ovulation; the incidence was 55.2%, 23.8%, 21.3%, and 10.5% at the first, second, third, and fourth ovulations postpartum, respectively. Moderate and high milk yield significantly increased the risk of silent ovulation at second (odds ratio [OR]=2.7 and 1.2; P=0.04) and third and/or fourth ovulations (OR=6.7 and 12.9; P=0.03). Based on survival analysis, silent ovulations at the first, second, third, and/or fourth ovulations were associated with 28% (hazard ratio [HR]=0.72), 55% (HR=0.45), and 47% (HR=0.53) reductions in pregnancy rate, respectively, and 41% (HR=0.59), 66% (HR=0.34), and 65% (HR=0.35) reductions in artificial insemination (AI) submission rate. Cows with at least one silent ovulation (with the exception of the first ovulation) had a longer interval from calving to first AI (72 vs. 54 d, P<0.001) and to achievement of pregnancy (133 vs. 80 d, P<0.001). In conclusion, approximately one third of the ovulations (based on milk progesterone concentrations) in Holstein cows within 90 d postpartum were silent. Silent ovulations at the second to fourth ovulations were associated with high milk yields and at all ovulations were associated with impaired reproductive performance.