This study aimed to improve estrus management in Japanese Black cows by investigating the effects of controlled internal drug release (CIDR) treatment on luteal function including luteolysis and estrous characteristics. Cows with normal estrous cycles were divided into three groups, and CIDR treatment was initiated on day 5 (Group 1), 10 (Group 2), or 16 (Group 3) of the estrous cycle and continued for 12 days. Plasma progesterone (P4) levels, luteal diameter, and luteal blood flow (LBF) were monitored during treatment, and correlations among these parameters were analyzed. Additionally, the interval from CIDR removal to estrus, and the incidence of standing (ST) behavior during estrus were evaluated. All luteal parameters changed in a cycle-dependent manner, irrespective of CIDR treatment. Significant correlations were observed between P4 levels and both luteal diameter and LBF. The interval from treatment withdrawal to estrus was significantly longer in Group 1 (4.6 ± 2.2 days) than in Groups 2 (2.1 ± 0.4 days) and 3 (2.2 ± 0.7 days). Silent heat, defined as estrus without ST behavior, occurred in 27.3% of cows in group 1 and 10.0% in Group 2, but not in Group 3. These findings suggest that initiating CIDR treatment in the early luteal stage and removing it just before or during luteolysis, causes variability in estrus timing after treatment and increases the risk of silent heat. When implementing estrus synchronization using CIDR in the field, both the timing of treatment initiation and removal should be carefully considered.
HOJO et al. (2026) studied this question.