Association between circulating progesterone during the luteal phase and estrous activity detected by automated activity monitoring in dairy cattle

Abstract Text: The objective of this study was to evaluate the association between circulating progesterone (P4) concentration during the luteal phase (LP) and estrous activity detected by automated activity monitoring (AAM). The hypothesis was that a shortened LP would decrease the intensity of estrus expression. A total of 48 cows at the UBC Dairy Center were assigned to one of two treatments: a short (n=23) or a normal P4 (n=25) LP. The short LP was achieved by injecting 25 mg of prostaglandin F2α at d 7 of the cycle; normal LP cows were not treated. Ultrasound of the reproductive tract was performed at day 0, 2, 5, 7, 9, and 11 of the cycle, then daily until ovulation to identify and measure the dominant follicle. Blood samples were taken on the same days to measure serum P4. Cows were equipped with two activity monitoring devices: Heatime HR System (SCR, Israel) and SmartDairy (BouMatic, USA). The peak activity score (Heatime), peak of relative activity (%, SmartDairy) and duration of the estrus (both systems) following the LP were recorded. Linear regression models were used to calculate marginal means (LS mean ± SEM). As expected, P4 at d14 of the cycle differed between the short and normal LP (2.9 ±0.4 vs 7.6 ±0.4 ng/ml, respectively; P<0.01). The number of estrus events detected was not different between treatment groups for Heatime (short: n=15, normal: n=20, P=0.25) or for SmartDairy (short: n=15, normal: n=16, P=0.93). Using the Heatime system, the peak activity score differed between the short and normal LP groups (73.3 ±4.0 vs 84.3 ±3.5; P=0.05) and the duration of estrus tended to differ (10.8 ±1.0 vs 13.1 ±0.9 h; P=0.09). Using the SmartDairy system, neither the peak activity (434 ±39 vs 465 ±38 %; P=0.57) nor the duration of estrus (8.0 ±1.2 vs 10.0 ±1.2 h; P=0.25) differed. There was no difference in serum P4 concentration (0.89 ±0.14 vs 0.95 ±0.13 ng/ml; P=0.75) or in ovulatory follicle size (18.7 ±1.4 vs 18.1 ±1.4 mm; P=0.78) on the day of estrus. The number of cows that ovulated was not different between short and normal LP groups (short: n=14, normal: n=18; P=0.41).  The shortened LP moderately reduced the intensity of estrus expression but did not affect ovulatory follicle size or ovulation. Decreased LP duration or peak circulating concentration of P4 may reduce the intensity of estrus-related activity.

Keywords: automated activity monitoring system, estrus detection, luteal phase