The acute effect of bull presence on plasma profiles of luteinizing hormone in postpartum, anoestrous dairy cows

The objective of this study was to investigate whether bull exposure affects LH profiles in postpartum, anoestrous dairy cows. Eight cows between 10 and 17 days after parturition were used. On Day 1, blood samples were taken at 10 min intervals for 8 h. On Day 2, blood sampling continued at 10 min intervals and after 2 h a bull was introduced behind a fence, and blood sampling continued for another 8 h. Time of resumption of luteal activity was between 25 and more than 80 days after parturition for these animals and was not related (P > 0.1) with frequency of LH pulses, amplitude of pulses and basal LH concentration on either Day 1 or Day 2. In 6 of the 8 cows, average and basal LH concentration were greater (P < 0.001) during the 8 h of bull presence (0.56 ± 0.33 and 0.39 ± 0.26 ng/ml, respectively) compared to the 8 h without a bull (0.50 ± 0.30 and 0.35 ± 0.24 ng/ml, respectively). Pulse amplitude did not differ (P = 0.85) between Day 2 (0.45 ± 0.24 ng/ml) or Day 1 (0.45 ± 0.14 ng/ml). LH pulse frequency was greater (P < 0.1) on Day 2 (5.3 pulses/8 h) compared to the Day 1 (4.6 pulses/8 h). In conclusion, fenceline bull exposure early postpartum seems to have an acute effect on LH-release in anoestrous dairy cows. Whether sustained bull exposure can hasten first ovulation after calving through an effect on LH release in dairy cows is an interesting area of research.