Reindeer bull introduction affects the onset of the breeding season

Reindeer are seasonally polyestrus, short day breeders, with estrous cycles of approximately 20 days in length. The objective of this study was to investigate the effects of reindeer bull exposure on the onset of the reindeer cow breeding season and to investigate whether cows with calving experience responded differently than cows with no previous reproductive experience. During year 1, blood samples were collected weekly beginning 14 July and continuing until 15 September (n = 8) or 30 September (n = 8) in order to determine the onset of the breeding season, based on ovarian function, with no bull present. Plasma was stored frozen for later assay of progesterone (P(4)) following the conclusion of sample collection. Eight randomly selected cows were allowed to breed with a bull during year 1. The mean onset of ovarian activity in the first year was 15 September (range: 8-29 September). The bull was removed from cows more than 2 months prior to the start of the experimental period during year 2 and housed at a separate facility approximately 2 km distant. Blood samples were collected during year 2 from 14 cows 3x weekly beginning on 11 August (day 1) and continuing until 29 September (day 50) and plasma was stored frozen for later assay of P(4) following the conclusion of sample collection. On day 6, cows were divided into two groups such that group 1 (early bull exposure; EBE), consisted of four cows that had calved the previous spring and four cows that had no reproductive experience (n = 8). Group 2 (late bull exposure; LBE), consisted of three cows that had calved the previous spring and three cows that had no reproductive experience (n = 6). EBE experienced bull introduction on day 13, 23 days earlier than the average onset of ovarian activity during year 1. LBE experienced bull introduction on day 46, 10 days later than the average onset of ovarian activity during year 1. Progesterone concentrations were analyzed by ANOVA procedures for repeated measures. Bull presence was not requisite for the onset of ovarian activity during either year. Previous reproductive status had no effect on the onset of ovarian activity within EBE (P = 0.61) or within LBE (P = 0.92). Time of bull introduction had a significant effect on the onset of ovarian activity when EBE was compared to LBE (P<0.001). The first sustained increase in mean P(4) concentration above 1 ng/ml occurred on day 25 in EBE reindeer and day 41 in LBE reindeer. EBE reindeer initiated ovarian activity 12 days after bull introduction while LBE reindeer initiated ovarian activity 5 days before bull introduction. All cows penned with the bull conceived during both breeding seasons, demonstrated by production of live calves during the subsequent spring. Cows bred during year 1 all calved within a 9 day-period. During year 2, EBE displayed a more synchronous calving period compared to LBE (P<0.05). Results indicate that bull management affects the onset of the breeding season in reindeer cows, regardless of previous reproductive experience.