Postpartum automated estrus alert phenotypes in seasonal-calving, pasture-based dairy cows

The objectives of this retrospective observational study were to (1) characterize postpartum estrus phenotypes in spring-calving, pasture-based dairy cows; (2) evaluate associations between cow-level factors and the timing of return to estrus; and (3) evaluate associations between pre-breeding estrus phenotypes and first-service pregnancy per artificial insemination (P/AI). Individual cow data were available from 4,586 lactations across 19 herds in 2022 and 2023. All cows were monitored with a neck-mounted activity monitoring device (SenseHub, MSD Animal Health). The first automated estrus alert occurred on 34 ± 16 d postpartum (mean ± SD); 25%, 51%, and 24% of cows were classified as early (≤21 d), intermediate (22-42 d), or late (>42 d), respectively. First-lactation cows had a lower hazard of first estrus compared with multiparous cows. Cows in the medium and high tertiles for genetic merit for fertility traits had a greater hazard of first estrus and achieved greater first-service P/AI compared with cows in the low genetic merit for fertility tertile. Only a small proportion (2%) of interestrus intervals were between 2 and 7 d. Most interestrus intervals were normal length (18-24 d), although short (8-17 d) and prolonged (>24 d) intervals were more common after the first estrus compared with subsequent estrus events. Cows that expressed at least one estrus before first service had greater P/AI (57% vs. 47%), and cows inseminated following a normal interestrus interval had greater P/AI (58%) than those inseminated after a long (50%) or short (54%) interestrus interval. These results demonstrate that most (76%) pasture-based dairy cows in our analysis had at least one automated estrus alert within 6 wk postpartum and that both parity and genetic merit for fertility traits influenced the postpartum interval to return to estrus and reproductive performance. Automated activity monitoring systems provide a useful tool for characterizing pre-breeding estrus phenotypes and identifying cows with delayed estrus or atypical estrous cycles to inform targeted reproductive management.