Lunar cycle influence on first-service fertility in dairy heifers: effects of age, region, season, and year of insemination

Abstract

A retrospective analysis was conducted to investigate whether the phase of the lunar cycle (LC) is associated with fertility at first service (FFS) in 21,527 Holstein heifers from a commercial dairy operation in Uruguay. Artificial inseminations (AI), performed following estrus detection, took place between 2017 and 2019 in herds located across two regions (central and east/west) during winter and spring. The LC (29.52 days) was split into 30 periods of 0.98 days each. Data were analyzed using a generalized linear mixed model (GLMM) that included region, year, age (≤17 vs. >17 months), season, and lunar day as fixed effects, along with their interactions. Bull and technician were modeled as random effects. The LC phase (P<0.0001), season (P<0.0001), age (P=0.0006), region (P=0.006), and marginally the year (P=0.052) affected FFS. Significant interactions were found between LC and season, region, and year (P<0.0001), but not with age (P=0.3988). FFS varied markedly throughout the LC, particularly in spring and during the last 10 days of the cycle. In winter, this variation was more moderate (48.6–70.2%) compared to spring (20.0–76.5%). FFS was more stable across the last two-thirds of the cycle in the east/west region, whereas in the central region, a pronounced decline was observed during the waxing phase. In 2018 and 2019, greater instability in FFS was observed compared to 2017. In conclusion, the LC phase was associated with FFS in dairy heifers, although the underlying mechanisms remain unknown. Furthermore, its effect was modulated by season, region, age, and year. These findings suggest the potential to incorporate lunar rhythms into reproductive management strategies to improve fertility outcomes.