Influence of the lunar cycle on first service conception rate in dairy cows: effects of region, year, season, insemination method, age, and days in milk

Abstract

A retrospective analysis was conducted to assess whether the phase of the lunar cycle (LC) is related to first service conception rate (FSC) in 72,006 Holstein cows from a commercial dairy operation in Uruguay. Inseminations performed between 2017 and 2019, either timed artificial insemination (TAI) or after estrus detection (CAI), took place in farms located in three regions (central, eastern, and western) during the winter and spring seasons. The LC (29.52 days) was divided into 30 periods of 0.98 days. The analysis was performed using a generalized linear mixed model (GLMM), which included region, year, season, insemination method, age (≤34> months), days in milk (DIM; ≤90> days), and LC as fixed effects, along with their interactions. Bull and inseminator effects were considered random factors. LC (P<0.0001), DIM (P=0.006), age (P=0.0003), and year (P<0.0001) significantly affected FSC, while region (P=0.13), season (P=0.1), and insemination method (P=0.16) did not. Significant interactions between LC and season (P<0.0001), year (P<0.0001), age (P<0.01), and DIM (P=0.0003) were detected. FSC showed a non-uniform distribution throughout the lunar cycle, with notable differences between individual days. Using day 1 as a reference (FSC similar to the average: 38.8%), significant differences were observed with days 13 (44.6%) and 23 (27.9%). Cows aged ≥34 months and with ≥90 DIM exhibited greater variability in FSC across the lunar cycle, with more pronounced peaks and drops than younger cows and those with fewer DIM. Lunar cycle relationship with fertility was observed across all three years, with differences in the magnitude and timing of peaks and troughs. FSC varied significantly across the days of the lunar cycle, and this effect was modulated by season, DIM, age, and year. Understanding how the lunar cycle relates with FSC and interacts with multiple factors may help optimizing fixed-time artificial insemination programs in dairy herds.