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This study investigated associations between prenatal and early postnatal heat stress exposure and first-lactation performance in 3182 Holstein Friesian cows born between 2016 and 2022. Early-life periods were defined as the first (T1), second (T2), and third (T3) trimesters of gestation and the postnatal period (0-3 months). Heat stress was quantified as the number of days with THI ≥68 and cumulative THI load. Linear mixed-effects models assessed associations with 305-day milk, fat and protein yield, age at first insemination (AFI), age at first calving (AFC), services per conception (SPC), and first-service conception rate (FSCR). Heat stress during T1 was significantly associated with lower 305-day milk yield (-6.65 kg per additional day with THI ≥68; 95% CI -10.44, -2.87, p < 0.001), protein yield (-0.22 kg; 95% CI -0.34, -0.10, p < 0.001), and fat yield (-0.16 kg; 95% CI -0.31, -0.01, p = 0.040). No significant associations with production traits were found for T2, T3, or postnatal exposure. Prenatal heat stress during T2 and T3 was associated with later AFI and AFC. Each additional day with THI ≥68 increased AFI and AFC by 0.20 and 0.19 days in T2 (p < 0.001) and by 0.12-0.13 days in T3 (p ≤ 0.032). No associations were observed for T1 or postnatal exposure. SPC and FSCR showed no associations with early-life heat stress exposure. These findings indicate that prenatal heat stress at different stages of gestation was differentially associated with first-lactation milk yield, AFI and AFC.
Baccouri et al. (Wed,) studied this question.