203 Effects of prenatal heat stress on subsequent reproductive outcomes in replacement Holstein heifers up to the first lactation

The negative direct impact of heat stress (HS) on cattle production and reproduction is well documented. Although HS during last trimester of gestation has a lifelong impact on offspring performance, the carryover effects of HS around conception are understudied. We hypothesized that HS around conception (affecting oocyte and early embryo development) and during late fetal development impairs offspring reproductive performance. Thus, our aim was to evaluate the association between HS exposure in these periods and the subsequent reproductive outcomes of replacement heifers through a retrospective cohort study. The data set included records of 1206 female Holsteins, born between 2016 and 2021 in the same farm, that were monitored until 150 days in milk (DIM) into their first lactation. HS was considered when the mean daily maximum temperature-humidity index (THI) in each period was ≥72 (mean ± SD). On that basis, three groups were established: (1) HS 2 months before and 45 days after conception (HS_CON: conception from July to September [THI = 74 ± 2] and dry-off from February to May [THI = 65 ± 4]); (2) HS during the two dry-off months (HS_DRY: conception from November to January [THI = 59 ± 3] and dry-off from June to September [THI = 75 ± 2]); and (3) thermoneutral conditions (NO_HS) during either conception (THI = 63 ± 5) or dry-off (THI = 62 ± 7), encompassing remaining months. Reproductive outcome variables were first analyzed in univariate analyses with independent variables for offspring (HS group, birth year, dam’s parity, birth weight, passive immune transfer failure, diarrhea, pneumonia, weaning average daily gain, HS at AI, semen type at AI, calving difficulty, calf sex, calf breed, and postpartum diseases). Final generalized linear models were constructed (GLIMMIX, SAS) after backward elimination of noninfluential variables. No differences in conception rate (CR) in heifers at first AI were found. However, HS_CON heifers had a higher odds of multiple AI attempts (≥3) per pregnancy compared with NO_HS heifers (OR = 1.96; P = 0.01) and greater age at first calving compared with NO_HS and HS_DRY heifers (726.8 ± 3.7 vs. 717.9 ± 1.5 days and 719.1 ± 2.1 days [mean ± SE]; P = 0.01). After the first calving, CR at first AI was lower in HS_CON cows compared with NO_HS and HS_DRY cows (25.4% vs. 40.7% and 47.5%; P < 0.01). HS_CON cows also had a higher odds of multiple AI attempts (≥3) per pregnancy compared with NO_HS and HS_DRY cows (OR = 1.46 and 2.16; P < 0.01). Consequently, in the first lactation, HS_CON cows were more likely to remain open by 150 DIM compared with NO_HS and HS_DRY cows (OR = 1.83 and 2.49; P < 0.01). In conclusion, exposure of pregnant cows to HS around conception impairs the subsequent reproductive outcomes of replacement heifers until at least their first lactation, while HS exposure during dry-off seems to have no impact on these parameters. Thus, these results could contribute to establishing an efficient heifer replacement plan by choosing heifers according to the period they were conceived.

Funding for this study was provided by MCIN/AEI/10.13039/501100011033 and European Union Next GenerationEU/PRTR (RYC2021-033574-I and TED2021-129764B-I00).