139 Heat stress has a deleterious effect on bull semen quality and subsequent embryo development

Abstract

High ambient temperature induces an increase in bovine body temperature above the physiological homeothermic point, leading to impaired reproductive performance. Belgian Blue bulls are more susceptible to heat stress compared with most other cattle breeds. Therefore, the aim of this study was to investigate whether high ambient temperature affected bull semen quality and subsequent embryo development. For this purpose, semen samples were collected from six Belgian Blue bulls during August (14-28 days after three consecutive warm days with temperature-humidity index between 63.8 and 83.7) and March 2016 (temperature-humidity index between 35.9 and 47.4). After semen collection, volume, sperm concentration, and motility of fresh semen were assessed. Furthermore, frozen sperm samples were used to assess the motion parameters and morphological abnormalities using computer-assisted sperm analysis, viability and reactive oxygen species (hydrogen peroxide and superoxide) production using flow cytometry, and acrosome integrity and chromatin condensation using fluorescence microscopy. Afterward, blastocysts were produced (r = 4) by conventional in vitro methods for assessment of embryo development (Wydooghe et al. 2014 Reprod. Fertil. Dev. 26, 717-724; https://doi.org/10.1071/RD13043). Cleavage rate was determined 48 h after fertilisation, and the blastocyst rates were determined on Days 7 and 8 postinsemination. Moreover, Day 8 blastocysts underwent differential staining in order to determine the numbers of the inner cell mass, trophectoderm, total cell number, and apoptotic cells ratio. The data set was analysed using the GLM procedure of SAS (SAS Institute Inc.). Normal distribution was checked using the UNIVARIATE procedure, and the Shapiro-Wilk test and arcsine square root transformation were used when required. Furthermore, Duncan's test was applied to determine the significant differences (P = 0.05). In fresh semen, samples from the non-heat-stressed (NHS) group showed a higher sperm concentration compared with samples from the heat-stressed (HS) group (P = 0.05), whereas semen volume and motility were not affected by heat stress (P > 0.05). In frozen-thawed semen, total and progressive motility and straight-line velocity were lower in the HS group compared with the NHS group (P = 0.05), whereas the generation of H₂O₂, percentages of aberrant chromatin condensation, total morphological abnormality, spermatozoa with bent tails, and distal protoplasmic droplets were higher in the HS group compared with the NHS group (P = 0.05). The cleavage rate and blastocyst rates on Days 7 and 8 were higher (P = 0.05) in the NHS group (70.4 ± 1.13, 25.4 ± 1.84, and 40.3 ± 1.15, respectively) compared with the HS group (62.8 ± 1.49, 15.4 ± 1.56, and 23.3 ± 1.84, respectively). However, there were no significant differences in hatched or hatching rate of two treatments (P > 0.05). The total cell number and trophectoderm were higher in NHS-derived blastocysts than in HS-derived blastocysts (P = 0.05), whereas the apoptotic cells ratio was lower (P < 0.001) in NHS blastocysts (2.16 ± 0.48% vs. 5.21 ± 0.52%). In conclusion, these findings show that elevated ambient temperature during summer as a consequence of climate change can lead to decreased quality of fresh and frozen-thawed bull spermatozoa and subsequent embryo development.