Seasonal variation in bull semen quality demonstrates there are heat-sensitive and heat-tolerant bulls

Abstract

Using semen data from 1271 ejaculates (79 different bulls, 11 different breeds) we have investigated the variability of semen quality in cattle living in sub-tropical conditions. Modelling shows definitive evidence of seasonal variation. Semen quality from the same bulls had a 90% “pass rate” for cryopreservation purposes in winter, dropping to less than 50% in summer. Notably, individual bulls could be classified as either “heat-tolerant” (produce good quality spermatozoa all year regardless of temperature) or “heat-sensitive” (only produce good quality sperm in summer). Nominal logistic regression demonstrated when temperatures reach 30.5 °C, 40% of heat-sensitive bulls fail a semen analysis 17 days later. At 34 °C, the proportion of bulls failing reached 63%. Ratifying this, the purposeful heating of bulls to 40 °C for 12 h showed that individual animals had different degrees of heat-sensitivity. Using historical temperature data, we then modelled how many days/decade bulls would be subject to heat-events. Beginning from 1939 to 1949, on average, the area in which bulls were kept recorded 19, 7 and 1 day over 38 °C, 39 °C and 40 °C respectively. This number steadily increases and of last decade (2010–2010), the numbers of days per decade over 38 °C, 39 °C and 40 °C jumped to a staggering 75, 39 and 15 respectively. These data show the urgent need to identify heat-tolerant bulls as future sires. Such variation likely explains why the veterinary bull breeding test often fails to accurately predict bull breeding potential.

The fertility of a sub-population of stallions is negatively affected by ambient climatic conditions, mediated through DNA damage

“…Some stallions did not present biologically meaningful relationships between climatic conditions, fertility, or DNA damage, whereas others presented consistent, strong relationships. This is in keeping with studies in cattle that have also reported differences in the heat tolerance of individual bulls (Netherton et al, 2022;Vogler et al, 1993;Vogler et al, 1991;Walters et al, 2005). Netherton et al (2022) reported that ambient temperatures greater than 34°C resulted in poor sperm production in 63% of bulls assessed, with the spermatozoa of the remaining 37% unaffected.…”

“…Spermatogenesis in the stallions lasts approximately 57 days, followed by a further seven days of epididymal storage (Johnson et al, 1997). Heat related insults, whether via systemic stress or direct, affect germ cells differently depending on their developmental stage at the time of insult (Houston et al, 2018;Netherton et al, 2022). As such, the timing of heat-induced subfertility can inform on the stages of germ cell development most vulnerable to damage (Love and Kenney 1999).…”

The Impact of Testicular Hyperthermia, Its Physiological Relevance and the Ability of Antioxidants to Overcome the Condition

“…However, these same authors did note that bulls found in the hottest part of the country (north region) were less likely to pass a semen test. In support of this, we have reported definitive evidence of seasonal variation within bulls taken from central QLD, Australia [82] where temperatures in summer are exceedingly hot.…”

“…We found large changes in semen quality following intervention. Some bulls dropped to below 50% normal forms, whilst others maintained their baseline 80% average throughout 12 weeks of testing [82]. Thus, it is clear that there is a spectrum of thermo-sensitivity within individual animals.…”

Seasonal variations in semen traits of exotic cattle bulls of different age groups in the subtropical region

“…They have reported on the optimal season for optimal outcomes; nevertheless, contradictory findings have also been published. 8,9 According to the study of Shawki et al, 10 statistical analysis revealed variations in the impact of season on semen parameters. In the tropics, seasonal fluctuations in cattle fertility are common.…”