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

• Published in: Scientific reports Vol. 12; no. 1; pp. 15322 - 11
• Main Authors: Netherton, Jacob K., Robinson, Benjamin R., Ogle, Rachel A., Gunn, Allan, Villaverde, Ana Izabel S. Balbin, Colyvas, Kim, Wise, Ced, Russo, Tylah, Dowdell, Amiee, Baker, Mark A.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 12.09.2022; Nature Publishing Group; Nature Portfolio
• Subjects: 631/158; 631/158/1745; 631/158/670; 704/172; Breeding; Cryopreservation; Heat; Humanities and Social Sciences; multidisciplinary; Science; Science (multidisciplinary); Seasonal variations; Semen; Summer; Temperature tolerance
• ISSN: 2045-2322; 2045-2322
• DOI: 10.1038/s41598-022-17708-9

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.