Angus bulls voluntarily access shade during hot weather, reducing scrotal subcutaneous temperatures and improving sperm quality.

Abstract

The objective of this study was to determine the relationship of bull location (shade versus no shade), scrotal subcutaneous and ambient temperatures, and sperm quality. Six Angus bulls (4 to 5 y) were randomly allocated into 2 groups of 3 bulls each, housed in 2 outdoor pens, with 1 containing a shed (~3.5 × 6 m and 2.5 m high, 1 open side) to provide shade. Semen was collected by electroejaculation once weekly for 9 wk. The percentage of time a bull voluntarily accessed shade for ≥ 15 min (observed with a game camera) increased with the ambient temperature and ranged from 7.6% to 86.7% for ambient temperatures of < 25°C and > 33°C, respectively. During the 10 hottest days, scrotal subcutaneous temperature (measured hourly with an implanted data logger) in the bulls without access to shade (control group) was directly associated with ambient temperature. Conversely, bulls with access to shade had lower (P = 0.001) scrotal subcutaneous temperatures during high ambient temperatures, particularly when they accessed shade. During the 4 hottest days, these bulls voluntarily accessed shade most of the time from 12:00 noon to 5:00 p.m. (peak ambient temperatures). For total sperm morphological abnormalities and acrosome integrity, there were group effects (P = 0.001 for each), plus a time effect for acrosome integrity (P = 0.009). For total and progressive forward sperm motility, there were group effects (P = 0.001 and 0.023, respectively). For sperm motility kinetics, which were measured with computer-assisted sperm analysis (CASA), [average path velocity (VAP), curvilinear velocity (VCL), straight line velocity (VSL), straightness of track (STR), and linearity of track (LIN)], there were also group effects (P = 0.005, 0.011, 0.010, 0.020, and 0.046, respectively). In summary, during hot weather, bulls voluntarily accessed shade, which significantly lowered scrotal subcutaneous temperatures and improved sperm quality.