Relationship between scrotal infrared temperature patterns and natural-mating fertility in beef bulls.

The infrared temperature pattern (IRT) of the scrotal surface was recorded for 73 yearling beef bulls and a color video thermogram of the pattern of each bull was recorded. The average scortal surface temperature, temperature at the top and bottom of the scrotum, scortal temperature gradient, and thermal class (normal, questionable, or abnormal scortal surface thermal pattern) were recorded for each thermogram. Thirty-seven bulls had a normal temperature pattern (51%), 20 had a questionable pattern (27%), and 16 had an abnormal temperature pattern (22%). Bulls exhibiting abnormal scrotal temperature patterns had lower (P < .05) percentages of sperm exhibiting normal head and tail morphology and had a higher (P < .01) percentage of sperm with proximal droplets than did bulls with normal or questionable thermogram patterns. Thirty bulls with acceptable testis size and semen quality and representing the three thermal classes were each exposed single-sire to approximately 18 heifers during a 45-d pasture breeding period. Pregnancy rate was lower (P < .01) for bulls with abnormal scrotal temperature patterns (68 +/- 4%, n = 8) than for bulls with normal (83 +/- 4%, n = 13) and questionable temperature patterns (85 +/- 4%, n = 9), and pregnancy rate was related significantly to all four major characteristics (surface, top, and bottom temperatures and temperature gradient) of scortal thermograms. Data indicated that bulls with abnormal scortal temperature patterns exhibited a reduced ability to maintain an effective thermal gradient from top to bottom of the testes and that bulls with abnormal scrotal temperature patterns achieved reduced pregnancy rates when used for natural mating.