U.S. Department of Agriculture: Agricultural Research Service, Lincoln, Nebraska


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Published in Beef Research Program Progress Report (1993) No. 4 (Part 1): 86-89


The scientific literature offers only sparse information on mechanisms controlling reproductive function in beef bulls and indicates that there are very few consistent relationships between the commonly evaluated male reproductive characteristics and variations in natural-mating fertility of beef bulls. Current techniques for evaluating and predicting reproductive potential of bulls are ineffective, and the beef cattle industry selects breeding males on the basis of appearance, growth rate, size, and other factors of little relationship to reproductive potential. Reproductive merit is five times more important economically than is growth performance and at least ten times more important than product quality for the average cow-calf producer. However, little selection pressure for fertility has been applied to beef sires (bulls) in North America because of the relative inaccuracy of methods available for evaluation and prediction of breeding potential and fertility in beef bulls. For example, detailed semen evaluation in groups of yearling (less than 18 mo of age), physically-sound beef bulls rarely eliminates more than 5% as potential breeding sires, yet single-sire mating using bulls from the 95% that passed semen evaluation still results in dramatic variation in sire fertility. Many producers obtain little or no information on the reproductive status of their bulls prior to use in natural-mating programs, and this is particularly true for the use of young breeding bulls (Le., yearlings) in the beef cattle industry. The lack of effective means for selecting males with superior fertility is due primarily to two factors: 1) the lack of information on basic measurable characteristics of male reproduction that are related to sire fertility, and 2) the cost and difficulty of obtaining accurate fertility data on individual sires.

Development of valid knowledge of the limiting mechanisms in male reproduction and establishment of effective, reliable techniques for evaluating the characteristics of male reproduction that are related to fertility are prerequisite to improving the productivity of the livestock industry. It is known that thermoregulation in the testes is essential for sperm production. For normal spermatogenesis in the bull, the testes in the scrotum must be maintained at a temperature approximately 5° to 8°F lower than normal body temperature (101°F). Adverse effects of elevated testicular temperature on sperm production, semen quality, and male fertility have been documented for many species of domestic animals. Recently, other researchers have shown that the surface temperature of the scrotum is highly correlated with deep testicular temperature, and that infrared thermograms (images of radiated heat emission) of the scrotal surface provide accurate information about testicular thermoregulation in domestic species. However, the relationships between scrotal thermography (infrared thermography of the scrotal surface) and various aspects of semen quality and fertility remain unknown in the beef bull. The objectives of the following study were to evaluate the potential usefulness of scrotal thermography as a tool for predicting the natural-mating fertility of yearling beef bulls, and to obtain standard breeding soundness information for comparison to scrotal thermography and bull fertility data.