Heat stress due to a combination of temperature, humidity, direct sunlight and wind speed can negatively affect bull fertility. Although heat stress can affect the health and welfare of cattle several different ways, bulls are particularly sensitive in that even moderate heat stress can negatively affect fertility.

Bull anatomy is specifically designed to keep testicular temperature cooler than the core body temperature. Special blood flow removes heat from testicles, and the scrotum thickness and position can be altered by muscles that respond to environmental temperature. But even though the bull has mechanisms to regulate testicular temperature, heat stress events can overcome those protections. 

Heat stress events as short as two days can disrupt testicular tissue, resulting in negative effects on sperm motility, shape and function. The degree of sperm damage and the length of time the damage persists depends on the intensity and length of the heat stress insult. 

Mild testicular damage may only affect certain stages of sperm cell development and result in relatively few days where sperm production is affected. Even with moderate heat stress, not all testicular tissue is affected to the same degree so that some sperm cells are damaged, but others develop normally. More severe or prolonged heat stress can result in damage to a high percent of testicular tissue and up to 70 or more days of disturbed sperm production. 

Sperm cells damaged by heat stress are not only less likely to successfully fertilize an egg, even successful fertilization by a heat-damaged sperm cell is less likely to develop past the early embryo stage. The combination of lower sperm production, a greater number of abnormal sperm and the reduced viability of early embryos all combine to make heat stress one of the most important causes of low bull fertility. 

Determining fertility

When a veterinarian examines bull semen under a microscope to evaluate sperm cell motility and structure, the defects found due to heat stress are the same as the defects due to other causes of stress and testicular damage. 

In other words, the veterinarian can identify that a bull has poor sperm motility and a high percentage of sperm cells with abnormal structure — but the cause of those problems cannot be determined with the microscopic exam. In addition, how long it will take for a bull to recover fertility after an insult (if ever) cannot be determined from a single examination. 

Because sperm production is a continual process, on any given day millions of sperm cells are completing their 70-day development process, millions are just starting the process and millions more are somewhere in the middle of their development. 

If a severe heat stress event lasts several days, sperm cells at many different stages of development are affected, and the type of damage to a specific sperm cell varies depending on the development stage at the time of heat stress. By retesting a bull every few weeks after a testicular insult, veterinarians can provide a much better prediction of how long it will take a bull to recover fertility than after the first breeding soundness examination. 

Although most bulls that suffer a heat stress-induced decrease in fertility will eventually improve, the recovery period make take anywhere from a week or two up to two months or more depending on the severity of the insult.  

Editor’s note: Robert L. Larson is a professor of production medicine and executive director of Veterinary Medicine Continuing Education at Kansas State University in Manhattan, Kan.