Considering the attention reproduction gets in the cattle business, many people seem so transfixed by the cows and heifers that they end up overlooking an equally important part of the equation: The bull. Given that sperm provided by the bull contains half of the genetic material required to make the calves, more attention should be given to the ins and outs of the masculine element and how it is maintained.
The Bull’s Job
One of the most important duties that the bull fulfills is to produce numerous, healthy sperm cells. The lack of which yields inferior offspring, or perhaps even no offspring. The work and detail involved in maintaining a quality sperm-making environment in the testicles involves the regulation of several factors. Such factors include the sweat glands located on the surface of the scrotum, the action of muscles near and around the testicles and the structure of the blood vessel network supplying the scrotum and testicles. The concerted effort of all of these key players guarantees a favorable environment for the sperm-making process.
The importance of testicular thermoregulation in bovines cannot be understated. Spermatogenesis, the sperm-making process, is a temperature-sensitive process which requires that the testicles be a few degrees cooler than core body temperature, with the normal core body temperature of the average bovine being approximately 101.5°F. If the testicles reach temperatures at or above this, the sperm-making process is disrupted, leading to the failure of the sperm to fully mature and may even cause them to die. The making of sperm is a process that can take anywhere from 45 to 60 days in bovines. Protective mechanisms are required by the bull to ensure that the testicles are kept at suitable temperatures so sperm production may proceed as normal.
How Bulls Keep it Cool
One method bulls use to control testicular temperature is scrotal sweating. The sweating of the scrotum allows excess heat from its surface to be expelled through evaporation. Research indicates that the number and size of the sweat glands dotting the skin of the scrotum is greater than that of any of the other area of the body. It has also been shown that the number and volume of sweat glands increases towards the lower end of the scrotum. The fact that greater sweat gland size and numbers are found on the scrotum leads to the conclusion that fine control over testicular temperature is most likely influenced by its variable sweat gland density.
Research has determined that in normal bulls the scrotum is warmer nearer the top (closer to the body) than at the bottom, and the testicles themselves are warmer near the bottom than at the top. This opposing gradient in temperature is apparently due to the differences in the blood vessel supply network of the scrotum and testicles. These opposing temperature gradients complement one another and help ensure that the temperature of the testicles remains uniform.
And If They Can’t…
Bulls that are unable to maintain these temperature control mechanisms experience numerous reproductive problems. When the temperature of the scrotum is raised from about 93.2°F to 97.7°F, sperm samples are shown to have a greater proportion of defects. Sperm numbers decrease in bulls that experience this temperature change, with sperm numbers decreasing steadily for several weeks afterwards.
Research conducted at the U.S. Meat Animal Research Center in Clay Center, Nev., determined that bulls who failed to maintain normal scrotal/testicular temperatures had greater amounts of flawed sperm and suffered greatly in their ability to impregnate females. Fertility testing showed that bulls with abnormal scrotal temperature patterns had pregnancy rates 15-17 percent lower than bulls that had normal testicular temperatures, revealing that such bulls are sub-standard for the purposes of natural mating.
The causes of some problems, however, are not due to any inborn attribute of the bull, but are simply a result of environmental factors. A key environmental factor that often crops up in the cattle raising business here in southwest Missouri is fescue toxicosis. Consumption of fescue grass is associated with reduced fertility rates in livestock due to the fact that the majority of fescue is infected with an endophytic fungus (a fungus that lives in concert with the plant) that produces ergot alkaloid compounds that can be detrimental to the health of the animal consuming it.
Research that has been done has found that bulls consuming toxic levels of endophyte-infected fescue have higher than normal body temperatures when compared with bulls that consume no fescue. In addition, these bulls experience increases in testicular temperature. Testicle size also decreases for bulls that consume toxic amounts of endophyte infected-fescue as well, indicating that the testicles sweat more and produce less sperm.
Strangely, semen from bulls that consume high levels of infected fescue is found to have sperm numbers similar to, and in some cases higher than, the bulls fed the control diet. This implies that bulls that consume infested fescue do not experience marked effects on the mobility, shape, and numbers of their sperm. However, given the increase in body and testicular temperature observed in fescue-fed bulls, it is well worth noting that this makes the bulls more susceptible to heat stress during times of higher environmental temperatures.
The regulation of testicular temperature is a very necessary quality for any bull to have, and it is important to better understand the factors that affect it given that the bull is an all-too-important component in reproduction.
This paper was written in conjunction with the 2010 Missouri State University School of Agriculture Farm Animal Physiology Class.