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. As a result, any prolonged exposure of the testicles to temperatures at or above body temperature can destroy the sperm and leave the bull infertile for up to two months while new sperm are being made. This period of infertility can be costly, especially if it occurs at or near the breeding season. For these reasons, 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 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.
Muscles also play an important part in maintaining proper testicular temperatures in bulls, namely the tunica dartos and the cremaster muscle. The tunica dartos and the cremaster muscle are located directly underneath the skin of the scrotum and on the spermatic cord, respectively. The tunica dartos works to control wrinkling of the scrotal skin, thereby controlling the amount of heat lost. The cremaster acts to raise and lower the testicles in the event that they require warming or cooling.
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. Decreases in sperm mobility and increases in the number of sperm with irregular shapes are also observed in bulls experiencing higher testicular temperatures. Increases in testicular temperature have such an affect as to disrupt the sperm-making process at all stages. Since sperm do not undergo further change after they have fully matured, flaws acquired as a result of temperature damage are irreversible, and, more often than not, render the sperm unable to accomplish their only job: To find and fertilize an egg cell. With a greater proportion of damaged sperm, fertility is likely to be affected.
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.
Fescue Toxicosis Gets Some Blame
The causes of some temperature regulation 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 occurs in cattle here in the Ozarks 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 together with the plant) that produces ergot alkaloid compounds that can be detrimental to the health of the animal consuming it.
The compounds produced by the endophytic fungus have negative effects on the reproductive hormone profiles, estrous cycles, and ovulation processes of the female bovine. Unfortunately, research is lacking in regards to how these toxins affect the reproduction of the bull. 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 infected fescue do not experience marked effects on the mobility, shape or 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. Since spermatogenesis is such a temperature sensitive process this implies that bulls affected by fescue toxicosis could have reduced fertility, given that body temperatures are elevated along with outside temperatures.
Clearly more research is needed in order to clarify the full range of affects that infected fescue imparts on the reproductive capabilities of the bull, especially since it is abundantly clear that fescue toxicosis causes such difficulties for females.
A bull with over-heated testicles will simply not do when one wishes to secure a higher probability for a successful breeding season. Producers should also consider the added temperature increases caused by fescue toxicosis.
For a list of references used in this paper visit www.ozarksfn.com.
This article was written in conjunction with the 2010 Missouri State University School of Agriculture Farm Animal Physiology Class.
