How to manage beef bulls for fertility
By Joe C. Paschal
Nov. 12, 2016 at 5:18 p.m.
In South Texas and the Gulf Coast, the fertility of beef bulls can be greatly affected by the hot, humid environment.
The effect on bull fertility reduces the quality and quantity of semen produced, which in turns reduces the number of pregnancies and subsequent calves born and sold at weaning.
This is the reason why the Brahman breed was developed in and for those areas.
The Bos indicus breeds that were used to create the modern American Brahman were originally from mostly hot and humid regions of India. The resulting cross of those breeds was adapted to hot climate, was resistant to some diseases and expressed some tolerance to internal and external parasites.
Crosses of Brahman with other non-Bos indicus breeds generated high levels of hybrid vigor but also passed along the adaptability to hot and humid climates to their offspring.
In time other breeds, which have been termed "American breeds" (beef cattle breeds that have some Brahman or Bos indicus genetics blended with other breeds: the Beefmaster, Braford, Brangus, Red Brangus, Santa Gertrudis, and Simbrah,) were created, mostly in the same regions as the Brahman.
These breeds were developed to increase muscularity and marbling while supplying replacements without additional crossing.
Several other breeds that are not from the Southern U.S. have been raised in the Gulf Coast and South Texas for many generations that have become somewhat acclimated (adaptation takes a lot longer).
I know of a lot of purebred breeders of cattle that originated in cooler climates whose cattle do well in our hot and humid climate. Generally, they are lighter-colored cattle, red or white, that have been here many cattle generations, but there are some black ones, too. This is an important consideration on the cow side that is often overlooked in the bull.
Body temperature in animals is controlled through a complex process that involves observable actions such as panting, seeking shade or standing in deep water but also ones that are harder to observe such as increased heart rate and the dilation of blood vessels near the surface of the skin, which cools it.
In bulls, the scrotum is often totally exposed not only to sunlight and temperature but to reflected radiation from bare ground. A bull's cooling system keeps the scrotum and the testicles cooled along with the semen being produced and stored by lowering it away from the heat produced by the bull's body.
However, this often exposes it to more heat from sunlight and reflected from the ground. Good ground cover obviously reduces that but can deflect cooling from air movement. The time period from late spring until early fall can reduce the semen quality of bulls, both old and young. Breeding in late winter until early to mid-spring is more desirable from a semen quality standpoint.
Damage from long-term heat reduces the quality and quantity of semen but doesn't destroy it totally. A short-term effect of very high heat often doesn't affect semen quality until a few weeks later. Mature sperm cells can be destroyed, but the main effect is on those being produced or to be produced in the next few weeks.
Bulls will recover, but when these incidents occur in short breeding seasons or at the beginning of a breeding season, they can have a real negative effect on calf crop. This is also the reason for the recommendation against a summer breeding season.
Bulls reach puberty or sexual maturity around the same time under the same conditions as the females of their breed or cross. Usually it is a combination of age (around 14 months for most non-Bos indicus breeds, a couple of months later for Bos indicus breeds) and weight (about 65 percent of their mature weight).
There can and will be some bulls that are earlier or older, lighter or heavier.
As in females, where I recommend scoring of their reproductive tract to evaluate their fertility, I highly recommend tracking the scrotal circumference of young (12-month-old and older) bulls. As a young bull develops sexually, their scrotal circumference increases. In a group of bulls of similar age treated or developed similarly (on grass or on feed), those bulls that have greater scrotal circumferences will tend to reach puberty at an earlier age.
This will also be seen in the daughter of these bulls as well. It does not mean that they will be more fertile or even breed a cow - those need to be evaluated by passing a breeding soundness examination or fertility test during breeding season.
Never buy a bull that hasn't passed a fertility test and, more importantly, hasn't had a trichomoniasis test. Usually, these should be done 45-60 days before breeding season begins to allow for retesting (for fertility) or replacement.
Next month: Discussion on diseases, herd health, internal and external parasites, body condition and breeding ratios.
Joe Paschal is a livestockspecialist a tTexasA&M AgriLife Extension in Corpus Christi. He can be contacted at 361-265-9203 or email firstname.lastname@example.org.