Spring and early summer before breeding is breeding soundness exams (BSE) time for young and mature bulls. Especially with the last two years of drought, every calf on the ground will be critical this year for beef herds, and bull management is an important part of the equation.
When doing BSEs, Bob Larson, DVM, PhD, Dipl. ACVPM, Dipl. ACT, Kansas State University, has different priorities and concerns when evaluating yearling bulls compared to evaluating mature bulls. “Yearling bulls have not established their fertility by successfully getting cows pregnant and they have only recently reached puberty and may or may not be sufficiently sexually mature to be successful breeders,” Larson explains. “They are at risk for young bull problems such as persistent frenulum and penile warts.”
On the other hand, young bulls have had less opportunity to become injured or to develop structural or degenerative problems compared to mature bulls. “Mature bulls have proven their ability to achieve pregnancy in exposed cows in the past, but because of time and age, they may have musculoskeletal injury or degeneration, the penis or prepuce may have become injured, the testicles or epididymis may have experienced infectious, toxic, or degenerative insults, and disease of any major body system can reduce the ability or desire to successfully mate cows,” Larson notes.
We look for different things in different ages of bulls, agrees Dave Rethorst, DVM, Kansas State University. “But the fact remains that we must do a complete exam on each and every bull,” Rethorst says. “This includes measuring and palpating testicles, palpate epididymis, rectal exam, extend penis and then semen evaluation. This is done after looking at condition, feet, legs and eyes. Usually condition, feet/ legs, movement are done before the bull actually gets to the chute.”
Preparing the young bull
Before their first BSE, young bulls need to be in good shape. Larson says young bulls should be in a body condition score of about 6 (on a 9-point scale) prior to the start of their first breeding-season. “I expect bulls to lose weight as the breeding season progresses and they need to have some energy reserve stored as body fat.”
The drought and subsequent forage shortage may lead to some under-conditioned young bulls this year. Poor body condition can affect young bulls in a number of ways. “First, the onset of puberty is influenced by both age and weight,” Larson says. “Bulls that should be old enough to be producing viable sperm will be delayed if they have not received adequate nutrition.”
Second, even short-term feed restriction results in stress and stress will reduce testosterone production which is critical for normal sperm cell development. Rethorst says semen morphologyon poorly conditioned bulls will display proximal droplets and separated heads. While not becoming over-fat, young bulls should be steadily gaining weight up through the start of their first breeding season.
On the other hand, over-conditioned young bulls may be at higher risk for musculoskeletal problems and are at higher risk of sperm morphology abnormalities. Rethorst says he see more separated heads in over-conditioned bulls, along with laminitis.
One reason over-conditioned bulls may produce abnormal sperm is that fat accumulation in the neck of the scrotum decreases the body’s ability to keep the testicles cooler than the core body temperature. “In addition, over-conditioned bulls are considered to be at higher risk of decreased libido, lameness, and rapid weight loss early in the breeding season,” Larson adds.
Growing bulls need good quality forage as a diet base. In addition, high-energy diets hasten the onset of puberty and do not decrease future fertility as long as the bulls do not become overconditioned, Larson says. And, he says, don’t forget the salt which is critical for cattle diets.
“Young bulls need adequate protein and energy to reach desired BCS,” Rethorst says. “Proper levels of copper, zinc, selenium and manganese are necessary, particularly zinc as it is critical to testicular function and foot health. The vitamin A requirement of the testicle is higher than other body tissues. This is of particular concern with the drought conditions we have been experiencing.”
Based on soil type, forage species, and feed attributes, many pastures, harvested forages, and concentrate supplements provide adequate minerals for growing bulls. “In some situations, soils and the plants grown on them can be deficient in important minerals and strategic supplementation of trace and macro minerals in those geographic areas is essential,” Larson says.
Larson recommends that bulls follow the same vaccination protocol (with the exception of brucellosis vaccination) as females within the herd of the same age. In other words, yearling bulls should be vaccinated at the same time and with the same products as yearling heifers (except no brucellosis vaccination). For Rethorst, the absolute minimum is modified-live viral vaccination for IBR and BVD, plust campylobacter and leptospirosis. “Deworming and fly control decisions should be based on climate, time of year, and type of housing (drylot vs. pasture) or timing of movement to new pastures,” adds Larson.
BSEs on young bulls
A group of bulls developed together are expected to reach puberty on average between 9 and 10 months of age. But reaching puberty is not the same thing as achieving acceptable fertility and is far from reaching sexual maturity. “A bull that has just reached puberty will have low numbers of sperm in an ejaculate with only 10% or a little better progressive motility and many sperm with abnormal morphology – in other words, they will not pass a BSE,” Larson says.
As bulls continue to mature past puberty, sperm production, motility, and morphology all gradually improve over at least the next four months. “Even bulls that will achieve excellent fertility when they are older will not pass a thorough BSE near the time of puberty; therefore, young bulls should be evaluated as near to the time they will be expected to breed as practical,” Larson says.
Most of the young bulls Rethorst has evaluated over the years were done at 12-13 months of age. “This would let me check late January- and February-born bulls in late February or early March for March sales,” he explains. “I would end up doing some 11-month old bulls and usually got along with them fairly well. If I got into bulls less than 11 months old, I would find more maturity issue defects, proximal droplets, separated heads, etc.”
For producers selling yearling bulls, this means that young bulls will be examined relatively close to the time they are offered for sale. If young bulls did not receive a thorough BSE prior to purchase, the producer should have the bulls evaluated relatively close to the start of the breeding season, but early enough to allow time to find replacements for any bulls that fail the BSE.
Frame size in itself doesn’t factor into whether or not Rethorst passes or fails a bull on a BSE, but it becomes part of the discussion when he is working with a client on bull selection. “For the most part, I want a moderate- framed bull with good depth of body and muscling,” he says. “There are places for the small- and large-framed bulls depending on what kind of cows they are going on and what the producer is trying to do.”
Selection criteria of bulls based on genetic contribution to the value of the calf crop is greatly dependent on whether or not heifers from the bull will be retained in the herd as replacements, and how the offspring will be marketed. “When heifers will be retained in the herd, both minimum and maximum expression of attributes such as mature size, growth potential, and milking ability must be established,” Larson says. “For offspring destined for growth to slaughter weight, a bull’s genetic contribution to growth potential, mature size, and potentially carcass characteristics will impact the value of the bull as a breeding animal.”
Rethorst’s list of importance for a young bull BSE is as follows: 1) physical soundness, 2) optimal scrotal circumference, 3) >70% normal spermatozoa, and 4) adequate motility.
Scrotal circumference is of particular importance when evaluating young bulls because it is an excellent predictor of puberty (even better than age or weight), Larson says. Bulls of all breeds reach puberty when scrotal circumfer ence reaches about 25 to 27 cm.; therefore, young bulls that greatly exceed this measurement when evaluated are assumed to have reached puberty several weeks or months earlier. “In addition, because bulls that reach puberty at younger ages are more likely to have heifer offspring that reach puberty at a young age, selecting for yearling bulls with large scrotal circumference will ensure that heifer progeny can reach puberty at a young enough age to become pregnant early in their first breeding season,” Larson says.
It is critical that yearling bulls have a structurally sound musculoskeletal system as well as normal testicles, accessory sex glands, penis, and prepuce. In addition, bulls must be healthy and able to pass a thorough physical examination. Many young bulls have recently reached puberty and may fail a BSE due to excessive numbers of sperm with abnormal morphology. But a high percentage of these bulls will mature rapidly enough to be fertile by the start of the breeding season. In contrast, bulls with small scrotal circumference, musculoskeletal problems, and severe illness are unlikely or unable to become satisfactory breeders.
Both Larson and Rethorst agree that bull temperament should also be taken into consideration when evaluating bulls. “Injury due to bulls is a leading cause of death and injury on cattle ranches,” Larson says. “Bulls that are difficult to handle or aggressive to humans should not be considered as breeding animals.”
“Bulls with poor attitudes sire calves with poor attitudes,” Rethorst adds. “Not only are there injury considerations but there are also performance considerations”
Failing young bulls
The most common reason young bulls fail a BSE is semen morphology and an immaturity type profile. “These bulls are deferred,” notes Rethorst. “Many times these bulls can be re tested in as little as two weeks and produce a satisfactory ejaculate. It will take several retests on some bulls to get a good sample.” Rethorst says on yearling bulls he will rarely cull before three tests.
Larson says yearling bulls with structural problems such as being post-legged will not improve with time. Similarly, bulls that are old enough to have reached puberty but that do not have adequate scrotal circumference are not likely to catch up with their peers and should be culled. A persistent penile frenulum can be easily corrected with a simple surgery, but the client should be informed that this condition is suspected to have a heritable component. Penile warts are a common condition of young bulls that will cause them to fail a BSE, but with correction and complete healing will not affect fertility.
Common sperm cell morphology problems in young bulls include proximal droplets, distal midpiece reflex, decapitated sperm, and sperm head defects. “For many bulls these defects are due to the relatively low testosterone produced by peri-pubertal bulls and these defects will decrease or disappear as the bull matures,” Larson says. “But, one cannot assume that young bulls with these defects will be fertile by the start of the breeding season because some may have very delayed sexual development and others may have development problems that will prevent them ever becoming a satisfactory breeder.” Yearling bulls with seminal vesiculitis will not pass a BSE, but with time, many of these bulls recover and become satisfactory breeders.
If a young bull passes a thorough physical exam, including measurement and palpation of the testicles, but fails the BSE due to excessive numbers of sperm with morphology abnormalities, it can be re-evaluated closer to the start of the breeding season. “Young bulls that pass a BSE a few weeks after failing an initial BSE are considered to be satisfactory breeders that are displaying the rapid pace of increasing sexual maturity that follows the onset of puberty,” Larson says.
Preparing the mature bull
At the start of the breeding season bulls cannot have any flaws. They must be able to produce abundant fertile sperm as well as being healthy and sound so that they can deliver that sperm to the reproductive tract of every cow that expresses estrus, Larson says.
Mature bulls should enter the breeding season in about a body condition score of 6 on a 9-point scale. This level of condition provides some energy reserve so that even as he loses weight due to the high activity of a breeding season, a bull is not as likely to become overly thin.
Mature bulls have lower nutritional requirements for their size than other cattle on the ranch. However, to maintain their body weight, forage quality and availability must be adequate. Good quality forage provides an adequate diet in many situations, but producers must be prepared to supplement bull diets if forage quality is poor or unavailable.
Production of testosterone by the Leydig cells of the testicles is critical for normal sperm production and maturation. Even short-term reductions in feed intake will cause stress in bulls which greatly reduce testosterone production and subsequently reduce production of normal sperm cells, Larson says. Longer-term negative energy balance results in even more severe effects on fertility.
Although the musculoskeletal development problems associated with over-conditioned growing bulls is not a concern with mature bulls, fat accumulation in the neck of the scrotum and excessive weight for the amount of exercise demanded are problems of being over-conditioned in mature bulls.
Because of the importance to testosterone levels and subsequently to sperm production that bulls remain in a positive energy balance, rapid weight loss is not desirable even if bulls are over-conditioned, Larson cautions. “Over-conditioned bulls should be managed to slowly reduce their body weight over many weeks by gradually reducing the energy density of their diet.”
BSEs in the mature bull
If mature bulls are examined too far ahead of the breeding season, bulls that pass a BSE will have an extended time at risk when they may experience health, nutritional, or injury insults that reduce their fertility. On the other hand, bull evaluations performed too close to the start of the breeding season may not allow adequate time to find suitable replacements for any bull that fails. “My general recommendation is to determine how many days a producer needs to confidently find, obtain, and evaluate replacements for bulls that fail the breeding evaluation; and then schedule the herd BSEs as close to that date as possible,” Larson recommends.
Rethorst likes to do BSEs 30-60 days ahead of the breeding season. “However, as early as some of the bull sales are, if a producer wants to buy at a specific sale I could end up doing bulls 90-110 days before breeding season.”
Larson recommends that mature bulls receive the same vaccinations at about the same timing as the vaccination strategy for mature cows in the herd. Deworming bulls is usually administered when handled near the time of turn-out to the breeding pasture. The timing of the breeding season will dictate whether or not fly control should be initiated at the time of the BSE. “Bulls should be observed for indications of heavy fly populations during the breeding season and fly control used as needed,” he says.
Failing the mature bull
Common reasons for a mature bull to fail a BSE are feet and leg problems that limit his ability to adequately cover the breeding pasture, injury to penis or prepuce, and testicular degeneration that can be detected on semen evaluation or testicular palpation/ultrasound or documentation of decreased scrotal circumference. “Many of these conditions are not likely to improve quickly enough to expect resumption of successful breeding within the upcoming breeding season,” Larson says. “Some temporary testicular insults may recover quickly; but a bull that fails to demonstrate improved breeding potential within three to four weeks after failing a BSE is not likely to be useful soon.”
Once a bull has calves that he can evaluate, Larson will only cull a bull for genetic reasons based on the performance of his calves, not his own characteristics. He notes that although not directly a measure of culling criteria at the time of a BSE, a NAHMS (National Animal Health Monitoring Service) survey of U.S. beef producers found that the most important culling criteria for bulls from the herd were: infertility, physical unsoundness, disease, poor growth performance of calves, and dangerous temperament.
Once the breeding season starts, it is important to observe bulls frequently so that problems with feet, legs, penis, prepuce, or overall health can be detected early and the bull removed from the breeding pasture. Early in the breeding season, fertile replacements must be added to the breeding pasture to keep the number of cycling females per fertile bulls from getting too high.
The old bull
Some producers may keep their good bulls around as long as they can. Rethorst has seen a fair number of 7-8 year-old bulls in terminal sire programs if they remain sound and are siring good calves. Larson says often bulls are removed from breeding groups after four years to avoid inbreeding as their heifer offspring enter the mature breeding herd, and three years if heifers are exposed to the same bulls as the mature herd.
The most common reason mature and older bulls are culled from the herd are problems with feet/legs, injury to penis or prepuce, other disease problems, temperament, and testicular insult/degeneration.
Whether to cull an old bull might be hard for some to decide. “I probably would keep him as long as he is fertile if I can keep him from breeding his daughters,” Larson says.
Sidebar: Trich testing bulls
Trichomoniasis is a venereally-transmitted disease, therefore, if young bulls have not mated any females they cannot have become infected with the organism and therefore, usually do not need to be tested, says Bob Larson, DVM, PhD, Dipl. ACVPM, Dipl. ACT. However, “There will be seedstock producers who trich test young bulls as a marketing tool,” adds Dave Rethorst, DVM.
Because trichomoniasis is a venereal disease, bulls can only become infected by mating an infected cow. Larson says most bulls do not need to be tested for trichomoniasis; however, any bull with even a small risk of being exposed to infected cows should be evaluated after sexual rest three or more times at weekly intervals or until the veterinarian is are confident that he is not infected.
“If a producer knows there is trich in his area or in the area where his pastures are, this is a discussion they need to have with the herd veterinarian,” Rethorst says. “For example, there are areas in Kansas where I would definitely test routinely. With the advent of PCR testing, we can be 97-98% sure a negative test result is truly negative.”
Rethorst adds that this assumes proper pre-test sexual rest, good sample collection technique and proper sample handling.