On-farm dairy bulls have been a part of the dairy industry since cows have been milked, even with the advent of sophisticated breeding programs and the use of artificial insemination (AI). But to be effective, dairy bulls have to be healthy and reproductively sound, which can be a challenge for dairy producers and their veterinarians for a variety of reasons.

Michael Overton, DVM, MPVM, Veterinary Medicine Teaching and Research Center, University of California-Davis, Tulare, Calif., says that in his area of California, many dry lot dairies rely exclusively on bulls. “However, in free-stall dairies, I think that more producers are reducing the number of bull pens in the herds and adding an extra AI pen,” says Overton. “Surprisingly, I see an increasing number of dairies that use bulls actually using AI in many of the pens behind the bulls. They will breed any cows that they find in heat and hope that the bulls settle the rest.”

Overton says there are different reasons why this is done: 1) producers aren’t satisfied with the bull results and want to try to “help them out” or 2) they use the bulls as insurance in case their heat detection is shoddy.


Infertile bulls are economically inefficient by being fed for no gain (other than slaughter for beef) and decreased reproductive efficiency.

Karen Jacobsen, DVM, MS, Farm Animal Resources & Management, LLC, Athens, Ga., has seen an increase in dairy bull use on her clients’ operations. “Many dairies, particularly the larger ones, have increased their use of bulls,” she says. “When heat detection is less than optimal, bulls can solve the problem rather quickly. Bulls are far better at heat detection than any human,” says Jacobsen. “In other cases, it is a labor issue. Some dairies have become frustrated by the labor and time demands of AI and timed-AI programs.”

Bulls remain a viable option for many dairies that do not raise their own replacements or that are expanding or new. Expanding or new dairies often use bulls early during the busy, hectic time and then gradually introduce AI into the program. “Bulls are often seen as an easier way of getting cows pregnant, opposed to active management with AI,” says Overton. “However, I think that more and more are starting to understand that bulls require active management to achieve good reproductive performance.”

Biosecurity and bulls
Dairies that rely on bulls primarily or exclusively tend to purchase bulls from good AI herds with excellent genetics, while herds that use them for clean-up may use some of their own home-raised bulls. “Most producers recognize that using natural service bulls is a real crapshoot with genetic merit and would prefer to use breeding bulls that are out of AI sires and dams,” says Overton.

“Genetic merit is an important point for any dairy using bulls,” says Jacobsen. “When you run the pregnancy rate numbers, some dairymen using bulls only for cleanup have been surprised to discover that most of their cows were actually getting bred by the bulls. One herd I work with was actually over 80 percent bull bred and thought they were primarily an AI herd!”

You can help clients reduce the risk of incoming diseases when they purchase bulls by giving them a list of negative tests and vaccines the bull should have prior to arrival on the farm. All bulls should be tested for Tritrichomonas fetus and Vibrio at the time of the breeding soundness evaluation. In addition, dairies that are working to improve their bovine viral diarrhea virus (BVDV) status are also testing bulls for persistant infection by an ear-notch test.

“Based on current information, we are not able to clear Leptospira serovar hardjo-positive bulls by treating or vaccination and therefore, in a herd that is working to limit the impact of this disease, consideration should be given to testing the bulls as well,” suggests Overton.

Of course, if dairies are on any kind of disease cleanup/control program, any test that would be done on cows entering the herd would generally apply to bulls, as well. This should include being sure that bulls are Johne’s negative by fecal culture. Unfortunately, many dairies are inconsistent with health screening and segregation of bulls.


Heat stress and lameness are two common problems that can affect bulls.

As far as vaccinations, Overton and Jacobsen say bulls should receive most of the same vaccinations as the cows, with a few exceptions. Overton recommends a modified-live IBR, BVDV, PI3, BRSV, 5-way leptospirosis vaccine, repeated in three to four weeks. Also included should be a 7- or 8-way clostridium, as well as two doses of a Vibrio vaccine repeated in three to four weeks.

“I think that if dairymen truly understand the risks that bulls may pose, they will at least start to do the basics, starting with breeding soundness evaluations and testing for the presence of Tritrichomonas fetus and Campylobacter fetus venerealis,” says Overton.

Bulls should also be dewormed and deloused and this repeated three weeks after the first application.

Breeding soundness exams
In the beef world, hardly any rancher would think of using a young bull that hadn’t had, and passed, a breeding soundness exam (BSE). On dairies, however, this can be more of a challenge in many circumstances. “Most dairies assume that the answer to bull problems is to simply add more bulls,” says Overton. “However, economic models would suggest that this is an incorrect assumption, especially considering the possibility that an infertile mature bull may dominate subordinate, healthy bulls.”


Michael Overton, DVM, MPVM,  recommends having a “working” and a “resting” group of bulls.

Jacobsen adds that some clients are frustrated because of the high rate of failure of bulls on BSEs, despite good fertility. “Perhaps many veterinarians err on the side of caution when failing a bull on a BSE, but when too many of the bulls fail, the dairymen get frustrated,” says Jacobsen. Jacobsen notes that many bulls fail a BSE before semen is even collected and evaluated. Reasons for failure or deferred classification include feet and leg problems, ocular problems, seminal vesiculitis, abnormal testicles, inadequate scrotal circumference and other general health problems.

Obstacles to breeding soundness exams include not having chutes designed to handle bulls and makeshift equipment used. “On the dairies where we have done BSEs, we have usually used the farm’s trim chutes that are sturdy enough but present some issues with access to the penis and prepuce,” says Overton.

Breeding soundness exams for your clients who are using bulls is a role many veterinarians could fill with a little training in the proper techniques of a BSE (see BSE sidebar), including physical evaluation and semen collection/evaluation. Basic equipment needed to do BSEs includes electroejaculators, scrotal circumference tape, a microscope, etc.

Infertile bulls
Especially without BSEs, having infertile bulls and not knowing it creates problems. There are two main problems with the use of infertile bulls. First, a dairy is feeding a bull for no gain, other than the income derived from the extra pounds of beef produced. Second, and probably of larger economic consequence, is the potential for decreased reproductive efficiency.

“I typically find that bull-bred herds have lower overall pregnancy rates than a typical AI-bred dairy,” says Overton. “We don’t know how much of the decreased efficiency might be overcome by ensuring that all bulls are in good physical condition and of acceptable normal fertility.”


Karen Jacobsen, DVM, MS, says non-slip surfaces are important to protect bulls from injuries and lameness.

It’s nearly impossible to find out if one or more bulls that are in with a group of cows is fertile or not. Jacobsen says the pregnancy rate of the group could give you some idea. “But this is difficult if a client is rotating bulls frequently,” she says. “Visual observation of libido and physical problems are, of course, helpful but won’t detect all problems.”

Bull health problems
Good vision is extremely important for bulls. Bulls often identify cows in estrus by locating the small, active riding cows that are often located away from the other cows. In a dry lot or pasture dairy, bulls must be able to see these cows to improve the odds of actually servicing and impregnating them.

Bulls are susceptible to the same ocular problems that plague cows, especially pinkeye. Unfortunately, bulls are often overlooked until problems get severe. “If diagnosed early enough, typical treatment approaches such as systemic oxytetracycline should work well,” says Overton. “The key is to observe the bulls on a daily basis, but most dairy workers are happy to tiptoe around the bulls from a safety perspective.”

The largest health problem in dairy bulls is lameness and the causes are similar to cows. While bulls don’t have the same level of dry matter intake that lactating cows have, they are eating the same rations. Bulls are often placed into lactating pens directly from a pasture or dry lot where the previous ration was much less energy dense. Once on these rations, bulls rapidly gain weight and may become fat. Fat bulls are less tolerant to heat stress and are more likely to ejaculate defective sperm in periods of hot weather, and they also tend to have lower libido during heat stress.

Jacobsen adds that rumen acidosis can become a problem, especially when rotating bulls onto and off of rations that are very different from the lactation ration. The combination of high energy, high protein, overconditioning and constant walking and standing on concrete combine to create laminitis and other lameness issues. In free-stalls, bulls often don’t fit in the beds and thus are even more prone to developing lameness due to the inability to comfortably lie down (see lameness sidebar). It has also been suggested that pain from lameness can result in testicular degeneration affecting sperm quality.

Non-slip concrete surfaces are important to prevent lameness in bulls, says Jacobsen. “The surface of concrete is important to prevent slipping. Grooved concrete or grooved rubber mats can help with bull footing. Fighting and slipping on concrete con-tribute to lameness issues.” She adds that bulls tend to work best in herds with pasture housing or bedded packs, and bulls in free-stall housing need adjacent dry lots for exercise and relief from concrete.


Bulls depend on their eyesight to visually target cows, but eye problems can plague them.

In the South, Jacobsen says other strategies to help prevent and mitigate bull lameness include nutritional management minimizing ration changes, avoiding rocks in cooling ponds, frequent monitoring and care of bull feet problems, and clearing rocks from dirt areas, especially when bulls’ feet are wet from cooling ponds.

Managing bulls
Keeping bulls healthy on the dairy can be accomplished in part by using them in a rotational system. Overton recommends using two bull groups – a “working” group and a “resting” group. The two groups are rotated every two to three weeks. “The idea is to try and improve libido by providing a period of sexual rest followed by reintroduction into a novel group of cows,” says Overton. This is called the Coolidge effect.

“In my large herds, bulls are rotated in and out every week,” Jacobsen adds. A few hours once a week are scheduled for moving bulls. This works well. A separate fenced paddock is used for the bulls on these herds.”

When bulls are being rested, they are placed on a low-quality pasture or open dry lot and fed a ration that is lower in energy, calcium, gossypol and protein but contains some of the same ingredients as the lactating diets. “Interestingly, we have had better success keeping the bulls on the lactating ration, just a lower amount, with free-choice hay available,” says Jacobsen. “We have more bull feet problems when the rations are abruptly changed than with this method.” Each time the bulls are rotated, they can be evaluated for lameness or other health problems and trimmed/treated as needed. “Most bulls never see a trim chute, and I think that trimming can be used to reduce the impact of lameness in many instances,” says Overton.

In some parts of the country where cold stress is an issue, bulls need to be provided with some dry bedding and wind breaks. Heat stress is definitely a problem on most dairies, and bulls need to be cooled just the same as cows. “Remember that sperm are very sensitive to high ambient temperatures and even short-term exposure to very high temps can lead to temporary infertility,” says Overton.

Elevated scrotal temperatures adversely affect both epididymal sperm and testicular sperm by reducing sperm chromatin stability. Testicular sperm is most sensitive to heat stress effects, and this explains why bulls may be infertile for long periods of time even after the heat-stress period is over.

“Cooling ponds or ‘bull baths’ can be a good way to cool a bull and his testicles,” Jacobsen says. “However, these should not be used when rotating into concrete-floored free-stalls, as wet feet are more susceptible to concrete’s abrasiveness.”

Most bulls are sold after one year of service due to either lameness or behavioral problems (see human safety sidebar). “In my opinion, on most dairies, no bulls should be used for more than one year due to the tendency to become aggressive,” says Overton.

For maximum efficiency, a general guideline for stocking density of bulls to cows is at a ratio of one bull per 12-20 non-pregnant cows, depending on the age, fertility and housing system. In open pasture or dry lots with older, fertile bulls, 20 cows is probably fine, says Overton.

In free-stall facilities, bulls must deal with overcrowding and concrete and must rely on more direct cow contact to find cows in estrus. “Usually in this setting, we are using younger bulls with less lameness and need to stock more toward one per 12-15 head,” Overton notes. “Of course, if you are turning cows in with bulls after having received an early lactation series of prostaglandin injections, a higher stocking density is needed to ensure that all cows are serviced.”

However, more bulls is not always better. In some cases, a large dominant mature bull may keep younger sub-ordinate bulls away or they may fight among themselves over a particular cow. This can result in bull injuries and missing other cows.

So if your client is using bulls, consider BSEs, lameness mitigation and good vaccination programs to help increase the efficiency of the bulls. Alternatively, if bulls don’t seem to be doing the job, weigh the economics of keeping and feeding potentially dangerous bulls against the use of a strictly AI program.

For more information on dairy bull management, see:

  • Breeding soundness evaluation: physical assessment, Gary D. Warner, DVM, 2004 Proceedings of the AABP.
  • Selection and management of natural service sires in dairy herds, Michael W. Overton, DVM, MPVM, Carlos Risco, DVM, Dipl. ACT, Joseph C. Dalton, PhD, 2003 Proceedings of the AABP.

Watch glossypol and bulls

Cottonseed products contain various levels of gossypol and can cause an
increase in the number of abnormal sperm (midpiece abnormalities) or when fed at even higher levels, render bulls infertile, says Michael Overton, DVM, MPVM. Bulls are more sensitive to gossypol than cows, and ordinary feeding levels of 5-6 pounds of whole cottonseed, while not causing any problems in lactating cows, may potentially affect breeding bulls, even adjusting for the lower level of dry matter intake predicted for bulls.

Karen Jacobsen, DVM, MS, adds that it doesn’t seem to be a problem if whole cottonseed is kept below 6 pounds/head/day in the cow ration. “Cottonseed meal is less of a problem than whole cottonseed because the gossypol in the meal is bound. Cottonseed hulls are not an issue,” she adds. “The gossypol is present in the seed, not the hull or fuzzy lint.”

“Of course, a lot depends on the type of cotton product being fed, but I try and limit the cottonseed or cotton byproducts to less than 10 percent of the total diet if using natural service bulls,” adds Overton.

Bull lameness

One of the biggest lameness problems for bulls in confinement systems on concrete is excessive wear of the sole, particularly at the toe on the outside claw of the rear feet. “They tend to get excessive wear and thinning of the sole as a result of increased weight-bearing on rear claws when mounting cows during breeding. Eventually this thinning of the sole leads to bruising of the underlying corium, white line separation and exposure of the underlying corium and, ultimately, abscess formation in the white line region of the toe,” explains Jan Shearer, DVM, MS, University of Florida.

“Separation of the white line is sometimes complicated by the extension of infection through the corium and into the 3rd phalanx. Involvement of the 3rd phalanx can lead to a chronic problem that is difficult to deal with,” he adds.

Shearer says some dairymen in Florida have been using bulls for a week then resting them two or even three weeks. This can be difficult because it requires a lot of bulls. “But if you work them any more than that on wet abrasive concrete, they may develop thin-sole problems very quickly. The thin-sole problem from excessive wear has become a major problem in some herds using bulls. If you can, minimize the exposure of bulls to concrete and be sure to rotate them as necessary to avoid thin-sole problems.”

Because bulls are housed with lactating cows, they have access to the high-energy feedstuffs that can induce laminitis and lead to the production of poor-quality claw horn. This exacerbates the thin-sole problem since poorer-quality horn is softer and wears faster. Laminitis is a disease that results from disrupted blood flow in the corium. When blood flow to the corium is disturbed, less nutrients and oxygen diffuse into the lower layers of the epithelium (filled with keratinocytes destined to cornify and become horn). Deprived of oxygen and nutrients, the rate of keratinization of horn cells is reduced leading to softer and poorer-quality horn.  

Bull lameness is a combination of housing factors associated with rough concrete, the constant exposure to wet manure slurry and moisture, mounting behavior associated with breeding cows, and nutrition and feeding management practices that contribute to laminitis. “It becomes very complicated,” says Shearer. “I’d guess that most bulls leave herds primarily because of lameness, not because they are not doing their job.”

Human safety

Bulls can be extremely dangerous to humans on the farm. Farm workers are injured or killed every year by bulls, which is one reason not to use them. But for those who choose to use bulls, and the veterinarians who work with them, Michael Overton, DVM, MPVM, offers a few safety strategies.

“I know several veterinarians that carry bear repellent with them when palpating cows in open dry lots to spray at bulls if threatened,” says Overton. In addition, facilities should be built to provide escape areas both in terms of pass-throughs and ground clearance (to allow for a drop and roll if needed). No bull should be used once he has displayed any signs of aggression toward dairy workers.

“I think that it takes a combined effort from veterinarians, dairymen and farm workers to identify aggressive bulls and remove them from the herd as soon as possible,” says Overton. “Veterinarians need to demand that aggressive bulls be removed rather than risk their lives to palpate cows.”

Breeding soundness exams

Breeding soundness exams are critical to assess fertility and suitability of young dairy bulls. At the American Association of Bovine Practitioners in 2004, Gary Warner, DVM, Elgin Veterinary Hospital, Elgin, Texas, notes the six steps to a proper breeding soundness exam (abbreviated). For more information, see Breeding Soundness Evaluation: Physical Assessments, 2004 AABP Proceedings.

Six steps of a BSE

1. Observe unrestrained bull’s movements, physical structure, disposition and body condition. Bulls with body condition below 5 or above 7 on a 9-point scale should be scrutinized closely. Over-conditioned bulls should be evaluated closely for fatty deposits in the scrotal neck.

2. On a properly restrained bull (mechanical squeeze chute is preferred), perform a rectal examination of internal reproductive organs, including the prostate gland and seminal vesicles. Palpate for cysts and abscesses (seminal vesiculitis). Examine/identify rumen, omentum, peritoneum, small intestine and internal inguinal rings. Be aware of adhesions. Note structures other than spermatic vessels and the ductus deferens that pass through the inguinal rings. Inguinal rings larger than a three-finger size are abnormal.

3.Visually assess both scrotal and sheath structure, noting irregularities such as previous insult, scarring dermatitis or fluid swelling. Palpate scrotal contents, sheath and penis. Individually palpate testicles, note painful or fibrotic areas. Palpate all portions of the epididymis. Note fibrosis or cystic areas. Palpate scrotal neck for adhesions, hydroceles, intestinal protrusion or vessel varicosities. Palpate spermatic vessels and cremaster muscle.

4. Measure scrotal circumference from the Society for Theriogenology guidelines:

Age less than or equal to 15 months

Minimum scrotal circumference (cm)    30)

Age 15 to18 months:

Minimum scrotal circumference (cm)    31

Age 18 to 21 months:

Minimum scrotal circumference (cm)    32

Age 21 to 24 months

Minimum scrotal circumference (cm)    33

Age more than or equal to 24 months

Minimum scrotal circumference (cm)    34)

5. Collect a semen sample via electroejaculation. Collect when the ejaculated fluid turns from clear to cloudy white/milky, which is the sperm-rich fraction.

6. Examine the semen sample with a bright field or phase contrast microscope. Also needed are semen stains (eosin and nigrosin), glassware and warming trays. Knowledge of sperm cell morphology and classification is needed. The use of new methylene blue stain will improve the ability to detect white blood cells in the semen.