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Role of the bull in poor pregnancy outcomes

published: September 18th 2020
by: Sandy Johnson, Kansas Extension Beef spe

When the number of cows pregnant is far below expectations, poor reproductive performance by both cows and bulls must be considered.  Keep in mind that in some cases multiple issues may contribute. The focus of this piece will be on the bull.
    Pregnancy rate is typically defined as the number pregnant of the total number exposed to bulls with a given time period noted (AI pregnancy rate, season-long pregnancy rate).  Conception rate is the number pregnant express-ed as a proportion of those inseminated.  In natural service settings, conception rate is rarely known.
    Benchmark data can serve as a baseline for average performance.  The CHAPS database shows an average season-long pregnancy rate of 93.7% and 63% calved in the first 21 days of the calving season.  The length of the breeding season is not part of the data set but, based on calving data reported, most would be at least 60 days.  Similar data are not available for yearling heifers. Even with use of best management practices to ensure their reproductive success there are likely still sub-fertile heifers remaining and season-long pregnancy rates may be somewhat lower than mature cows.
    When shorter breeding seasons are used, lower pregnancy rates are expected to a point.  Pregnancy rates of 80% or better have been achieved in well managed herds in a 30-day breeding period that started with estrus synchronization on fifth day of the breeding season.
Troubleshooting poor bull breeding performance –
    A review of data and management practices can help identify the reason(s) for low pregnancy rates.  The following questions are designed to work through possibilities.
    What has been the trend in pregnancy rate and calving distribution over time?  A gradual downtrend could be due to a slowly developing disease process.  Identification of a trend may avoid a more shocking 40 to 60% drop in pregnancy rate in a severe disease outbreak.  Both bull and cow fertility are acceptable when 60% or more of calves are born in the first 21 days of the calving period,
    What breeding activity was observed during the breeding season?  Activity should start high and drop off during the course of the season and remain low.
    Did all bulls have a breeding soundness exam (BSE) pre-breeding?  Were any bulls borderline on passing any segments?  Bull fertility can and will change overtime, nevertheless, information from a pre-breeding, breeding soundness exam is a starting point when bull fertility is questioned.  A complete exam should include a physical evaluation, palpation and semen evaluation.  If a low pregnancy rate is noted at weaning time, a second BSE may be warranted, however, if the bull tests as a satisfactory breeder at that point, he may still have had an insult that decreased fertility earlier in the breeding season.  For example, semen production can be damaged if a bull ran a fever from foot rot.  Semen production would take about 60 days to return to normal.
    Was complete intromission observed for each bull? There is not a good test for libido or mating ability so monitoring activity is the only way to ensure successful mating occurs.  One of the challenges with libido tests is that bull performance changes as experience is gained.  Virgin bulls should not be put in the situation where they have lots of cows in heat at their first turnout as might occur with synchronization with natural service.  Keep in mind an injury during the breeding season may keep a dominant bull from mating but he may still be able to intimidate a less dominant bull.
    What was the bull to female ratio and size/type of pasture?  Standard recommendations are for bull to female ratios of 1 female per month of age for bulls under two years of age and 25-30 females for mature bulls with consideration for pasture size and terrain.  There are no specific guidelines related to pasture size and terrain adjustments.  Studies identifying sires of calves in multi-sire breeding pastures consistently show a wide range in number of calves sired.  As of yet, we have no way to identify in advance the sires that seem to dominate in this way.  Some bulls can handle much more than the standard recommendations and those producers choosing to do so should use all risk management tools to minimize potential problems.  Bulls should be in planned breeding groups prior to turnout so a pecking order and any injury in the establishment occurs before the start of the breeding season. In multiple sire pastures, bulls will establish a pecking order which will change as the bulls age.  A single sire breeding pasture has no fights between bulls, but since there is no backup, a BSE and monitoring during the season are crucial.
    Was there any mixing with neighboring herds?  It is important to make a record of dates any cattle crossed fences (either way) as it could represent a disease exposure or potential time of injury.  You may not know at the time that the neighbor’s bull brought you a gift of disease and the small bit of information about the date of the visit may end up being very important in diagnosing the problem.
    Did bull body condition change during the breeding season?  It is not uncommon for bulls to lose body condition during the breeding season, however a bull with a body condition of 4 or less (1=emaciated, 9=obese) will have poorer semen quality than moderate conditioned bulls.  A record of bull body condition score at the time of a BSE and at turnout provide a reference point for any change.  Snap a picture on your phone and you can have a ready reference.
    What biosecurity practices are in place?  Pur-chased bulls should be tested negative for a persistent infection of Bovine Viral Diarrhea (BVD) and isolated for 30 to 45 days prior to exposure to the herd.  Avoid use of non-virgin bulls and only consider their use after a negative Trichomonas test and consideration of other potential disease issues.
    Are bulls vaccinated appropriately for your location and disease challenges?  Bulls should re-ceive annual vaccinations of IBR/BVD, Campylo-bacter (Vibro), and Lepto-spirosis as directed by your local veterinarian.
    A review of all these issues may still leave the cause of poor fertility unknown or only speculative.  However, tracking these details of bull management each year will increase awareness of potential problems and hopefully reduce the likelihood of fertility issues.


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