Many commercial producers and purebred seed stock cattlemen often start to question the appearance of lots of droplets appearing on the semen evaluation forms of young bulls.
It can lead to frustration when bulls have to be retested. These droplets collectively are probably the most numerous defects we as veterinarians see when performing breeding soundness exams (semen evaluations) in the spring. It may help producers to have a better understanding of what the presence of proximal and distal droplets represent.
There is no doubt, on average, we see many more droplets in young immature bulls that are just reaching sexual maturity (puberty). To start with, these bulls are less active. Especially when bulls are not housed close to cycling females, these droplets in semen really are a very frequent occurrence.
One must realize a few important facts. When maturing in the testicles epididymis (little knob at the bottom of the testicles), 100 per cent of the sperm start with a proximal droplet. This is wiggled or shook off as the sperm matures and travels through the canals within the epididymis. Because the sperm with droplets are not mature, fertility is affected.
The story on droplets
There are essentially two types of distal droplets — those severely affected (pathologic) and those that will be shook off later (physiological). In a study by Dr. Jacob Thundathil at the University of Calgary Veterinary Medicine (UCVM), he found the percentage of droplets in ejaculated sperm declines in the time between collection and when it is chilled to be frozen and put into straws for artificial insemination.
This time interval is only three or so hours. If we extrapolate this, the same thing must happen with natural breeding. As the sperm is swimming up the vagina and into the uterus, a number of these distal droplets are shook off and left behind. As a result the distal droplets aren’t near as serious a defect as the proximal ones.
As mentioned, all sperm when formed start with a proximal droplet and during maturation in the canals of the epididymis, these become distal droplets and then are shed in the normal chain of events. If we do see lots of proximal droplets, they are often associated with other sperm defects such as head defects in a number of cases.
Droplets are really the main cause of many young bulls failing their first semen evaluation, especially when tested at a very young age. That’s why it is not recommended to test a beef bull before 12 months of age. Bison mature even much later, so 18 months for them is the minimum age. On average, the timing or age factor is why only about 50 per cent of 12-month-old bulls pass. The passing rate is elevated to 75 per cent by the time they are 14 to 15 months old. Many very young bulls that fail on initial semen tests pass in subsequent tests one to two months later.
It is difficult for veterinarians to predict ahead of time which young bulls will improve greatly. The only way is through retesting them. In my opinion, it does help if young bulls are housed close to cycling females. They will ejaculate off the senescent semen, which ultimately speeds up the improvement in semen evaluation.
Droplets disappear with age
With the veterinarian documenting the morphological defects on the semen evaluation form, you can see over time if improvement is happening. The most dramatic change toward improvement I have seen involved a young bull with essentially 100 per cent proximal droplets, but over several months the percentage kept decreasing and eventually he had excellent semen as a two-year-old. He was a show bull and the question always comes up about whether he was too fat, or inactive or there was some other reason, which may have added to his issue for sure.
When we encounter lots of droplets in mature bulls (which is much rarer) veterinarians usually are cognizant of other defects that accompany the droplets to determine the overall prognosis.
Again, the proximal droplets are a much more severe an issue than the distal droplets, realizing a certain percentage are shed before the sperm contact the egg in a natural breeding situation.
As producers evaluate semen reports, if most of the defects in a young bull are distal droplets and he still passes the semen test (overall defects are less than 30 per cent) there is a very good chance those defects will lessen with maturity and usage.
Over the years we have upped the ante by testing bulls at younger ages. In some instances bull sale dates have remained the same yet the purebred producers are calving later which translates into veterinarians testing younger and younger bulls.
When selling bulls off the farm, test as bulls as late or as mature as possible. The warm spring weather, with cows cycling close by, makes for an easier time in evaluating sperm, as opposed to the dead of winter when many bulls are little more than one year of age. Even one to two weeks older makes a big difference.
If semen evaluation is done later, the pass rate will be higher and it will be easier on everyone including the bulls. Don’t ask your veterinarian to evaluate bulls less than one year of age. Waiting will benefit both you and the bulls.