How do you know when your bull is in its most fertile state? Is your heifer ready to birth a calf? What parameters do you need to keep in mind when selecting breeding stock?
The answer to these questions comes from Harrison Dudley, an associate veterinarian that provides a livestock veterinary service based in North Carolina. With over 11 years of experience, Dudley consults on everything from reproductive evaluations to bull breeding soundness and serologic exams.
When it comes to head selection, before doing a soundness exam, Dudley focuses first on pheno/genotype selection points. “Most of my producers are going to come in at weaning time and have some benchmarks for females that they decided to keep back from selling,” Dudley said.
Reasons for this vary, including the structure of the feet and legs, the body type, weight, hairstyle and any information about its parents.
Dudley pointed out that it’s important to be aware that owners are more likely to hold onto the bigger, prettier heifers. However, he said, “just because you kept her at weaning time doesn’t mean that she’s going to continue to grow, nor does it mean that she needs to stay in your herd long term.”
“I don’t like to keep a heifer that’s not at 55% of her mature weight when it gets to be yearling time,” Dudley said. When it comes to breeding his own heifers, he wants to make sure they are at least 700 pounds, preferably closer to 800, before he’ll put one with a bull. However, he said, a small cow could also mean she is just more efficient.
At the end of the day, Dudley wants to make sure he has a continued phenotype in his herd that will last between six and 10 years. Therefore, “if she has any kind of docility problems, if her feet start to look bad, she doesn’t shed out her calf hair very well and stays real furry, she’s probably not going to stay in my herd.”
When conducting a soundness exam, Dudley recommended first making sure the heifers have an appropriate weight and body condition. “The other two aspects include a pelvic exam of both the reproductive organs and pelvic measurements. This is to make sure that the birth canal is an appropriate shape and size to be able to have a calf,” Dudley said.
“The goal,” he continued, “is to get heifers pregnant as early in the breeding season as possible so they can calf successfully between 24 to 26 months of age. If you have prominent influence in your herd, the timeframe should be later.”
Dudley starts with an exam of the cow’s uterus and ovaries to give a “reproductive track score” (on a scale of 1 – 5). A high score indicates the heifer’s uterine horn diameter is 35 mm or greater and there’s an active CL (corpus luteum) on one ovary. “If she’s not yet gone through puberty, or has a very low score, the track could be around 20 millimeters in diameter, very flaccid and have no tone,” he said. A score of 3 is a median number which indicates large-sized follicles and that the heifer is close to ovulation.
The next part of the exam involves a Rice pelvimeter, a measuring tool that is inserted rectally and then spread to attain a vertical and a horizontal measurement of the pelvic canal.
“We try to position this in the most narrow place so we can get a minimum measurement, giving us a number that is correlated with the lowest possible chances of her having a difficult birth at her first calving,” said Dudley.
The size of the pelvic area doesn’t always determine if the cow will have trouble birthing a calf. Other factors include the health and viability of the heifer at capping. Dudley noted studies where the prediction of an uneventful birth is only 80% – 85% when based on the heifer’s pelvic measurements.
The cost of an assessment can be about $6 to $8 a head, according to Dudley. However, he added, “the return on that investment is an earlier born, more uniform calf crop rather than heifers that breed late or have trouble. If you can avoid even one dystocia or one dead calf, it quickly and easily pays for itself.”
Breeding soundness exams (BSEs) for females are normally done one time only, and that is to assess their “first potential achievement of pregnancy because at that point, if they’ve successfully bred a calf, the breeding soundness is pretty much going to stay the same,” Dudley said.
In contrast, he recommended testing a bull every time it’s going to be turned out on cows. “We look at structures and semen viability. We don’t typically test testosterone levels, so just because he passes a breeding soundness exam doesn’t necessarily mean that he can complete the job in the first place.”
Because young bulls don’t know how to be efficient, they wear themselves out before being able to cover many females. As a precaution, before allowing his bulls to breed, Dudley prefers to wait until they weigh around 1,200 pounds and are at least 15 months old. Even then, he expects them to breed maybe 10 to 15 animals in a 70- to 90-day period.
At turn out and early in the breeding season, it’s important to monitor a bull’s body condition. “This is because in the thick of a breeding season, bulls will be much more focused on breeding than they will on maintaining their own nutritional status. They will forego grazing often if cows are cycling, especially the young bulls,” Dudley said. Have an appropriate supplementation program or nutrition plan so that the bull can do his part of the breeding process from the beginning of the season to the end. If he loses more than 300 pounds in the first 45 days, his fertility is going to drop precipitously.
To get a semen sample, Dudley uses an electrode ejaculator machine probe, which mimics the type of contraction necessary for a natural ejaculation. This method produces a higher proportion of seminal and prostatic fluid. When examining the concentration of both, Dudley said it’s up to the individual to decide if it’s adequate.
He noted that BSE tests are the only way to find out if a bull has the hardware to do the job. “A lot of things can affect whether he’s going pass it or not,” Dudley said. For example, it depends on the bull’s condition when the ejaculate is collected and/or whether the bull has been abstinent for the few days prior to the test.
When examining the sperm sample, the sperm should be swimming in a straight line and contain a minimum of 30% progressively motile sperm. “For morphology, you want 70% of the cells determined to be normal confirmation, meaning the head, midpiece and tail are put together right,” he said. “You need to count a minimum of 100 sperm to see if they have all three parts. The kinds of defects tell you something about the prognosis of the bull. If he has a lot of head or midpiece abnormalities, that would suggest there’s a kind of testicular problem. If he has a lot of tail abnormalities that could be something as simple as heat or cold stress, or he just hasn’t ejaculated in a long time.”
The cure for this, Dudley said, is to collect a second sample to make sure that you get those two tubes flushed out and you get down to some normal sperm.
by Jessica Bern