Reproductive expert outlines the influence of nutritional management on bull fertility
During the Range Beef Cow Symposium, held Nov. 10-11 at the Event Center at Archer in Cheyenne, University of Georgia (UGA) Department of Animal and Dairy Science Associate Professor Dr. Pedro Fontes presented his research on the impact of diet on bull fertility.
Fontes received his Doctor of Veterinary Medicine from São Paulo State University and his PhD in Physiology of Reproduction from Texas A&M University. His current research focuses on increasing profitability and sustainability of cattle operations by developing new reproductive management strategies and assisted reproductive technologies.
His talk, titled “The Influence of Nutritional Management on Bull Fertility” outlined his research and existing literature on the impact of over-conditioned bulls on fertility and how bulls influence embryonic development, pregnancy establishment and even post-natal growth.
UGA study
According to Fontes, UGA observed 700 bulls from three different veterinary practices. The bulls were sorted into the top and bottom 10 percent, with a middling 80 percent, based on subcutaneous back fat thickness (BFT) from an ultrasound.
The top 10 percent had a minimum BFT of 0.55 inches. These bulls were found to fail the breeding soundness exam (BSE) at a much higher rate than the other bulls. This was based off of semen quality alone and controlled for other factors which may affect the BSE.
Fontes and his team then attempted to recreate this in a controlled study.
In the preliminary study, herd bulls were fed two diets – one with a goal average daily gain (ADG) of four pounds and another to maintain body weight. After 67 days on feed, their semen was preserved using cryopreservation and in vitro fertilization (IVF) was performed.
During the IVF process, the same number of oocytes were fertilized but the number of blastocysts – early-stage embryos which have survived long enough to be transferred – were lower.
This study was then recreated with young bulls which, unlike herd bulls, are less likely to gain fat on a high-gain diet. Forty-four half sibling bulls were fed a 114-day diet.
The moderate-gain diet had a target ADG of 2.7 pounds and the high-gain diet had a target ADG of four pounds.
The bulls on a high-gain diet had a decrease in sperm motility, making them less eligible for cryopreservation. They also had a lower percentage of sperm with intact plasma membranes, meaning this sperm wasn’t as resilient and responded to stressors worse.
IVF was performed with these bulls, and the over-conditioned bulls fertilized fewer eggs and had fewer blastocysts six to seven days after fertilization, which is when labs generally send out samples. The number of trophectoderms – cells which eventually become the placenta – were also decreased, and embryos from over-conditioned bulls also had delayed embryonic development.
“The diet the bull is exposed to alters sperm biology in a way which influences post-fertilization growth. Not only that, it also affects post-natal growth and metabolism,” says Fontes.
Overnutrition leads to excess fat, which leads to impaired thermoregulation in the scrotum and testes, as shown on infrared cameras. This disruption in natural cooling is harmful to sperm, which then performs worse in fertilization and growth.
Further research
There is expected to be more research on this topic in beef cattle in coming years, but similar studies exist in other mammals, namely dairy cattle, sheep and humans.
Dairy cattle are excellent for nutritional studies because they are weaned at birth and can have their diets controlled much easier.
An Irish study on Holsteins revealed high nutrition only hastened the age of puberty when offered in the first six months of life, but the window of developmental plasticity closed after that.
A study from North Dakota State University revealed lambs sired by obese rams experienced a 12 percent decrease in ADG compared to fitter rams.
Male obesity in humans has been shown to lead to increased scrotal temperature, sperm DNA fragmentation, germ cell apoptosis or cell death, as well as a decrease in embryo production.
When couples with an obese male attempt IVF, they experience a decreased fertilization rate, fewer embryos are transferable and, of those embryos, there is a lower rate of generating a clinical pregnancy.
Often, doctors will recommend men institute nutritional changes.
Like humans, over-conditioned bulls will experience insulin resistance, commonly referred to as type 2 diabetes.
Studies in rodents show obese sires induce a disruption in the metabolism of offspring.
More research is being done now on beef bulls and how their diet affects reproduction and offspring, but existing studies and research done in other mammals supports keeping bulls in shape will lead to more effective pregnancies and better development of offspring.
Fontes continues to research the impacts, but so far, nutritional and dietary management has led to leaner bulls performing better on all fronts.
In explaining the impact of his research, Fontes states, “In the context of the beef industry, we have some sires that might generate thousands of offspring in a single breeding season. Depending on how we’re feeding those bulls, we might be optimizing the post-natal performance of offspring.”
Erin Rees is a corresponding writer for the Wyoming Livestock Roundup. Send comments on this article to roundup@wylr.net.
