While reading some industry information, I was reminded about an article Dr. Burris wrote for Cow Country News a few years ago. The focus of his article was to treat your herd bulls like an athlete; keep them fit and in great working shape. As always, it was a super article and is still relevant. Recently, more research has been done on bull fitness and fertility that is quite interesting.

We have known for years that over-conditioning bulls is detrimental to their fertility. When bulls are overfed and their body condition score get excessive (> 7), fat begins to build up in the scrotum and in the spermatic cord. Fat is an excellent insulator and this buildup of fat in the neck of the scrotum leads to an increase in scrotal temperature. For optimum sperm production, the testis needs to be about two degrees cooler than body temperature and this buildup of fat especially in the neck of the testis (around the spermatic cord) can lead to abnormal sperm development. When these fatter bulls are subjected to a breeding soundness exam, they are more likely to fail due to an increase in abnormalities with sperm morphology and motility. More work from Dr. Pedro Fontes at the University of Georgia also indicated that bulls with more backfat were more likely have defects in the development of sperm and to fail a breeding soundness exam.

Dr. Fontes has completed some fascinating work extending our knowledge on the impact of bull condition on fertility. He recently used IVF to exam the ability of sperm from moderately- and over-conditioned bulls to fertilize an oocyte resulting in the proper development of an embryo. His research demonstrated that if an oocyte was fertilized by an over-conditioned bull the resulting embryo was less likely to continue to develop. His work suggests that bull diet and condition can negatively impact the ability of an embryo to grow and may lead to increases in early embryonic mortality. His studies examined both mature and young, developing bulls and the results did not vary. Interestingly, in this work sperm morphology and motility were similar between over-conditioned bulls and moderately conditioned bulls suggesting the reduced embryonic survival may run deeper than simple changes in sperm development. Truly fascinating work.

Over-conditioning not only impacts sperm production, but it also reduces the bull’s interest in breeding cows. Research from Australia indicated that over-conditioned bulls also have lower libido, and their serving capacity was significantly lower than moderately-conditioned bulls.

Research from Canada clinched the nail on the head. The goal of this research was to identify factors associated with the male that impacts pregnancy in pasture situations. Considerable data on the bulls was collected including scrotal circumference, a wide variety of sperm traits, and back fat thickness. These 277 bulls of British and Continental breeds were turned out with over 9,000 cows and pregnancy was assessed after the end of a 70-day breeding season. Of all the measurements taken, backfat thickness of the bull had the highest significant correlation with failure to breed. Basically, fatter bulls got fewer cows pregnant.

So, as Dr. Burris advised years ago, keep your bulls fit and think of them like athletes. Also, we are entering bull buying season so find bulls that not only meet your herds genetic needs but also are in proper body condition. If a bull’s BCS exceeds 6, check to see if his scrotum looks blocky and full of fat and avoid purchasing him. Once you get your bull home, manage his diet and exercise to keep him in a BCS of 5-6. The pregnancy rate of your herd may depend on it!