Sire overconditioning is a prevalent issue in the beef industry. Because pre- and post-weaning growth are important in the cattle business, we often place considerable emphasis on identifying bulls with superior genetics for weaning and yearling weights. To help us identify these superior bulls, high-energy diets are often utilized to allow young bulls to express their genetic potential. Nevertheless, bulls are often not only fed high-energy diets to express their genetic potential at the time of yearling weight collection, but also until the time of sale, resulting in overconditioned bulls. While “fat sells” when it comes to selling bulls, research has shown that excessive conditioning can negatively affect semen quality.
An observational study evaluating over 1,600 breeding soundness exam records revealed that only 45% of extremely overconditioned bulls (with a body condition score [BCS] of at least 4 on a scale of 1 to 5) passed the breeding soundness exam, compared to 81% of bulls with an adequate BCS of 3. Moreover, when an experimentally controlled study compared the impact of feeding young bulls a moderate-gain diet primarily based on forage (control) to a high-energy diet comprising 80% concentrate and 20% forage (high-gain) for approximately 160 days, similar results were observed. Bulls fed the high-gain diet were heavier and had greater backfat thickness. However, they exhibited a greater percentage of secondary sperm defects and reduced sperm motility compared with the control bulls. Additionally, high-gain bulls had greater scrotal circumference and suboptimal regulation of scrotum surface temperature, which may be linked to increased fat deposition in the neck region of the scrotum, potentially explaining the decrease in semen quality.
Our research group at the University of Georgia has been trying to leverage new technologies, such as computer-assisted sperm analysis and flow cytometry, to gain a deeper understanding of how high-energy diets affect the fertility of beef bulls. We are also currently exploring the consequences of these diets on embryo development and pregnancy establishment.
In a recent study, bulls that had previously passed a breeding soundness exam were randomly divided into two groups. One group received a diet designed to achieve a target average daily gain (ADG) of 4 pounds per day (high-gain), while the other group was kept on a diet to maintain their bodyweight (control). These bulls were individually fed their respective diets for 67 days, and semen was collected and frozen at the end of the feeding period. This frozen semen was then used to produce embryos using in vitro fertilization (IVF). We were particularly interested in evaluating post-fertilization consequences of high-energy diets because studies in humans have shown that IVF outcomes are negatively impacted when the male partner is overweight.
The study’s results indicate that a similar effect of overconditioning on embryo development occurs in beef bulls. When IVF outcomes were evaluated, we observed that the percentage of embryos produced relative to the number of oocytes (female eggs) tended to be lower in high-gain bulls compared to control bulls. Moreover, the percentage of embryos produced relative to the number of cleaved oocytes (eggs that have undergone fertilization and started cell division) was significantly reduced when semen from high-gain bulls was used compared to control bulls. Collectively, these results indicate that overconditioning not only affects semen quality, as previously shown in other studies, but also has a negative influence on embryo development after the sperm is able to successfully fertilize the egg.
In summary, while high-energy diets should be used to enable young bulls to express their genetic potential for post-weaning growth, it’s crucial to avoid overfeeding them to an extent that fertility is compromised. This can be accomplished by closely evaluating body condition scores while developing bulls. Bulls should be managed to have a body condition score of 6 on a 1-to-9 scale before the beginning of the breeding season. This ensures they have sufficient energy reserves to handle a busy breeding season while minimizing the risk of reduced fertility associated with overconditioning. Additionally, it is important to keep in mind that bulls can lose over 100 pounds during the breeding season. Adapting young bulls that were developed in a predominantly concentrated diet to a forage-based diet before turning them out to breed cows can help mitigate these losses and optimize bull performance during the breeding season.
References omitted but are available upon request by sending an email to the editor.
