New Bulls Benefit from Nutritional Support and Monitoring

 Ensuring that bulls are in good body condition and physically fit for the upcoming breeding season is a key to reproductive success, according to North Dakota State University (NDSU) Extension specialists.

Most bulls are developed by seedstock producers and development strategies vary by operation.

“While there are a lot of different rations used to develop bulls, some type of high concentrate ration is typically used to determine the genetic potential for growth among herd mates,” says Janna Block, NDSU Extension livestock systems specialist based out of the Hettinger Research Extension Center. 

“A common complaint of bull buyers is that bulls lose condition quickly when they get home. This is often the result of failing to adjust bull rations gradually from a high-energy ration to a forage-based diet,” Block adds.

The consequences of poorly transitioned bulls can include increased stress, digestive disturbances, body condition losses, and impacts on sperm quality and quantity.

Feed considerations

In beef cattle, the rumen is the primary fermentation chamber of the stomach. It contains billions of microorganisms that break down feed consumed by the host animal. Different species of microbes have different functions and preferred food sources. Some digest starch and sugars, while others digest fiber. The numbers and proportion of each species vary based on diet composition. Therefore, when a high-energy diet is provided to the animal, the rumen microbial population consists mostly of starch-digesting bacteria. When the diet is changed abruptly, microbial populations are out of balance with the feed source.

“It can take four to six weeks for the population of fiber-digesting bacteria to build up after concentrates are removed from the ration,” says Block. “Stepping down bulls from a high plane of nutrition through gradual reductions in grain combined with increased amounts of forage is necessary to stabilize the rumen microbial population and avoid digestive issues and rapid weight loss.”

Zac Carlson, NDSU Extension beef specialist, recommends that bull buyers get detailed information about the previous ration and try to start with similar feedstuffs.

“A diet consisting of 80-90% of the previous amount of concentrate can be offered at first, and then reduced by 10% every few days until the final diet formulation is reached,” says Carlson.  “If information about the previous diet is not available, high-quality grass hay and 4 to 6 pounds of concentrate can be provided.”

Total ration crude protein (CP) should be between 10% and 11%, with energy (total digestible nutrients or TDN) levels at 60 to 70%. Producers should have feeds analyzed for nutrient content and develop a ration, advises Carlson. Because the process of semen production takes around 60 days, it is ideal if the transition to a forage-based ration is completed at least 90 days prior to the breeding season.

BCS considerations

“Body condition scoring (BCS) is another important component in developing an appropriate nutrition program for bulls,” says Block. “Bulls should have a BCS of 5.5 to 6.5, depending on the length of the breeding season, cow-to-bull ratio and other factors.”

In the BCS range of 4 to 7, there is a difference of approximately 8% body weight between each score. A yearling bull weighing 1,200 pounds with a BCS of 6 would weigh about 100 pounds less as a BCS 5. If there are 70 days until the breeding season starts and the bull needs to gain one condition score, he would need to gain approximately 1.4 pounds each day. 

When bulls are in a BCS of 6, no ribs will be visible, the back will appear slightly rounded and there will be some evidence of fat in the brisket and tail head. If bulls are thin, with BCS of less than 5, they will need to be on an increasing plane of nutrition to achieve desired performance. In this situation, additional concentrate may be needed initially; however, bulls will still need to be transitioned to a high-forage diet prior to turnout.

“Some bulls will be in excess condition after purchase and may need to lose fat and add muscle prior to breeding, but it is important to remember that they should still be gaining weight, says Block. “If bulls are in excess body condition after the sale, the goal is to reduce condition slowly to avoid issues.”

Nutritional monitoring

“It is important to monitor nutritional status of growing bulls closely for the first several seasons of use,” says Carlson.

After their first breeding season, bulls should be managed to weigh 65% to 75% of their estimated mature weight by the time they turn 2. Bulls may need to gain significant amounts of weight to overcome weight lost during the first breeding season and allow for continued growth. 

If a bull with a mature weight of 2,000 pounds weighs 1,200 pounds at turnout and loses 200 pounds during the breeding season, he would have to gain 400 pounds between the end of breeding season and his second birthday (Goal weight of 1,400 pounds [70% of 2,000 lbs.] – 1,000 pounds [weight at the end of the breeding season] = 400 pounds).

“Pulling yearling bulls early or using them for only a limited time during the breeding season can help reduce total weight loss and provide additional time to regain lost weight,” says Carlson. “Mature bulls will also need to regain weight lost during the breeding season, but their nutrient requirements are lower than growing bulls.”

“Feeding bulls to optimize fertility is not only dependent on protein and energy,” Block adds. “Minerals and vitamins play an important role in spermatogenesis and can also impact semen quality. In particular, copper, zinc and phosphorus have a direct role in fertility, and forages are often marginal to deficient in these minerals.”

Mineral analysis of forages and other feeds will allow producers to select a mineral that works best with the feeding strategy. Producers should also supplement Vitamin A should when feeding harvested forages, whether through a mineral supplement or an injection. Vitamin A deficiency may impact spermatogenesis. 

The cost of bulls and their potential genetic contribution to the cow herd indicates that they should be managed to optimize performance. For information on assessing body condition and managing bulls, please refer to NDSU Extension Ag Hub resources available at https://www.ndsu.edu/agriculture/ag-hub/ag-topics/livestock/beef/breeding-genetics/assessing-body-condition-and-managing-bulls. Tips on biosecurity, animal health and general management of yearling bulls can be found in the NDSU Extension publication AS2011, available at https://www.ag.ndsu.edu/publications/livestock/bull-management-after-the-sale. Contact your county NDSU Extension agent for additional information.