Bull's eye: targeting nutritional management of bulls

Jan 24, 2014
by WLJ

How did your bulls perform last year? Did you get the calf crop you were striving for?

While many factors impact a successful breeding season and subsequent calf crop, nutritional management of bulls is a crucial part of a successful operation and it starts long before they are turned out with the cows. Without adequate nutrition, the time and effort that producers put toward carefully selecting sires that would potentially add value to their operation through an improved calf crop, could go unnoticed. Fortunately, nutrition is a factor that can be altered to achieve optimal performance from your bulls.

For bulls, three nutritional phases should be considered in order to maximize their performance; conditioning prior to breeding season, nutritional management during the breeding season, and management after the breeding season is over. There are some important goals to prepare bulls for each of these phases, as well as key points for success within these phases that producers should keep in mind.

Prior to breeding

The goal prior to breeding season that most producers should strive for is to present their bulls to a cow herd in the best condition possible. Getting bulls ready to work so time, energy and money are not wasted should be a top priority at least 60 days prior to breeding. Any necessary adjustments to body condition and/or the ration being consumed by the bulls should occur within this time period.

The ideal body condition for a bull about to be turned out should provide the animal with enough reserves to last him through the breeding season, but not be so excessive to be detrimental to semen quality. This detriment can occur when excess energy is fed which can reduce semen quality and servicing capability due to surplus fat deposits in the scrotum, insulating the testicles and increasing testicular temperature. Therefore, bulls entering breeding season with a body condition score of around 5.5 to 6.5 (on a scale of 9 points) should ensure optimal performance.

It is not only crucial to permit enough time for a bull to reach the proper body condition, but it is also important to allow the animal to overcome any nutritional stresses from necessary diet transitions that may have been reproductively detrimental.

For example, if a bull were zinc deficient, this could have a negative effect on sperm production and potency, and adequate time would be needed to allow spermatogenesis to produce quality sperm. Since spermatogenesis requires 60 days for the sperm population to turnover, this is the minimum amount of time needed prior to a bull entering the breeding season.

Other minerals such as phosphorous can play key roles in reproductive success. Ensuring an adequate supply especially during fall and winter (prior to the breeding season) is essential because phosphorous is not typically presented in suitable amounts in dry or harvested forages. Providing bulls with year-round access to a high quality mineral and vitamin supplement to allow for effective growth and optimal breeding performance is recommended.

A key point that producers should be aware of when nutritionally managing their bulls is the nutrient requirements for growing and mature bulls, which can be found in Table 1.

Both younger and mature bulls need to be considered when conditioning prior to the breeding season. Younger bulls that are coming from a developmental phase, which involves a higher plane of nutrition, need to be “let down” or acclimated to more of a maintenance diet that is typically forage-based and will allow them to enter the breeding season in good condition. Acclimation to this new forage-based diet should be done over time to avoid digestive upset, which can lead to reproductive issues.

More mature bulls should be evaluated based on their physical condition and placed on a diet that will allow them to reach or maintain a body condition that will be adequate for breeding before the season begins.

If possible, bulls should be introduced to the pasture they will be grazing during the breeding season (or a similar diet) for 7-10 days prior to being turned out. This will allow time for the rumen microbial population to adjust to the fresh forage diet and will alleviate transitional stress before joining the rest of the herd.

During the breeding season

Nutritional management prior to breeding season is critical due to the limited potential to do so during the breeding season. During this time, bulls are typically on the same plane of nutrition as the rest of the herd.

Mature bulls use nutrients primarily to support body maintenance, whereas younger bulls need nutrients to support growth, as well. Therefore, younger bulls may require additional supplementation during the breeding season in order to maintain their body condition.

It is not unheard of for bulls to lose 100-300 pounds over the course of a breeding season, depending on age, level of activity, length of season, and condition prior to the breeding season. Brief nutritional intervention may become necessary if a bull gets too thin during the breeding season and becomes inefficient in his ability to service the cow herd. However, removing a working bull during the breeding season can lead to an extended calving season and less uniformity throughout a calf crop; two very good reasons to ensure a bull is in optimal physical condition prior to entering the breeding season.

After the breeding season

As mentioned, bulls can lose a significant amount of weight and body condition during the breeding season, and evaluating their condition after the season can help determine which animals may need supplementation to bring them back to working condition.

Mature bulls emerging from the breeding season in good condition can typically be maintained on an all-forage diet. However, younger bulls tend to lose more body condition and should be provided supplemental nutrients to meet their needs and regain what has been lost.

In this case, not only do younger bulls have to regain what weight was lost during the breeding season, they still have to achieve their mature body weight by next season.

Failing to properly feed developing bulls can lead to irreversible testicular damage and, in mature bulls, can decrease sperm production. Nutritionally managing bulls after the breeding season has ended is important to ensure they will continue to have long and productive lives.

Proper nutritional management of bulls during their on and off seasons can optimize their ability to serve your cow herd and remain reproductively successful for years to come. When accurate nutrition is delivered, producers have the ability to provide the best opportunity for bulls to fulfill their genetic and reproductive potential. — Courtney Verzosa, Anipro/Xtraformance Feeds