by Benjamin Williamson, Penn State

In the United States, the majority of beef cows are bred by natural service, i.e., a bull. Most of those cows exist in spring-calving herds, resulting in breeding bulls being placed with cows in late spring into the summer months. Thus, spring becomes bull-buying season as operators plan for their next year’s calf crop.

A number of criteria can be used to select a bull. Of course, breed is one very broad consideration for bull selection. But expected progeny differences (EPDs), actual performance, ratios, lineage and the “eye” test all come into factor in selecting sires that match your management and marketing goals.

Managing bulls postweaning to gain 2.5 to 3 lb. per day will provide optimal results for bulls entering the breeding season.

However, all these evaluation tools become available when the bull is close to or ready for sale. This means there are 14-16 months of potential influential inputs for that bull prior to sale. Not only that, but how the bull will function is influenced by how he was developed. Breeding soundness exams (sometimes referred to as BSE) evaluate several physical and seminal characteristics to ensure that a bull is fit for breeding season and will have the ability to breed cows. The focus of this article will be on nutrition and bull growth for developing herd sires.

Any cattle feeder will tell you, it is not necessarily the faster-growing cattle, but rather the most feed-efficient cattle, those that can gain more with less feed, that make them more money. Feed efficiency is difficult to measure. Therefore, gain has long been a metric of success in the cattle industry.

In some bull-development programs, gain has also become the metric of success — prioritized, perhaps, more than proper bull development. In some cases, this has meant that the top-selling bull on sale day gained 6 pounds (lb.) a day. However, rapid growth and proper bull development can be two different sides of a coin.

If we consider just one metric of the breeding soundness exam — for example, scrotal circumference — an interesting tale of bull development unfolds. Scrotal circumference has long been used as an indicator of daughter fertility. That is, bulls with a greater scrotal circumference will produce daughters with improved fertility when compared to daughters from bulls with a smaller scrotal circumference. Due to the ease of measuring scrotal circumference and this indication of fertility, it is regularly reported in bull sale books. However, scrotal circumference can be affected by bull growth rate (Menegassi et al., 2019) and can be a poor indicator of bull fertility (Barber and Almquist, 1975).

Recent data suggest that to generate a bull that will be successful at breeding cows, gains of 2.5 to 3 lb. per day should be targeted (Barth, 2012; Kastelic, 2013). While scrotal circumference can be increased by developing growing bulls on an energy-dense diet (Barth, 2012), this increased energy to bulls can increase fat deposition in the scrotum. Fat in the scrotum surrounding the testes increases testicular temperature and can increase the risk for improper spermatogenesis (Walker, 2020). Because it takes six to eight weeks for sperm to mature in the testes, this added heat stress, due to fat deposition, can have longer-term effects, potentially reducing conception rates in the early breeding season. Barber and Almquist (1975) reported a decrease in progressively motile sperm and in the percentage of live sperm in bulls with an increasing weight per day of age.

While scrotal circumference still has value in selecting bulls, emphasis on appropriate bull gains needs to go hand in hand with observing mature scrotal circumference. The No. 1 function of bulls is to service cows, and sperm quality is critical to that service.

There are times when energy-dense diets may be more appropriate during bull development. For example, bulls undergo a rapid period of testes growth from 25 to 37 weeks of age (Barth, 2012). Increasing nutrient delivery to bulls during this time frame may increase mature testes size. One easy method of delivery at this age may be creep-feeding bulls prior to weaning.

More recent data suggest the energy density of the dam’s diet may also influence the bull’s development. While the field of fetal programming is relatively new to animal science, we have known for many years that fetal nutrition is important. Bulls from dams fed energy-dense diets may have reduced fertility.

Therefore, bull development begins in utero. Effects of early-life nutrition can enhance (creep-feeding) or hinder (dam’s nutrition and postweaning) bull fertility. Managing bulls postweaning to gain 2.5 to 3 lb. per day will provide optimal results for bulls entering the breeding season.

Producers looking to improve the quality of their genetic programs will use EPDs to make their bull selection. However, even a bull with the best EPDs may be challenged to breed or function. Producers should use all available resources and select sires specifically that meet their operational goals. However, proper nutritional development of bulls will ensure not only his ability to breed, but his longevity as a herd sire.

Editor’s note: Benjamin Williamson is an instructor of animal science at Penn State. [Lead photo by Michelle Kunjappu.]