In the United States, the majority of our 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.
There are a number of criteria that can be used to purchase a bull. Of course, breed is one very broad consideration for bull selection; but, Expected Progeny Differences (EPD), actual performance, ratios, lineage, and the "eye" test all come into factor in selecting sires that match your management and marketing goals.
However, all these evaluation tools become available when the bull is close to or ready for sale. This means there are 14 to 16 months of potential influential inputs for that bull prior to sale. Not only that, but how the bull will function to do his job is influenced by how he was developed. Breeding soundness exams 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 specifically on nutrition and bull growth for developing herd sires.
Any cattle feeder will tell you that it is not necessarily the faster growing cattle but 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, over proper bull development. In some cases, this has meant that the top selling bull on sale day gained 6 pounds 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 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 catalogs. However, scrotal circumference can be impacted 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 pounds 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 6 to 8 weeks for sperm to mature in the testes, this added heat stress due to fat deposition can have longer term impacts, 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 while observing mature scrotal circumference need to go hand-in-hand. The number 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 suggests that the energy density of the dams diet may also influence the bulls 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.
Source: psu.edu
Photo Credit: gettyimages-steveoehlenschlager
Categories: Pennsylvania, Livestock, Beef Cattle, Weather