Bulls are often managed as one group even though there may be large differences in age, size, and body condition. For commercial cow-calf producers, feeding programs usually include feeding purchased bull(s) from delivery until the breeding season and bulls retained from one breeding season to the next. Seedstock producers are the primary individuals managing the development program of bulls being sold. Only about 35%, 60%, and 95% of 12, 14, and 16 month old bulls, respectively, are reproductively mature and produce good quality semen (Barth, 2000). Sperm quality and quantity continues to increase for several months after the initiation of semen production. However, simply because a bull can produce semen does not translate directly to fertility. The age and weight at which puberty occurs also varies greatly among breeds and level of nutrition during development however, research with various breeds suggests that a practical indication of eminent puberty is when scrotal circumference is between 27 and 29 cm (Lunstra et al., 1978). In addition, bulls that are undernourished at a young age may never develop appropriately when compared to bulls that are properly fed (VanDemark et al., 1964). This is especially important for young bulls since they are still growing.įurthermore, extremely low energy can delay puberty and potentially impair sperm production.

This weight loss should come from energy stored as fat (condition) rather than muscle tissue. Ideally, bulls should be in a condition score of 6 (9 point system) since bulls normally lose about 100 to 200 pounds during the breeding season. Highly fitted or excessive conditioned bulls may fatigue rapidly, contributing to fewer cows serviced. High levels of energy can increase weight, height, and scrotal circumference without effecting age at puberty or first mating, showing nutritional effects on bull development without affecting sexual development. The age and weight that puberty occurs is affected by feeding different energy levels of feed.

The most commonly used definition of puberty in bulls is when an ejaculate collected via electroejaculation contains a minimum of 50 x 10 6 total sperm with at least 10% progressive motility. Third, libido and social dominance influence a bull's ability and desire to service cows. Second, physical characteristics, such as scrotal circumference, mating ability, and semen quality play a role in a bull's fertility. First, a bull must be developed properly and have reached puberty to be fertile. There are several factors that influence bull fertility. Producers spend the necessary time to find quality bulls based on EPDs, but how is the bull managed from purchase to breeding or between breeding seasons can affect the longevity of a bull? Has the nutrient requirements of the bull(s) been met during these periods or are the bulls placed into a pasture to be forgotten about until the next breeding season? Since greater than 90% of the beef cows in the United States are bred by natural service, it is important that bulls be managed to optimize breeding performance.

The weaning weight is influenced by the bull selection. The number of cows bred during the breeding season plays the largest role in percent calf crop weaned, and percent calf crop weaned is the single most important factor influencing profitability in beef operations. Therefore, bull selection can be the most powerful method of genetic improvement in the herd, but bulls with low fertility, structural problems and low libido reduce the percent calf crop weaned. Additionally, bulls influence their daughters' production in the cow herd. Each cow produces one calf per year however, bulls should contribute to the calf crop by 25 to 60 times via siring 25 to 60 calves. In addition, the herd bull influences overall herd fertility more than any other single animal, and loss of fertility by a bull can cause substantial loss to a potential calf crop. Proper management and nutrition of bulls is essential to ensure cow/calf producers maximize reproductive efficiency and genetic improvement of the calf crop. Julie Walker 1, George Perry 1, Russ Daly 2 and Ken Olson 1ġ Departments of Animal and Range Sciences and 2 Veterinary Science December 1, 2 and 3 2009, Casper, Wyoming Bull Management and Nutrition

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