Managing bulls for high conception

The management of bulls has a large impact on herd reproduction. Bulls with low fertility decrease conception rates, which leads to low pregnancy rates, increased calving spans, reduced throughput of weaners and animals meeting market specifications, and consequently reduced farming profits.

Common sources of low bull fertility and conception include:

• bulls in poor condition two months prior to mating (when semen is produced)

• insufficient effective bulls for the number of cows in the mating herd

• use of too many bulls, which encourages fighting and is wasteful

• mixed ages of bulls in mating groups, or mixing bulls shortly before or during mating, which can affect conception while social dominance is being established

• large mating paddocks where bulls and cows become separated

• over-fat bulls (condition score 4.0–5.0) and unfit bulls due to lack of exercise (over-fatness can interfere with the heat exchange function of the testicles resulting in infertility with low sperm output)

• transporting bulls for some distance close to the start of mating

• venereal disease (eg vibriosis, trichomoniasis).

Body condition score is a key factor when monitoring the general health and nutritional well-being of bulls. The ideal condition for a bull prior to mating is condition score 3.0 (range of 2.5-3.5).

Prior to mating, bulls need to be assessed as physically sound, not carrying reproductive infectious diseases and having acceptable levels of libido and semen quality.

What to do to ensure a successful mating:

• planning mating groups eight weeks before joining and running bulls together before mating to allow social groupings to establish (mixing bulls shortly before or during mating can reduce conception rate due to distraction when fighting, and possible injury to bulls)

• ensuring access to at least one replacement bull (more in larger herds) from four weeks before mating begins

• vaccinating bulls with a BVD vaccine

• ratio of 1 bull to 40-50 cows

• multiple sire mating with bull rotation

• use smaller flatter paddocks (if possible) for mating

A successful mating relies on cows calving at the right time of year – this ensures the period between calving and coming on heat is 70-80 days – this then allows a calving interval of 365 days. Hence, it is far better to have the cows calving in the middle of spring rather than late winter...food for thought!