Guidelines for managing bulls to achieve high conception
Carefully consider the number of bulls allocated to mating groups or herds. Insufficient bulls for the number of cows in a herd can lead to lower pregnancy rates and reduced throughput of animals meeting market specifications.
Bulls must be monitored closely during mating. General guidelines for bull ratios are:
• Intensively managed southern herds – maximum 2 bulls/100 cows
• Extensive grazing, ie pastoral zone – maximum 4 bulls/100 cows
• The minimum is 2 bulls/100 cows or per herd
Single sire joining is widely practised in the southern beef industry and particular care needs to be taken to achieve high conception rates at every joining. Single bull mating reduces the risk of bull injury from fighting, but increases the potential for low calving percentages due to infertility or sudden loss of service ability. The following guidelines are suggested to reduce the risk:
• Assess all bulls every year prior to mating and only use those that pass assessment guidelines (as described in the Bull physical soundness box over the page).
• Join each bull to a maximum of 50 cows.
• There is no minimum, but joining sound bulls to less than 40 cows is wasteful and increases costs of bull purchases.
• Observe all herds weekly during the joining period to ensure that the bull is working and has not been injured during mating.
• Have bulls in reserve (at least 20%) to replace injured bulls as soon as they are identified and consider extending the joining period in that herd by one week (or the period between observations).
The management of bulls has a large impact on herd reproduction. Bulls with low fertility produce low conception rates. This leads to low pregnancy rates, reduces the throughput of weaners and animals meeting market specifications, and consequently reduces enterprise profitability. Additional costs may be incurred because of:
• The need to replace bulls more often; and
• The potential spread of infectious diseases that may reduce the fertility of cows and increase enterprise costs through treatment and/or eradication of the diseases.
Common sources of low bull fertility and conception include:
• Bulls in poor condition two months prior to mating when the semen is produced;
• Insufficient effective bulls for the number of cows in the mating herd;
• Using too many bulls, which encourages fighting and is wasteful (but be sure to retain access to a replacement bull);
• Mixed ages of bulls in mating groups or mixing bulls shortly before or during mating, which can affect conception while dominance is being established;
• Large mating paddocks where bulls and cows become separated;
• Over-fat bulls (condition score 4 or 5) and unfit bulls due to lack of exercise. Over-fatness can interfere with the heat exchange function of the testicles resulting in infertility;
• Transporting bulls for some distance or feeding high grain supplements close to the start of mating.
