Guidelines to allocating bulls to mating groups
When allocating bulls to mating groups, reduce the risk of inbreeding and dystocia in heifers by:
-Mating bulls with the highest EBV for calving ease to heifers;
-Possibly mating bulls from a breed with a lower mature size to heifers; -Minimising inbreeding through preventing the mating of bulls with their daughters or with cows that have a common parent (half-brothers and half-sisters).
If your strategy is to mate bulls from a breed with a lower mature size to heifers, have a sound knowledge of EBVs for calving ease so that calving difficulties are reduced. Multibreed EBVs will become available for some breeds so it will be possible to compare genetic differences in birth weight between bulls across these breeds. This strategy also results in crossbred progeny, so be prepared to manage and market these animals differently.
Inbreeding is of less concern when you regularly cull older bulls and introduce new genetically unrelated bulls into the herd, possibly from different sources.
These tactics will address the common breeding problems of:
-Increased costs from the labour in supervising calving, and deaths associated with dystocia in heifers when they are mated to bulls that produce large calves (that is, low EBVs for calving ease);
-Inbreeding reaching unacceptable levels resulting in reduced expression of productive traits and increased susceptibility to genetic disorders that are controlled by recessive genes;
-Failure to achieve the desired progress toward your breeding objective. If dystocia is a management problem, you need to:
-Supervise calving closely to reduce the adverse consequences of dystocia in heifers
-Select bulls for future mating with heifers by using the previous year’s levels of calving difficulty as a reference point.
And if inbreeding is considered to be an issue:
-Ensure the male parentage of all calves: born is recorded to avoid future inbreeding;
-Consider bringing unrelated bulls in from different sources if this is a sensible option.
What to measure and when
-Calving ease EBV for bulls allocated to weaner heifers or birth weight EBVs when calving ease EBVs are not available; and
-Male parentage of all cows in the herd.
Focus on these every year when bulls are selected and purchased and when bulls are allocated to mating groups.
Commonly used genetics terms
Breeding objective relates to the goal of the breeding program and to the traits that need to be improved to contribute to the overall enterprise objective, which is presumed to be primarily economic gain.
BreedObjectTM is a software package that can calculate a ‘dollar index’ value for animals specific to the breeding objective for your herd.
Estimated breeding value (EBV) is the estimate of an animal’s breeding value or ‘genetic worth’ for a particular trait. The estimation can be based on (phenotypes of) the animal itself and/or its relatives, for the same and/or different traits to the trait of interest. This is made possible by knowing the genetic relationships between animals, and the genetic correlations between, and the heritabilities of traits.
The bull and cow each contribute a random sample of their genes to their offspring, half from each, meaning that half of the EBV of each parent is the contribution to their progeny.
