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Birth Weight
Calf birth weight is a good indicator trait for calving difficulty. If calving difficulty is a problem in the herd, and calving ease EPDs are not available, selection of breeding animals for lighter birth weight may be an effective strategy to improve direct calving ease. However, single trait selection for lighter birth weight or shorter gestation intervals may reduce calf viability[1] and growth rate from birth to weaning.
Phenotype
Obtaining an accurate measure of birth weight (in pounds of calf) using a high-quality digital scale is important for producing meaningful calving difficulty EPDs. However, it is not always feasible to obtain birth weights this way. Digital scales for measuring calf birth weights can cost thousands of dollars. Even with good mechanical scales, trying to catch a calf when the mother is being protective, and lifting that calf is a challenge for many producers and their ranch hands. There are reports of producers estimating the calf weights using visual inspection and this can be seen in many breed association birth weight datasets. Experience has shown these data are low accuracy, at best, and should not be submitted to breed associations and used in selection decisions.
Adjusted Value
Both sex of calf and age of dam influence birth weight of the calf. BIF recommends the use of additive rather than multiplicative age of damage adjustment factors because research indicates that they are more appropriate<ref>[NEED CITATION].
Birth weight adjustments for the age of dam can differ from one breed to another. Some breed associations have developed adjustments using their own data. All breed associations are encouraged to develop their own age of damage adjustment factors for birth weight.
Adj. Birth Wt. = Birth Wt. + Age of dam Adj.
Significant amounts of unfavorable heterosis have been observed in birth weight and should be accounted for if adjusting birth weights.
Contempory Group
Contemporary grouping for Birth Weight
- Breeder-Herd Code
- Year
- Season (January-June, July-December)
- Sex (Bull, Heifer)
- Birth Management Code
- Service Type (Embryo Transfer Calves)
Genetic Evaluation
Birth weight EPDs are generally produced using a multi-trait animal model that includes weaning weight and yearling weight (usually fit as is often fit as post-weaning gain).
While a maternal effect has been consistently observed on birth weight, it is always small with low heritability. Most analyses ignore the maternal effect and produce only additive direct genetic effect EPDs for birth weight.
Usage
In all situations, birth weight should be considered as only an indicator trait. In no situation is it an economically relevant trait. When ERT EPDs are available, actual birth weights or birth weight EPDs should never be considered in a selection decision.
- ↑ [NEED REFERENCE HERE]
