First of all season and age of dam effect birth weights.
Second, ET birthweights are unreliable because the uterine environment of the recip dam is not an indication to that animals breeding value of the calf. The calf is not usually related to the recip.
Third, the ECONOMICALLY RELAVANT TRAIT is calving ease. That is the trait that influences whether you are pulling calves, loosing cows and calves, or sleeping through the night.
Animal breeding and EPDs are not hocus pocus, they are based on millions of observations and sound science.
Here is what the Beef Improve Federation Guidelines say:
"For traits for which they are available, EPDs most efficiently combine phenotypic
performance data, records from all related animals, and information from genetically
correlated traits to predict the relative performance of future progeny of candidates for
selection. Information from these sources is optimally combined into a single predictor
of genetic merit. Therefore, there is no advantage in using information from sources
that contribute to the EPD when the EPD itself is available. For example, if the EPDs for
________________ BIF Guidelines 68 ________________
birth weight are available on two candidates for selection, then these statistics provide a
more accurate indication of the difference in birth weights of future progeny than the
actual or adjusted birth weight records of the candidates themselves. Likewise, if the
selection criterion is calving ease and calving ease EPDs are available, then
consideration of birth weight EPDs is unwarranted."
"Birth weight. Calf birth weight in relation to dam weight is a good indicator of calving
difficulty. If calving difficulty is a problem in the herd, 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 and growth rate from birth to weaning.
Both sex of calf and age-of-dam influence birth weight of the calf. The factors for
adjusting birth weights for differences caused by age-of-dam are given in Appendix 3.1.
BIF recommends the use of additive rather than multiplicative age-of-dam adjustment
factors for weaning weight, because research indicates that they are more appropriate.
Birth and weaning weight adjustments for age-of-dam can differ from one breed to
another. Some breed associations have developed adjustments using their own data.
These should be substituted for the BIF standard whenever possible. All breed
associations are encouraged to develop their own age-of-dam adjustment factors for
birth and weaning weight.
Adj. Birth Wt. = Birth Wt. + Age-of-Dam Adj.
BIF age-of-dam categories are listed below:
Classification of Age-of-Dam
Age-of-Dam Range (days) Age-of-Dam (years)
Less than 1,004
1,004 to 1,338
1,339 to 1,703
1,704 to 3,561
3,562 to 3,926
3,927 to 4,292
4,293 to 4,657
More than 4,657
2
3
4
5 to 9
10
11
12
13
Season of birth also has an important influence on birth weight. Research indicates that
summer- and fall-born calves have lighter birth weights than spring-born calves.
Adjusted birth weight ratios calculated within season/sex contemporary group can be
________________ BIF Guidelines 18 ________________
used to compare calves from the same herd born in different seasons. It is
recommended that adjusted birth weight ratio be computed as follows:
Adj. Birth Wt. Ratio = Individual Adj. Birth Wt. _ X 100
Group Average Adj. Birth Wt."
http://www.beefimprovement.org/library/06guidelines.pdf