Selection indexes for improving growth rate in holestein heifers with minimum concomitant increase in birth weight

الملخص EN

The aim of this paper was to improve pre and post weaning growth rates while keeping birth weight increase at minimum level to alleviate risk of dystocia.

A total number of ten selection indexes were derived from data on Holstein heifers raised at an Egyptian State-owned farm, from 1988 through 1994.

These data represent records of 552 progeny of 25 sires and 459 dams.

The traits considered as sources of information in the indexes were birth weight (BW), weaning weight (WW), yearling weight (YW) and the average daily gains during the periods separating birth (B) from weaning (W), (DGB-W), weaning from yearling (Y), (DGW-Y), and birth from yearling, (DGB-Y).

In constructing the indexes the aggregate genotype included DGB-W and DGW-Y.

Phenotypic and genetic parameters were estimated from a multigrain animal model where year and season of birth effects were considered as fixed and the additive direct genetic effects as random.

Selection index with BW, WW and YW (I1) had comparable accuracy with the index including the two segmented daily gains (I6), but with greater increase in BW, WW and YW.

Excluding BW from I1, to form I3, resulted in an increase of 17% in BW, while accuracy and improvements in WW and YW remained the same.

Ignoring both BW and WW from I1, to form I5, is expected to reduce the increase in BW by 0.10kg but with reduction in the increase in WW and YW by 0.50 and 0.10 kg, respectively.

Results indicated that in case of populations where a level of increase of less than 0.7 kg in BW would be tolerated in each round of selection, the three indexes including YW, (I1, I3 and I5), are recommended.

However, breeders using I5 would benefit from its being a single trait index that can be less expensive and easier to apply.

In case of populations that have already reached optimal BW, restricting I1 to form I1 (BW), genetic gain in BW is limited to zero, but at the expense of reduction in improvement of 35% in WW and 10.5% in YW as compared with the unrestricted form (I1).