Effect of Within-Herd Variance and Herd Mean Production on Response to Selection Within Herd

Lactation records of 1,032,438 Jersey and 1,490,909 Holstein cows on official DHI test from 20,380 and 33,858 herdyears, respectively, were used to compute herd-year means and within-herd-year standard deviations for mature equivalent fat percent, fat, and milk.Within-herdyear means and standard deviations were used to stratify records into five classes for each trait.Regressions of individual daughter's modified contemporary deviation on sire's PD were calculated for each of the 25 subclass combinations of within-herd-year mean and standard deviation.For milk and fat yield, low-low and high-high mean-variance subclasses contained more herd-years than low-high and high-low mean-variance subclasses.This was less evident for fat percent.Regression coefficients ranged from .66 to 1.41, .69 to 1.41, and .78 to 1.34 for fat percent, fat, and milk, respectively, for both breeds.Estimates of regressions indicated that coefficients increased with increases in within-herd-year variance but were nearly constant across herd-year means.