Genetic correlations in the feed conversion complex of primiparous cows at a recommended and a reduced plane of nutrition

An experiment at the Agricultural University of Norway provided data to estimate genetic parameters of roughage intake (RI), fat-corrected milk yield (FCM), smoothed BW (SW), weight change (WC), and energy balance (EB). Weekly measurements were averaged in four consecutive 6-wk periods after parturition for each of 331 primiparous Norwegian cows of 20 sires. Amount of concentrate fed was adjusted according to stage of lactation and cows were randomly assigned to a normal or a low level. Animals were given ad libitum access to grass silage. Multiple-trait animal models with all additive genetic relationships incorporated were applied to the subsets of all averages of one trait and all averages in one period. Each model contained for every trait 165 observations, 24 mo-year seasons of calving, and 313 additive genetic effects. (Co)variance components were estimated by an EM-REML algorithm. The heritability of RI increased with negative energy balance from .25 to .86, whereas the estimates were approximately .2 for FCM, .6 to .7 for SW, and .03 to .4 for WC and EB. The correlations between RI and FCM were .37 to .58 phenotypically and -.11 to .88 genetically; between RI and SW they were .57 to .73 phenotypically and .7 to 1 genetically; between FCM and SW they were -.06 to .35 phenotypically and -.33 to .70 genetically; and between WC and EB they were .52 to .92 phenotypically and -.09 to .88 genetically. Correlations between WC and EB and other traits were inconsistent over periods and had very high SE.