GENETIC-PARAMETERS IN THE FEED CONVERSION COMPLEX OF PRIMIPAROUS COWSIN THE 1ST 2 TRIMESTERS

Feed conversion has been described as a ''milk yield-tissue balance-ap petite complex'' in which energy inputs from feed intake and tissue ba lance impose a limit on milk yield. An experiment at the Agricultural University of Norway provided data to estimate genetic parameters of v ariables in this ''complex.'' Fat-corrected milk yield, NE from rougha ge (NER) and BW were measured weekly on 353 first-lactation, dual-purp ose Norwegian cows of 20 sires. Weekly weight change (WC) was computed from BW. Residual energy intake (RES) was calculated from energy bala nce (EB) corrected for WC. Amount of concentrate fed was adjusted acco rding to stage of lactation and cows were randomly assigned to a norma l or low level. Animals were given ad libitum access to grass silage. Data from the first and second trimesters of lactation were analyzed s eparately using an animal model containing fixed effects of 145 season s in weeks, 24 subclasses of stages of lactation by levels of concentr ate, and linear regression on age at calving and random effects of 512 animals and permanent environmental effects for the 353 animals with records. Cows fed at the low level of concentrate consumed more rougha ge and lost more weight but produced less milk. Catabolism of adipose tissue was poorly reflected by weight change. Heritability estimates f or NER were .32 in the first trimester and .20 in the second; repeatab ility was .60. For EB, the respective estimates of heritability were . 14 and .06, whereas repeatability was .55. No additive genetic variati on was found for RES.