Genetic parameters in the feed conversion complex of primiparous cows in the first two trimesters

Feed conversion has been described as a "milk yield-tissue balance-appetite complex" in which energy inputs from feed intake and tissue balance impose a limit on milk yield. An experiment at the Agricultural University of Norway provided data to estimate genetic parameters of variables in this "complex." Fat-corrected milk yield, NE from roughage (NER) and BW were measured weekly on 353 first-lactation, dual-purpose Norwegian cows of 20 sires. Weekly weight change (WC) was computed from BW. Residual energy intake (RES) was calculated from energy balance (EB) corrected for WC. Amount of concentrate fed was adjusted according to stage of lactation and cows were randomly assigned to a normal or low level. Animals were given ad libitum access to grass silage. Data from the first and second trimesters of lactation were analyzed separately using an animal model containing fixed effects of 145 seasons in weeks, 24 subclasses of stages of lactation by levels of concentrate, 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 roughage and lost more weight but produced less milk. Catabolism of adipose tissue was poorly reflected by weight change. Heritability estimates for NER were .32 in the first trimester and .20 in the second; repeatability was .60. For EB, the respective estimates of heritability were .14 and .06, whereas repeatability was .55. No additive genetic variation was found for RES.