GENETIC COMPONENTS FOR MILK-PRODUCTION OF TARENTAISE, HEREFORD, AND TARENTAISE X HEREFORD COWS

The objective of this study was to estimate genetic components for mil k production of Hereford (HH), Tarentaise (TT), and Hereford x Tarenta ise or Tarentaise x Hereford(HT) cows under range conditions at the No rthern Agricultural Research Center, Havre, MT. Milk production of 494 cow-calf pairs was estimated by the weigh-suckle-weigh procedure from 1989 to 1994. Milk production and cow and calf weights were measured at 40 d (early) and 120 d (late) of lactation. The mating scheme was a 3 x 3 design in which HH, HT, and TT cows were mated with HH, HT, and TT sires, producing calves that were varying percentages of Hereford and Tarentaise. Data were analyzed by least squares procedures accordi ng to a model that included year, age of cow, sex of calf, linear regr ession on calf age, and linear regressions on coefficients that coded for various genetic components. These genetic components were breed in dividual, breed maternal, breed grand maternal, individual heterosis ( calf heterosis), and maternal heterosis (cow heterosis). Coefficients for genetic components were such that breed effects estimated TT - HH and regressions on heterosis values estimated the full effect of heter osis. The breed individual genetic component was significant only for calf early and late weights; Tarentaise calves were lighter. Breed mat ernal was significant for most traits, and these effects were positive for milk production (2.16 and 3.77 kg/d for early and late, respectiv ely) and calf weights but negative for cow weights. Individual heteros is was not significant for early milk production but was significant f or late milk production (1.05 kg/d) and calf weights (2.3 and 8.1 kg f or early and late, respectively). Maternal heterosis was significant f or most traits and increased early milk production (1.25 kg/d), calf w eights (4.5 and 5.8 kg for early and late, respectively), and cow weig hts (18.1 and 6.0 kg for early and late, respectively).