ADDITIVE AND NONADDITIVE DIFFERENCES IN MILK-PRODUCTION OF 2-YEAR-OLDDEVON, HEREFORD AND RECIPROCAL-CROSS HEIFERS

Data from a complete two-breed diallel of the Devon and Hereford breed s were used to estimate additive and non-additive effects for milk yie ld and composition. A sample of 40 straightbred and crossbred heifers were milked four times during their lactation by measuring milk letdow n resulting from oxytocin injection. Milk yield, milk fat and protein percentages, heifer weight, body fat score and calf weight were record ed. Heterosis for daily milk yield was 1.33, 1.30, 0.94 and 0.80 kg/da y at the four stages of lactation, respectively and represented an ave rage heterosis of 32%. There was no observed heterosis for persistence of lactation, or composition percentages. No additive or reciprocal d ifferences were observed for daily milk yield or milk fat and protein percentages. Higher body fat scores later in lactation were associated with decreased daily milk yield. Crossbreeding the Devon and Hereford breeds was observed to increase average daily milk yield without alte ring milk composition percentages.