| Popis: |
Milk yield and quality were measured on 139 Angus, Brahman, and reciprocal-cross cows grazing common bermuda grass or endophyte-infected tall fescue for 4 yr to evaluate interactions of direct and maternal breed effects and heterosis for these traits with forage environment. Milk yield was estimated by method of milking machine, and milk fat, protein, and somatic cell counts were evaluated in a commercial dairy laboratory. Monthly estimates were made beginning on an average d 61 of lactation and continued monthly for six estimates in 3 yr and five estimates in 1 yr. Data were averaged over month within year, and the model included sire breed, sire in sire breed, dam breed, forage, and age averages. Somatic cell counts were transformed using natural logarithms prior to analyses. Forage effects for milk yield were dissimilar among sire breed x dam breed subclasses (P < .10), resulting in higher levels of heterosis on common bermuda grass than on tall fescue. Maternal breed effects for milk yield favored Angus on bermuda grass (P < .05) but not on tall fescue, whereas direct breed effects were similar on both forages and favored Brahman. Milk fat was reduced on tall fescue compared to bermuda grass by an average of .6% (P < .01), and direct breed effects were similar across forages and averaged 1.04% (P < .01) in favor of Brahman. Heterosis and maternal breed effects for milk fat were not important. There was little evidence of direct and maternal breed effects or heterosis for milk protein or somatic cell counts. These data suggest that heterosis for milk yield is larger on common bermuda grass than on tall fescue and that grazing endophyte-infected tall fescue is detrimental to milk fat. |
