Genotype × Environment Interactions in Horn Fly (Diptera:Muscidae) Resistance in Angus, Brahman, and F1 Reciprocal Crossbred Cows on Common Bermudagrass or Endophyte-Infected Tail Fescue1

Abstract Weekly horn fly ( Diptera: Muscidae) counts were recorded on 125 Angus, Brahman, and F 1 reciprocal crossbred heifers grazing endophyte-infected tall fescue or common bermudagrass during the fly seasons in 1990 and 1991. Direct heterosis, maternal breed, and direct breed effects for average fly infestation were estimated as linear contrasts among sire breed × dam breed least squares means in these heifers. Heterosis differed between forages ( P P P =.88) flies per head per d on common bermudagrass and tall fescue, respectively. Maternal breed effects were not significant on either forage. Direct breed effects favored the Brahman, were larger on common bermudagrass compared to tall fescue ( P P P Bos indicus × Bos taurus crossbreeding systems was effective in substantially lowering horn fly infestation, but the efficacy of the genetic effects depended on the forage environment.