Effects of aphids, barley yellow dwarf, and grassy weeds on grazed winter wheat

The effects of grazing winter wheat, Triticum aestivum L., on abundance of aphids, barley yellow dwarf virus (BYDV), and grassy weeds (primarily cheat, Bromus secalinus L.), and on grain yield and yield components were determined during crop years 1999-2000 and 2000-2001. Cheat and aphids were suppressed or enhanced to acquire a range of aphid and grassy weed infestations in each field. During the 1999-2000 season, grazing reduced aphid abundance (as much as 87%) and BYDV levels (as much as 70%), but often promoted greater abundance of cheat. Stepwise regression analyses of 1999-2000 growing season data indicated that grazing had little to no effect on yields or yield components when aphids were part of the wheat system. In contrast, grazing was significantly correlated with reduced yields when aphids were not present during the 2000-2001 season. These findings may suggest that the negative effects of grazing are not detectable when aphid abundance is reduced as cattle feed on winter wheat.