MONOCYTE IMMUNE CELL RESPONSE AND COPPER STATUS IN BEEF STEERS THAT GRAZED ENDOPHYTE-INFECTED TALL FESCUE

A 3-yr study was conducted to evaluate immune response and Cu status o f yearling beef steers as a consequence of grazing tall fescue (Festuc a arundinacea Schreb.) infected (E+) with the endophyte fungus Neotyph odium coenophialum ([Morgan-Jones and Gams] Glenn, Bacon, and Hanlin). During a preliminary study in 1994, 24 weanling Angus and Angus x Her eford steers were blocked by breed and weight (initial BW 271 kg; SD 2 5) and were randomized to E+ and low endophyte (E-) fescue in pastures at Glade Spring, VA. Grazing began in April and was discontinued in J uly. In 1995 and 1996, 24 weanling Angus and Angus x Hereford steers ( initial BW 249 kg, SD 20 and 240 kg, SD 15, respectively) were randomi zed to the E+ and E- pastures at Glade Spring during each year. Grazin g began in April and continued until September in 1995 and October in 1996. In 1994, steers that grazed E+ fescue exhibited lower(P < .05) p hagocytic activity, major histocompatibility complex (MHC) class II ex pression, ceruloplasmin, and serum Cu than steers that grazed E- tall fescue. During 1995, steers grazing E+ fescue had lower (P < .05) phag ocytic activity and MHC class II expression than steers that grazed E- fescue. In 1996, one-half of the steers within each paddock received a Cu oxide bolus at the beginning of the grazing season. During 1996, phagocytic activity was lower (P < .01) and MHC class II expression te nded (P < .07) to be lower in steers that; grazed E+ tall fescue than in steers that grazed E- tall fescue. Copper supplementation increased (P < .05) MHC class II expression in July regardless of endophyte sta tus over nonsupplemented steers. Steers that grazed E- tall fescue had higher (P <.05) plasma or serum Cu concentrations than steers that; g razed E+ tall fescue in each year of the study. These data indicate th at the endophyte compromised the immune function of grazing steers, an d the data suggest a relationship with depressed Cu status.