Genetic x environment interactions on blood constituents of Angus, Brahman, and reciprocal-cross cows and calves grazing common bermudagrass or endophyte-infected tall fescue

Over a 2-yr period, effects of genotype and forage on blood metabolites, en zymes, and minerals were determined in Angus, Brahman, Angus x Brahman (sir e x dam), and Brahman x Angus cows, and 129 calves from these cows sired by Hereford bulls. Cows and calves continuously grazed either common bermudag rass or endophyte-infected 'Kentucky-31' tall fescue pastures throughout th e year. Blood samples were collected via jugular venipuncture in April, Aug ust, October (weaning), and November (after 30 d in a feedlot) of each year . Plasma urea N concentrations of cows and calves were affected by forage ( P < 0.01) and breed (P < 0.05). Plasma cholesterol and FFA concentrations o f cows were affected by forage (P < 0.01) and breed (P < 0.05). In calves, antibody titers to infectious bovine rhinotracheitis virus were not affecte d by forage but were affected by breed. Serum inorganic P concentrations of calves and cows were affected by forage (P < 0.05). Serum P concentrations and alkaline phosphatase activity of calves were affected by breed (P < 0. 05). Calves grazing bermudagrass had higher (P < 0.05) serum concentrations of Fe and total iron binding capacity (TIBC). There was evidence of matern al heterosis for concentrations of free fatty acids, cholesterol, aspartate aminotransferase, Ca, Mg, alkaline phosphatase, ceruloplasmin, Fe, and TIB C. There was evidence of grandmaternal effects for plasma concentrations of urea N, cholesterol, Ca, P, Mg, and alkaline phosphatase. These results su ggest that calves and cows grazing tall fescue are generally on a lower pla ne of nutrition than those grazing bermudagrass and that Brahman x Angus an d Angus x Brahman crossbred cows and their calves seen to be more tolerant of the negative effects of tall fescue than the average of their purebred c ontemporaries.