Tall Fescue Copper and Copper-Zinc Superoxide Dismutase Status in Beef Steers Grazing Three Different Fescue Types1

Copper status of steers (n = 24; 258 kg initial BW) grazing tall fescue (Festuca arundinacea Shreb.) pastures was measured for 2 consecutive grazing seasons. The forages tested included 'Kentucky-31' wild-type endophyte (Neotyphodium coenophialum)-infected tall fescue and endophyte-free tall fescue, and 'Quantum' tall fescue infected with endophyte AR542, a non-ergot alkaloid-producing strain. Forage Cu concentration was similar (P = 0.20) across treatments and averaged 4.4 mg/kg. Steers consumed less (P < 0.01) Cu in yr 1 than in yr 2, and a period × treatment interaction (P < 0.01) was observed. Stepwise regression indicated that differences in DMI accounted for 88% of the variation in Cu intake. Serum Cu concentration was not different among treatments (P > 0. 22). In yr 1, liver Cu concentrations of cattle grazing endophyte-free tall fescue were greater (P < 0.05) than for cattle grazing endophyte-infected tall fescue, whereas liver Cu concentrations of steers grazing 'Quantum-AR542' were intermediate. No differences were detected in liver Cu concentrations among treatments in yr 2 (P = 0.86). Enzymatic activity of Cu-Zn superoxide dismutase (SOD) and relative Cu-Zn SOD mRNA abundance did not differ (P > 0.33) among treatments in either year. These results suggest that the tendency for less Cu intake of steers grazing endophyte-infected tall fescue and 'Quantum-AR542' (P ≤ 0.10) was related to the smaller DMI on these pastures. The smaller Cu intakes likely contributed to differences in liver Cu. Endophyte status of forage did not affect Cu-Zn SOD enzymatic activity or relative mRNA abundance.