BOVINE PLASMA VITAMIN-A RESPONSES TO SELECTED NUTRIENTS, MONENSIN, AND ENDOPHYTIC FESCUE

Previous research at this station adapted a maximal dose response (MDR ) method of evaluating vitamin A status and utilization for use in bee f cattle. This method was used in two experiments. In this first exper iment, forty-eight crossbred steers (average weight, 284 kg) were fed diets supplemented with salt, monensin or both, and injected with vita min E, zinc or selenium. Steers receiving monensin had higher (38.5 mu g/dl, monensin; 31.0 mug/dl, controls) initial plasma concentrations o f vitamin A (P = .14). However, monensin did not affect post-dosing (M DR) vitamin A concentrations. None of the other dietary treatments or injections affected either pre- or post-dosing concentrations of vitam in A in the plasma. In a second experiment 23 lactating multiparous be ef cows (average weight, 500 kg) grazing either fungal endophyte-infec ted or endophyte-free tall fescue were used to assess possible influen ces of infected fescue upon vitamin A metabolism. Fungal endophyte inf ection did not affect either pre- (44.9 mug/dl, end-noninf.; 47.7 mug/ dl, end-inf.) or post-dosing (57.2 mug/dl, end-noninf.; 59.3 mug/dl, e nd-inf.) vitamin A concentrations.