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 beef cattle. This method was used in two experiments. In this first experiment, forty-eight crossbred steers (average weight, 284 kg) were fed diets supplemented with salt, monensin or both, and injected with vitamin E, zinc or selenium. Steers receiving monensin had higher (38.5 micrograms/dl, monensin; 31.0 micrograms/dl, controls) initial plasma concentrations of vitamin A (P = .14). However, monensin did not affect post-dosing (MDR) vitamin A concentrations. None of the other dietary treatments or injections affected either pre- or post-dosing concentrations of vitamin A in the plasma. In a second experiment 23 lactating multiparous beef cows (average weight, 500 kg) grazing either fungal endophyte-infected or endophyte-free tall fescue were used to assess possible influences of infected fescue upon vitamin A metabolism. Fungal endophyte infection did not affect either pre- (44.9 micrograms/dl, end-noninf.; 47.7 micrograms/dl, end-inf.) or post-dosing (57.2 micrograms/dl, end-noninf.; 59.3 micrograms/dl, end-inf.) vitamin A concentrations.