EFFECTS OF PREVIOUS GRAZING TREATMENT AND CONSUMPTION OF LOCOWEED ON LIVER MINERAL CONCENTRATIONS IN BEEF STEERS

Twelve Hereford steers (average BW = 231 kg) that had previously graze d native rangeland (Range)or irrigated winter wheat pasture (Wheat) we re allowed to graze locoweed-infested rangeland from April 1 to June 9 , 1994 (six steers/previous grazing treatment). Relative consumption l evel of locoweed and other forage classes was measured as observed bit es per steer, Liver biopsy and whole blood samples were obtained from each steer before and after grazing. Liver samples were analyzed for s everal minerals by inductively coupled plasma-atomic emission spectros copy, and whole blood samples were analyzed for Se. Liver concentratio ns of Ba (P < .001), Cd (P < .001), Ca (P < .01), Cr (P < .01), Ni (P < .001), Na (P < .01), and V (P < .001) were greater and concentration s of Mn (P < .09), P (P < .01), and K (P < .07) were less in Wheat tha n in Range steers. Liver concentrations of Fe, Mg, S, and Zn and whole blood Se concentrations did not differ (P > .10) between the two grou ps. Liver concentrations of Cr (P < .04) and Mn (P < .001) were less, and Fe concentrations were greater (P < .01), in samples taken after g razing than in samples taken before grazing of locoweed-infested range . Whole blood Se concentrations decreased (P < .01) from the beginning to the end of the grazing period, but this effect was not related (P > .15) to locoweed consumption. Changes in liver concentrations of min erals were compared relative to consumption levels of all forage class es in the locoweed-infested range. Liver concentrations of Cu decrease d (r(2) = .45; P < .02) as the percentage of bites consumed as locowee d increased, but concentrations after grazing locoweed-infested range were still within normal ranges. Changes in liver concentrations of ot her minerals were not related (P > .15) to consumption of locoweed. Th ese data indicate that previous grazing history can have significant e ffects on liver mineral stores and that, under our conditions, consump tion of locoweed by grazing beef steers altered liver Cu concentration s. Toxic effects of locoweed consumption would likely occur before Cu deficiency would be induced by grazing locoweed-infested range; hence, supplementation of Cu would seem unlikely to alter the course of loco weed toxicosis.