CHROMIC OXIDE CONTAMINATION OF PASTURE PREVIOUSLY USED IN MARKER STUDIES

Fecal output of range cows was determined during 2 periods of summer ( period I) and late summer (period LI) grazing using a constant release intraruminal Cr2O3 bolus, Chromic oxide contamination was determined by analyzing forage for Cr2O3 and by obtaining fecal samples from cows prior to bolusing, Control cows were also monitored along with the ex perimental cows during grazing periods, The overall herd least squares mean for fecal output during period I was lower than the expected val ue by 48%. Forage during period I contained an average of 55.7 mu g Cr 2O3 g(-1) of forage or about 45% of the daily dose of the bolus. Forag e during period II contained an average of 38.3 mu g Cr2O3 g(-1) of fo rage or about 29% of the daily dose of the bolus, Our results indicate that comparisons of fecal output least squares means by period of the year can be biased by Cr2O3 contamination of forage in pastures which have been previously used for marker studies.