Animal Variation in Chromium Sesquioxide Excretion Patterns of Grazing Cows1

The diurnal patterns for excretion of chromium sesquioxide (Cr2O3) by 40 mature lactating Angus cows were characterized with the Fourier series: ŷ = AO+ A1sin (ct) + A2cos (ct) + A3sin (2ct) + A4cos (2ct) where ŷ = mg Cr2O3/g fecal DM at time t, AO= mean fecal Cr2O3concentration, c = the fundamental period (radians/hour) and t = time (hours). The period c was circadian and was fixed at .1309 radians/hour corresponding to a 24 hr fundamental period. Cows grazed either high quality (fescue-legume) or low quality (fescue) pastures. Within pasture quality, size of cow was intentionally confounded with pasture (two pastures for each quality). Within-cow standard deviation was .45 mg Cr2O3/g fecal DM. In a general analysis, pasture quality (P<.0001), cow weight (P<.02) and percent butterfat (P<.03) influenced AOmuch more than other Ai(P>.10). Pasture quality interacted with calf weaning weight (P<.02) and percent butterfat (P<.01) in its effect on A1. Calf weaning weight and percent butterfat were associated with variation in A1(P,.05) for cows grazing fescue but not for those grazing fescue-legume pastures. Other Aiin the Fourier series were not affected by variables considered in this study.Each cow's model was estimated at t = 8:00 am and 5:00 pm to evaluate these sampling times for determining average fecal Cr2O3concentration. Regression of AOvalues on the means of data from these evaluations gave a b value of .97 ± .020 (R2= .98).