Animal Variation in Chromium Sesquioxide Excretion Patterns of Grazing Cows1

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Abstract
The diurnal patterns for excretion of chromium sesquioxide (Cr2O3) by 40 mature lactating Angus cows were characterized with the Fourier series: ŷ = AO + A1 sin (ct) + A2 cos (ct) + A3 sin (2ct) + A4 cos (2ct) where ŷ = mg Cr2O3/g fecal DM at time t, AO = mean fecal Cr2O3 concentration, 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 AO much 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 Ai in 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 Cr2O3 concentration. Regression of AO values on the means of data from these evaluations gave a b value of .97 ± .020 (R2 = .98). Copyright © 1978. American Society of Animal Science . Copyright 1978 by American Society of Animal Science