|VAN SOEST, PETER - Cornell University - New York|
|Hall, Mary Beth|
Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/13/2020
Publication Date: N/A
Interpretive Summary: Markers for liquid digesta are used in dairy cattle research to measure how quickly liquid and nutrients pass from the rumen. Such markers used in research are assumed to be largely chemically and biologically unreactive so that they do not cause changes in the digesta or animal. Two commonly used markers, CrEDTA and CoEDTA, were tested to evaluate whether they were inert under reducing conditions possible in the rumen. Although CrEDTA showed slight dissociation, CoEDTA became substantially reduced and dissociated, indicating that it is not inert. Previous work with dairy cows had shown that CoEDTA altered the fatty acid profile of milk; the present study provides better understanding of why that occurred. These results indicate that CoEDTA should not be used as a marker in studies with ruminants. Reexamination of the suitability of specific liquid digesta markers for research is advised.
Technical Abstract: Ideal digesta markers used for feeding studies are inert, unabsorbable, and move with the digesta they are intended to mark. Both chromium (III) (Cr) and cobalt (III) (Co) salts of ethylenediaminetetraacetic acid (EDTA; CrEDTA and CoEDTA, respectively) are used as markers of liquid digesta in dairy cattle research. A small portion is absorbed and excreted in urine, but the markers are assumed to remain unreactive and ionically bound in the digesta and animal. The degree to which these colored salts remain bound in solution can be determined through spectrophotometric measurement at their wavelength of peak absorbance (ABS). The objective of this in vitro study was to evaluate whether CrEDTA and CoEDTA dissociate under reducing conditions that could be experienced in the rumen. In a complete block design with 2 replicate analytical runs and samples in duplicate within run, approximately 26 mg/L of Cr from CrEDTA or Co from CoEDTA were incubated in a 26 mL reaction volume containing 20 mL of Goering and Van Soest medium without tryptone, 3 mL of CoEDTA or CrEDTA solutions, or water (reagent blanks), and 3.0 mL comprised of water with 0, 0.25, 0.50, 0.75, or 1.00 mL reducing solution (RedSol). After incubation for 0.5 h at 39°C, ABS were read at wavelengths of 535, 465, and 560 nm the peak wavelength for EDTA salts of Co(III), Co(II), and Cr(III), respectively. Mean reagent blank values were subtracted from Co- and CrEDTA data. The ABS data at peak wavelengths were analyzed by marker in models that included RedSol with analytical run as a random variable. Contrasts were used to detect linear through quartic effects of RedSol. Samples with RedSol had redox potentials of -250 to -328 mV, which are within the range of reported ruminal measures. As RedSol increased, CoEDTA showed a cubic decline of 75% in ABS at 535 nm and a quadratic four-fold increase then 60% decline at 465 nm. These responses indicate a reduction of Co(III) to Co(II), and subsequent dissociation of Co(II)EDTA. The ABS of CrEDTA at 560 nm showed a tendency for an 8% linear decrease as RedSol increased. Wavescans from wavelengths of 330 nm to 700 nm showed CrEDTA retaining its 2 characteristic peak pattern as RedSol increased, whereas CoEDTA curves deformed entirely. We conclude that CoEDTA is not a stable, inert digesta marker under reducing conditions achievable in the rumen and is therefore unsuitable for use in studies with ruminants. Reexamination of the suitability of available liquid digesta markers is advised.