The in vitro reduction of sodium [³⁶Cl]-chlorate in bovine ruminal fluid

¹ Department of Animal and Range Sciences, North Dakota State University, Fargo, North Dakota 58105
² United States Department of Agriculture Agricultural Research Service, Food and Feed Safety Research, 2881 F&B Road, College Station, Texas 77845
³ United States Department of Agriculture Agricultural Research Service, Biosciences Research Laboratory, 1605 Albrecht Boulevard, Fargo, North Dakota 58105-5674

^* To whom correspondence should be addressed. E-mail: david.j.smith{at}ars.usda.gov.

	Abstract

Sodium chlorate effectively reduces or eliminates the numbers of gram-negative pathogenic bacteria in the gastrointestinal tracts of live cattle. Limitations to the in vivo efficacy of chlorate are its rapid absorption from the gastrointestinal tract and its presumed reduction to chloride within the gastrointestinal tract. We hypothesized that chlorate would be reduced via ruminal bacteria in a ruminal in vitro system, and that the reduction of chlorate would be influenced by the dietary forage:concentrate ratio, thus, 4 ruminally-cannulated steers were fed 20 or 80% concentrate diets in a cross-over design. Ruminal fluid was collected in 2 periods and dispensed into in vitro tubes containing sodium [³⁶Cl]chlorate sufficient for 100 or 300 ppm final chlorate concentrations. Tubes were incubated for 0, 1, 4, 8, 16, or 24 h; autoclaved control ruminal fluid, fortified with sodium [³⁶Cl]chlorate, was incubated for 24 h. Chlorate remaining in each sample was measured by liquid scintillation counting after [³⁶Cl]chloride was precipitated with silver nitrate. A preliminary study indicated that chlorite, a possible intermediate in the reduction of chlorate, had a half-life of approximately 4.5 min in live ruminal fluid; chlorite was, therefore, not specifically measured in ruminal incubations. Chlorate dose did not affect in vitro DM digestion (P 0.11) whereas in vitro DM digestibility was decreased (P 0.05) by 80% forage content. By 24 h, 57.5 ± 2.6% of the chlorate remained in 100-ppm incubations, whereas 78.2% ± 2.6 of the chlorate remained in the 300-ppm incubations. When data were expressed on a concentration basis (ppm), diet had no effect (P 0.18) on chlorate reduction; however, when chlorate reduction was expressed on a percentage basis, chlorate reduction tended to be greater (P 0.09) at h 8 and 16 in incubations of the low concentrate diet. Chlorate remaining in autoclaved controls at 24 h was intermediate (P < 0.01) between chlorate remaining in "live" ruminal fluid samples incubated for 0 and 24 h. Attempts to isolate chlorate respiring bacteria from 2 sources of ruminal fluid were not successful. These data indicate that microbial dependent and(or) chemical dependent reduction of chlorate occurs in bovine ruminal fluid, and that dietary concentrate had a negligible effect on chlorate reduction.

Key Words: bovine, chlorate, ruminal fluid