Factors that determine rates of cyanogenesis in bovine ruminal fluid in vitro.

Strained ruminal fluid was collected from cattle fed five diets at two locations to determine in vitro rates of cyanogenesis from the glycosides amygdalin, prunasin and linamarin. Rates of dissociation for the corresponding aglycones, benzaldehyde cyanohydrin and acetone cyanohydrin, also were determined. Hydrogen cyanide (HCN) in ruminal fluid was determined with a modified method of HCN analysis that independently measured the overall rate of cyanogenesis and the nonenzymatic dissociation of cyanohydrins, the intermediate products in the degradation of cyanogenic glycosides to HCN. Rate of dissociation of cyanohydrins in ruminal fluid was pH-dependent, with high rates of dissociation (as expressed by the rate constant or half-life of the reaction) occurring at pH greater than 6 and slower rates at pH 5 to 6. Cyanohydrin dissociation was most rapid when cattle were fasted for 24 to 48 h and ruminal pH was high; rate of dissociation was much slower during feeding and digestion. When the glycosides were examined, highest rates of cyanogenesis (mg HCN.liter-1.s-1) were observed after a 24-h postprandial period. Hence, cattle are most susceptible to poisoning by cyanogenic plants when the pH of ruminal fluid is elevated (for rapid dissociation) and also when the activity of microbial beta-glucosidase is adequate for rapid hydrolysis of glycosidic bonds. Rates of cyanogenesis were higher when ruminal inocula were from cattle fed fresh alfalfa or cubed alfalfa hay rather than grain or long hay. Rates of HCN production were slowest using inocula from cattle fed grain; rates for the three glycosides were negligible at the 3 and 6 h postprandial sampling times.(ABSTRACT TRUNCATED AT 250 WORDS)