Further enhancement of 3-nitropropanol detoxification by ruminal bacteria in cattle

Abstract

Ruminal fluid obtained from cattle was used to determine the effects of different diets and supplements on the in vitro degradation of 3-nitropropanol, the miserotoxin aglycone of certain Astragalus species such as timber milkvetch (Leguminosae). Nitroethane, an analog of the aglycone that is much less toxic to mammals, was effective as a supplement when given on alternate days. It enhanced the in vitro rates of nitropropanol degradation in all experiments, by 70–170%. In agreement, daily doses of timber milkvetch at a level equivalent to 3 mg nitropropanol kg−1 body weight also enhanced nitropropanol degradation, suggesting an adaptation to the aliphatic nitro group by ruminal bacteria. The forage protein content appears to contribute to the microbial efficacy and this was verified when diets were supplemented with soybean meal. The protein supplement increased nitropropanol degradation by 37–44% and, as expected, it also promoted cellulose digestion in vitro. The effect of nitroethane supplementation on the absorption of nitropropanol from the rumen was examined when treated and control groups were challenged with timber milkvetch. Plasma levels of 3-nitropropionic acid, the oxidized form of nitropropanol which is detected in the blood, were significantly reduced in response to nitroethane supplementation, and rates of nitropropanol degradation by ruminal microbes were concomitantly enhanced. These combined results further elucidate the factors that control and contribute to nitropropanol detoxification and suggest possible methods of prevention of timber milkvetch poisoning under rangeland conditions. Key words: Timber milkvetch, toxic plant, detoxification, 3-nitropropanol, ruminal bacteria, cattle