Analysis of phenolic compounds in relation to the prediction of digestibility and toxicity of ammonia-treated cereal straws

Abstract

Thirty-five samples of cereal straws were collected from farms in southern Spain before and after treatment with ammonia (35 g NH 3 kg −1 straw) in sealed plastic-covered stacks. Seven of the treated straws produced symptoms of acute toxicity (hyperexcitability) when fed to sheep and cattle. Total phenolic content (TP), phenolic material solubilised by water extraction of straws at 70 °C (SP) and residual, water-insoluble phenolics (RP) were measured by the acetyl bromide method. No significant differences were found between the mean TP or RP content of untreated (136 ± 9 g kg −1 straw and 112 ± 9 g kg −1 straw, respectively) and treated straws (145 ± 21 g kg −1 straw and 107 ± 9 g kg −1 straw, respectively). Ammonia treatment did not significantly increase the water solubility of the phenolic fraction of the non-toxic straws examined. The mean SP content of toxic straws however, was found to be three to four times greater (91 g kg −1 straw) than non-toxic treated straws (24 g kg −1 straw), this difference being reflected in the TP values which were approximately 30% higher in the toxic samples. A significantly higher proportion of added nitrogen was fixed in the toxic straws compared with their non-toxic counterparts. Limited analysis of SP fractions from toxic straws by thinlayer chromatography and gas chromatography-mass spectroscopy failed to reveal any compound which co-chromatographed with reference imidazoles including 4- O-methyl imidazole. Thin-layer chromatography fractionation showed qualitative differences between SP fractions from toxic and non-toxic straws but the compounds involved were not identified. Results obtained suggest that soluble phenolics do not provide a means of predicting the digestibility of ammonia-treated straw but that a substantially elevated soluble phenolic content is indicative of potential toxicity.