Effect of gaseous ammonia on barley straws showing different rumen degradabilities

Abstract

Straw samples from five cultivars of spring barley (Hordeum vulgare L; Golden Promise, Golf, Klaxon, Heriot and Doublet), selected to show a range of rumen degradabilities, were examined to measure the effect of gaseous ammonia on aspects of their cell wall composition and degradability after treatment. Cellulose degradability of the untreated straws, as measured by the nylon-bag method, ranged from 41.0% for Golden Promise to 57.1% for Doublet. The extent of improvement in degradability following ammonia treatment was 12.5% units for Golden Promise and only 2.5% units for Doublet, showing that the effect of ammonia was more pronounced for materials of lower inherent degradability (r= −0.774). Straw from three of the five cultivars was dissected into its botanical parts, and the dry matter content and digestibility of the fractions were determined. Leaf material formed a higher fraction of dry matter in Doublet (0.48), the most degradable cultivar, than in Golden Promise (0.30), the least degradable straw. Half of the observed difference in degradability between Doublet and Golden Promise could be attributed to differences in dry matter distribution between botanical fractions. For untreated straw, degradability was weakly, negatively correlated with the proportion of arabinose residues substituted with alkali-labile linkages at position 0–5 (r = −0.690), and positively correlated with the moisture retention (water-holding capacity) of straws (r = 0.838), water-soluble dry matter content (r = 0.922) and water-soluble phenolic content (r = 0.791). The proportion of pentose residues carrying alkali-labile linkages was only slightly decreased followign ammonia treatment. In the case of the 0–5 position of arabinose residues the original values were reduced by 2.4–15.9%. Ammonia treatment had little effect on the capacity of the straw to retain moisture, and did not significantly increase the proportion of water-soluble dry matter or phenolics. Cellulose degradability, or the improvement in degradability, of ammonia-treated straw was not correlated with any of the physical and chemical characteristicds of staw considered above.