Chemical composition, digestibility in vitro and biodegradability of grass hays oven-treated with different amounts of ammonia

Abstract

This study examined the effects of thermo-ammoniation (0, 15, 25 or 35 g ammonia kg −1 dry matter (DM) on the chemical composition, organic matter digestibility in vitro (OMD [vt]) and cell wall degradability by cellulase (CWD) of cocksfoot, ryegrass and timothy hays, each prepared at three stages of plant maturity. For each variety, absolute increases in total nitrogen, ammonia nitrogen and soluble non-ammonia nitrogen in response to ammoniation were large, and of similar magnitude between varieties and between cuts. However, incremental effects were small for applications above 15 g kg −1 DM. In contrast, concentrations of 4-methylimidazole were, with one exception, more than doubled when treatment was raised from 15 to 35 g ammonia kg −1 DM, and related to the amount of water soluble carbohydrate (WSC) in the untreated hays. Each increment of ammonia stimulated the release of aromatic compounds (principally phenolic acids) from cell walls. OMD [vt] was most strongly influenced by CWD and the proportions of cellulase undegradable cell walls, cell walls and total nitrogen (percentage of variance accounted for (VAR) = 74, 79, 59 and 62 respectively). Of the cell wall constituents, CWD was most closely related to the concentrations of p-coumaric acid and total phenolic acids (VAR=69 and 68% respectively). The validity of using gravimetric methods to determine OMD [vt] and CWD in ammoniated hays can however be questioned. To limit the production of potential toxins, it is suggested that ammoniation should be restricted to crops with a low content of WSC, and that ammonia should be applied in minimal amounts, commensurate with requirements for preservation or upgrading under relatively low temperature conditions.