Ammonia treatment of straw via the hydrolysis of urea. II. Additions of soya bean (urease), sodium hydroxide and molasses; effects on the digestibility of urea-treated straw

Abstract

Samples of barley straw were sprayed with solutions of urea (U), urea plus a dry addition of soya bean meal (50 g/kg straw dry matter (DM)) (UB) alone or with sodium hydroxide (15 g/kg straw DM) (UBS), or urea plus molasses (50 g/kg straw DM) (UM). Water was added to lower the DM content of the straw to 700 g kg −1, and the level of urea applied was 70 g/kg straw DM. A further quantity of straw was treated with aqueous ammonia (40 g/kg straw DM). Two-kilogramme lots of each treated straw were sealed in polythene bags and stored for 40 days at 18°C. From the levels of gaseous ammonia in the bags on Day 40, there were indications that the rate of hydrolysis of urea was decreased by treatment UBS, but was increased by molasses addition (UM) compared with straw treated with urea alone (U). The addition of soya bean meal (UB) as a source of urease had no effect. After 48 h incubation in the rumen of steers, samples from Treatments UBS, UB and UM were not significantly different to samples from Treatment U in DM degradability. The DM degradability (%) of a control, untreated straw or samples of straw treated with urea (U) or ammonia (A) was 41.4 < 47.1 < 59.1, respectively ( P < 0.001). Large quantities of straw (200 kg) were treated as for U, UBS, UM and A. The straw was stored in polythene bags for 40 days when the mean ambient temperature was 5.5°C. Diets of treated straw and dried grass (1:1 on a DM basis) were offered to sheep and digestibility was determined. There were no differences in DM digestibility between diets made up with dried grass and straw (U, 0.64; UBS, 0.65; UM, 0.65; A, 0.65). The DM digestibility of dried grass plus untreated straw was 0.56. Apparent DM digestibility of the straw alone in the above diets was 0.49, 0.51, 0.52, 0.51 and 0.34, respectively. Both ammonia and urea treatment significantly ( P < 0.001) increased the digestibility of straw.