Influence of Urea Treatment and Soybean Meal (Urease) Addition on the Utilization of Wheat Straw by Sheep

The effect of ammoniation with urea and with soybean meal (SBM) as a source of urease on the nutritive value of wheat straw was evaluated in sheep. Twenty-four male Najdi lambs were used in a 3${\times}$2 factorial design, in which the animals were allocated to three straw treatments: 0% urea-treated (NT), 6% urea-treated (UT) and 2.2% urea-supplemented (US) straws. Each straw treatment was either supplemented or non-supplemented with 70 g SBM $kg^{-1}$ straw during the treatment time with urea, giving a total of six straw treatments. Each of these treatments was individually fed ad libitum to 4 lambs, together with 300 g of barley grain/head/day. Total N content of UT and US straws increased significantly (p<0.001) as compared to NT straw. The degree of urea hydrolysis, either with or without SBM addition, was nearly similar. Lambs fed either UT or US straw based diets had significantly (p<0.01) and numerically (p>0.05) higher straw DM intake (g $d^{-1}$ $kg^{-1}$ $BW^{0.75}$), compared to those fed NT straw based diet. Apparent DM or OM digestibilities increased significantly (p = 0.014) in lambs fed UT diet, and numerically (p>0.05) in lambs fed US diet as compared to those fed NT diet. Fiber (CF, NDF, ADF, cellulose and hemicellulose) digestibility increased to a similar magnitude, averaging 20.2 (p<0.001) and 7.8% (p<0.07); this corresponds to 35 (p<0.001) and 51% (p<0.001) in N digestibility and approximately 78 (p<0.017) and 105% (p<0.002) in N retention, for UT and US diets, respectively, as compared to NT diet. However, the UT diet had higher (p<0.01) fiber digestibility over the US diet. Addition of SBM tended to improve (p = 0.09) straw DM and digestible OM intakes, while significantly increasing (p<0.001) total and digestible CP intakes across all diets. Lambs fed on US diet had higher ruminal ammonia N than those fed on UT (p<0.05) or NT (p<0.001) diets. However, ruminal pH and molar proportion of the volatile fatty acids did not differ (p>0.05) among the treatment diets. This study suggests that US and UT treatments, particularly the latter, improved straw intake, digestibility and N utilization by lambs compared to NT treatment. On the other hand, addition of SBM as a source of urease had a negligible effect on urea hydrolysis.