IN VIVO AND IN VITRO EVALUATIONS OF UREA-BASED PRODUCTS IN CATTLE FEEDING

abstractita

The main purpose of the present thesis was to improve the knowledge about the aspects of urea utilization in cattle diets in respect to rumen fermentation and animal performance. The first experimental part of the thesis consists of feeding trial with Italian Simmental fattening bulls (Exp. 1) and in vitro trial with continuous culture fermenters (Exp. 2). The aim was to study the effects of dietary substitution of soybean meal (SBM) with urea (U) within medium (M) and low (L) levels of intestinal digestible protein (PDI) on animal performance and on in vitro rumen fermentation characteristics. In Exp. 1, the average daily gain (ADG) of Italian Simmental finishing bulls was satisfactory (1.32 kg/d), however, both ADG and feed efficiency tended (P<0.10) to be higher in M diets. Slaughter traits were unaffected by dietary treatments. In Exp. 2, large differences in the ammonia concentrations were found between M and L diets over different sampling times (P<0.01 at T0, T1, and T2; P<0.05 at T3). In vitro DM digestibility tended to be higher for U diets (P<0.10). Molar proportions of butyrate were higher in M and U diets (P<0.01). Although the in vitro environment lowered diversity and richness of bacteria population and total VFA concentration (P<0.01), it allowed the growth of the major bacterial phyla found in vivo. The second experimental part consists of three in vitro trials. The aim was to evaluate the capacity of urea-treated cereals (barley and corn) for slow-release ammonia properties in vitro. In Exp. 1, milled untreated (CTR), urea-treated (UT) and urea-added (UA) samples were incubated in the rumen liquid and sampled for ammonia at 0, 0.5, 2, 4, 6, 8 and 24 h. The UT cereals released less ammonia than the UA (P<0.05), while the CTR showed the least ammonia concentrations (P<0.01). In Exp.2, CTR and UT samples in three physical forms: whole, coarsely and finely milled were incubated in distilled water for 1, 2, 4, 6 or 8 h, and then analyzed for N content. The N solubility of UT and CTR cereals changed according to the physical form (20.2 vs. 9.6%, 31.9 vs. 13.2% and 43.7 vs. 15.2% for the whole, coarse and milled, respectively, P<0.01 of interaction). In Exp.3, CTR, UT and UA milled samples were incubated in rumen fluid first for 96 h to determine a halftime (t1/2) and second, for t1/2 to measure gas production, degradability, microbial N, VFA and ammonia. In barley, UT and CTR treatments produced more gas than UA incubation, while in corn, UT and UA produced less gas than CTR (P<0.01). The highest microbial N was found for UT samples in both cereals (P<0.01). A lower butyrate concentration was detected for UT samples (P<0.01). The third experimental part consists of two in vitro gas tests. The aim was to test in vitro rumen gas production technique in differentiating slow-release urea (SRU) products. In the Exp. 1, five progressive urea doses (0, 25, 50, 75 and 100 mg) added to corn meal (CM) were tested. Rise in urea doses depressed gas production at each of measurement times (P<0.01). Above 50 mg of urea dose, a decrease (P<0.01) of apparent substrate degradability (ASD) was observed. In Exp. 2 nine SRU products added to CM in amounts equivalent to 50 mg (on N basis) of urea (CM + SRU), CM alone and CM added with urea (CM + U) were incubated. Gas production had the highest values for CM and the lowest for CM + U substrates (P<0.01) while for all CM + SRU mixes gas production varied between CM and CM + U values. Overall, despite a tendency for lower ADG in finishing bulls fed L diets, no detrimental effects neither on slaughter traits nor on in vitro rumen fermentation were detected. High moisture cereal grains treated with urea might be an opportunity to safety include urea in fattening diets, given their slow releasing properties and their capacity to improve microbial N synthesis found in vitro. Finally, a new approach to evaluate a variety of SRU products was suggested for consideration.