Effect of nitrogen source in high-concentrate, low-protein beef cattle diets on microbial fermentation studied in vivo and in vitro

In Exp. 1, four Holstein heifers (112 +/- 5.5 kg EW) fitted with ruminal ca nnulas were used in a 4 x 4 Latin square to evaluate the effects of N sourc e on ruminal fermentation and urinary excretion of purine derivatives. A 2 x 2 factorial arrangement of treatments was used; the factors were the type of protein source (soybean meal, SBM, vs a 50:50 mixture of fish meal and corn gluten meal, FMCGM) and the partial substitution of protein source by urea (with vs without). Heifers were allowed to consume concentrate and bar ley straw on an ad libitum basis. Barley straw:concentrate ratio (12:88) an d average ruminal pH (6.25) were not affected (P > 0.05) by treatment. Rumi nal NH3 N concentration and urinary excretion of purine derivatives were no t affected (P > 0.05) by supplemental N source. In situ CP degradability of supplemented SBM was very low (50%). In Exp. 2, eight dual-flow continuous -culture fermenters were used to study diet effects on microbial fermentati on and nutrient flow, using forage:concentrate ratio, solid and liquid pass age rates, and pH fluctuation to simulate in vivo conditions. The treatment containing SBM without urea reached the greatest total VFA concentration ( P < 0.01), molar percentage of acetate (P < 0.05), and NH3 N concentration (P < 0.05), followed by treatments with partial substitution of protein sou rce by urea, and finally by the treatment containing FMCGM; True OM digesti on tended to increase (P = 0.13) in treatments containing SBM. These result s suggest that amino N from SBM and NH3 N concentration stimulated nutrient digestion. Microbial protein synthesis was lowest in treatments with FMCGM and without urea, indicating that rapidly available N limited microbial gr owth. The low GP degradability of SBM observed may have contributed to the limitation in N supply for microbial growth. Efficiency of microbial protei n synthesis increased in treatments containing urea (P < 0.05). Protein sou rce affected total (P < 0.05) and essential AA (P <less than> 0.10) flows, which were greater in treatments containing FMCGM. Partial replacement of p rotein supplements by urea did not affect total and essential AA flows. Bec ause mean dietary protein contribution to total N effluent was 46%, the AA profile of supplemental protein sources had a great impact on total AA flow and its prove.