IN VITRO UTILIZATION OF NPN SOURCES BY INCREASING LEVELS OF CORN STARCH IN STRAW BASED DIETS

This study was conducted to investigate the effect of replacement of 50% cottonseed meal (CSM) nitrogen with various non protein nitrogen (NPN) sources i.e. urea (CU), biuret (CB) and diammonium phosphate (CD). The four energy sources were: wheat straw with no corn starch (WS), WS + 20% corn starch, WS + 30% corn starch and WS + 40% corn starch. These substrates were fermented with rumen liquor to measure in vitro dry matter digestibility (DMD), bacterial count and ammonia nitrogen (NH3-N) concentrations. The protein sources provided 2% nitrogen (12.5% CP). The control substrate contained CSM as the sole source of nitrogen and ground wheat straw as the sole sources of energy. The in vitro DMD increased to 49.10, 40.06 and 31.52% in substrates containing CB, CU and CD compared to 23.10% for CSM (P<0.01). Similarly, supplementation of straw with 20, 30 and 40% corn starch gave 24.31, 38.03 and 45.48% DMD compared to 23.10% for control (P<0.01). Increase of corn starch from 20 to 30% resulted in 13.72 units higher DMD which increased to a mere 7.45 units when the corn starch was raised from 30 to 40%. The interactions between nitrogen sources and starch levels revealed that substrate CB x 40% corn starch yielded 70.73% DMD, followed by 49.66% DMD with CU x 40% starch (P<0.01). The NH3-N increased due to 50% replacement of CSM with NPN sources on isonitrogenous basis. It was maximum with CU as nitrogen source, followed by CD, CB and CSM. The differences among the four nitrogen sources were significant (P<0.01). The substrates containing CU resulted in highest bacterial counts of 33.78x108 compared to 20.41x108, 17.06x108 and 11.34x108 for CB, CSM and CD, respectively (P<0.01). Addition of corn starch up to 20 and 30% to straw based substrates increased the bacterial counts to 23.25x108 and 23.12x108 and 40% corn starch yielded 15.58x108 bacterial counts which was significantly (P<0.01) lesser than 17.06x108 for substrates containing 0% corn starch. Bacterial count of 33.72x108 was significantly higher than 8.51x108 and 19.72x108 at 0 and 48 hours fermentation, respectively. The ruminal NH3-N concentration was highest (16.97 mg/dl) for CU, followed by 15.75, 14.31 and 12.98 mg/dl for CD CB and CSM, respectively. The ruminal NH3-N concentrations were 15.69, 15.14, 14.18 and 12.98 mg/dl for 30, 20, 40 and 0% corn starch supplementation in the substrates, respectively. All these values were significantly (P<0.01) different from each other. During in vitro fermentation the NH3-N progressively increased till 2 hour of fermentation and declined thereafter.