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

Institute of Animal Nutrition and Feed Technology, ¹Department of Physiology and Pharmacology, University of Agriculture, Faisalabad, Pakistan; ²Dairy Cattle Research Division, National Livestock Research Institute, Korea; ³Islamabad College for Girls, Islamabad, Pakistan

Abstract

    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.

Key words: in vitro, NPN, corn starch, digestibility, ammonia nitrogen concentration, bacterial population.