29 Indian Journal of Animal Nutrition Indian J. Anim. Nutr. 2011. 28(1) : 29-33 Calibration of In Vitro Gas Production Technique Using Digital Pressure Gauge: Air Versus Fermentative Gases Seyed Hadi Ebrahimi 1 Pankaj Jha & Madhu Mohini Division of Dairy Cattle Nutrition, National Dairy Research Institute (NDRI), Karnal, Haryana -132001, India. ABSTRACT This experiment was aimed to compare air with fermentative gas for calibrating an in vitro gas production system using digital pressure gauge. Serum bottles (100 ml) containing 45 ml of in vitro medium were injected with air and fermentative gas produced from incubation of starch (S gas), starch and cellulose (S + C gas) and cellulose (C gas). The pressures in the bottles were read after equilibrating the vials overnight at 39 °C for each gas, calibration line was constructed by linear regression of gauge reading (in psi) versus volume fraction (Fv) of added gas, where Fv = milliliters added gas/milliliters bottle headspace volume. For all four gases (air, S gas, S + C gas and C gas), there was a linear relationship between volume and pressure (r 2 = 0.99). There was no significant difference between slopes of calibration curve obtained from air, S gas, S + C gas and C gas. Results of present work indicated that calibration of in vitro gas production system using air can give reliable regression equation applicable for measurement of total gas volume of wide ranges of substrates. Key words: Calibration, In vitro gas production technique, Digital pressure gauge INTRODUCTION Studies with live animals (in vivo) for nutritional evaluation of feeds are time-consuming, laborious, and expensive and require large quantities of feed. The in vitro gas production technique has gained general recognition over the last decades, being used increasingly for feed evaluation, to investigate mechanisms of microbial fermentation and for studying the mode of action of anti-nutritive factors, additives and feed supplements. The technique is based on the assumption that the gas produced in batch cultures is just the consequence of the fermentation of a given amount of substrate (France et al. 2000). There are basically two approaches for measuring gas volumes. The first approach implies measurement of gas collected at atmospheric pressure and its volume determined directly (Menke et al. 1979; Menke and Steingass 1988). Calibrated glass syringes of 100 ml capacity were used for incubating 200 mg substrate with 30 ml buffered rumen liquor (BRL) and gas volume were determined by difference between the positions of piston initially and after 24 h incubation. However, calibrated glass syringes are expensive and assuming 30 ml of BRL needed for incubation, there is always limitation for incubation of 200 mg substrate because gas produced from incubation of concentrate and compound feed might be higher than the capacity of syringes. Therefore, to Indian J. Anim. Nutr. 2011. 28(1) : 29-33 Corresponding Author : E-mail : ebrahimish@gmail.com