Determination of nutritive value of forages in south Texas using an in vitro gas production technique

The objectives of this study were to use the in vitro gas production ( IVGP) technique to evaluate the pattern and parameters of anaerobic fermentation of forages from south Texas pastures throughout the year to ( i) obtain empirical relationships between the IVGP technique fermentation parameters and chemical composition of the forages and ( ii) develop equations to compute total digestible nutrients ( TDN). During four consecutive years ( 2006- 2009), forage samples were collected monthly ( n = 39) at the King Ranch, TX, and chemical analyses and IVGP were obtained. For 2006, 2007, 2008 and 2009, the average lag times, h, were 6 47 +/- 0 54, 7 75 +/- 0 65, 7 49 +/- 2 01 and 5 44 +/- 1 46, and the average ratio of millilitre of gas per milligram of dry matter was 0 41 +/- 0 11, 0 34 +/- 0 09, 0 34 +/- 0 07 and 0 26 +/- 0 10 respectively. There was a moderate negative correlation ( r =) 0 53) between lignin and neutral detergent fibre ( NDF) and a moderate positive correlation ( r = 0 58) between crude protein and NDF digestibility. The predicted fractional passage rate ( kp) by the large ruminant nutrition system model using the level 2 solution was on average 0 0366 h) 1. The average computed TDN assuming a kp of 0 04 h) 1 was 55 9%. We concluded the IVGP technique may be used to predict TDN values of warm- season forages.