The aim of the present study was to evaluate the effect of drying temperature on in vitro digestibility of organic matter (DOM), metabolizable energy (ME) and net energy for lactation (NEL) values, and drying efficiency of wheat dried distillers’ grains (WDDG), applied a tower heat-dryer system, using mixed rumen micro-biota obtained from Holstein steers. The technology was developed to dry WDDG when it was flew constantly through a tower (10 m length) against a current of pressure of heated air at 185 ºC (LAT) or 246 ºC (HAT) with a pressure suction cooling. Dried samples were obtained from the lowest and the highest applied temperature, then ground to pass through a 1-mm screen and subjected to a gas production technique. Rumen content was collected from four ruminally fistulated steers (420±13 kg, body weight) and strained through 4 layers of cheesecloth. The laboratory handling of rumen fluid was carried out under a continuous flow of CO2. In vitro incubation of the samples was done using a manual pressure transducer technique. Approximately 200 mg of each sample was weighed into a 120 ml serum bottle (n= 8). The bottles were pre-warmed at 38.6 °C before the injection of 30 ml rumen fluid-buffer mixture (10 ml rumen fluid and 20 ml buffer solution) into each bottle followed by incubation in a water bath at 38.6 °C. Gas pressure was recorded after 2, 4, 8, 12 and 24 h of incubation and converted to gas volume using experimentally determined calibrated curve. Metabolizable energy, NEL and DOM values of the samples were calculated using following equations: ME (MJ/kg DM)= 1.56 +0.1390 GP +0.0074 XP +0.0178 XL; NEL (MJ/kg DM)= 0.1010 GP +0.0051 XP +0.011 XL; DOM (g/kg DM)= 14.88 +0.8893 GP +0.0448 XP +0.0651XA. Where GP is net gas produced after 24 h of incubation (ml/0.2 g DM), and XP, XL and XA are crude protein, crud fat and ash content of the feed (g/kg DM), respectively.

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