The effects of manure age on emission of H2S and required level of nitrite or molybdate to control these emissions were investigated in the present work. Molybdate mediated control of H2S emission was also studied in semi-pilot scale open systems, and in specifically designed chambers which simulated swine production rooms. With fresh 1-, 3- and 6-month old manures average H2S concentration in the head-space gas of the closed systems were 4856 +/- 460, 3431 +/- 208, 1037 +/- 98 ppm and non-detectable, respectively. Moreover, the level of nitrite or molybdate required to control the emission of H2S decreased as manure age increased. In the semi-pilot scale open system and chambers, average H2S concentration at the surface of agitated fresh manure were 831 +/- 26 and 88.4 +/- 5.7 ppm, respectively. Furthermore, 0.1-0.25 mM molybdate was sufficient to control the emission of H2S. A cost study for an average size swine operation showed that the cost of treatment with molybdate was less than 1% of the overall production cost for each market hog. (C) 2009 Elsevier Ltd. All rights reserved.