In Australia, enteric methane from ruminants represents 11% of total agricultural GH emissions. Practical ways of reducing methane emissions from ruminants in Australia are required to enable the livestock industries to reduce their carbon footprint. When rumen microorganisms ferment feed organic matter, they generate the reduced cofactor NADH which is in equilibrium with rumen H₂. In ruminants, the H₂ is normally removed by the reduction of CO₂ to form methane. However, NO₃⁻ (present in some fresh pasture forage) has a higher affinity for H₂ than CO₂ and, when it is present, H₂ is first used in the reduction of NO₃⁻ to NO₂⁻ and NO₂⁻ to NH₃ thereby reducing the production of methane from CO₂. After reviewing the literature and considering the potential for NO₂⁻ poisoning, Leng (2008) concluded that supplementation of ruminants with NO₃⁻ (as an alternative to urea) is entirely feasible. This study therefore examined digestion, microbial growth and methane production in sheep given a diet of chaffed oaten hay supplemented with isonitrogenous amounts of KNO₃, urea, or both N sources. Eight Merino wethers (38.6 kg, SE 2.4; aged 3 years with long-established rumen fistulas) were housed in metabolism cages in 2 rooms (15-20°C, continuous lighting) and allocated to 2 treatment groups. Two isonitrogenous diets based on chaffed oat hay were prepared. A diet with 4% added KNO3 was prepared by sprinkling a solution of KNO₃ onto the hay while it was stirred in a rotary feed mixer. Another diet (0% KNO₃) was similarly prepared using a urea solution so that 5.54 g N was added per kg hay for both diets. The sheep were gradually acclimated to the NO₃⁻ containing diet over 18 days. The daily ration (1 kg/d air-dry feed) was delivered to both groups of sheep in equal portions each hour by automatic feeders during a 4-day digestibility trial, and every 2 h while the sheep were in respiration chambers to determine their methane output.