Nitrate supplementation has been shown to be effective in reducing enteric methane emission from ruminants, but there have been few large-scale studies assessing the effects of level of nitrate supplementation on feed intake, animal growth, or carcass and meat quality attributes of beef cattle. A feedlot study was conducted to assess the effects of supplementing 0.25 or 0.45% NPN in dietary DM as either urea (Ur) or calcium nitrate (CaN) on DMI, ADG, G:F, and carcass attributes of feedlot steers (n = 383). The levels of NPN inclusion were selected as those at which nitrate has previously achieved measurable mitigation of enteric methane. The higher level of NPN inclusion reduced ADG as did replacement of Ur with CaN (P < 0.01). A combined analysis of DMI for 139 steers with individual animal intake data and pen-average intakes for 244 bunk-fed steers showed a significant interaction between NPN source and level (P = 0.02) with steers on the high-CaN diet eating less than those on the other 3 diets (P < 0.001). Neither level nor NPN source significantly affected cattle G:F. There was a tendency (P = 0.05) for nitrate-supplemented cattle to have a slower rate of eating (g DMI/min) than Ur-supplemented cattle. When adjusted for BW, neither NPN source nor inclusion level affected cross-sectional area of the LM or fatness measured on the live animal. Similarly, there were no significant main effects of treatments on dressing percentage or fat depth or muscling attributes of the carcass after adjustment for HCW (P > 0.05). Analysis of composited meat samples showed no detectable nitrates or nitrosamines in raw or cooked meat, and the level of nitrate detected in meat from nitrate-supplemented cattle was no higher than for Ur-fed cattle (P > 0.05). We conclude that increasing NPN inclusion from 0.25 to 0.45% NPN in dietary DM and replacing Ur with CaN decreased ADG in feedlot cattle without improving G:F.

It is unclear how nutritional conditions (below or above maintenance) affect Merino follicle characteristics in sheep with different estimated breeding values (EBVs) for wool production but with similar EBVs for fibre diameter and liveweight. This was addressed in our study. Twenty castrated male Merino sheep were selected from a commercial flock, 10 with high EBVs for wool production (F+) and 10 with low EBVs (F–). The animals were offered a diet providing 1.2 times their metabolisable energy requirement for maintenance (1.2 M) for 4 weeks. After 4 weeks’ acclimatisation, five sheep from each EBV group were offered a diet of providing 0.8 times their metabolisable energy requirement for maintenance (0.8 M), and the other five from each EBV group were offered the 1.8 M diet for 5 weeks. On Day 35, one skin biopsy was taken from a shaved area on the left mid-side of each animal under local anaesthesia (subcutaneous injection of 2 ml 2% (w/v) Lignocaine), using a trephine (1.5 cm diameter). The skin sample was stored in 37% (w/v) buffered formalin (pH = 7) for skin histology analysis. Skin samples were processed by CSIRO (Armidale, NSW). The histology methods were essentially those described by Maddocks and Jackson (1988).

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.

The effects of dietary nitrate on DM digestion, rumen volatile fatty acid concentrations, microbial protein outflow, rumen water kinetics, and methane production were studied. Eight rumen-cannulated sheep were acclimated to a diet consisting of chaffed oaten hay supplemented with either 4% KNO₃ or 0% KNO₃ but made iso-nitrogenous by the addition of urea. Nitrate supplementation did not affect blood methaemoglobin concentration, DM intake, whole tract or ruminal DM digestibility and the sheep appeared healthy at all times throughout the acclimation and experimental periods. Nitrate did cause changes in rumen fermentation consistent with its acting as a high-affinity hydrogen acceptor, i.e. there was a tendency towards a lower molar percentage of propionate in the rumen volatile fatty acids, and higher molar ratio of acetate to propionate. Methane yield (MY,Lmethane/kgDMintake) was reduced by 23% in KNO₃-supplemented sheep (P<0.05) and these sheep tended to have a shorter mean fluid retention time in the rumen (MRT). There was a significant association between MRT and MY, such that a shorter MRT was associated with a lower MY. The results confirmed that the presence of nitrate in the diet lowers enteric methane production even though there was considerable between-animal variation in gut kinetics and methane production.