Simple SummaryRegardless of the production system adopted, ruminant livestock contribute to greenhouse emissions that are associated with climate change. Among the greenhouse gases, enteric methane produced from the rumen is of the greatest concern because it is the largest single source of livestock emissions. Among the different dietary strategies examined to decrease methanogenesis in ruminants, the use of tannins shows promise, but has received only moderate attention. However, tannins are abundant in both tropical and temperate plants and so are widely available globally and may be an economical approach for livestock producers to mitigate enteric methane emissions. This review explores the challenges and opportunities of using dietary tannins to reduce enteric methane emissions from ruminants.AbstractThere is a need to reduce enteric methane (CH4) to ensure the environmental sustainability of ruminant production systems. Tannins are naturally found in both tropical and temperate plants, and have been shown to consistently decrease urinary nitrogen (N) excretion when consumed by ruminants. However, the limited number of in vivo studies conducted indicates that the effects of tannins on intake, digestibility, rumen fermentation, CH4 production and animal performance vary depending on source, type, dose, and molecular weight (MW). There are two main types of tannin in terrestrial plants: condensed tannin (CT; high MW) and hydrolysable tannin (HT; low MW). Consumption of CT and HT by ruminants can reduce N excretion without negatively affecting animal performance. High MW tannins bind to dietary protein, while low MW tannins affect rumen microbes, and thus, irrespective of type of tannin, N excretion is affected. The structure of high MW tannin is more diverse compared with that of low MW tannin, which may partly explain the inconsistent effects of CT on CH4 production reported in in vivo studies. In contrast, the limited number of in vivo studies with low MW HT potentially shows a consistent decrease in CH4 production, possibly attributed to the gallic acid subunit. Further in vivo studies are needed to determine the effects of tannins, characterized by MW and structural composition, on reducing CH4 emissions and improving animal performance in ruminants.

of the Lethbridge Research and Development Centre, AB, Canada, for their invaluable technical assistance. We also thank R. Merrill and the metabolism barn staff for their care of the experimental animals. 2 Lethbridge Research and Development Centre contribution number is 38718065.

Sustainability of animal agriculture requires efficient use of energy and nitrogen (N) by ruminants fed high-forage diets. Thus, there is a need to decrease methane (CH 4 ) emissions and prevent excessive N release into the environment. Therefore, this experiment examined the long-term effects of feeding hydrolyzable tannin (HT) with or without condensed tannin (CT) on animal performance, rumen fermentation, N use, and CH 4 production in beef cattle fed a high-forage diet. A total of 75 weaned crossbred steers (292 ± 4.1 kg) were grouped by body weight (BW), housed in individual pens, and randomly assigned to 1 of 5 dietary treatments (15 animals/treatment) in a completely random design. The animals were fed a basal diet of alfalfa:barley silages (50:50; dry matter [DM] basis) with a crude protein content of 17.1% and supplemented with HT extract (chestnut, CN) or a combination (50:50) of HT and CT extracts (quebracho, Q) in a powdered form at different levels of dietary DM. The treatments for determining animal performance and N use were control (no tannin), 0.25% CN, 1.5% CN, combination of CN and Q at 0.125% each (0.25% CNQ), and CN and Q at 0.75% each (1.5% CNQ) of dietary DM. The treatments for the CH 4 measurement were control, 1.5% CN, and 1.5% CNQ of dietary DM. The first 84 d of the study were used to measure animal performance, rumen fermentation, and N use, and the next 30 d were used to measure CH 4 emissions with the tracer gas technique. There were no effects of treatment on DM intake (DMI), BW, average daily gain, and gain: feed (P ≥ 0.10). The plasma urea N concentration was greater (P < 0.05) for 1.5% CN and 1.5% CNQ than those fed 0.25% CNQ (120.9 and 120.4 vs. 111.7 mg/L, respectively), but not different (P > 0.05) from animals fed control or 0.25% CN (117.2 and 117.5 mg/L, respectively). Tannin inclusion did not affect rumen pH, total volatile fatty acid concentration, proportions of acetate and propionate, and total protozoa populations (P ≥ 0.16). Tannin, irrespective of type or dose, decreased (P < 0.01) ruminal ammonia concentration. Tannin type and dose did not affect (P = 0.54) daily CH 4 production (154 ± 5.9 g/d) but 1.5% CNQ tended to decrease CH 4 yield compared with control (20.6 vs. 22.0 g/kg DMI; P = 0.094). HT from CN alone or in combination with CT from Q can be added at a low (0.25% DM) or high (1.5% DM) level to a forage-based diet to decrease ruminal ammonia concentration in growing beef cattle fed a high-protein diet without adverse effects on animal performance. A combination of HT and CT at a concentration of 1.5% dietary DM also tended to decrease CH 4 emissions without negatively affecting performance.