Effect of Oilseed Source on Ruminal Fermentation and Methane Production of a Grass-Legume Diet in Continuous Culture

Addition of oilseeds to pasture-based ruminant diets has been shown to decrease enteric CH₄ emissions. However, little research has directly compared the effect of oilseed source on ruminal fermentation and CH₄ production. A 4-unit continuous culture fermentor system was used to test 4 oilseeds supplemented to a 45% orchardgrass (Dactylis glomerata L.) + 45% sericea lespedeza (Lespedeza cuneata Don. cv. Auburn Grazer) diet. Oilseeds (supplemented at 10% of total DM fed) were: soybean (SOY), sunflower (SUN), canola (CAN), and a mixture of equal proportions of SOY, SUN and CAN (MIX). Diets were randomly assigned to fermentors in a 4 × 4 Latin square design. Feeding occurred 4 times daily throughout 4, 10-d periods using 7 d for adaptation and 3 d for collection. Effluent samples were analyzed for pH, VFA, and DM, OM, ADF and NDF digestibilities. Gas samples were recorded using a photoacoustic gas analyzer for total daily CH₄ production. Data were analyzed using the GLIMMIX procedure of SAS with treatment as a fixed effect and fermentor and period as random effects. No differences (P > 0.10) were found for pH (7.10) among oilseeds. Total VFA concentration was greatest (P < 0.01) in SOY (46.0 mmol/L), with no differences (P > 0.05) among other oilseeds (38.2 mmol/L). There were no differences (P > 0.05) among oilseeds in molar proportions of acetate, propionate, isobutyrate, butyrate, isovalerate and valerate. No differences (P > 0.10) were found for apparent and true DM and OM digestibilities, or apparent ADF and NDF digestibilities. Total daily CH₄ production and total CH₄ production per gram of nutrients fed or per gram of digestible nutrients fed were greatest (P < 0.001) in SOY. The SOY produced daily 68, 81, and 85% greater (P < 0.001) amounts of CH₄ (80.1 mg/d) compared to SUN, CAN and MIX, respectively. No differences (P > 0.05) were found between SUN, CAN and MIX for total daily CH₄ production or total CH₄ production per gram of DM, OM, ADF and NDF fed or per gram of digestible OM fed. The SOY produced greater (P < 0.001) amounts of CH₄ per gram of digestible DM fed or per gram of digestible ADF and NDF fed than MIX, with SUN and CAN showing intermediate levels. Addition of oilseeds such as SUN and CAN to a grass-legume diet is a potentially environmentally-friendly feeding strategy to reduce enteric CH₄ emissions without negatively impacting ruminal fermentation.