Methane emissions from sheep fed fresh brassicas (Brassica spp.) compared to perennial ryegrass (Lolium perenne)

Four forage brassicas, kale (Brassica oleracea L. cv. Kestrel), turnip (B. campestris L. cv. Appin), rape (B. napus L. cv. Titan) and swede (B. napus L. cv. Dominion) were compared with perennial ryegrass (Lolium perenne L. cv. mixture of Delish and Banquet) to examine their potential to mitigate CH4 emissions from sheep.

Sixty healthy, one-year-old Romney cryptorchids (liveweight 36±1.0kg) were randomly allocated to 5 groups of 12. The sheep were initially adapted to their diets for 2wks on paddocks and 1wk in indoor pens, then fed at 1.5 times their metabolisable energy (ME) maintenance requirement during measurements.

Apparent total tract digestibility (n=5) and ME (n=5) of brassicas and ryegrass, and rumen fermentation parameters (n=10) were measured, and CH4 emissions (n=9) were determined using open circuit respiration chambers. Correlations between nitrate, sulphur, sulphate, glucosinolates and S-methyl cysteine sulphoxide (SMCO) contents of forages and CH4 emissions were determined.

Methane emissions/unit feed dry matter intake (g CH4/kg) from sheep were kale 19.8, rape 16.4, swedes 16.9, turnips 20.6 and ryegrass 22.0. Compared with ryegrass, rape and swedes reduced CH4 yield by 23% and 25%, respectively. Total tract digestibilities were higher for brassicas than for ryegrass: 25% higher for DM, organic matter (OM) and crude protein, and 12–16% higher for neutral detergent fibre (aNDF) and acid detergent fibre (ADF).

The ME values (MJ/kg DM) were kale 12.7, rape 13.2, swedes 14.1, turnips 12.1 and ryegrass 9.4. Digestibility (P<0.05) and ME (P<0.001) of the forages were correlated to CH4 yield. The correlation coefficients were −0.570 for OM digestibility, −0.505 for NDF digestibility, −0.469 for ADF digestibility and −0.699 for ME.

Although the ratio of acetate to propionate in the rumen of sheep fed brassicas was lower than that measured from those fed ryegrass, this ratio was not correlated to CH4 emissions. There were large variations in nitrate, sulphur, sulphate, glucosinolates and SMCO amongst the forage brassicas and ryegrass, but these parameters could not explain reduced CH4 emissions.

Feeding brassica forages to sheep reduced emissions/unit DM intake and digestible OM with the reduction being particularly large for forage rape and swedes. Thus these forages may be a viable option for CH4 mitigation from pastoral based sheep production systems. However the mechanisms underlying the reduction need further investigation.This paper is part of the special issue entitled: Plant Bioactive Compounds in Ruminant Agriculture – Impacts and Opportunities, Guest Edited by A.Z.M.

Salem and S. López, and Editor for Animal Feed Science and Technology, P.H. Robinson.