Abstract
The objectives of this study were to examine long-term effects of feeding forage rape (Brassica napus L.) on methane yields (g methane per kg of feed dry matter intake), and to propose mechanisms that may be responsible for lower emissions from lambs fed forage rape compared to perennial ryegrass (Lolium perenne L.). The lambs were fed fresh winter forage rape or ryegrass as their sole diet for 15 weeks. Methane yields were measured using open circuit respiration chambers, and were 22-30% smaller from forage rape than from ryegrass (averages of 13.6 g versus 19.5 g after 7 weeks, and 17.8 g versus 22.9 g after 15 weeks). The difference therefore persisted consistently for at least 3 months. The smaller methane yields from forage rape were not related to nitrate or sulfate in the feed, which might act as alternative electron acceptors, or to the levels of the potential inhibitors glucosinolates and S-methyl L-cysteine sulfoxide. Ruminal microbial communities in forage rape-fed lambs were different from those in ryegrass-fed lambs, with greater proportions of potentially propionate-forming bacteria, and were consistent with less hydrogen and hence less methane being produced during fermentation. The molar proportions of ruminal acetate were smaller and those of propionate were greater in forage rape-fed lambs, consistent with the larger propionate-forming populations and less hydrogen production. Forage rape contained more readily fermentable carbohydrates and less structural carbohydrates than ryegrass, and was more rapidly degraded in the rumen, which might favour this fermentation profile. The ruminal pH was lower in forage rape-fed lambs, which might inhibit methanogenic activity, shifting the rumen fermentation to more propionate and less hydrogen and methane. The significance of these two mechanisms remains to be investigated. The results suggest that forage rape is a potential methane mitigation tool in pastoral-based sheep production systems.
Highlights
Methane (CH4) accounts for 37.4% of total anthropogenic greenhouse gas (GHG) emissions in New Zealand [1], and 85% of this is from enteric fermentation in the digestive tracts of grazing ruminants
Details of the experimental animals, forages and feeding, the protocols describing measurement of CH4 emissions, digestibility and ME measurements, rumen fluid sampling and sample processing, the determination of rumen liquid and particulate passage rates, in situ dry matter (DM) degradation kinetics, methods for determining the nutritional composition of the forages, methods for measuring nitrate, sulfate, glucosinolate and SMCO concentrations, methods for the assessment of rumen microbial community composition, and the statistical analyses used in this study are all described in S1 Text
The CH4 yield (g/kg DM intake) from the forage rape-fed lambs was 30% smaller (P
Summary
Methane (CH4) accounts for 37.4% of total anthropogenic greenhouse gas (GHG) emissions in New Zealand [1], and 85% of this is from enteric fermentation in the digestive tracts of grazing ruminants. Some means to mitigate enteric CH4 emissions have been proposed, including manipulation of the rumen microbes using inhibitors or vaccines, modifying the fermentation by supplying H2 sinks as feed additives, animal selection for low CH4 emitting genotypes, and livestock systems improvement [2,3,4,5,6,7,8]. Lambs fed forage rape emitted 25% less CH4 per unit of dry matter intake compared to ryegrass [15]. This result was observed in a single, short term trial only, and no information is available on the persistence of the CH4 reduction elicited by feeding forage rape to sheep
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