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Low GHG feeds: latest research shows that forage rape can make a real impact

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Could forage rape, a brassica crop, hold the key to understanding what and how to feed animals to reduce their greenhouse gas emissions?

A wide search has been carried out to date for animal feeds that reduce GHGs. For a long time everything studied either had no effect or contained some sort of inhibitory compound, but was impractical as an animal feed. For example, garlic has been shown to reduce methane emissions, but is not an agronomically viable forage crop. Grain is known to lead to reduced emissions, but a large proportion of the diet needs to be grain (>65%) to see a reduction. Not a viable option for NZ farmers. Some fresh forages evaluated in New Zealand, such as white clover, have given inconsistent responses when fed to sheep.

Researchers had begun to think that any type of fresh animal feed would result in the same amount of methane for the same amount of dry matter. That was until some species of brassicas were studied and gave a surprising result.

Field trials have shown that diets based on 100% forage rape, a type of brassica, can reduce methane emissions per unit of feed from sheep by as much as 30%. Currently the reason for this is not fully understood. Brassicas contain compounds that could act as potential inhibitors of methanogenesis, but all the data so far suggest they are not causing the reduction in methane. It seems that the reduction is due to the way that the feed is being fermented in the rumen.

David Pacheco and Sunny Sun, with their team, have been following up on this observation with the aim of working out what it is about the brassicas that leads to a drop in methane. Once this piece of the puzzle has been solved, the goal is to be able to identify and develop feeds and feeding practices that result in reduced GHG emissions and can be counted in the national greenhouse gas inventory.

The research team has been investigating how increasing levels of forage rape in the diet affects methane production. In contrast to grain, preliminary observations from indoor trials with forage rape suggest a linear reduction in methane yield as the proportion of forage rape in the diet increases. If confirmed, this result would mean that feeding of forage brassicas can lead to a reduction in methane emissions at a larger range of dietary inclusion levels than previously thought. The results also suggest that reductions in methane observed in brassicas may occur through a different mechanism to those elicited by feeding grains.

The research is being extended to include more brassica species, as all those studied to date appear to have reduced methane emissions to some degree. One prediction is that the decrease is due to readily fermentable carbohydrates present in the brassicas. This suggests that researchers should look at other forage crops with this characteristic. Therefore, one of the next feeds to be studied more closely is fodder beet. This crop has been growing in popularity in recent years and the results of any GHG trials will be of interest to NZ livestock industries. Results will also help to inform the search for new feeds and feeding strategies that can be included in the NZ inventory.

While the team expertise is focused on methane, the project team is linking with other experts to have an integrated, wider view of what the impact is for whole GHGs. For example, Pacheco's team has also been looking at the impacts of brassica feeds on nitrous oxide emissions from urine deposited onto soil by grazing animals. This is being done in conjunction with the nitrous oxide experts in AgResearch and funded by MPI through its Sustainable Land Management and Climate Change (SLMACC) programme. To bring all these findings together a parallel project led by Stewart Ledgard, also part of the SLMACC research fund is using the data generated to do a full life cycle analysis of alternative feeds, like brassicas, that require cultivation and application of fertilisers. These processes can also produce GHGs, and their impacts need to be known as well.


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