Saturday, March 30, 2024

Digesting the trial results

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Before encouraging farmers to adopt brassica feeding to reduce methane emissions the effects on animal production also need to be understood. AgResearch science team leader David Pacheco said a lamb finishing trial – comparing weight gains on fodder rape to ryegrass – found the lambs grazing brassicas grew faster. 
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“A properly managed brassica crop can deliver equal or better animal performance than a ryegrass pasture.” 

When animals digest forage rape it promotes a fermentation type in the rumen that leads to less hydrogen being produced, compared to a typical ryegrass pasture. Hydrogen is required to produce methane, a process occurring during fermentation of the feed in the animal’s rumen. Forage rape contains more carbohydrates that are rapidly fermented in the rumen. 

“When we feed the forage rape and see these reductions, the microbes that live in the rumen look very much like the microbes that you get if you feed a grain diet,” Pacheco said.

He said grain also reduces methane significantly but the amount of grain required to reduce methane is large, making this option too expensive and impractical. 

“In some livestock systems feeding grain will be hard to implement.

The idea will be to find a feed that gives fermentation like brassica that can be grazed and is reasonably easy to incorporate into our farming systems.”

The first experiments measured methane emissions from sheep grazing 100% forage rape, compared with sheep fed solely on ryegrass. Of course not every farmer uses 100% forage rape so it also makes sense to test methane emissions from a combination of feeds.

Pacheco said trials combining varying proportions of ryegrass and forage rape in the diet still resulted in lower methane emissions. He said that was different to grain which, when combined with other feed, does not start reducing emissions until it makes up about 70% of the diet. 

“To our surprise the reductions in methane were more linear and in proportion to the amount of brassica they were eating. “

At 50-75% of the diet it was very clear that there was a significant reduction.

“This is encouraging because it means a potential reduction would be achievable even if you don’t feed 100% forage rape.”

“We now need to look at combinations of forage rape with feeds other than ryegrass. There is an infinite number of combinations of feed and we cannot measure them all.”

It may be possible to test these combinations through computer-based simulations of rumen function in future, rather than physical trials.

While initially the work focused on winter brassica cultivars the research team has also evaluated summer brassicas.

Summer varieties of forage rape, turnip and swede resulted in less dramatic methane reductions of 10-15%. The reasons for the difference between winter and summer responses are unknown.

Recently, the effect of forage rape fed to cattle was evaluated to make sure the response was not just specific to sheep. These methane reductions were slightly higher at about 40% less methane a unit of feed eaten compared to ryegrass pasture.

Pacheco said it was too soon to prescribe how the potential to reduce methane emissions by feeding brassica could be applied onfarm. 

“We are putting all the data together including measurements that we have done on nitrous oxide emissions from urine patches when animals are fed forage rape. 

“We will look at where the information gaps are and if the greenhouse gas balance sheet is positive. We will need to make sure that if brassicas are considered a mitigation tool for greenhouse gases, reductions can be achieved profitably.”

The AgResearch team has involved a collaboration of animal, forage and soil scientists and technical staff to do this research.

Food for thought

Pastoral Greenhouse Gas Research Consortium manager Mark Aspin said the results of the brassica trials have been encouraging. With the feed base being 85-90% pasture brassicas will likely only ever be a small proportion of an animal’s diet, he said.

Aspin said the funding partners have invested up to $3 million into research to identify low greenhouse gas feeds during the past five years and with agriculture producing almost half of NZ’s total greenhouse gas emissions it was important to look for ways to reduce this carbon footprint.

While a certain feed might reduce emissions from livestock there were other factors to be considered – including costs of growing the crop and the carbon emissions during the planting process, such as from machinery use – while always delivering productivity for farmers.

“The use of crops by farmers is driven by productivity more than greenhouse gas production. We don’t expect this to change unless there are dramatic changes in the cost of carbon and therefore we are always mindful of the production impacts.”

It is also important to collect sound scientifically-based information for farmers including the impact on carbon emissions so that they are able to make informed decisions in the future when carbon cost may be a consideration, Aspin said.

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