Fresh grass can lead to less methane gas

Photo: Wageningen University & Research


Cows that eat grass produce methane. Studies by Wageningen Livestock Research show that methane emissions can be lower for cows that graze in fields compared with cows that eat silaged grass. It turns out that putting cows out to pasture is important not just for reasons of tradition, but also because it can play a role in the reduction of methane and ammonia emissions.

They are a standard news item on the Dutch TV: clips of cows frolicking in the field after they have been let out after a long winter in the barn. And most people like seeing cows in the fields. For 400 years, cows grazing in pastures have been a source of inspiration for Dutch landscape artists. It is an image that is inextricably linked with the traditional Dutch countryside. But it is not only that cherished countryside that is making cows in the fields an interesting option again; the government is aiming for circular agriculture across the board, whereby farmers source their feed from their own farms wherever possible. This means that fresh grass grown on their own land will become more important for dairy farms.

Old grass creates more gas

Cows naturally cough and burp, and when they do they release the powerful greenhouse gas methane (CH4). Emissions of greenhouse gases are usually expressed as CO2 equivalents to enable comparisons. The agricultural sector in the Netherlands is responsible for total greenhouse gas emissions of 17.9 megatons in CO2 equivalents. Of that total, 7.2 megatons is due to methane emissions from cows. So if those emissions could be reduced, that would have a big impact.

Meadow grass in morning dew. Foto: Shutterstock

Methane is formed as a result of the digestion of grass in the rumen. The amount of methane that is produced varies from cow to cow, but depends on the quality of the grass as well. One aspect of that quality is what is termed the NDF value. NDF is a measure of the grass structure, what you could call the woodiness of the grass — its fibre content and digestibility. “We’re fairly confident that a higher NDF value in silage leads to higher methane emissions in cows,” says Cindy Klootwijk, a researcher at Wageningen Livestock Research. “Put simply, it boils down to older grass meaning higher methane emissions. That has to do with the digestion of the fibres in the rumen.”

Sensors in a feeding station

The question is how changes in the NDF values affect methane emissions in grazing cows. To investigate this, WUR carried out a study in the past year on cows put out to pasture on the Dairy Campus in Leeuwarden. A fixed number of cows were allowed to graze in clearly defined plots with the same species of grass at different times of the year.

Put simply, it boils down to older grass meaning higher methane emissions

Each plot had mobile, covered concentrate stations known as Greenfeeds that are able to record the methane emissions of individual cows. The cows visited the Greenfeed stations about seven times a day for a tasty snack. Sensors in the station recorded the methane content of the exhaled air. It turned out that regardless of the season, the methane emissions were lowest in the cows that were put out to pasture. Incidentally, the cows in the field and in the barn were fed the same amount in concentrates to eliminate any possible effect this might have on the methane emissions. What makes the provisional results from the experiment interesting for Klootwijk is that it suggests different mechanisms are in play with fresh grass compared to silage when predicting their effect on methane emissions. “In this study, we performed a lot of measurements of the grass quality but we also looked at how exactly the grass is broken down in the rumen. Now we can examine these associations in combination with the methane emissions we measured.”

You can wake up WUR researcher Bert Philipsen for grass in the middle of the night. He explains why grass should be at the core of dairy farming: outdoor grazing reduces the cost price, and improves the quality of water, soil, and air. It also boosts the taste of meadow grass, which leads to more milk from home soil.

According to Klootwijk, grass has long had a low priority in the feed regimes in dairy farming. “You could easily adjust the supplements with soymeal or maize silage if you needed to correct for the grass protein content or the cow’s energy requirements. But now that the emphasis is on closed cycles where possible, grass will need to play more of a key role on dairy farms. That is why we ran this experiment. We had research questions such as how to influence the grass quality, when the best time is to harvest the grass, what the optimum grass length is and at what time of day you should harvest the grass. We hope that studying these questions will let us find out how to reduce methane emissions.” However, reducing methane emissions is not the only challenge facing dairy farmers. Ammonia emissions also need to come down. Spring pasture grass contains fewer woody fibres and therefore scores best in terms of methane emissions, but young grass also contains more protein and that can boost nitrogen surpluses and ammonia. Summer grass, which has been growing for longer and contains more fibres, leads to higher methane emissions but less ammonia.

The solution may lie in grasslands rich with herbal plants

Klootwijk acknowledges that it is a complex problem. “You really want to minimise both methane and ammonia emissions, so you need to find precisely that quality of grass where the structure means lower methane emissions and the protein content means lower ammonia emissions. That’s very challenging for your grass management. We are searching for integrated solutions that can reduce both ammonia emissions and methane emissions.”

What, how and when: it all has an effect

That solution may lie in grasslands rich with herbal plants. “We sowed ribwort plantain seeds last spring. That plant contains tannins that bind with protein in the rumen, causing less protein to be broken down. And less protein breakdown means less ammonia. The ribwort plantain also contains substances that have the potential in principle to reduce methane emissions.” Back to the cow for a moment. A covered barn measuring 3,000 square metres is not the same as a field of grass measuring 5 hectares. Could the cows’ natural grazing behaviour in a field affect what happens in the rumen?

This sensor in the feeding station measures methane emissions. Photo: Wageningen Livestock Research

Cows snacking from the feeding station. Photo: Wageningen Livestock Research

“That is a really interesting question.” Klootwijk simulates a cow grazing by making a grinding motion with her hand and wrist. “In the field, a cow pulls up the grass with a twist of its tongue. The relevant aspect for methane emissions is that when grazing in a field, the cow itself selects the grass and the depth to which it grazes. Once we start bringing freshly mown grass to the barn, it’s us who are choosing the cutting height, and the cows eat everything.”

Furthermore, the quality of a blade of grass is not the same along the entire length of the blade. So whether the cow eats just the tip or the entire stalk will also affect methane emissions. In addition, there is a strong day effect in grass, says Klootwijk. “The sugar content is higher and the protein content lower in the evening. That means the time of day when you mow also affects the grass quality and consequently methane emissions. All these factors could influence the digestion process in the rumen and the resulting methane emissions. So it’s not simply a question of whether to put cows out to pasture, give them fresh grass in a barn or feed them silage. We’re interested in how you can influence methane emissions by playing with the quality: how should you harvest the grass, at what time of day and how tall should it be to minimize the methane emissions?”

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