Every year, farmers choose the genetic make-up of the next generation of cattle by selecting semen from a catalogue showcasing the nation’s finest breeding bulls.

Typically, they’re looking for high milk production in the bull’s daughters, and cows of the right size and attributes for their farm – at a price they can afford.

But this year, farmers can also select lower-methane cattle from the menu, as well as ones that produce less river-polluting nitrogen for each litre of milk.

A note of caution: scientists haven’t yet found genetic markers identifying the climate-friendlier cows from the rest, so herd improvement company Livestock Improvement Corporation (LIC) has used an animal’s efficiency at turning its food into milk as a proxy for its environmental hoofprint. But the offer is part of a gradual shift towards trying to breed a national herd with less impact.

LIC supplies the semen used to breed most of New Zealand’s dairy herd, a process that happens via artificial insemination. The new index assigns bulls a score from 1 to 10, based on their progeny’s estimated methane output and nitrogen excretion.

In the absence of direct genetic markers, an animal’s efficiency at converting its own food into food for people is a fairly good proxy for the animal’s climate impact per litre of milk or kilogram of butter, says Tony Fransen, LIC’s environment and welfare manager.

“Potentially, we could have a pack of butter sitting on the shelf in Tokyo that has a lower footprint than another pack from Australia or the U.S., so having the labelling to calculate the impact at a product level could be quite useful for us,” he says.

Government estimates suggest every kilo of dry food a cow eats produces 21 grams of methane – so the more a cow must chow to make the same amount of milk, the higher its likely climate impact.

As well as methane from the snout, there’s an environmental double-whammy from nitrogen wasted from a cow’s rear end. When nitrogen-laced cattle urine lands on paddocks, particularly in the wet season, it can end up in waterways, and it also makes a powerful greenhouse gas, nitrous oxide.

By breeding more efficient herds, farmers might find they can shrink the number of cows on their farm and still get the same amount of milk, says Fransen – which could help meet water quality standards in polluted catchments.

Meanwhile, researchers are working on finding genetic markers for identifying lower-methane cows directly, to predict their output more accurately.

A small pilot trial looking for a link between the methane cows produce and their genetics is underway in the Waikato, run by LIC and another breeding and genetics company, CRV.

The taxpayer-funded New Zealand Agricultural Greenhouse Gas Research Centre (NZAGGRC) has funded the trial, which will test bulls by luring them with snacks so they place their heads in a bucket-like contraption called a Greenfeed. The machine sucks away their breath to see if some animals are producing less methane than others. If it works, a bigger trial of 300 bulls should start in February.

The story of New Zealand’s methane emissions is one of rising efficiency, but rising impact.

Since 1990, Fransen estimates LIC’s breeding bulls have lowered their output of methane per kilo of milk solids by 13 per cent, purely through breeding to increase efficiency. He hopes finding genetic markers for methane will accelerate the process.

However over the same period, government statistics show New Zealand’s overall methane output has risen, because the national herd has increased with dairy conversions. (Sheep, which were displaced by cows, have a lower methane output per animal than cattle).

The Ministry for Primary Industries’ Jennie Marks told an NZAGGRC seminar agriculture emissions had increased 17 per cent since 1990, making up 48 per cent of emissions today.

Agriculture Minister Damien O’Connor was asked whether the government would delay bringing farming into the Emissions Trading Scheme (ETS), while research continued in areas like methane-lowering feed supplements. So far, more than $100 million has been spent researching how to shrink farming emissions, including trialling low-methane feeds and a hoped-for methane vaccine.

Agriculture will enter the ETS in 2025, but only a small fraction of its emissions will need to be offset or paid for.

“We’ve got five years,” O’Connor said. “The fact we bring it in at only 5 per cent obligation is a huge concession.”

He said incorporating agriculture into the ETS was hugely important to trade negotiations with the likes of the United Kingdom and the European Union. “Don’t underestimate the positives of this.”

The hunt for lower-methane genes is already well underway for sheep. State-owned farming company Pamu has been breeding sheep specially selected to be lower-methane, in Rotorua.

After a few generations of selective breeding, sheep can produce ten per cent lower emissions than their high-emitting relatives, previous studies by AgResearch have found.

The challenge now is to transfer those gains to cattle. Cows have a higher methane output than sheep but also cost more to study. Not only do the animals themselves cost more, they typically need to go in bigger full-body respiration chambers to measure their gases.

Fransen says, if cows gain their own genetic markers for methane, it would be quick to translate the results into dairy herds. “While sheep have the advantage of initial measurements, once we get those for cattle, we’ll have the advantage of being able to roll out through our AI programmes (quickly),” he says.

“The (LIC) tool is already set up so if there was genetic value for methane emissions, I reckon within 24 hours I could have it in the tool,” says Fransen.

Meanwhile, lower-nitrogen cows could also become a reality.

Lincoln University researchers have confirmed farmers could breed cows that pee less nitrogen, a benefit CRV Ambreed claimed could add up to improvements of 10-12 per cent by 2025.

In a potential bonus for farmers, the cows seemed to convert the otherwise wasted nitrogen into milk protein.

While new opposition leader Todd Muller has bristled at suggestions of shrinking the national herd, previous Lincoln research found having fewer cows and feeding them better lowered nitrogen run-off hugely, for similar profit, a finding backed up by other studies trialling less intensive farming methods.

As for the new options in the catalogue, Fransen says he isn’t expecting an instant change in the dairy herd, but seeing the options will get farmers thinking.

“I’m expecting farmers to see it, become aware of it, be ready to ask questions and then next year they might make decisions around it.”