Plantain works in distinct ways: Its high water content means a cow's urine is more diluted, enabling the nitrate to spread over a greater area of pasture; the nitrogen consumed is portioned differently in the animal's body so less makes it into the urine; and mechanisms within the crop's root system locks more of the nitrate up in the soil.
While research has suggested cows eating plantain could lead to a reduction of leaching from dairy pastures by as much as 50 per cent, DairyNZ's Adam Duker, who is heading up the Tararua project, says it is too early to yet give definitive answers.
But he says the initial signs are promising: "We wouldn't be jumping on this if it wasn't showing potential. We think it may be possible ultimately to achieve reductions of between 30-40 per cent.
"This is a farmer-led initiative," he says. "They recognise they need to make changes and we see plantain as a 'new technology' that could, along with other methods, help achieve significant reductions.
"Ten years ago, plantain was used splutteringly by farmers as a productivity driver (it is able to source moisture at deeper soil levels than other grasses) and because of its health benefit for animals."
But recent research by DairyNZ, AgResearch and Massey and Lincoln universities has shown plantain may offer unique possibilities in reducing the nitrate leaching problem on farms.
The project has developed over the last seven years, involving significant investment and partnership between the agricultural sector and government. The research has moved from the laboratory to the paddock and ultimately the aim is to take it catchment scale as farmers and researchers learn what is possible and practical.
Duker, DairyNZ's catchment engagement leader, says while there are still unanswered scientific questions about why plantain is such an effective tool in mitigating nitrate loss, DairyNZ is working with 21 farmers in Tararua to see if the results can be successfully applied on a larger farm and catchment scale.
He says they have set a target of having plantain make up 30 per cent of the diet of cows between January and May.
Able to be grown either as a special-purpose crop or as a pasture mix with grass or clover, plantain allows farmers to maintain the pasture-based system for which New Zealand dairy farming is known.
Duker says while plantain has been proven in a research sense to reduce the nitrate loss on farms, the project is also aiming to establish how it works practically on farms.
Dairy farming is a major contributor to the Tararua district economy and the area has been chosen because it is a catchment at high risk from nitrate leaching. Duker says about 95 per cent of nitrate loss in the district comes from various agriculture land uses.
Within the Tararua district there are 135 farms unconsented under the Horizon Regional Council's 'One Plan' which requires all dairy farms to limit nitrate loss. The project will initially involve 21 of these farms and Duker is hopeful all will ultimately take part.
"The farmers we are working with want to show the council they are committed to finding a solution," he says.
Duker says research has shown that if farmers can achieve the target of plantain making up 30 per cent of a cow's diet, reductions in nitrate loss of up to 50 per cent could be achieved.
"In a farm context, a more realistic figure is suggested to be in the order of 20 per cent. When coupled with good management practices, this could achieve a 30-40 per cent reduction."
Nitrate can have damaging effects on the health of waterways. It has been shown to lead to excessive phytoplankton growth which, by depleting water of oxygen, potentially causes the death of some species; it can cause toxic blooms, allow invasive weeds to thrive and adversely impact human recreation in rivers.
