Plantain has been pushed as a solution to nitrogen leaching on farms. Photo: Supplied
A new study is debunking research that suggested the common grazing herb plantain could mitigate nitrogen leaching on farms.
The science review published in the New Zealand Journal of Agricultural Research examined 15 years of peer-reviewed research into the grazing herb and pasture species.
In the past decade, there has been a push for farmers to take up plantain in their paddocks to reduce nitrogen leaching from their livestock into the land and surrounding waters.
Urine from livestock is a source of nitrate or nitrogen leaching on farmland, which is associated with reduced water quality in New Zealand's rivers and lakes.
With greater impetus to improve environmental outcomes while maintaining milk production in farming, a stream of industry-backed and science-led research ensued.
The new study found some projects did not stand up to scrutiny in a New Zealand farming context, due to insufficient evidence, methodology or data interpretation.
One example was the calculation of the nitrogen concentration in an average pasture-fed dairy cow urine patch.
One formula of this concentration had been cited extensively across industry reports, including the Waikato Regional Council and the New Zealand Agricultural Greenhouse Gas Research Centre just this year, and in other plantain leaching studies.
However, the figure was generated from a study of lactating cows on a dairy farm in North Carolina in 1993 - excluding environmental variables in a pastoral farm context.
"The applicability of this figure to New Zealand pasture-fed dairy cows requires verification since it has major implications regarding the amount of N [nitrogen] lost through leaching vs the amount of N recycled," the report said.
Co-author Dr Tony Conner of Lincoln Genetics said there was little evidence the recommended 30 percent visual plantain in mixed pastures would have a direct benefit in terms of reducing leaching - and higher levels of the plant could contribute to poor animal health.
Meanwhile, one research paper tested nitrogen leaching from a 100 percent plantain diet.
Conner, a pastoral scientist formerly with AgResearch, said plantain helped dilute the animals' urine - though a similar effect was seen with other pasture species, such as ryegrass.
"[Plantain] got a lot of momentum early on," Conner said.
"I think ... initially [with] some of the experiments, a lot of hype was placed around them, and they looked really promising. But when you drill down to some of the key early papers, the experimental design wasn't the best, particularly some of the work around the lysimeters that were used."
Field lysimeters are cylinder containers filled with soil that study how water and material travel through the soil.
"They weren't really comparing apples with apples and the effects that were attributed to plantain may not necessarily have been due solely to plantain," he said.
"Field and lysimeter experiments to evaluate the effect of plantain in pasture on nitrate leaching are fraught with difficulties," the report said.
"Controlling variables such as water, winter growth, species composition, management, plantain persistence, establishment, and weed/clover ingress are problematic, and all can produce confounding data that fail to demonstrate a beneficial N leaching effect of plantain under farm conditions."
Conner said all that hype early on may not have been justified.
"There was a lot of political pressure on scientists to find a solution, a lot of commercial pressures on companies to have a solution that they can translate into products," he said.
"I feel it got caught up in a little bit of tunnel vision and enthusiasm to get through to where we are now, finding that some of the evidence is being challenged."
Plantain could still be considered among other pasture species in a cow's diet, he said.
"There is still a role of plantain in mixed pastures - we're not denying that. It does have a role, but not necessarily as a solution to nitrogen mitigation."
The authors proposed a re-examination of current recommendations around the use of plantain - to include other pasture management practices and species.
Industry body DairyNZ said it was focused on plantain to help reduce nitrogen loss because research had indicated it was working, but acknowledged farmers needed a range of options to manage healthy waterways.
A spokesperson said the group welcomed scientific debate in building more sustainability and resilience into dairy farming.
"While this is one paper presented from a certain perspective, equally there is research that demonstrates the value of plantain in helping to reduce nitrogen loss.
"Over four years, cumulative nitrogen loss at the Massey farmlet trial has been on average 26 percent lower from plantain pastures making up 18-47 percent of the sward and 12-30 percent of the diet, compared to a perennial ryegrass-white clover control.
"Over two years at Lincoln, cumulative nitrogen loss has been 17 percent lower from plantain pastures making up 15-30 percent of the sward."
Both parties agreed more research should be undertaken across other pasture species that also have potential to reduce nitrogen loss, the DairyNZ spokesperson said.
"However, the review does not reflect the full scale of evidence highlighting plantain's efficacy from a range of research projects, and in some cases has incorrectly interpreted research findings, and incorrectly reported the way in which DairyNZ has recommended plantain use. "
The Plantain Potency and Practice programme was undergoing its scheduled independent, mid-term review by MPI to test the robustness of the science and findings emerging from the programme.
DairyNZ was committed to finishing the programme, the spokesperson said.