NZ rivers may benefit from research into new pasture feed – but further tests are needed.
The use of the herb plantain in dairy farm pastures – likened to giving "lollies" to cows – has been shown in one test to reduce nitrate leaching by up to 90 per cent.
New research by a Massey University-led team has shown that incorporating the plant in a cow's diet could significantly help reduce the amount of nitrate leaching into New Zealand's waterways.
Many farms are already applying management practices that reduce nitrate run off, but these studies give farmers more options and tools.
An initial trial, run over two months last year in the Tararua district of the Manwatu-Wanganui region, recorded a drop in nitrate leaching of 90 per cent. A follow up test, held over a longer nine-month period, resulted in a reduction of 66 per cent.
A researcher involved in the project, Soledad Navarrete, a post-doctoral fellow at the university, says the first tests, which took place in March and April 2017, "suggested plantain had the potential to reduce nitrate leaching by 90 per cent through a lower concentration of nitrate in a cow's urine."
Subsequent testing between September 2017 and June this year also showed significant gains resulting in the 66 per cent drop.
Navarrete put the variability between trials down to the fact the first tests were carried out before winter in 2017 - a time when nitrate leaching is high - while the second trial was undertaken over a longer period.
But Navarrete says many more questions need to be answered before scientists will feel confident in recommending its widespread use on New Zealand dairy farms: "We need to better understand what proportion of plantain is necessary in the sward [pasture] to provide the greater impact; the testing we are planning in next year's grazing season ought to give us this."
The Massey research is one of a number of plantain studies taking place around the world. In Tasmania, the leader of a project on dairy farms there, Adam Langworthy, says the crop is not only nutritious, it is also extremely palatable.
"For the cows, it's like having lollies sprinkled through the paddock," he says. "It helps to increase their consumption as they eat more pasture while searching for the plantain."
The Tasmanian project, which is partly funded by Dairy Australia, is, like the Massey trial, looking to establish the optimal sowing methods and rate of coverage of the crop per hectare.
And in the Netherlands a scientific article written by the plant sciences department at Wageningen University says including herbs like plantain in pastures has been shown to not only reduce nitrate leaching, but to reduce methane emissions by livestock.
Plantain works in several ways to reduce nitrogen leaching. 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 eaten is proportioned differently in an 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.
Navarrete says three pastures were used in the Massey trial each with different mixes – plantain only, a plantain-clover mix and ryegrass-white clover. Sixty cows grazed in each over eight consecutive days. The proportion of plantain used was 85 per cent (plantain only) and 65 per cent (plantain-clover mix).
Researchers measured pasture production, the nitrous oxide loss from the pasture and the nitrogen content in animal urine, faeces, blood and milk. During winter 2017 drainage water was collected and analysed.
"These results demonstrated large reductions in nitrate leaching," Navarrete says. "The study showed that plantain decreases the urinary nitrate and urea concentration of cows when compared to cows fed ryegrass. The incorporation of plantain in a cow's diet appeared to reduce the nitrate leaching."
Navarrete says the next step is to determine how much plantain an animal should be eating. To test this, the Massey team is preparing pastures for grazing next year by varying the proportion of plantain in each. It has yet to decide on the proportions but Navarrete says it will possibly be at levels of 20, 40 and 60 per cent.
Nitrate can have damaging effects on the health of waterways. Significant amounts can lead to excessive algal and aquatic plant growth which, by depleting water of oxygen or changing habitat, can reduce the number of sensitive species living in that waterway.
It has also been associated with toxic blooms in lakes, allowing invasive weeds to thrive and impact human recreation in rivers.
The Massey study is running at the same time as a farmer-led initiative in Tararua in which DairyNZ is working with 21 farmers to see if the results translate to larger catchment scale.