Dairying and Peat Soils

June 2018

Understanding dairy production systems on peat soils in Waikato

All peat soils have been classified as high risk for effluent irrigation but farmer-led research has shown that there are significant differences between well developed, consolidated peat and relatively undeveloped peat.

A Sustainable Farming Fund project that looked at N and P movements in Waikato peat soils showed a dramatic response to N on raw peat and more typical responses on consolidated peat. There are also differences in leaching losses as measured by lysimeters and in drainage water.

Practical guidelines for assessing the risk status of peat paddocks have been developed, allowing farmers to categorise their farms’ soils and identify safer sites for irrigation of effluent. Farms with sufficient low-risk areas for irrigation may require less effluent storage, leading to considerable savings in costs. Damming drains in summer helps to stop the peat drying out and oxidising and supports pasture persistence by keeping the water table close to the root zone.

Liming trials have been started to determine whether this suppresses insect pests and helps pastures to persist.

There are 94,000ha of peat soils in Waikato of which 75,000ha are farmed. Dairy farmers are faced with strict rules around timing and extent of effluent irrigation. Sufficient storage is required during wet weather when irrigation would result in ponding and loss of nutrients to groundwater and waterways through runoff. This is not permitted under Waikato Regional Council rules.

Peat soils are generally classified as being high risk, meaning that both storage ponds and effluent irrigation areas need to be large and therefore expensive. However, some farmers on peat disagree with the blanket “high risk” categorisation, arguing that peat soils vary considerably in content and character depending on the parent raw material and on how long the area has been farmed. Some areas have been in pasture for 50 years or more and it is argued that these consolidated peat soils behave more like mineral soils and constitute a less risky irrigation environment.

Raw peat soils are very high in fibrous organic matter, very acid and are subject to a fluctuating water table that can be high in winter and spring. Consolidated peat that has been farmed for many decades is less acid, has more biological activity in the top 100 mm and is generally well drained and may be less subject to high water table levels for long periods.

Ian Taylor, a dairy farmer at Gordonton near Hamilton, farms well developed consolidated peat. In 2012 he was faced with the prospect of having to construct a huge effluent storage pond. Little research had been done on nutrient losses from peat and so, with the help of DairyNZ, AgResearch, Ballance Agrinutrients and the Waikato Regional Council, a research proposal was put together. MPI, through the SFF, funded the project that looked at the ways nutrients from fertilisers and effluent move through different peat soils and the ongoing effects of those nutrients reaching waterways.

Phil Irvine, DairyNZ regional leader for North Waikato, says that the farmer-led project involved a number of farms on consolidated peat that had been developed for over 40 years. These were contrasted with Orini Downs Station where the soils are younger and the peat is fairly raw and has a lot of fibrous material. “The initial trials involved applying increasing amounts of nitrogen. Pasture response in spring on consolidated peat tapered off after 25kg/ha but on the undeveloped peat it increased almost linearly up to 75kg,” he says.

“So the consolidated peat appears to behave much more like a mineral soil where there is a significant background supply of N and pasture growth is already high. On the undeveloped peat there is little in the way of N reserves and so it responds well to putting on more N but we didn’t know whether this would increase N leaching losses, so lysimeter trials and drain water analyses were then undertaken.”

Leaching of N, P and E.coli through the soil profile on each farm was determined using lysimeters based at AgResearch, and annual losses were estimated by analysing water from the farms drains.

  • Annual N loss per hectare from the undeveloped peat was about one fifth that of developed peat, as expected. Ammonia-N levels in almost all samples were excellent or satisfactory according to Waikato Regional Council guidelines.
  • Annual P loss per hectare from the undeveloped peat was considerably higher than from developed peat. This requires further investigation. Was it due more to the very wet winter or to the low anion storage capacity (ASC) of the raw peat as shown by soil tests.   This may have implications for P fertiliser practices on raw peat soils.
  • Levels of E. coli in the majority of water samples from both farms were “excellent” or “ satisfactory” according to WRC guidelines.

Unfortunately the recent winter and spring were particularly wet and the results may not be typical of a normal year, so the group has been granted funding for a further year’s lysimeter measurements.

Harry Rich, Equity Manager of Orini Downs that runs a dairy herd of 2200 cows on relatively undeveloped peat, says that knowing how much phosphate they are losing has led to the use of more organic and slow release forms of P fertiliser. “We have had access to Biophos from Fonterra, which is a product from the wash water at dairy plants, and we are hoping that it will reduce the phosphate loss if we apply it a little and often. The other thing that may help is using RPR because its breakdown is a lot slower,” he says.

“We have also realised that a stronger sward will retain nutrients better and so we need to keep the moisture level in the peat close to the root zone in summer so that pastures will survive better. Last year and this year we put something like 30 dams in the drains to retain water. We have also learnt from the trial work that if we can keep the water table up, there will be less oxidation and loss of peat soil.”

On maize crops strip tilling has been used to reduce oxidation and loss of peat caused by cultivation. A machine cultivates a strip 100 to 150 mm wide and sows seeds, disturbing only a small portion of the ground. Harry says that they have increased the amount of maize grown from 50 to 70ha and may grow more as an alternative to PKE. They are also considering growing maize on the same areas for several years with crops of short rotation annuals between maize harvest and sowing.

On well developed peat near Gordonton, dairy farmer Ian Taylor is looking at reducing N losses by storing more spring effluent and using it to irrigate crops. Ian was the original instigator of the project and was able to get some of his property reclassified as low risk and so reduce the required storage capacity, but in the end he opted for a pond that was almost double the required size.

“Rather than go for the absolute minimum I was keen to build in contingency on the basis that the rules may change in the future, and also after last winter and spring having been so wet I'm now thinking about building a stand-off area, so the larger pond will cater for that as well,” says Ian. “Once the new pond is in use this spring I will have some new cropping options such as irrigating the effluent on to the cropping paddocks maybe in November and December, which I couldn't do without having storage. It means I can use the effluent at a more appropriate time of the year on a crop that would benefit from having both nutrients and water heading into the summer. With the pressure that is coming on all farmers now I can see that I need to be operating at best practice in terms of effluent application, and that means not putting effluent on when the ground is saturated under the conditions like we had last year.”

Ian has also had a farm environment plan drawn up by Farm Source Sustainability Advisers and he says that has raised his level of awareness of practices that he needs to reconsider or change. For example, being aware of run-off from around the farm dairy and from races and high traffic areas into waterways close by. Mitigating that isn't going to be easy, he says, and there may need to be a grass or other buffer between the race and the drain to filter out nutrients before they get into the waterway.

Phil Irvine believes that the project is a good example of how, with a bit of outside technical help and facilitation, farmers can get together to solve local problems.

“There have been some very positive outcomes including practical tools that peat farmers can use to classify their paddocks and manage water and effluent. One is a guide to determining the risk status of peat soils, which Landcare Research was contracted to produce. Another is a simple yet very practical tubular float developed by Ken Oliver, which sits in the peat and shows the level of the water table,” says Phil. “Ongoing work should clarify nutrient losses and how to mitigate them, and also come up with ways of improving pasture persistence.”