R. M. Monaghan

Nutrient management in New Zealand pastures— recent developments and future issues

Abstract In this publication we review recent research and understandings of nutrient flows and losses, and management practices on grazed pastoral farms in New Zealand. Developments in nutrient management principles in recent years have seen a much greater focus on practices and technologies that minimise the leakage of nutrients, especially nitrogen (N) and phosphorus (P), from farms to the wider environment. This has seen farm nutrient management planning shift from a relatively small set of procedures designed to optimise fertiliser application rates for pasture and animal production to a comprehensive whole‐farm nutrient management approach that considers a range of issues to ensure both farm productivity and environmental outcomes are achieved. These include consideration of factors such as multiple sources of nutrient imports to farms, the optimal re‐use and re‐distribution of nutrient sources generated within the farm (such as farm dairy effluent), identification of the risks associated with applying various nutrient forms to contrasting land management units, and an econometric evaluation of farm fertilisation practices. The development of nutrient budgeting and econometric decision support tools has greatly aided putting these more complex whole‐farm nutrient management systems into practice. Research has also identified a suite of mitigation systems and technological measures that appear to be able to deliver substantial reductions in nutrient losses from pastoral farms. However, issues of cost, complexity, compatibility with the current farm system, and a perceived uncertainty of actual environmental benefits are identified as key barriers to adoption of some of these technologies. Farmers accordingly identified that their main requirement for improved nutrient management planning systems was flexibility in how they would meet their environmental targets. The provision of readily discernible information and tools defining the economic and environmental implications of a range of proven management or mitigation practices is a key requirement to achieve this.

The impacts of nitrogen fertilisation and increased stocking rate on pasture yield, soil physical condition and nutrient losses in drainage from a cattle‐grazed pasture

Abstract The effects of increasing nitrogen (N) fertiliser inputs, and associated cattle stocking rates, on pasture yield and composition, soil physical quality and nutrient losses in drainage were measured in an experiment on permanent white clover/ryegrass pastures in eastern Southland, New Zealand. Treatments were established on hydrologically‐isolated replicate plots (900 m2) where pastures received annual fertiliser N inputs of 0, 100, 200 or 400 kg ha–1 and were grazed throughout spring, summer, and autumn of each year by non‐lactating dairy cattle. Our aim was to determine if N fertilisation of cattle pastures led to the deterioration of pasture or soil quality, or to the excessive loss of nutrients in drainage over the 3–4 years after such land management started. Pasture and soil monitoring showed that N fertilisation and increased stocking rate resulted in large, but variable, increases in pasture yield, with little discernible effect on soil physical condition, as evidenced by twice‐yearly measurements of soil bulk density, percentage of soil pores >300 μm, soil macroporosity (volumetric percentage of pores >30 μm), hydraulic conductivity, and air permeability. A cyclical pattern of spring soil compaction followed by recovery over summer, autumn, and winter was evident in the 0–5 cm soil layer within all treatments. Mean annual pasture responses to applied fertiliser N were 14.8, 12.9, and 9.1 kg DM kg–1 N applied in the 100, 200, and 400 N treatments, respectively, with greater responses observed in spring than in autumn in 3 out of 4 years. N fertilisation significantly increased losses of nitrate‐N and Ca in drainage but had no significant effect on K, Mg, Na, sulphate‐S, Cl, and P drainage losses. Within the context of the potential for enriching groundwater supplies of domestic drinking water, these losses suggest that annual fertiliser N inputs should not exceed approx. 170 kg N ha–1 yr–1 at this site. Considered from the perspective of potential surface water enrichment with P and N promoting nuisance weed and algal growth, losses of N and P in drainage water exceeded currently accepted guidelines, especially for N. The responses measured in this study represent a system that has recently undergone an improvement in soil fertility along with a change from sheep to cattle grazing. We thus caution that our findings pertain to short‐term changes in soil and plant responses and may not accurately reflect those in a system that has been in long‐term (>20 years) equilibrium.

Prioritisation of farm scale remediation efforts for reducing losses of nutrients and faecal indicator organisms to waterways: a case study of New Zealand dairy farming.

The international competitiveness of the New Zealand (NZ) dairy industry is built on low cost clover-based systems and a favourable temperate climate that enables cows to graze pastures mostly all year round. Whilst this grazed pasture farming system is very efficient at producing milk, it has also been identified as a significant source of nutrients (N and P) and faecal bacteria which have contributed to water quality degradation in some rivers and lakes. In response to these concerns, a tool-box of mitigation measures that farmers can apply on farm to reduce environmental emissions has been developed. Here we report the potential reduction in nutrient losses and costs to farm businesses arising from the implementation of individual best management practices (BMPs) within this tool-box. Modelling analysis was carried out for a range of BMPs targeting pollutant source reduction on case-study dairy farms, located in four contrasting catchments. Due to the contrasting physical resources and management systems present in the four dairy catchments evaluated, the effectiveness and costs of BMPs varied. Farm managements that optimised soil Olsen P levels or used nitrification inhibitors were observed to result in win-win outcomes whereby nutrient losses were consistently reduced and farm profitability was increased in three of the four case study farming systems. Other BMPs generally reduced nutrient and faecal bacteria losses but at a small cost to the farm business. Our analysis indicates that there are a range of technological measures that can deliver substantial reductions in nutrient losses to waterways from dairy farms, whilst not increasing or even reducing other environmental impacts (e.g. greenhouse gas emissions and energy use). Their implementation will first require clearly defined environmental goals for the catchment/water body that is to be protected. Secondly, given that the major sources of water pollutants often differed between catchments, it is important that BMPs are matched to the physical resources and management systems of the existing farm businesses.

The effectiveness of a granular formulation of dicyandiamide (DCD) in limiting nitrate leaching from a grazed dairy pasture

Abstract The effectiveness of a granular formulation of dicyandiamide (DCD) in limiting nitrate leaching from a grazed dairy pasture in southern New Zealand is reported. Treatments were an untreated Control managed as standard farm practice, and+DCD with two or three applications of DCD per annum at a rate of 10 kg active ingredient (a.i.) ha‐1 per application. Each treatment had hydrologically‐isolated plots 12 m wide × 15m long with separate mole‐pipe drainage systems from which drainage waters were collected and analysed for nitrate and ammonium over a 4‐year period. Pasture production, grass N uptake, grass nitrate‐N concentrations and DCD losses in drainage were also measured over this period. The application of DCD showed a clear and consistent trend in reducing concentrations of nitrate‐N in autumn and early winter drainage. On an annual basis, the application of DCD reduced the amounts of N lost in drainage by between 21 and 56%, depending on the year of study. Calculated mean annual losses of nitrate‐N in drainage over the 4‐year period were 12.9 kgNha‐1 from the control and 6.8 kg N ha‐1 from the DCD treatments (P < 0.05), with the greatest losses in the May‐July period. The application of DCD had no significant effect on annual or seasonal pasture production across all measurement years, with pasture yields in the DCD‐treated plots being less than 1% greater than those observed in the control plots. The application of DCD had little consistent effect on the botanical composition of the sward. Cost‐benefit analysis suggests that the small pasture responses observed at this site would not cover the costs of applying DCD unless there were additional benefits such as a carbon credit for reduced nitrous oxide emissions. The application of DCD had the additional benefit of lowering grass nitrate‐N concentrations on 22 of the 31 measurement dates. Between 2 and 16% of the DCD applied annually to the +DCD treatment was lost in drainage, representing c. 7% of applied DCD over the 4 years of measurement. Based on measured soil temperatures at this site and the observed monthly pattern of N loss in drainage, it is suggested that the scheduling of two autumn applications of DCD (e.g., at the March and May grazings) is the most effective strategy for minimising N losses in drainage at this site.

Land-use impacts and water quality targets in the intensive dairying catchment of the Toenepi Stream, New Zealand.

Abstract Water quality monitoring in Toenepi Stream, New Zealand, started in 1995 in a study of dairy farming influences on lowland stream quality and has continued since then with brief interruptions. Surveys have provided information about changes in farm and soil management practices as they relate to environmental sustainability. Although average water quality in Toenepi Stream has changed little during 1995–2004, there have been some notable improvements. Water clarity measured by black disc has improved from 0.6m to 1.5m, and median ammonia‐N and nitrate‐N concentrations have declined by 70% and 57%, respectively. The frequency and magnitude of extreme concentrations have declined—most notably for nitrogen (N) forms, which also had decreased mean values. Specific yields for suspended solids (SS) and phosphorus (P) forms in 2002–04 were 47–67% of 1995–97 values, mainly because of lower water yields. Reduced specific yields for N forms in 2002–04 (34–37% of 1995–97 yields) were also attributable to lower mean concentrations in stream water. Faecal bacteria concentrations have not abated and are on average 2–3 times recommended guideline values for contact recreation. Fewer dairy farms and an increased proportion irrigating dairyshed effluent to land, rather than discharging it to the stream via two‐pond systems, were likely causes of improvement in water quality. Water quality targets were developed for Toenepi Stream to achieve contact recreation criteria for the Piako River (downstream) and for intrinsic habitat values for Toenepi Stream. A range of mitigation measures has been formulated to meet these targets, but substantial uptake of sustainable farming practices is needed to improve water quality in Toenepi Stream.

Minimising surface water pollution resulting from farm‐dairy effluent application to mole‐pipe drained soils. II. The contribution of preferential flow of effluent to whole‐farm pollutant losses in subsurface drainage from a West Otago dairy farm

Abstract To evaluate the role of artificial drainage systems in the transfer of nutrients and faecal organisms from soil to waterways, mole‐pipe drainage flows were monitored from two large (27 × 40 m), hydrologically isolated field plots that were part of a long‐term dairy pasture in West Otago, New Zealand. One plot was grazed only whilst the other plot was spray irrigated with untreated dairy‐shed effluent on seven occasions, shortly following grazing events in spring, summer or autumn. Monitoring throughout the 4‐year study showed that relatively large amounts of ammonium‐N, total‐P, and Escherichia coli (E. coli) bacteria were transported through the artificial drainage system via direct drainage of effluent following three of the seven effluent irrigation events. These effluent drainage events occurred when the maximum depth of effluent application exceeded the soil water deficit measured in the 0–45‐cm layer. A pronounced non‐uniform pattern of effluent application was observed, with areas to the outside of the irrigator run effectively receiving double the average application depth, and at an instantaneous rate greater than 100 mm h−1. Although the volumes of effluent transported in drainage flow were relatively small, the concentrations and loads of ammonium‐N, total‐P, and E. coli bacteria in the resulting drainage were large. Based on the measured irrigation uniformities and soil water balance calculations, a simple model of effluent flow through mole‐pipe drained soils was developed. Model outputs indicated that two key management strategies could avoid or reduce the transport of pollutants from these effluent‐treated soils, specifically (i) increasing irrigator groundspeed and (ii) storing effluent when soil conditions are wet. Failure to implement these management practices is likely to result in the delivery of large amounts of pollutants to surface waterways when flows in the receiving surface waters are low and temperatures relatively high.

Soil phosphorus concentrations to minimise potential P loss to surface waters in Southland

Abstract Losses of soil‐derived P in overland flow induced by artificial rainfall were measured from six pastoral soils under a range of soil Olsen P concentrations. These soils were selected as being typical of the major soil groups that cover much of lowland Southland. Our objectives were to establish the magnitude and patterns of soil P release to overland flow under a wide range of soil Olsen P concentrations, established by amendment with either P fertiliser or dairy manure. The incorporation of superphosphate or manure into soils increased the soil Olsen P concentration as well as the concentration of P fractions in overland flow. There appeared to be no difference in the trend of P loss relative to Olsen P regardless of the form of P amendment. The magnitude of P loss appeared to be influenced by soil pedological origin, with lower dissolved reactive P (DRP) concentrations measured in overland flow from the Brown soils, compared to the less weathered Recent and Pallic soils. Regression analyses indicated that DRP concentrations in overland flow exceeded 0.02 mg DRP litre−1 when soil Olsen P values were greater than 51, 24, 10, 9, 5, and 20 mg kg−1 for the Woodlands, Waikiwi, Mataura, Northope, Pukemutu, and Waikoikoi soils, respectively. Given that eutrophication is likely to be encouraged when DRP concentrations in overland flow exceed this value of 0.02 mg litre−1, we suggest that soil Olsen P concentrations should be kept at or as close to the agronomic optimum as possible to minimise potential soil‐derived P losses and still maintain pasture productivity. Economic analysis indicated that significant savings could be made by keeping to the agronomic optimum, avoiding the unnecessary increase in soil Olsen P concentration and consequent increased environmental risk from P loss.

Nitrogen and phosphorus losses in overland flow from a cattle‐grazed pasture in Southland

Abstract In response to local concerns about the off‐site impacts of intensive cattle grazing on surface water quality, a field study was conducted to quantify N and P losses in overland flow from a cattle‐grazed pasture in eastern Southland. Overland flow from drained and undrained areas of a silt loam soil was collected over a 3‐year period using a surface guttering system connected to 0.5‐litre tipping buckets for recording flow volumes. Results indicated that total overland flow was between two to seven times greater from the undrained than from the drained soil. Losses of N and P in overland flow from drained soils were low relative to that measured in subsurface drainage from these soils. Much of the overland flow, and overland N and P loss, occurred during winter and spring, rather than summer and autumn when annual P fertiliser applications were made. Overland flow and losses of N and P were greatest from the undrained soil receiving 400 kg N ha−1 year−1 and stocked at the equivalent of 3.1 cows ha−1. These N and P losses in overland flow were greatest within the 1‐ to 2‐week period following spring grazing, rather than the period shortly following P fertiliser applications. These increased losses were probably caused by a combination of (i) the return of nutrients via dung and urine, and (ii) the decline in soil structural condition often observed during grazing under wet soil conditions. Managements targeted to minimise soil‐treading damage during spring, and thus maintain good soil infiltration, are therefore suggested as helpful strategies for minimising N and P losses via overland flow, particularly from heavy, poorly‐drained soils in Southland.

Potential phosphorus losses in overland flow from pastoral soils receiving long-term applications of either superphosphate or reactive phosphate rock

Abstract The forms and concentrations of P in overland flow were measured from intact pastoral soils obtained from the Winchmore long‐term P fertiliser trial. Treatments under evaluation were soils that received either 0, 188, 250 or 376 kg superphosphate ha−1 yr−1, or 175 kg reactive phosphate rock (RPR) ha−1 yr−1. The objective was to determine the magnitude of potential P transfers from soil to water following P fertilisation, and to determine if losses were different following RPR fertilisation compared with superphosphate. Overland flow was induced by the application of artificial rainfall at 15 mm h−1, maintained for 1 h after flow commenced. Concentrations of dissolved reactive P (DRP) and total P (TP) mirrored the long‐term application rates, although prior to a fresh application of P, soils with P applied in RPR form lost more P during an event than soils with the same rate of P applied as superphosphate. After a fresh application of RPR and superphosphate treatments, up to 5.4 mg TP litre−1 was lost in flow from the 376 kg superphosphate ha−1 yr−1 treatment, while P in flow from soils fertilised with RPR were commonly c. 0.11 mg litre−1, but still greater than from the unfertilised control soils (0.02 mg litre−1). Regression analysis indicated that DRP concentrations in flow from the fertilised soils were elevated above that lost before fertiliser application for a period of approximately 60 days. These results support earlier studies that demonstrate the greater risks of incidental P losses from soluble P fertilisers such as superphosphate (up to 60 days), and conversely the potential environmental benefits from RPR fertilisation of soils “at‐risk” of P loss (e.g., where much overland flow occurs such as in very wet soils and near stream channels). However, if good management practice is followed then the difference in P loss between superphosphate and RPR treated soils should be minimal over a period of a year.

Nitrogen and phosphorus losses in mole and tile drainage from a cattle-grazed pasture in eastern Southland

Abstract An experimental system for monitoring drainage outflows from mole‐ and tile‐drained plots is described, and nitrogen (N) and phosphorus (P) losses in drainage are reported for Year 1 of a 4‐year study examining nutrient losses in drainage from a pasture in Southland. Twelve plots (0.09 ha), grazed by non‐lactating dairy stock, were artificially drained by installing a mole and tile drainage network. A metering station was used to monitor drainage flow rate from six of these plots using a V‐notch weir and a shaft encoder system. Drainage water samples were collected on a flow proportional basis using either an automated water sampler triggered by the flow monitoring system, or by manual collection during daylight hours. The amount of nitrate‐N lost in drainage water in this first year of study was 25 kg N ha–1, resulting in a volume‐averaged nitrate‐N concentration of 6.9 mg N litre–1. Although this is a significant loss of potentially plant available N, the average nitrate‐N concentration of the drainage water was below the 11.3 mg N litre–1 standard adopted by the New Zealand Ministry of Health for acceptable nitrate levels in drinking water. Mean dissolved reactive P and total P concentrations in drainage waters were 23 and 74 μg P litre–1, respectively. Analysis of forms of P showed 61 % of the total P lost in the drainage was in the form of particulate P, which may reflect the recent introduction of mole and tile drainage to this site.