Stewart Ledgard

Nitrogen inputs and losses from clover/grass pastures grazed by dairy cows, as affected by nitrogen fertilizer application

Nitrogen (N) inputs and outputs were measured over 3 years in a trial with four farmlets (each with 16 randomly-allocated 0·4 ha paddocks) on permanent white clover/ryegrass pastures which were grazed throughout the year by dairy cows near Hamilton, New Zealand. Three farmlets were stocked at 3·3 cows/ha and received nominal rates of N fertilizer (urea in 8–10 split applications) of 0, 200 or 400 kg N/ha per year. A fourth farmlet with 4·4 cows/ha received 400 kg N/ha per year and was supplemented with maize grain during the first two years. Nitrogen balances were calculated, with [sum ]N inputs[ape ][sum ]N outputs. Annual inputs from N 2 fixation were 99–231 kg N/ha in the 0 N farmlet, but declined to 15–44 kg N/ha in the 400 N farmlets. The main N outputs (in kg N/ha per year) were in milk (72–126), nitrate leaching (20–204), and transfer of N via cow excreta from pastures to lanes and milking shed (54–92). Gaseous losses by denitrification (3–34) and volatilization (15–78) were smaller than the other N outputs but increased significantly with N fertilizer application. In the maize-supplemented farmlet, N outputs in milk were 31% higher than in the corresponding non-supplemented 400 N farmlet, whereas leaching losses averaged 17% lower during the 2 years of supplementation. In the N-fertilized farmlets, estimated N balances were influenced by inclusion of the transitional N processes of immobilization of fertilizer N into the soil organic N pool (estimated using 15 N at 42–94 kg N/ha per year) and the contribution from mineralization of residual clover-fixed N in soil not accounted for in the current estimates of N 2 fixation (estimated at up to 70% of measured N 2 fixation or 46 kg N/ha per year). However, these processes were counteracting and together were calculated to have only a small net effect on total N balances. The output of N in products (milk, meat and feed) relative to the total N input averaged 26% in the 400 N farmlets, and is compared to that measured for commercial intensively-managed dairy farms in England and the Netherlands (14–20%). The 0 N farmlet, which was reliant on N 2 fixation as the sole N input, was relatively very N-efficient with the milk production being 83% of that in the 400 N farmlet (at 3·3 cows/ha) and the N output in products relative to total N input averaging 52%.

Nitrogen cycling in low input legume-based agriculture, with emphasis on legume/grass pastures

Low input legume-based agriculture exists in a continuum between subsistence farming and intensive arable and pastoral systems. This review covers this range, but with most emphasis on temperate legume/grass pastures under grazing by livestock. Key determinants of nitrogen (N) flows in grazed legume/grass pastures are: inputs of N from symbiotic N2 fixation which are constrained through self-regulation via grass/legume interactions; large quantities of N cycling through grazing animals with localised return in excreta; low direct conversion of pasture N into produce (typically 5–20%) but with N recycling under intensive grazing the farm efficiency of product N: fixed N can be up to 50%; and regulation of N flows by mineralisation/immobilisation reactions. Pastoral systems reliant solely on fixed N are capable of moderate-high production with modest N losses e.g. average denitrification and leaching losses from grazed pastures of 6 and 23 kg N ha−1 yr−1. Methods for improving efficiency of N cycling in legume-based cropping and legume/grass pasture systems are discussed. In legume/arable rotations, the utilisation of fixed N by crops is influenced greatly by the timing of management practices for synchrony of N supply via mineralisation and crop N uptake. In legume/grass pastures, the spatial return of excreta and the uptake of excreta N by pastures can potentially be improved through dietary manipulation and management strategies. Plant species selection and plant constituent modification also offer the potential to increase N efficiency through greater conversion into animal produce, improved N uptake from soil and manipulation of mineralisation/immobilisation/nitrification reactions.

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.

Nitrogen fixation by white clover in pastures grazed by dairy cows: Temporal variation and effects of nitrogen fertilization

Effects of rate of nitrogen (N) fertilizer and stocking rate on production and N2 fixation by white clover (Trifolium repens L.) grown with perennial ryegrass (Lolium perenne L.) were determined over 5 years in farmlets near Hamilton, New Zealand. Three farmlets carried 3.3 dairy cows ha−1 and received urea at 0, 200 or 400 kg N ha−1 yr−1 in 8–10 split applications. A fourth farmlet received 400 kg N ha−1 yr−1 and had 4.4 cows ha−1.There was large variation in annual clover production and total N2 fixation, which in the 0 N treatment ranged from 9 to 20% clover content in pasture and from 79 to 212 kg N fixed ha−1 yr−1. Despite this variation, total pasture production in the 0 N treatment remained at 75–85% of that in the 400 N treatments in all years, due in part to the moderating effect of carry-over of fixed N between years.Fertilizer N application decreased the average proportion of clover N derived from N2 fixation (PN; estimated by 15N dilution) from 77% in the 0 N treatment to 43–48% in the 400 N treatments. The corresponding average total N2 fixation decreased from 154 kg N ha−1 yr−1 to 39–53 kg N ha−1 yr−1. This includes N2 fixation in clover tissue below grazing height estimated at 70% of N2 fixation in above grazing height tissue, based on associated measurements, and confirmed by field N balance calculations. Effects of N fertilizer on clover growth and N2 fixation were greatest in spring and summer. In autumn, the 200 N treatment grew more clover than the 0 N treatment and N2 fixation was the same. This was attributed to more severe grazing during summer in the 0 N treatment, resulting in higher surface soil temperatures and a deleterious effect on clover stolons.In the 400 N treatments, a 33% increase in cow stocking rate tended to decrease PN from 48 to 43% due to more N cycling in excreta, but resulted in up to 2-fold more clover dry matter and N2 fixation because lower pasture mass reduced grass competition, particularly during spring.

Fate of 15N labelled urine on four soil types

A field lysimeter experiment was conducted over a 406 day period to determine the effect of different soil types on the fate of synthetic urinary nitrogen (N). Soil types included a sandy loam, silty loam, clay and peat. Synthetic urine was applied at 1000 kg N ha-1, during a winter season, to intact soil cores in lysimeters. Leaching losses, nitrous oxide (N2O) emissions, and plant uptake of N were monitored, with soil 15N content determined upon destructive sampling of the lysimeters. Plant uptake of urine-N ranged from 21.6 to 31.4%. Soil type influenced timing and form of inorganic-N leaching. Macropore flow occurred in the structured silt and clay soils resulting in the leaching of urea. Ammonium (NH4+–N), nitrite (NO2-–N) and nitrate (NO3-–N) all occurred in the leachates with maximum concentrations, varying with soil type and ranging from 2.3–31.4 μg NH4+–N mL-1, 2.4–35.6 μg NO2-–N mL-1, and 62–102 μg NO3-–N mL-1, respectively. Leachates from the peat and clay soils contained high concentrations of NO2-–N. Gaseous losses of N2O were low (<2% of N applied) over a 112 day measurement period. An associated experiment showed the ratio of N2–N:N2O–N ranged from 6.2 to 33.2. Unrecovered 15N was presumed to have been lost predominantly as gaseous N2. It is postulated that the high levels of NO2-–N could have contributed to chemodenitrification mechanisms in the peat soil.

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.

An analysis of environmental and economic implications of nil and restricted grazing systems designed to reduce nitrate leaching from New Zealand dairy farms. I. Nitrogen losses

Abstract Nitrate leaching is perceived to be a serious consequence of dairy farming due to the uneven return of N in small concentrated urine patches. Management systems in which the direct deposition of urine is avoided throughout the year (nil grazing) or during autumn/winter when the risk of nitrate leaching is highest (restricted grazing) could potentially reduce nitrate leaching. However, possible disadvantages of such systems include a reduction in the clover content of pastures, increases in gaseous losses, and increases in capital and/or operating costs. This paper examines some of the effects of nil and restricted grazing management systems for dairy farming on N flows and losses to the environment. The estimates of N losses are based on the results of a long‐term farmlet study under conventional grazing, on data for an average New Zealand farm, and on literature information. The analysis showed that in nil grazing systems nitrate leaching losses may be reduced by 55–65% compared with conventional grazing systems, and by 35–50% in restricted grazing systems. For nil grazing systems, however, total N losses were 10–35% higher than under conventional grazing because of increased gaseous losses. The total N losses from restricted grazing systems were similar (‐10 to +5%) to those from conventional systems. The analysis showed the potential benefit of a restricted grazing system as a management tool to reduce nitrate leaching losses, especially in areas where contamination of ground and surface waters is ol particular concern.

Fate of urine nitrogen on mineral and peat soils in New Zealand

A field lysimeter experiment was conducted over 150 days to examine the fate of synthetic urinary nitrogen (N) applied to peat and mineral soils, with and without a water table. At the start of the winter season, synthetic urine labelled with 15N, was applied at 500 kg N ha−1. Plant uptake, leaching losses and nitrous oxide (N2O) fluxes were monitored. Total plant uptake ranged from 11% to 35% of the urine-N applied depending on soil type and treatment. Plant uptake of applied N was greater in the presence of a water table in the mineral soil. Nitrate-N (NO3--N) was only detected in leachates from the mineral soil, at concentrations up to 146 μg NO3--N mL−1. Presence of a water table in the mineral soil reduced leaching losses (as inorganic-N) from 47% to 6%, incrased plant uptake and doubled apparent denitrification losses. In the peat soils leaching losses of applied urine-N as inorganic-N were low (<5%). Losses of N as N2O were greater in the mineral soil than in the peat soils, with losses of 3% and <1% of N applied respectively after 100 days. Apparent denitrification losses far exceeded N2O losses and it is postulated that the difference could be due to dinitrogen (N2) loss and soil entrapment of N2.

Nitrous Oxide Emissions from New Zealand Agriculture – key Sources and Mitigation Strategies

In most countries, nitrous oxide (N2O) emissions typically contribute less than 10% of the CO2 equivalent greenhouse gas (GHG) emissions. In New Zealand, however, this gas contributes 17% of the nation’s total GHG emissions due to the dominance of the agricultural sector. New Zealand’s target under the Kyoto Protocol is to reduce GHG emissions to 1990 levels. Currently total GHG emissions are 17% above 1990 levels. The single largest source of N2O emission in New Zealand is animal excreta deposited during grazing (80% of agricultural N2O emissions), while N fertilizer use currently contributes only 14% of agricultural emissions. Nitrogen fertilizer use has, however, increased 4-fold since 1990. Mitigation strategies for reducing N2O emissions in New Zealand focus on (i) reducing the amount of N excreted to pasture, e.g. through diet manipulation; (ii) increasing the N use efficiency of excreta or fertilizer, e.g. through grazing management or use of nitrification inhibitors; or (iii) avoiding soil conditions that favour denitrification e.g. improving drainage and reducing soil compaction. Current estimates suggest that, if fully implemented, these individual measures can reduce agricultural N2O emissions by 7–20%. The highest reduction potentials are obtained from measures that reduce the amount of excreta N, or increase the N use efficiency of excreta or fertilizer. However, New Zealand’s currently used N2O inventory methodology will require refinement to ensure that a reduction in N2O emissions achieved through implementation of any of these mitigation strategies can be fully accounted for. Furthermore, as many of these mitigation strategies also affect other greenhouse gas emissions or other environmental losses, it is crucial that both the economic and total environmental impacts of N2O mitigation strategies are evaluated at a farm system’s level.

Nitrogen inputs and losses from New Zealand dairy farmlets, as affected by nitrogen fertilizer application: year one

Inputs and losses of nitrogen (N) were determined in dairy cow farmlets receiving 0, 225 or 360 kg N ha-1 (in split applications as urea) in the first year of a large grazing experiment near Hamilton, New Zealand. Cows grazed perennial ryegrass/white clover pastures all year round on a free-draining soil. N2 fixation was estimated (using 15N dilution) to be 212, 165 and 74 kg N ha-1 yr-1 in the 0, 225 and 360 N treatments, respectively. The intermediate N rate had little effect on clover growth during spring but favoured more total pasture cover in summer and autumn, thereby reducing overgrazing and resulting in 140% more clover growth during the latter period.Removal of N in milk was 76,89 and 92 kg N ha-1 in the 0, 225 and 360 N treatments, respectively. Denitrification losses were low (7–14 kg N ha-1 yr-1), increased with N application, and occurred predominantly during winter. Ammonia volatilization was estimated by micrometeorological mass balance at 15, 45 and 63 kg N ha-1 yr-1 in the 0, 225 and 360 N treatments, respectively. Most of the increase in ammonia loss was attributed to direct loss after application of the urea fertilizer.Leaching of nitrate was estimated (using ceramic cup samplers at 1 m soil depth, in conjunction with lysimeters) to be 13, 18 and 31 kg N ha-1 yr-1 in a year of relatively low rainfall (990 mm yr-1) and drainage (170–210 mm yr-1). Drainage was lower in the N fertilized treatments and this was attributed to enhanced evapotranspiration associated with increased grass growth.Nitrate-N concentrations in leachates increased gradually over time to 30 mg L-1 in the 360 N treatment whereas there was little temporal variation evident in the 0 (mean 6.4 mg L-1) and 225 (mean 10.1 mg L-1) N treatments. Thus, the 360 N treatment had a major effect by greatly reducing N2 fixation and increasing N losses, whereas the 225 N treatment had little effect on N2 fixation or on nitrate leaching. However, these results refer to the first year of the experiment and further measurements over time will determine the longer-term effects of these treatments on N inputs, transformations and losses.