Combining Restricted Grazing and Nitrification Inhibitors to Reduce Nitrogen Leaching on New Zealand Dairy Farms

Effect of zeolite administration on nitrogen metabolism and excretion in lactating dairy cows offered pasture herbage

Context Due to high protein concentrations in pastures, dairy cows offered a pasture-based diet often consume excess nitrogen (N), which leads to high ruminal ammonia concentrations and excessive urinary N excretion, thereby contributing to pasture N leaching. Aims To study the effect of administration of natural zeolite on ruminal pH and ammonia production and N excretion in lactating cows offered an all-pasture herbage diet. Methods In a metabolism stall trial using a crossover trial design, rumen-cannulated Friesian cows were administered either zero (Control, n = 16) or 400 g/day of zeolite (Zeolite; n = 16). Zeolite was divided into two equal portions and administered directly into the rumen before feeding fresh-cut ryegrass-clover herbage at 07:30 and 15:30 hours. Cows were kept in the metabolism stalls for two measurement periods of 5 days each, with each period preceded by an adaptation/washout period of 2 weeks. Feed intake, milk yield, total urine and faecal outputs were measured daily. During the last day of each measurement period ruminal fluid and blood were frequently sampled. Key results Zeolite administered at 2.2% of dry matter intake (DMI) did not affect daily DMI. Moreover, milk yield and milk composition, including milk urea, were not affected by zeolite administration. In cows administered zeolite the mean 24-h ruminal ammonia concentration was reduced by 1.5 mmol/L (9%) and the ruminal pH pattern in zeolite-administered cows over 24 h was above that of Control cows, but the overall effect on pH was not significant. Zeolite had no effect on plasma urea, total urinary N excreted or faecal N. Of the total N excreted across the groups, 21.7, 50.6 and 27.7% was excreted into milk, urine and faeces respectively. Conclusions Zeolite administration reduced ruminal ammonia concentration but this did not result in reduced urinary N excretion in dairy cows offered pasture. Implications Dietary supplementation with zeolite may help to improve aspects of ruminal function in cows consuming pasture, but is unlikely to be an effective tool for reducing N leaching from pastures.

Benefits and Trade-Offs of Dairy System Changes Aimed at Reducing Nitrate Leaching

Simple Summary

Reducing inputs of nitrogen fertiliser and purchased feed, with an associated reduction in stocking rate on pastoral dairy farms resulted in less nitrate leaching. A co-benefit was a reduction in greenhouse gas emissions. The exception was the implementation of a wintering barn where nitrate leaching was reduced, but greenhouse gas emissions remained unchanged due to greater manure storage and handling. Emission reductions in the lower-input systems came at an average loss of profit of approximately NZ$100 per tonne CO₂-equivalent.

Abstract

Between 2011 and 2016, small-scale farm trials were run across three dairy regions of New Zealand (Waikato, Canterbury, Otago) to compare the performance of typical regional farm systems with farm systems implementing a combination of mitigation options most suitable to the region. The trials ran for at least three consecutive years with detailed recording of milk production and input costs. Nitrate leaching per hectare of the milking platform (where lactating cows are kept) was estimated using either measurements (suction cups), models, or soil mineral nitrogen measurements. Post-trial, detailed farm information was used in the New Zealand greenhouse gas inventory methodology to calculate the emissions from all sources; dairy platform, dairy support land used for wintering non-lactating cows (where applicable) and replacement stock, and imported supplements. Nitrate leaching was also estimated for the support land and growing of supplements imported from off-farm using the same methods as for the platform. Operating profit (NZ$/ha/year), nitrate leaching (kg N/ha/year), and greenhouse gas emissions (t CO₂-equivalent/ha/year) were all expressed per hectare of milking platform to enable comparisons across regions. Nitrate leaching mitigations adopted in lower-input (less purchased feed and nitrogen fertiliser) farm systems reduced leaching by 22 to 30 per cent, and greenhouse gas emissions by between nine and 24 per cent. The exception was the wintering barn system in Otago, where nitrate leaching was reduced by 45 per cent, but greenhouse gas emissions were unchanged due to greater manure storage and handling. Important drivers of a lower environmental footprint are reducing nitrogen fertiliser and purchased feed. Their effect is to reduce feed flow through the herd and drive down both greenhouse gas emissions and nitrate leaching. Emission reductions in the lower-input systems of Waikato and Canterbury came at an average loss of profit of approximately NZ$100/t CO₂-equivalent (three to five per cent of industry-average profit per hectare).

Multiple-stressor effects of dicyandiamide (DCD) and agricultural stressors on trait-based responses of stream benthic algal communities

Agricultural practices often result in multiple stressors affecting stream ecosystems, and interacting stressors complicate environmental assessment and management of impacted streams. The nitrification inhibitor dicyandiamide (DCD) is used for nitrogen management on farmland. Effects of leached DCD on stream ecosystems are still largely unstudied, even though it could be relevant as a stressor on its own or in combination with other agricultural stressors. We conducted two experiments in 128 outdoor stream-fed mesocosms to assess stressor effects on biomass, cell density, taxon richness, evenness and functional trait composition of benthic algal communities. First, we examined responses to a wide DCD gradient (eight concentrations, 0-31 mg L^-1) and two additional stressors, deposited fine sediment (none, high) and nutrient enrichment (ambient, enriched). Second, we determined algal responses to four stressors: DCD, sediment, nutrients, and reduced flow velocity. Here DCD treatments included controls, constant application (1.4 mg L^-1) and two pulsed treatments mimicking concentration patterns in real streams (peaks 3.5 mg L^-1, 2.2 mg L^-1). Sediment and nutrient enrichment were influential stressors in both experiments, with fine sediment having the most pervasive effects. In Experiment 2, reduced flow velocity had pervasive effects and stressor interactions were mainly restricted to two-way interactions. DCD had few, weak stressor main effects, especially at field-realistic concentrations (Experiment 2). At the highest concentrations in Experiment 1 (above levels observed in real streams), DCD effects were still rare but some significant stressor interactions occurred. Analyses of functional traits were helpful in identifying potential mechanisms driving changes in densities and community composition. These findings suggest that, while DCD on its own may be a minor stressor, it could have adverse effects on algal communities already exposed to other stressors, a scenario common in agricultural streams.

A framework for analysing nitrification inhibition: A case study on 3,4-dimethylpyrazole phosphate (DMPP)

Nitrification inhibitors show great potential to reduce nitrogen losses from agricultural systems and to improve nitrogen use efficiency. The most recently developed nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) is gaining popularity due to its benefits relative to other compounds. However, the behaviour of DMPP and its effect on nitrification in soils has been characterised using inconsistent and confusing terminology. Many studies have used the term half-life to describe the persistence of DMPP but used different experimental methods to derive it leading to highly variable results. We assessed how different methodologies in experiments may have contributed to the variability in the results using a framework that describes the behaviour of DMPP and its effect on nitrification in terms of: persistence, bioactivity and longevity. We show that deriving the persistence of DMPP using ¹⁴C labelling techniques is challenging because it requires consideration of other ¹⁴C pools in the soil. We also describe the limitations of soil inorganic nitrogen measurements to characterise the bioactivity and longevity of the inhibitory effect on nitrification. We conclude by proposing experiments that can facilitate the evaluation of the benefits of DMPP across broader scales. While this study focused on DMPP, the concepts presented here are equally relevant to other nitrification inhibitors.

Lateral spread affects nitrogen leaching from urine patches

Nitrate leaching from urine deposited by grazing animals is a critical constraint for sustainable dairy farming in New Zealand. While considerable progress has been made to understand the fate of nitrogen (N) under urine patches, little consideration has been given to the spread of urinary N beyond the wetted area. In this study, we modelled the lateral spread of nitrogen from the wetted area of a urine patch to the soil outside the patch using a combination of two process-based models (HYDRUS and APSIM). The simulations provided insights on the extent and temporal pattern for the redistribution of N in the soil following a urine deposition and enabled investigating the effect of lateral spread of urinary N on plant growth and N leaching. The APSIM simulation, using an implementation of a dispersion-diffusion function, was tested against experimental data from a field experiment conducted in spring on a well-drained soil. Depending on the geometry considered for the dispersion-diffusion function (plate or cylindrical) the area-averaged N leaching decreased by 8 and 37% compared with simulations without lateral N spread; this was due to additional N uptake from pasture on the edge area. A sensitivity analysis showed that area-averaged pasture growth was not greatly affected by the value of the dispersion factor used in the model, whereas N leaching was very sensitive. Thus, the need to account for the edge effect may depend on the objective of the simulations. The modelling results also showed that considering lateral spread of urinary N was sufficient to describe the experimental data, but plant root uptake across urine patch zones may still be relevant in other conditions. Although further work is needed for improving accuracy, the simulated and experimental results demonstrate that accounting for the edge effect is important for determining N leaching from urine-affected areas.

The performance of an efficient dairy system using a combination of nitrogen leaching mitigation strategies in a variable climate

An efficient dairy system, that implemented a combination of nitrogen (N) leaching mitigation strategies including lower N fertilizer input, standing cows off pasture for part of the day in autumn and winter (stand-off), and importing limited amounts of low protein supplements was evaluated over four consecutive years of a farmlet study. This efficient system consistently demonstrated a lower measured annual N leaching of 40 to 50% compared with a baseline system representing current practice with no mitigations. To maximize return from this system fewer cows but of higher genetic merit were used resulting in an average decrease in milk production of 2% and operating profit by 5% compared with the baseline system. The magnitude of the N leaching reduction from mitigation strategies was predicted in pre-trial modelling. Using similar mechanistic models in a post-trial study, we were able to satisfactorily predict the trends in the observed N leaching data over the four years. This enabled us to use the calibrated models to explore the contributions of the different mitigation strategies to the overall leaching reduction in the efficient system. In one of the years half of the leaching reduction was achieved by the 'input' component of the strategy (less feed N flowing through the herd from lower fertilizer use, less grass grown, and low-protein supplement use), while the other half was achieved by the stand-off strategy. However, these contributions are determined by the weather of a particular year. We estimate that on average stand-off would contribute 60% and 'input' 40% to the reduction. The implication is that farmers facing nutrient loss limitations have some current and some future technologies available to them for meeting these limitations. A shift towards the mitigations described here can result in a downward trend in their own N-loss metrics. The challenge will be to negate any reductions in production and profit, and remain competitive.