Grazed Dairy Pasture Research Articles

Inhibitors application time and pasture canopy capture regulate gaseous losses of urine-N

Technologies have been developed for the in-situ treatment of urine patches deposited by grazing livestock to mitigate nitrogen (N) losses using N transformation inhibitors. For this mitigation to be effective, close contact between the applied inhibitors and the N in the urine patch is required (similar to N-fertilisers coated with inhibitors). This research aimed to determine the proportions of urine-N that mixed with inhibitor at or exceeding the threshold concentration (inhibitor concentration at which the nitrification rate is reduced by at least 40%) when inhibitors were applied to simulated urine patches at 4, 24 and 48 h after synthetic urine application. Three commonly used nitrification inhibitors (NIs) [dicyandiamide (DCD), 3,4-dimethylpyrazole phosphate (DMPP), and 2-chloro-6-(trichloromethyl) pyridine (nitrapyrin)] were applied at 40 mL of inhibitor per urine-patch at two different concentrations. The field studies were undertaken in two dairy-grazed pasture soils with contrasting drainage. Large proportions of applied NIs (38%–59% DCD, 27%–58% DMPP, and 31%–58% nitrapyrin) were retained in the pasture canopy. In most cases, the inhibitor threshold concentration was present only within the top 0–20 mm of the soil, with only 16%–40% of the urine-N present. In some cases, the proportions of urine-N intercepted was 12%–15% higher when inhibitors were applied 4 h after urine application compared to delayed application of 24 and 48 h after urine application. Our results revealed that a substantial proportion of N in the urine-patch remained out of the reach of the inhibitor solution. This is possibly due to the small volume (40 mL per 2 L urine patch, 1:50) of the inhibitors applied, with up to 59% of inhibitor solution retained in the pasture canopy. The time delays (4 to 48 h) between the urine deposition and the inhibitor application could have also contributed to this poor physical mixing between inhibitor and urine. Increasing the volume of water applied with the inhibitor and assessing the effect of rainfall/irrigation on increasing urine-N and inhibitor mixing warrants further consideration.

Reduction of Nitrous Oxide Emissions from Urine Patches from Grazed Dairy Pastures in New Zealand: A Preliminary Assessment of ORUN® as an Alternative to the Use of Nitrification Inhibitor Dicyandiamide (DCD)

Agriculture plays a significant role in economic development and livelihood and is a key contributor to food security and nutrition. However, global concerns regarding the sustainability of the agricultural sector (mainly environmental damage) is linked to agricultural activities such as greenhouse gas (GHG) emissions, water pollution, and loss of biodiversity. The aim of this study was to assess the effectiveness of ORUN® (a formulated agricultural chemical mixture) to reduce N2O emissions from urine patches and to improve pasture yield, pasture N uptake, and soil mineral N concentrations. The field trials were conducted during the spring of 2015 on dairy urine patches at Massey University, New Zealand. Treatments consisted of control nil urine, control nil urine + ProGibb®, urine only, urine + ProGibb®, urine + ORUN®, and urine + ORUN PLUS® replicated four times in a randomized complete block design. At 31 days after treatment (DAT), analysis of soil samples in 0–5 cm soil profiles showed that urine + ProGibb® significantly (p = 0.0041) increased the soil nitrate concentration (121.40 kgN/ha) compared with 48.15 kgN/ha from urine only. The urine + ProGib® treatment produced significantly lower herbage N recovery (35% of applied N) compared with the urine only. Throughout the trial period, the urine patches treated with ProGibb® and ORUN® produced significantly higher N2O fluxes compared with urine only and urine + ORUN PLUS®, as well as higher surface soil nitrate and mineral N concentrations.

Net export of E. coli from a constructed wetland treating agricultural drainage

Monitoring of a constructed wetland receiving episodic pulses of subsurface drainage waters from grazed dairy pastures showed good nitrogen reductions, but moderate increases in median Escherichia coli concentrations. This unexpected increase of E. coli was initially postulated to be an artefact of the routine sampling approach, which was biased towards receding flows and likely missed high inflows of contaminated drainage. Intensive monitoring during storm events was therefore instigated using auto-samplers to quantify responses during rainfall-generated flow events. Determination of inflow and outflow E. coli fluxes and loads for ten storm events concluded that the wetland was consistently a net exporter of E. coli with increases ranging from 1.5 to 26-fold (median 10.2-fold). Whilst wildlife defecation in the wetland may account for a proportion of the rise in E. coli, it is likely that (consistent with associated genetic profiling studies) much of the increase is due to mobilisation of environmentally adapted strains of E. coli persisting and growing within the sediment, decomposing plant litter and organic detritus of the wetland. This raises interesting questions regarding the microbial ecology of E. coli and its utility as an indicator of faecal pathogens, and potential health risks from diffuse agricultural drainage after passage through wetlands.

Comammox Nitrospira Clade B is the most abundant complete ammonia oxidizer in a dairy pasture soil and inhibited by dicyandiamide and high ammonium concentrations.

The recent discovery of comammox Nitrospira, a complete ammonia oxidizer, capable of completing the nitrification on their own has presented tremendous challenges to our understanding of the nitrification process. There are two divergent clades of comammox Nitrospira, Clade A and B. However, their population abundance, community structure and role in ammonia and nitrite oxidation are poorly understood. We conducted a 94-day microcosm study using a grazed dairy pasture soil amended with urea fertilizers, synthetic cow urine, and the nitrification inhibitor, dicyandiamide (DCD), to investigate the growth and community structure of comammox Nitrospira spp. We discovered that comammox Nitrospira Clade B was two orders of magnitude more abundant than Clade A in this fertile dairy pasture soil and the most abundant subcluster was a distinctive phylogenetic uncultured subcluster Clade B2. We found that comammox Nitrospira Clade B might not play a major role in nitrite oxidation compared to the role of canonical Nitrospira nitrite-oxidizers, however, comammox Nitrospira Clade B is active in nitrification and the growth of comammox Nitrospira Clade B was inhibited by a high ammonium concentration (700 kg synthetic urine-N ha-1) and the nitrification inhibitor DCD. We concluded that comammox Nitrospira Clade B: (1) was the most abundant comammox in the dairy pasture soil; (2) had a low tolerance to ammonium and can be inhibited by DCD; and (3) was not the dominant nitrite-oxidizer in the soil. This is the first study discovering a new subcluster of comammox Nitrospira Clade B2 from an agricultural soil.

Simulated seasonal responses of grazed dairy pastures to nitrogen fertilizer in SE Australia: N loss and recovery

Evidence from farm level studies indicates that there is potential to improve nitrogen (N) fertilizer efficiency of Australian dairy farms. Increasing N fertilizer application rates to drive pasture dry matter production beyond an agronomic or economical optimum has the potential to result in detrimental environmental outcomes. Our study, using the biophysical whole-farm systems model DairyMod, modelled a range of N fertilizer rates on total N loss for five dairy sites through south-eastern Australia, using 18 years of historical climate. Nitrogen accumulation in plant biomass and soil N accumulation within and below the rootzone were estimated. Total N loss, in the form of volatilization, leaching, runoff and denitrification lost to the environment were also estimated. The reduction in N fertilizer inputs required to achieve 90% of relative yield (Y90), relative to maximum pasture production (Ymax), was > 50% across all sites and seasons. The associated reduction in total N loss when fertilizer was reduced from Ymax to Y90, varied between 34% and 74%, depending on site and season. Nitrogen recovery (proportion of N recovered in biomass relative to N fertilizer applied) exceeded 100% with lower N fertilizer rates (< 30 kg N ha-1 month-1) for most sites and seasons. Demand for N was high during spring due to high pasture growth and this was supported via N mineralization and legacy N build-up in winter. Nitrate leaching risk was highest in winter for the four temperate sites and autumn at the subtropical site. This study demonstrated the benefits of developing site and seasonal-specific N fertilizer best management practice guidelines that are both economical and environmentally beneficial. When considering whether to add more fertilizer, the value of additional pasture production needs to be weighed up against environmental N losses and the cost of additional N fertilizer to achieve this. The relationship between seasonal soil and climatic conditions and N loss and recovery were also examined for one rainfed site. As this study does not consider the externalities associated with N loss, recommendations need to be considered and amended in the context of location specificity and seasonal climatic conditions.

The effects of irrigation on carbon balance in an irrigated grazed pasture system in New Zealand

In New Zealand, increasing areas of dryland farming are being converted to irrigated farming, and there are conflicting findings whether this will lead to gains or losses of soil organic carbon (SOC). In this study, we used 2 years of eddy-covariance data from an irrigated, grazed dairy pasture in Canterbury, New Zealand and compared observed gas exchange fluxes to those from the process-based CenW model.CenW simulations agreed very well with observations for evapotranspiration, gross primary production, and ecosystem respiration rates with Nash-Sutcliffe model efficiencies ~0.8–0.9 for daily and weekly mean values. Model efficiency for net ecosystem productivity was ~0.7–0.8 for daily and weekly averages. The mean simulated net ecosystem carbon balance (NECB) over the 2 year period was 0.6 t C ha−1 y−1 which was comparable to the measured NECB of 1.0 ± 0.3 t C ha−1 y−1.The model also highlighted some processes where current understanding is incomplete. We found that gross primary production (GPP) was substantially reduced after grazing. While some GPP reduction was expected due to reduced leaf area, measured rates were only about half (reduced by a further ~30 kg C ha−1 d−1) of that expected due to reduced leaf area alone. This probably resulted from the combined effect of trampling damage by cattle, a minor contribution of foliage coverage by dung pats, and some additional leaf physiological effect when previously shaded parts of the foliage were suddenly exposed to full sun to which they were not fully adapted. GPP then gradually recovered over the subsequent 2 weeks.CenW was then used to compare SOC levels after 50 years of irrigated dairy farming with those under a non-irrigated system. We found that SOC responses to irrigation interacted strongly with imposed grazing patterns. When we used the same grazing regime under irrigated and non-irrigated conditions, the application of irrigation resulted in a small increase of 4 ± 3 t C ha−1 (mean ± standard error) over the range of weather conditions typical for this site. However, with this grazing management, the non-irrigated system had very low productivity (<0.1 t C ha−1 y−1 in animal products). When dryland pasture was combined with a grazing regime typical for rain-fed systems, simulated long-term SOC stocks were increased by 13.9 ± 1.5 t C ha−1 under the irrigated system. This highlights the importance of considering the whole management system rather than looking at individual practices in isolation.

Predicting ammonia volatilization from fertilized pastures used for grazing

Ammonia (NH3) volatilization from fertilised agricultural soils is driven by complex interactions between edaphic, climatic and plant canopy factors that can be difficult to measure or predict. We developed a simplified approach using default parameters in the DairyMod model to predict daily NH3 volatilization from urea applied to grazed dairy pastures. Several published datasets were used to validate the reliability of the model to reproduce key related soil processes in a whole farming systems framework. For the sites where monitoring for the experimental duration occurred, DairyMod simulated the main features of the observed NH3 emissions, with an overall predicted median of 4.1 kg/ha or 7% of applied N, compared to the measured median of 6.1 kg/ha for 12% of applied N fertilizer. There was an overall root mean square error (RMSE) of 0.9 kg NH3N/ha/d and an overall mean prediction error (MPE) of 0.5 kg NH3N /ha/d. However, there was high uncertainty in several of the datasets used which made it difficult to be conclusive about the validation. The simulation accuracy was improved using daily wind speed (collected on-site in field campaigns) as input to the evapotranspiration calculations. In cases of high certainty in the volatilization data, it was concluded that the model was useful for the analysis of N cycling in situations used for dairy farming without the need for a more complex mechanistic method with difficult-to-obtain parameters. DairyMod presents a simple but readily reproducible prediction of NH3 volatilization from urea application on pasture in intensive livestock farming systems compatible with the certainty of the model inputs and scale of model application. However, the collective understanding of NH3 volatilization in pasture based dairy systems is currently based on a limited number of often uncertain, short term plot studies in the absence of animals.

The mitigation of phosphorus losses from a water-repellent soil used for grazed dairy farming

Phosphorus (P) losses via surface runoff can be large. Soil water repellency (SWR) occurs in warm and dry conditions. If SWR coincides with P fertiliser applications, the risk of P loss by surface runoff may increase. Losses during hotter months can be particularly detrimental to surface water quality because warmer waters help algal growth. We hypothesized that, compared to highly water-soluble superphosphate, the application of P as a low water-soluble fertiliser would decrease P losses caused by SWR-induced surface runoff. A three-year trial was conducted on a grazed dairy pasture soil in the central North Island of New Zealand. The soil was classified as moderate to severely SWR by water droplet penetration time and molarity of ethanol tests. Phosphorus (40 kg P ha−1) was applied in autumn as superphosphate, serpentine super and as a new fertiliser product similar to di-calcic phosphate. These fertilisers had water solubilities of 85, 34 and 12 g P kg−1 fertiliser, respectively. Yields of P lost were <1 kg P ha−1 for the first year, but in the third year >8 kg P ha−1 was lost from the superphosphate treatment due to SWR-induced surface runoff within 21 days after fertiliser application. In contrast, P losses in the third year from lower water-soluble P fertilisers, serpentine super and the new product, were 2.2 and 1.6 kg P ha−1, respectively. The use of a low water-soluble P fertiliser is therefore recommended to prevent P losses in areas where SWR-induced surface runoff is likely and unavoidable.

Photogrammetry for assessment of pasture biomass

New tools are required to provide estimates of pasture biomass as current methods are time consuming and labour intensive. This proof-of-concept study tested the suitability of photogrammetry to estimate pasture height in a grazed dairy pasture. Images were obtained using a digital camera from one site on two separate occasions (May and June 2017). Photogrammetry-derived pasture height was estimated from digital surface models created using the photos. Pasture indices were also measured using two currently available methods: a Rising Plate Meter (RPM), and Normalised Difference Vegetation Index (NDVI). Empirical pasture biomass measurements were taken using destructive sampling after all other measurements were made, and were used to evaluate the accuracy of the estimates from each method. There was a strong linear relationship between photogrammetry-derived plant height and actual biomass (R2=0.92May and 0.78June) and between RPM and actual biomass (R2=0.91May and 0.78June). The relationship between NDVI and actual biomass was relatively weaker (R2=0.65May and 0.66June). Photogrammetry could be an efficient way to measure pasture biomass with an accuracy comparable to that of the RPM but further work is required to confirm these preliminary findings.

Reconciling annual nitrous oxide emissions of an intensively grazed dairy pasture determined by eddy covariance and emission factors

Estimates of regional and national nitrous oxide (N2O) emissions rely on emission factors (EFs) commonly derived from measurements using static chambers. These measurements can include high uncertainties and might obscure the quantification of N2O fluxes. Advances in micrometeorological eddy covariance technique (EC) now allow direct measurements of N2O fluxes at the field scale. Here, we compared N2O emissions calculated from site-specific EFs with N2O flux data derived from year-round EC measurements on an intensively grazed dairy pasture in the Waikato region, NZ. Annual N2O emissions of 7.30 kg N2O-N ha−1 yr−1 determined using gap-filled EC flux data were greater than N2O estimates of 3.82 kg N2O-N ha−1 yr−1 based on site-specific EFs for cattle urine (1.53%), cattle dung (0.24%) and urea fertiliser (0.16%). Likely reasons for this difference were that the EF approach did not take into account the seasonal variability of EFs, the effect of supplementary feed on cattle nitrogen (N) excretion and background N2O emissions (BNE). Including calculated emissions from supplementary feed N (0.92 kg N2O-N ha−1 yr−1) and BNE (1.09 kg N2O-N ha−1 yr−1) increased annual EF-based emissions to 5.83 kg N2O-N ha−1 yr−1. The site-specific EFs were established in spring 2017 and may not have adequately represented summer, winter and particularly autumn N2O emissions. The EF approach, therefore, did not fully account for the seasonal variability of N2O fluxes as measured by EC but, if quantified, could have led to further agreement between measurements. Using EC measurements to complement static chambers and EF approaches altered annual N2O emissions estimates from intensively grazed pastoral land. Hence, we conclude that N2O budgets derived from EFs need to better capture the effect of seasonal variability, supplementary feed and BNE.

Comparing the effectiveness and longevity of the urease inhibitor N-(2-nitrophenyl) phosphoric triamide (2-NPT) with N-(n-butyl) thiophosphoric triamide (nBTPT) in reducing ammonia emissions from cattle urine applied to dairy-grazed pasture soils

The objective of this laboratory incubation study was to assess the effectiveness and longevity of urease inhibitor N-(2-nitrophenyl) phosphoric triamide (2-NPT), along with the commonly used N-(n-butyl) thiophosphoric triamide (nBTPT), in reducing ammonia (NH3) emissions from cattle urine applied to pasture soils. It was hypothesised that 2-NPT would be effective as a longer-lasting inhibitor to reduce emissions after the deposition of urine during multiple grazing events. Two dairy-grazed pasture soils (Fluvisol, Rangitikei loamy sand (RLS) and Andosol, Egmont black loam (EBL)) with contrasting organic carbon levels and urease activity were used. The following treatments were applied to the soils at the start of the experiment: no inhibitor, nBTPT-low, 2-NPT-low, 2-NPT-medium and 2-NPT-high. Urine-N was applied to all the treatments at four stages: immediately before inhibitor application (Stage A), 29 days after inhibitor application (Stage B), 56 days after inhibitor application (Stage C) and 29 days and again 60 days after inhibitor application (Stage D); NH3 emissions were measured up to Day 31 after each urine application. The low, medium and high application rates of inhibitors were determined based on achieving 0.025%, 0.050% and 0.075% of quantity of urine-N applied in Stage A respectively. For the no inhibitor treatment, the proportion of total applied N in urine that was emitted as NH3 for the different stages ranged from 35.8% to 50.5% for RLS soil and from 14.2% to 26.7% for EBL soil. For Stage A, both inhibitors equally reduced NH3 emissions from applied urine in both soils (23.7–27.3% for the RLS and 20.6–27.2% for the EBL). For Stage B, significant reductions (4.2–13.4%) in NH3 emitted was observed only from RLS soil, with there being a significantly greater reduction from 2-NPT than from nBTPT, and 2-NPT continued to reduce NH3 emissions at Stage C (5.6–7.4%). There was no reduction in emissions during Stage D by either of the inhibitors. The results of this study suggest that 2-NPT can extend the longevity of urease inhibition and reduce NH3 emissions compared with the more commonly used inhibitor nBTPT in dairy-grazed pasture soils.

Simulated seasonal responses of grazed dairy pastures to nitrogen fertilizer in SE Australia: Pasture production

Many nitrogen (N) fertilizer recommendations for grazing livestock enterprises are based on cutting experiments, excluding the influence of recycled N in excreta. Grazing experiments are expensive to conduct, and so compromise on variables such as number of N fertilizer rates, replication and number of years of investigation. Biophysical modelling provides an efficient and effective approach to address many of the complexities of field studies. Our study, using the biophysical whole-farm systems model DairyMod, examined the effect of a range of N fertilizer rates on pasture production for five dairy sites through south-eastern Australia over 18 years under both cutting and grazing regimes. The study aims were to highlight the variation in pasture N responses between cutting and grazing experiments and compare results to current best management practice (BMP) guidelines for N fertilizer management. Annual and seasonal maximum and optimum pasture production, defined as 90% of maximum production, N fertilizer rate to achieve optimum pasture production and the slope of the response rate curve between two fertilizer application rates were estimated. For all five sites, at the lower N rates, there was a divergence in annual pasture production between the grazing and cutting management regimes. However, once N was no longer limiting pasture production for the cutting regime, annual pasture production under cutting and grazing converged. For most sites and seasons, current BMPs of applying between 20 and 50 kg N ha−1 post grazing will ensure efficient use of N applied, assuming soil moisture is not first limiting growth. However, this study has refined these recommendations across all sites and seasons. For some seasons and sites, there was high variability in pasture N response rate between years that need to be taken into consideration. At Elliott in Tasmania, an irrigated site, there was merit in increasing N fertilizer rates above the current recommendation above 50 kg N ha−1 post grazing during spring and summer. In contrast, at the rainfed sites of Ellinbank and Terang in Victoria, the recommendation would be to not apply N fertilizer during autumn and only in selected wetter summers.

Fertiliser strategies for improving nitrogen use efficiency in grazed dairy pastures

Evidence from farm level studies indicates that there is potential to improve nitrogen (N) use efficiency of the predominately pasture-based dairy farms in Australia. This is possible via several ways which includes modifying the timing and rates of N fertiliser applied to pasture. Traditionally fertiliser strategies have been based on a “recipe” approach where N fertiliser, primarily urea, is applied a set rate following grazing. The aim of this study was to compare the pasture dry matter response, N loss and response rate of fertiliser strategies which used increasing knowledge of plant and soil conditions in different ways. The study was conducted under grazing conditions using the biophysical model, DairyMod and repeated at several locations and farming systems in the dairy regions of Australia. In comparison to set rates this study showed that strategic approaches to N fertiliser have the potential to be more efficient in N use and lower both N inputs and N losses with little impact of pasture production. This was evident across all seasons and locations studied. Strategies that used the plant N status to trigger fertiliser timing and rates were more efficient and had lower environmental N losses than those that used fixed rates or soil N information. Fertilising per plant N requirements was the most efficient – and therefore should be the priority for development – particularly in view of the greater expense of fertilisers that are slow release. Precision fertiliser management strategies have the value in terms of reducing fertiliser use and loss during autumn and to a lesser extent in summer, with the least value in winter. However, for the strategies to be properly evaluated for pasture based dairy farms with grazing, a whole farm analysis needs to be conducted that incorporates other sources of feed. This is a necessary inclusion in any subsequent studies.

Can we maintain productivity on broad acre dairy farms during early transition from mineral to compost fertilization?

Composts are being increasingly used as an alternative to mineral fertilisers in production agriculture. However, some farmers are reluctant to utilise compost due to lack of consistent information regarding agricultural productivity and management. Here we explore the changes in soil chemistry, plant biomass production and nutrition during an 18 month transition period from mineral to compost fertiliser use. This was undertaken on plots excluding stock on two dairy farms in southeastern Australia. Biomass was not affected by compost application, demonstrating that productivity and nutrition were maintained during early stage transition. Soil chemical properties, including available nitrogen species, were influenced by compost, but changes fluctuated over the transition phase. Impacts of management practices, season and soil chemical influences on biomass production were explored using multivariate techniques. This analysis revealed that season and inherent soil chemical properties had the greatest influence on pasture biomass during early stage transition. Results are discussed in the context of a transition to compost-based nutrient management of grazed dairy pastures.

Duration-controlled grazing of dairy cows. 2: nitrogen losses in sub-surface drainage water and surface runoff

ABSTRACTThe annual quantities of nitrate-N (-N) and total N (TN) leached from mole and pipe drained dairy pastures on a Pallic soil, were reduced by an average of 52% and 42% (p < .05), respectively, by standing cows off pasture to ruminate and rest after grazing (Duration-controlled grazing, DC), compared to the standard grazing management (SG) of leaving cows at pasture between milkings. For the SG treatment, measured -N leached was 13, 8 and 21 kg N ha−1 and total N leached was 18, 13 and 26 kg N ha−1 in 2009, 2010 and 2011, respectively. Annual surface runoff contained <1 kg -N ha−1 y−1 and <3 kg TN ha−1 y−1. The deposition of urine in the autumn period had the greatest influence on the quantity of leached in winter drainage. DC grazing is a practical method to reduce N leaching from grazed dairy pastures.

Why copper and zinc are ineffective in reducing soil urease activity in New Zealand dairy-grazed pasture soils

Micronutrients copper (Cu) and zinc (Zn) have the potential to inhibit soil urease activity (UA) and reduce ammonia (NH3) emissions over long duration (8–12 weeks) but have not been tested for reducing NH3 losses from cattle urine deposited in dairy-grazed pasture soils. The objective of this study was to assess the effectiveness and longevity of Cu and Zn in reducing soil UA, for the use of these metals to reduce NH3 emissions from deposited urine by grazing cattle. A series of experiments were conducted to (i) assess the relationship between inherent Cu and Zn status and soil UA of New Zealand dairy-grazed pasture soils, (ii) determine the impact of Cu and Zn addition to pasture soils on soil UA and (iii) investigate how soil organic carbon (C) and other C-related textural and mineralogical properties such as clay content and cation exchange capacity influence the effectiveness of added Cu and Zn in reducing urea hydrolysis. The results showed significant positive correlations of soil total C and total nitrogen (N) with soil UA. However, there were no significant negative correlations of soil UA with inherent Cu and Zn levels. Similarly, addition of Cu and Zn to soil did not significantly reduce soil UA. However, when Cu was added to two different soil supernatants there was a significant reduction in hydrolysis of urea applied at 120 and 600 mg urea-N kg–1 soil. Additions of Zn achieved negligible or small reductions in urea hydrolysis after 120 and 600 mg urea-N kg–1 soil applications to soil supernatants. This result suggests that Cu can inhibit soil UA and urea hydrolysis in soil supernatants with potentially low C, clay and cation exchangeable base contents. However, the interaction of bioavailable Cu with labile soil organic C and clay particles leads to its inactivation, resulting in ineffectiveness in organic C-rich pasture soils. Although most of the added Zn did not complex and remained bioavailable, the observed levels of bioavailable Zn had limited effect on soil UA.

Soil properties impacting denitrifier community size, structure, and activity in New Zealand dairy-grazed pasture

Abstract. Denitrification is an anaerobic respiration process that is the primary contributor of the nitrous oxide (N2O) produced from grassland soils. Our objective was to gain insight into the relationships between denitrifier community size, structure, and activity for a range of pasture soils. We collected 10 dairy pasture soils with contrasting soil textures, drainage classes, management strategies (effluent irrigation or non-irrigation), and geographic locations in New Zealand, and measured their physicochemical characteristics. We measured denitrifier abundance by quantitative polymerase chain reaction (qPCR) and assessed denitrifier diversity and community structure by terminal restriction fragment length polymorphism (T-RFLP) of the nitrite reductase (nirS, nirK) and N2O reductase (nosZ) genes. We quantified denitrifier enzyme activity (DEA) using an acetylene inhibition technique. We investigated whether varied soil conditions lead to different denitrifier communities in soils, and if so, whether they are associated with different denitrification activities and are likely to generate different N2O emissions. Differences in the physicochemical characteristics of the soils were driven mainly by soil mineralogy and the management practices of the farms. We found that nirS and nirK communities were strongly structured along gradients of soil water and phosphorus (P) contents. By contrast, the size and structure of the nosZ community was unrelated to any of the measured soil characteristics. In soils with high water content, the richnesses and abundances of nirS, nirK, and nosZ genes were all significantly positively correlated with DEA. Our data suggest that management strategies to limit N2O emissions through denitrification are likely to be most important for dairy farms on fertile or allophanic soils during wetter periods. Finally, our data suggest that new techniques that would selectively target nirS denitrifiers may be the most effective for limiting N2O emissions through denitrification across a wide range of soil types.

Does variable rate irrigation decrease nutrient leaching losses from grazed dairy farming?

AbstractThe irrigation of grazed dairy pastures can be highly profitable, but can lead to the enhanced leaching of nitrogen (N) and phosphorus (P) and impairment of freshwater quality. A six‐year study was conducted to determine whether the use of variable rate irrigation (VRI), compared to uniform rate irrigation (URI), could decrease N and P leaching losses from a 143‐ha area under intensively grazed dairy cattle that had been partly hydrologically isolated by the installation of artificial drainage pipes. Median concentrations in drainage of NH4‐N, NO3‐N and total N were enriched after passing through the irrigated area under both URI and VRI. However, median concentrations of N species and filterable reactive P and total P in drainage downstream of the irrigated area were much less under three years of VRI than under three years of URI. After accounting for potential differences in flow, annual load estimates of N and P species at the downstream site under VRI were about 80–85% less than that lost under URI. Wider adoption of VRI technology could therefore decrease farm leaching losses and nutrient concentrations in receiving waterbodies compared to areas with widespread URI or flood irrigation.

Urine patch detection using LiDAR technology to improve nitrogen use efficiency in grazed pastures

In grazed dairy pastures, the largest N source for both nitrate (NO3−) leaching and nitrous oxide (N2O) emissions is urine-N excreted by the animals. Additional application of N on urine patches as fertilizer may increase these losses. Identification of urine patches could reduce N losses in grazed pastures through more efficient fertilizer application and improved fertilizer N use efficiency (NUE). The aim of this study was to determine if remote sensing using Light Detection and Ranging (LiDAR) technology could accurately identify urine patches in grazed pastures based on height variation of the grass canopy in close proximity. Synthetic cow urine (7gNL−1) was applied to two blocks (20m×20m) in a well-established pasture in Canterbury, New Zealand, which had no recent exposure to grazing animals or N fertilization. Urine patches were scanned weekly for five weeks. LiDAR based contour maps of the pasture were shown to accurately detect the asymmetric urine patches as well as calculate a percent area of urine based high N as early as one week after a simulated grazing event.

Trajectory and causes of decline in the botanical composition of dairy-grazed pasture in the Waikato

Abstract Herbage accumulation, botanical composition, tiller density and insect pest populations were monitored over 6 years for four perennial ryegrass functional types grazed by dairy cows in the Waikato. The aim was to identify genotypic and environmental factors contributing to ryegrass persistence failure in the upper North Island. Perennial ryegrass content of pastures declined as low as 60% of total herbage mass (HM) in summer of the first 3 years but recovered in autumn to at least 75%. However, following two successive severe summer/autumn droughts in Years 2 and 3, and subsequent grass grub larvae populations exceeding the damage threshold of 200/m2, by 4 years after sowing (autumn 2015), ryegrass had fallen to 20% of HM. This pattern was repeated in the following 2 years, and was not prevented by any combination of ryegrass functional type, endophyte, seeding rate, or best-practice dairy cattle grazing and soil nutrient management. The abiotic and biotic environmental stresses dominated all other factors. Keywords: pasture persistence, perennial ryegrass, drought, insects, black beetle, dairy systems