An investigation into measuring ammonia loss during the operation of a freestall dairy barn : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University, Palmerston North, New Zealand
The dairy sector in New Zealand (NZ) has undergone rapid intensification and transformation over the past few decades: the traditional pasture based grazing systems are continuously being replaced by high supplement feed input system like feed pads, herd homes and wintering barns. Practising duration controlled grazing; using temporary housing systems (naturally ventilated barns) can reduce urinary load to paddocks and N loss to water. There is concern that ammonia (NH3) loss to the atmosphere during housing, manure storage and re-application to pasture simply results in pollution swapping i.e. decreasing N loss to water while increasing the greenhouse gas emission footprint of dairying.
The main objective of this research was to develop cost effective techniques to monitor, mitigate and minimise the NH3 gas emissions from a duration controlled grazing system in the Manawatu region of NZ. The main components of dairy cattle (DC) grazing systems are feed, housing, storage and slurry reapplication to land. The first study focused on developing techniques to identify the hotspots contributing to gas emissions from all stages of the manure management chain. The efficiency of two commonly used techniques namely active and passive NH3 gas sampling was evaluated and modified. Commonly used 7 L dynamic chambers and large vacuum pumps were replaced by small 50 ml PVC tube acid scrubbers and small aquarium pumps. The acid scrubbers were successfully deployed for NH3 gas emission measurements from storage pond and slurry re-application to land. Simple diffusion sampling tubes (DSTs) were also developed and calibrated for long term measurements from storage ponds.
The second set of laboratory experiments aimed at studying the losses associated with the feed component. The effect of diet on urine N content excreted by dairy cows and the influence of urea N content of urine on the magnitude of NH3 emissions was studied by simulating excreta deposition on a barn floor in 1 L Agee jars. In NZ dairy cows are fed on pasture based production system, however when the pasture becomes limiting during summers then cows are fed on high supplement feed inputs like maize and hay silage. It was hypothesised that dietary manipulation would impact N excretion and NH3 emissions from excreta. For this experiment, a total of fifty four dairy cows were used. They were split into three groups (each group containing 18 cows) and fed on high crude protein (HCP, 25%), medium crude proteins (MCP, 18.5%) and low crude proteins (LCP, 13.5%) diets. Urine and dung samples were collected separately from each group of the cows. NH3 emissions from the slurry mixture were measured in vitro in a laboratory set up at room temperature (18 ºC - 24 ºC) for 6 days. The laboratory set up consisted of 11 Agee jars (1 L) with passive acid traps (10 ml 0.5 M H2SO4) contained in 50 ml pink tops. The slurry mixture was reconstituted at a standard rate of excretion by dairy cows at a ratio of 1 : 1.3 (dung: urine) by mixing the freshly collected urine and dung [(w/v); wet basis] in urine containers. The cumulative NH3 losses were reported based on the urea N applied and total Kjeldahl N applied to each Agee jar. The results showed that NH3 emissions reduced by 13 - 20% with decrease in dietary crude protein. It was concluded that manipulating the CP level in diet can reduce urinary N excretion from dairy cows and hence lower NH3 emissions.
A subsequent series of laboratory experiments were conducted using 1 L Agee jars to quantify NH3 losses from various hotspots in a naturally ventilated dairy cow barn to study the factors affecting NH3 emissions. The main sources of NH3 losses from the barn are excreta deposited on laneways, scraper lanes and slurry collection pits located underneath the barn. In Study 1, aged slurry samples were taken from different positions in the slurry pathway from channel grate to the storage pond. In Experiment 2 of study 1 (Ex-situ measurements), sources of fresh slurry were created (by mixing urine and dung) to represent the different depths of fresh slurry deposited in the free-stall barn’s laneways and under the grates in the transport channel. The NH3 emission rate from all slurry samples were measured in closed chambers. In study 2 (In situ measurements), a 3D sonic anemometer and NH3 acid traps were used to measure airflow rates and NH3 concentrations in the barns ventilation pathways. The barn’s estimated NH3 emissions calculated from the two contrasting studies were compared.
There is limited NZ data on NH3 gas emissions from a slurry storage pond receiving slurry from a wintering barn. NH3 gas emissions were monitored in the winter of 2017 (2nd June to 16th August) by modified integrated horizontal flux (IHF) methodology. The gas emission flux was measured using diffusion sampling tubes (DSTs) placed at sampling heights (0.25 m to 3.5 m) on an aluminium tower. The towers were mounted at 4 banks (N, S, E, W) of the pond and DSTs were changed every 72 or 94 h. The gas emission flux was found to be positively correlated to daily evapotranspiration rate (ETR) (R2=0.80) and this relationship was used to predict gaseous emission from a static pond.
The slurry is usually stored up-to 3 months before re-application to land. NH3 gas emission from the final stage of manure management was measured on two occasions; Summer 2015 and Autumn 2016. Slurry was incorporated in land through slurry surface spray and injection and losses were measured using IHF methodology. Slurry was applied at 81 kg N ha-1 for surface spray and 73 kg N ha-1 was injected into the soil through injection. Similarly the application rate for autumn application was 252 kg N ha-1 for surface spray and 233 kg N ha-1 for injection application. The percentage NH3-N losses were 2% from slurry surface spray and 1.4% from slurry incorporation through injection for summer application and 3% from slurry surface spray and only 1% from injected application of slurry in autumn.
On completion of NH3 loss measurements at all stages of manure management it was possible to construct a partial NH3 loss budget to illustrate the relative NH3 losses associated with the temporary housing of cows in a freestall barn, manure storage and reapplication to land. This simple analysis illustrated that the largest loss of NH3 can occur if there is a long storage phase of effluent in a open pond. Future research to mitigate NH3 losses created by housing cows should focus on the reduction of NH3 loss from ponds.