This page briefly describes how the following models were constructed:
• Pastoral
• Greenhouse emission model
• Cropping and horticultural model

Pastoral

The nutrient budget models for N, P, K and S in this version are derived from earlier versions of the model although modifications have occurred to reflect recent research results.

The underlying animal sub-model to assess animal intake is based on a metabolic energy intake model developed as part of the national methane inventory for MAF (Clark 2001).

The estimate of N leaching is determined primarily from calculation of the amount of N excreted in urine and dung, and factors for leaching which vary with form (urine or dung), animal type, soil group, drainage status and rainfall. Thus, the main determining factors on a farm are the amount of feed consumed (calculated from milk production or stocking rate and the energy intake model) and its N concentration (higher for N-fertilised pasture and differs between supplements). Direct leaching of fertiliser and effluent N is also accounted for, particularly when applied in high risk months for leaching. Leaching losses from effluent N applied to land from the farm dairy and winter feed pads are also estimated.

The P runoff/leaching model was derived from a model developed by McDowell et al. (2003).

The Ca, Mg and Na nutrient budgeting frameworks are based on the model reported by Carey and Metherell (2002).

The acidity model is based on that described by de Klein et al. (1997). The lime maintenance model is derived from this.

All sub-models have been validated against field measurements where such data exists.

Greenhouse gas emission model

The greenhouse gas inventory is based on models and algorithms used for New Zealand’s greenhouse gas national inventory, but with improvements to include on-farm management practices. Methane emissions are based on a metabolic energy intake model developed by Clark (2001). Nitrous oxide (N₂O) emissions are based on the New Zealand IPCC-based inventory, which includes the use of emission factors for direct N₂O losses from excreta, fertiliser and effluent, and indirect losses from leached N and volatilised ammonia. Leached N and volatilised ammonia are estimated from the associated N budget model. Methane and nitrous oxide emissions are converted to CO₂ equivalents.

Carbon dioxide (CO₂) emissions from fuel and electricity, processing and some indirect contributions (e.g. fertiliser manufacturing) are largely based on the data of Wells (2001). In many settings, default values are presented, but can be overridden by the user if required. Processing data for dairy factories was extracted from surveys. CO₂ emissions generally refer to total embodied emissions, that is, total CO₂ cost associated with the use and production of fuel, electricity, fertiliser, etc.

Cropping and Horticulture

The cropping model is based on monthly growth characteristics, and annual estimates are based on accumulated monthly estimates. In contrast, the horticultural model uses annual average values except for tree or vine age.

A simplified greenhouse gas model is used. For each crop this includes CO₂ lost from lime, N₂O emissions, and CO₂ emissions associated with fertiliser use and fuel use. This is not the total embodied CO₂ emissions from a crop, which would be higher.

References

Carey, P.L.; Metherell, A.K. 2002. Pastoral calcium and magnesium modules for the OVERSEER^® nutrient budget model. pp. 373-388. In: Dairy farm soil management. Occasional Report No. 15, Fertiliser and Lime Research Centre. Eds. Currie L. D. and Loganathan P. Massey University, Palmerston North.

Clark, H. 2001. Ruminant methane emissions: a review of the methodology used for national inventory estimations. Report for Ministry of Agriculture and Fisheries, Wellington.

de Klein, C.A.M.; Monaghan, R.M.; Sinclair A.G. 1997. Soil acidification: a provisional model for New Zealand pastoral systems. New Zealand Journal of Agricultural Research 40: 541-557.

McDowell, R. W., Wheeler, D. M., Monaghan R.M. 2003. A tool to determine relative P loss in New Zealand pastoral farming systems. ASPAC conference proceedings, Occasional Report, Fertiliser and Lime Research Centre, Massey University, New Zealand (in press).

Wells, C. 2001. Total energy indicators of agricultural sustainability: Dairy farming case study. MAF.