Murray C Hannah, Cameron J P Gourley, Kohleth Chia, Ivor M Awty
Agriculture Research and Development Division, Ellinbank Centre, Department of Economic Development, Jobs, Transport and Resources, Ellinbank, VIC 3821, AUSTRALIA. Murray.Hannah@ecodev.vic.gov.au.
The increasing use of nitrogen (N) fertiliser in pasture-based dairy systems is commensurate with a decline in N use efficiency and increase in N surplus. An improved ability to predict pasture yield response to applied N is a crucial first step in determining the production and economic benefits of N fertiliser inputs. Data and meta data on pasture yield response to N fertiliser were utilised from a database repository of Australian fertiliser trials. Despite the extent of the data, there was patchy availability of meta data, only two nitrogen rates applied in the majority of trials, skewed representation of states, regions and seasons, and likely selection biases arising from trial protocol. These data were analysed and a quantitative non-linear mixed effects model based upon the Mitscherlich function was developed. The model included fixed effects for state by season, phosphorus status and harvest type (initial or residual), and nested random effects for location and trial/sub-trial. The model may be useful in predicting pasture yield response to applied N fertiliser as a proportion of obtainable yield and can be scaled to absolute response using the fitted model estimates of maximal yield, classified by location and season and by P status and harvest type, or by specification of a target harvest yield.