Meta-analysis as a tool to improve robustness and reliability of yield response models to nitrogen fertilization

Valkama, E., Turtola, E.

MTT Agrifood Research Finland, Plant Production/ Soil and Plant Nutrition, FI-31600, Jokioinen, Finland
Fertilizer applications should be based on relevant yield response models and be economically justified. Yield responses to nitrogen (N) fertilizer usually vary among years, sites, crops and varieties that lead to numerous response curves, uncertainty in the models’ development, and difficulties in obtaining the economically optimal N rates (Nopt). To overcome these challenges the technique of meta-analysis was applied to improve the robustness and the reliability of the yield response models.

Meta-analysis is the statistical analysis of a large collection of results for the purpose of integrating the findings. In a meta-analysis, the result of each independent experiment is usually expressed as an index of effect (effect estimate). To produce a summary of the findings, these effect estimates are then combined across studies using a weighting procedure. Sub-groupings of experiments are often examined separately to determine whether experimental results differ among the categorical groups (e.g. decades, soil types, soil pH, cultivation zones and yield without N). When the statistically significant difference is found, the yield response models should be fitted separately for each categorical group.

We gathered 60 Finnish field experiments from the years 1947–2004 in order to develop the yield response models of cereals to nitrogen (N) fertilization. The Mitscherlich-type exponential models fitted the data better than the quadratic models. Among the studied categorical groups, yield without N fertilization (the “low”, “medium” or “high” group) was a key factor determining both the magnitude of yield response and models’ parameters. Taking this into account, from tens of independent experiments it allowed for obtaining only one response curve and one value of average Nopt for a certain cereal, yield group and the fertilizer-to-cereal price ratio (for Nopt).

Valkama_Malliseminaari_Helsinki_ 02 02 2012.pdf