Christel Richter

Nearest Neighbour Adjustment and Linear Variance Models in Plant Breeding Trials

This paper reviews methods for nearest neighbour analysis that adjust for local trend in one dimension. Such methods are commonly used in plant breeding and variety testing. The focus is on simple differencing methods, including first differences and the Papadakis method. We discuss mixed model representations of these methods on the scale of the observed data. Modelling observed data has a number of practical advantages compared to differencing, for example the facility to conveniently compute adjusted cultivar means. Most models considered involve a linear variance-covariance structure and can be represented as state-space models. The reviewed methods and models are exemplified using three datasets.

Statistical aspects of on-farm experimentation

This paper reviews options for the design and analysis of on-farm experiments. It covers both older approaches that have been popular since the Green Revolution, and more recent developments made possible by the availability of online monitoring systems as used in precision farming. The roles of randomisation as well as of geostatistical methods of analysis for these kinds of experiments are critically discussed. Two case studies are provided for illustration.

Strategy of statistical model selection for precision farming on-farm experiments

Nitrogen (N) fertilization implies two important issues: N enhances grain yields and quality, but applied in excess, nitrous oxide emissions and nitrate leaching may be induced. To reduce environmental impacts, spatial N variability in agricultural fields can be adapted using crop sensors. In on-farm experiments, sensor-based variable rate N application is compared to uniform N application, which is common agricultural practice. On-farm experiments (OFE) provide special considerations as opposed to on-station trials. In OFE, the experimental units in farmer-managed fields are considerably larger, which raises the question if soil heterogeneity may be fully controlled by the experimental design (random treatment allocation and blocking). Grain yield monitoring systems are used increasingly in OFE and provide spatially correlated data. As a consequence, classical analysis of variance is not a valid option. An alternative four-step strategy of statistical model selection is presented, generalizing the assumptions of classical analysis of variance within the framework of linear mixed models. Soil heterogeneity is preliminary identified in step 1 and finalized in step 2 using covariate combinations (analysis of covariance). Yield data correlations are handled in step 3 using geo-statistical models. The last step estimates treatment effects and derives the statistical inference. Analyses of three OFE revealed that different covariate combinations and geo-statistical models were needed for each trial, which involves higher analytical efforts than for on-station trials. These efforts can be minimized by following the steps provided in this study to find a best model approximation. Nevertheless, model selection in precision farming OFE will always accompany some uncertainty.

The effect of elevated temperatures on food conversion efficiencies of Coccinella septempunctata and Harmonia axyridis young adults feeding on Sitobion avenae

An analysis of existing data gained in a set of climate chamber experiments was conducted to compare food conversion efficiencies of young Coccinella septempunctata and Harmo-nia axyridis adults (Coleoptera: Coccinellidae) as natural enemies of Sitobion avenae (Homoptera: Aphididae) at elevated temperatures. The two species were treated at normal (Mean: 17.8°C, Max: 21.8°C, Min: 13.4°C) and elevated (Mean: 20.8°C, Max: 25.5°C, Min: 15.7°C) daily temperature profile from first instar to 10-day-old adult and fed S. avenae ad libitum. Elevated temperatures caused higher consumption rates in both coccinellids but different responses in weight gain and body fat accumulation within the first 10 days of adulthood. Harmonia axyridis males were poor converters of consumed biomass into body weight and H. axyridis females showed the lowest efficiencies for converting food into body fat content, particularly at elevated temperatures. These results show different temperature-dependent responses of C. septempunctata and H. axyridis and are discussed in relation to their different biology.