Detlef Ehlert

Laser rangefinder-based measuring of crop biomass under field conditions

Knowledge of site-specific crop parameters such as plant height, coverage and biomass density is important for optimising crop management and harvesting processes. Sensors for measuring crop parameters are essential pre-requisites to gather this information. In recent years, laser rangefinder sensors have been adopted in many industrial applications. In agricultural engineering, the potential of laser rangefinders for measuring crop parameters has been little exploited. This paper reports the design and the performance of a measuring system based on a triangulation and a time-of-flight laser rangefinder for estimating crop biomass density in representative crops under field conditions. It was shown that the mean height of reflection point is a suitable parameter for non-contact indirect measurement of crop biomass by laser rangefinder sensors. The main parameters for potential assessment were the coefficient of determination (R2) and the standard error (RMSE) for the relation between crop biomass density and the mean height of the reflection point in crop stands from oilseed rape, winter rye, winter wheat and grassland during the vegetation period in 2006. For the triangulation sensor, R2 was in the range from 0.87 to 0.98 and for the time-of-flight sensor in the range from 0.75 to 0.99 for both fresh matter and dry matter density. The triangulation sensor had a reduced suitability caused by masking effects of the reflected beam and because of limited measuring range. Based on the results of experiments and technical data, it was concluded that the time-of-flight principle has good potential for site-specific crop management.

Widescale testing of the Crop-meter for site-specific farming

A sensor for measuring crop biomass density has been designed and developed to meet the demands for practical use in site-specific farming. The mechanical sensor named ‘Crop-meter’ is based on the pendulum principle. The suitability and measuring stability of the Crop-meter has been confirmed under field conditions in different regions of Germany. Significant correlations were obtained between Crop-meter signals and soil electrical conductivity (R2=0.16−0.66) and grain yield (R2=0.42−0.57). To test the suitability of the Crop-meter for site-specific management, it was used to control variable application rates for nitrogen fertiliser, growth regulators and fungicides in real time. A small increase in yield (3.1%) as well as reduced application rates for agrochemicals (14.6% nitrogen fertilisers; 23.1% fungicides and growth regulators) were proved in large-scale trials.

Harvesters for short rotation coppice: current status and new solutions

Mechanization in the harvesting of short rotation coppice on farmed land is a prerequisite for the expansion of short rotation coppice cropping. Existing harvest lines can be classified as log, bundle, bale, or chip lines; the latter are widely considered the most economical. Despite the development of more than 20 different harvesting machines and assemblies for chip lines in the past decade alone, only a few have progressed beyond the prototype stage. In addition to special mower tools already available for conventional forage harvesters, there is a great need for low-cost tractor-mounted chip harvesters. Therefore, a novel working principle for a mower-chipper was developed and tested. In contrast to most other solutions, the trees remain in an upright position while mowing and chipping. The harvest machine has a compact single rotor bearing a circular saw blade with a diameter of approximately 1000 mm. Knives arranged radially are attached to the saw and offset above the disk with spacer blocks. To ensure even feeding of the mower-chipper unit, numerous passive and active machine elements were tested to find a reliable solution for practical application. With the investigated harvester, trees with stem diameters up to 15 cm and 10 m in height could be successfully harvested at very promising working speeds of 3–5 km/h. The Class P45 wood chips produced satisfied the CEN/TS 14961:2005 standard.

Biomass related nitrogen fertilization with a crop sensor.

In cereal crop production, approximately 30% of nitrous oxide (N2O) emissions and 47% of total energy inputs are attributed to the use of mineral N fertilizers. Furthermore, leaching of surplus N can contaminate ground water and promote eutrophication. In recent years, sensor-based systems for variable-rate N fertilization have been developed to optimize the ratio of grain yield to N application rate. Most commercially available sensors are based on opto-electronics; however, a mechanical crop biomass sensor (Crop-Meter) has been recently developed. Thus, the objective of this study was to evaluate the Crop-Meter for N application in cereal crops. The sensor was attached to conventional farm equipment, allowing on-the-go variable-rate N application and was evaluated during a three-year research project (2005-2007) on five large-scale farms in East Germany. In strip experiments, sensor-based crop biomass analysis and subsequent N fertilization contributed to a median increase in N efficiency of 14.4%. The observed increase in N efficiency was caused by a 10% to 15% reduction in N fertilizer and was not attributed to gains in yield. In field-scale experiments, a reduction in N of 17 kg N ha-1 was observed, which was similar to the value obtained in strip trials (21 kg N ha-1). Annual use of the biomass sensor on an area of 250 ha or more can increase profits of grain cultivation.

Measuring mass flow by bounce plate for yield mapping of potatoes.

There are reports about measuring mass flow in potato harvesters by X-ray with two wave lengths sources, by load cells in conveyors and by optical surveying with image analysis. In mechanics, an impulse is defined as the integral of force over time. If one succeeds in producing a constant velocity difference in the mass flow of potatoes, then there exists a directly proportional correlation between the integral force and the mass flow. A constant speed difference can be produced when the potatoes hit a plate on which the potatoes bounce almost vertically in the discharge trajectory of a conveyor belt. Under laboratory conditions, a rubber coated plate with a force measuring instrument was placed in different positions in the discharge trajectory of a conveyor belt running at 3 different speeds. For simulation of mass flow in steps of about 1 kg up to 40 kg, different masses of potatoes were placed on the conveyor belt. For additional simulation of vibrations in a potato harvester, the measurement device was moved horizontally, at five different frequencies and four amplitudes. The results showed a clear linear relationship between mass flow and force. For many parameter combinations, the quality of fit was more than 0.99 and, in some cases, standard error was less than 0.083 kg s−1. The results indicated that a bounce plate provides a basis for measurement in potato harvesters for yield mapping.

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.

Vehicle Based Laser Range Finding in Crops

Laser rangefinders and laser scanners are widely used for industrial purposes and for remote sensing. In agriculture information about crop parameters like volume, height, and density can support the optimisation of production processes. In scientific papers the measurement of these parameters by low cost laser rangefinders with one echo has been presented for short ranges. Because the cross section area of the beam increases with the measuring range, it can be expected that laser rangefinders will have a reduced measuring accuracy in small sized crops and when measuring far distances. These problems are caused by target areas smaller than the beam and by the beam striking the edges of crop objects. Lab tests under defined conditions and a real field test were performed to assess the measuring properties under such difficult conditions of a chosen low cost sensor. Based on lab tests it was shown that the accuracy was reduced, but the successful use of the sensor under field conditions demonstrated the potential to meet the demands for agricultural applications, Insights resulting from investigations made in the paper contribute to facilitating the choice or the development of laser rangefinder sensors for vehicle based measurement of crop parameters for optimisation of production processes.