Abstract
AbstractAgronomists have increasingly conducted experiments on-farm, in an attempt to increase the wider applicability (external validity) of their experimental findings and their relevance for agricultural development. This review assesses the way in which on-farm experimental studies address the scope or generalisability of their findings when based on a limited number of farms. A central question is how on-farm studies define the environment or research population in which the on-farm trial findings are valid, or are valuable for. Such an assessment is, of course, conditional on the (internal) validity of the experimental findings. We therefore first analyse how authors of on-farm experimental studies describe the factors that may shape experimental outcomes. As agronomic experiments often use ‘yield’ as dependent variable to assess treatment effects, we developed a procedure to score studies on their descriptions of yield-determining factors. Although experimental validity principally rests upon the reproducibility of the experiment and its findings, we found that on the basis of the information provided in published on-farm experimental studies, it is often difficult or impossible to reproduce the experimental design. Nutrient management, weed management and crop information are best described, whereas land preparation, field history and management of pests and water are rarely described. Further, on-farm experimental studies often compare treatments to a ‘farmer practice’ reference or control treatment which is assumed to be widely and uniformly practiced and known to the reader. The wider applicability or external validity is often poorly addressed in the reviewed studies. Most do not explicitly define the research population and/or environment in which (they expect) the experimental findings to work. Academic textbooks on agronomic experimentation are remarkably silent on both the internal and external validity of on-farm experimentation. We therefore argue for more systematic investigations and descriptions of the research population and settings to which on-farm experimental studies seek to generalise their findings.
Highlights
Agriculture is experimental by nature as it is geared towards improving farming results (Maat, 2011; Richards, 1985)
Following the emergence of Farming Systems Research (FSR) and Farmer Participatory Research (FPR) approaches in the 1980s, on-farm experimental studies have increasingly made it into high-ranking agronomy journals
Geographical distribution of on-farm experiments by topic The on-farm experimental studies considered in this review are distributed across SSA, except for a band stretching from Namibia and Botswana towards Chad and Sudan, where only few published on-farm studies were conducted
Summary
Agriculture is experimental by nature as it is geared towards improving farming results (Maat, 2011; Richards, 1985). Experiments are conducted by agronomists in two distinct locations: on research stations and in farmers’ fields. Research stations represent a highly controlled experimental environment, where single or multiple treatments and their interactions can be investigated holding other variables constant. In farmers’ fields, researchers have much less control over the experimental situation, which is embedded in a much wider and more variable bio-physical and socio-economic environment
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