The information content of indicators in intercropping research

This paper examines indicators and methods currently used in intercropping research. A review of papers in Experimental Agriculture and Journal of Agricultural Science (Cambridge) in the years 1990-1999 gave 50 papers on intercropping from which 72 experiments were examined. The objectives of experiments, apart from virtually all being concerned with crop yield under intercropping, were dominated by interest in management methods affecting intercropping benefit, with concerns about the economic analysis of intercropping benefits and of the sustainability of tested systems and in the mechanisms underlying intercropping effects. Concerns about stability and sustainability were not as central as might have been expected. Few experiments were mainly concerned with exploring the way in which species interact in mixture as an explanation of intercropping effects (e.g. through manipulation of the timing of planting or examination of root and shoot competition). Close to half the studies used the additive series design with pairwise, replacement series and response model designs each accounting for about 17% of cases. Only two studies measured at the level of the individual plant and only two studies included initial indicators of plant size or other biological starting point. Analysis of the combination of experimental design used, the measurements taken and the statistical analysis performed showed that while most experiments could provide some valid indicators and thus inferences on crop yields few of them could be used to address questions on the inter- and intraspecific interactions leading to the intercropping outcomes. The duration of experiments was 1 year for about half of experiments involving annual species only and about one-third of experiments involving at least one perennial species. The overall conclusion is that considerable care needs to taken to ensure that the indicators and experimental methods used are appropriate to the development and testing of sustainable systems based on intercropping. (C) 2001 Elsevier Science B.V. All rights reserved.