Aspects of the Design and Interpretation of Competition (Interference) Experiments

Abstract

The design and analysis of competition experiments should be based on an unambiguous objective. Recent criticisms of particular designs have been made without reference to objectives and may be misleading. Three common designs are discussed: additive, replacement series, and response surface. Additive designs are well suited to agronomic objectives; replacement series are useful for comparing pairs of species; response surface designs can be used for most objectives but may be unnecessarily complex. The published criticisms of additive and replacement series designs are argued to be acceptable limitations within the bounds of the objectives for which they are used. Concerns about these designs confounding density and proportion are irrelevant to the objectives for which they are most suited. The continued use of multiple comparison tests is argued to be illogical. Regression approaches to analysis are more relevant, many non-linear equations are now easy to fit to data and these can be used without the need for linearization. However, there are various pitfalls not adequately reported to date. In particular, error structures need to be checked carefully and over-elaborate equations should be avoided.

Habilidade competitiva relativa de arroz irrigado com arroz-vermelho suscetível ou resistente ao herbicida imazapyr + imazapic

“…Para analisar os dados das variáveis AF e MPAS da planta daninha (arroz-vermelho) e da cultura (arroz irrigado), foi utilizado o método da análise gráfica da produtividade relativa (Radosevich, 1987;Roush et al, 1989;Cousens, 1991). Esse procedimento, também chamado de método convencional para experimentos substitutivos, consiste na construção de diagrama com base nas produtividades relativas (PR) e produtividades relativas totais (PRT) nas proporções de plantas de 0, 25, 50, 75 e 100% da planta daninha e da cultura.…”

“…As interações competitivas entre plantas daninhas e culturas podem ser quantificadas por vários métodos experimentais, com destaque para o em série de substituição, o qual possibilita o estudo da competição inter e intraespecífica e permite indicar qual espécie é a mais competitiva (Cousens, 1991). A premissa desse tipo de experimento é de que as produtividades das associações possam ser determinadas em comparação às do monocultivo, mantendo constante a população total de plantas.…”

Competitive Ability of Canola Hybrids With Weeds

“…The RC represents the comparative growth of canola hybrids (X) in relation to the competitors turnip and/or ryegrass (Y); K indicates the relative dominance of one species over another; and A points which species is more aggressive. Thus, the RC, K and A indices indicate which species is more competitive and its joint interpretation more safely points to competitiveness of the species (Cousens, 1991). Canola hybrids X are more competitive than turnip and/or ryegrass Y when RC > 1, Kx > Ky and A > 0; on the other hand, turnip and/or ryegrass Y are more competitive than canola hybrids X when RC < 1, Kx < Ky and A < 0 (Hoffman & Buhler, 2002).…”

“…The data were analyzed by the method of variation graphical analysis or relative productivity (Roush et al, 1989;Cousens, 1991;Bianchi et al, 2006). This procedure, also known as a conventional method to substitutive experiments, involves the construction of a diagram based on the relative (RY) and total (TRY) yield or variations.…”

“…When the TRY is equal to unit 1 (straight line), there is a competition for the same resources; if it is greater than 1 (convex line), the competition is avoided. If the TRP is less than 1 (concave line), there is mutual impairment of growth (Cousens, 1991).…”