Yield-density equations and their application for agronomic research: a review

Plant population density (density) trials are time consuming, repetitive and cost intensive. The literatures indicate that density trials are more effectively and efficiently described when meaningful yield-density equations such as reciprocal linear and/or parabolic equations are applied to quantify data. Several of these types of equations and their derivatives such as competitive indices have been proposed. It is generally agreed that where response to density deviated from linearity, equations based on linear reciprocal of yield per plant and density can satisfactorily describe an asymptotic response. In certain cases when the harvest index is substantially affected by density and/or due to unfavourable growth conditions, equations that assumed parabolic responses may be more valid. The review by Willey and Heath (1969) on the quantitative response of yield to density appears to have remained a useful resource up to date. However, more recently, several investigations based on reciprocal quantitative relationships between yields and density and how this may be influenced by other agronomic practices were carried out. Yet, most of these substantial knowledge and progress on yield-density investigations appear to be scattered in published or unpublished works as no attempt has been made to collate them together. This paper reviews more recent progress on yield-density equations research and their application, and highlights needs for the applications of some of these equations for agronomic research with more emphasis given to intercropping.