Effect of population design and planting pattern on yield response of grain sorghum

Abstract

A range of grain sorghum cultivars were grown under irrigation of various population densities and planting pattern in three seasons. Yield responses are interpreted by a method allowing extrapolation to a wider environmental range. High populations (4 and 8 × 105 plant per ha) were always superior or equal to the standard population (2 × 105 plants per ha) generally used under irrigation. Whilst cultivar responses varied, no yield depressions were recorded at high population. Application of these results to other irrigated environments may be made with some confidence. This superiority of high population varied with season, and was related to variability of pre-anthesis growth. Seasonal conditions favouring rapid pre-anthesis growth resulted in a yield plateau at low populations (<2 × 105 per ha) (asymptotic response). In contrast, environments causing slow pre-anthesis growth (e.g. cool spring conditions) led to an increasing yield over the entire population range (hyperbolic response). Most of the yield differences were associated with differences in grain number, and hence with pre-anthesis environmental conditions. Maximum yield appeared to be the result of rapid canopy closure, whether achieved by high rated of growth of individual plants (asymptotic responses), or by high populations, when rates of growth of individual plants slow down (increasing hyperbolic response). These results contrast with responses to increasing population recorded for most species, where clear optimal ranges can be defined (parabolic responses).