Case history for weed competition/population ecology: barnyardgrass (Echinochloa crus-galli) in sugarbeets (Beta vulgaris).
Field trials were conducted on sandy loam in 1979-81 to evaluate the competitive relationship between spring-sown sugarbeet (cv. USH-9, SP-435 and SSE-1 in consecutive years), spaced at 15-20 cm apart after establishment, and E. crus-galli sown at 0.1-100 plants/m crop row. E. crus-galli at ≥10 plants/m row caused >80% sugarbeet root yield loss. The loss decreased to between 5 and 20% when E. crus-galli densities were 1 plant to every 2-3 m of crop. The relationship between crop yield and E. crus-galli density was described by a rectangular hyperbolic function. The economic threshold density was in the range of 1 E. crus-galli plant every 5-20 m of row. E. crus-galli growing at densities of <1 plant/m in sugarbeet showed only interspecific competition. Such individual E. crus-galli plants were estimated to produce from 30 000 to >200 000 seeds (av. of 100 000 seeds/plant). This equated to a seed rain of 4000-20 000 seeds/m from a density of 1 weed every 10 m of crop row. These data suggest that using single-year economic thresholds for E. crus-galli is not sound management practice, as low plant densities can produce large quantities of seed.