Relationships between Sunflower Plant Spacing and Yield: Importance of Uniformity in Spacing.

Sunflower (Helianthus annuus L.) production systems in semiarid regions often have poor seedling emergence and patchy stands leading to lower yield. We hypothesized plant spacing was positively related to individual plant yield and this relationship would strengthen as water deficits increased. In a 2-yr study in Colorado, individual plant spacing and associated seed was measured for different irrigation treatments. Highly significant positive linear and second-order polynomial regressions of individual plant spacing and yield (p < 0.001) were observed for all irrigation treatments in both years. However, given highly variable individual plant yield with spacing, regression explained no more than 16% of the variation. No consistent pattern in linear regression slope, y-intercept, or R2 values among irrigation treatments were noted, thereby not supporting the expectation that the relationship would be greater as water deficits increased, although rainfall events may have confounded the results. Leverage plots showed similar results as the regression equations but indicated plant spacing explained yield variation more than irrigation treatment. Regression tree analysis further confirmed that plant spacing was more important than irrigation treatment in explaining final plant yield. These results suggest there is a cost to final yield per unit area where large spacing results in lower yields even though these plants have the highest yields per plant of any spacing classes. [ABSTRACT FROM AUTHOR]