COMPETITION BETWEEN RIDOLFIA-SEGETUM AND SUNFLOWER

Studies on competition between Ridolfia segetum Maris. and sunflower ( Helianthemum annuus L.) were conducted at eight locations in southern Spain in 1990 and 1991, in order to define competition models and to e stimate from these economic thresholds as affected by crop inputs and potential yields. Competition losses in sunflower crops ranged from 19 % to 56% of weed-free yields. There were slightly better correlations between percentage sunflower reduction and weed density than with weed dry weight, (-0.66 and -0.59, respectively). The weed competitive ind ex, or sunflower crop dry weight reduction per unit dry weight of R. s egetum, was 1.09. The percentage yield losses due to weed density (NPR (t)) were fitted to multiple linear, quadratic, exponential and hyperb olic models. The hyperbolic equation, %RSY=100 (1+1/bNPR(t))(-1) wher e b=0.14 and is the R. segetum competitive ability index, had the lowe st error sum of squares (SSE), and gave the best biological explanatio n for the competition response. Early emergence (before mid-March) mad e weeds about 1.5 times more competitive than late emergence. The econ omic threshold to offset the cost of a shallow post-emergence tillage, assuming 70% control efficiency, ranged from about 2.5 plants m(-2) f or low-yielding crops (1200 kg ha(-1))to less than one plant m(-2) for higher-yielding crops (2800 kg ha(-1)).