MODELING THE ECONOMICS OF CONTROLLING NODDING BROOMRAPE (OROBANCHE CERNUA) IN SUNFLOWER (HELIANTHUS-ANNUUS)

Field studies were conducted at nine locations in southern Spain durin g 2 yr to develop models of nodding broomrape competition with sunflow er and to establish economic thresholds, At each location, 30 to 35 sm all plots, each consisting of three sunflower plants, were chosen at r andom, The infection severity (BIS, no, of emerged broomrapes per sunf lower plant) varied from 0 to 35. Plots were harvested at maturity to assess several sunflower and broomrape population variables, The perce nt sunflower yield reduction averaged over locations due to broomrape was estimated by the equation: % SYR= 1.7 x BIS (r(2) = 0.92). Crop yi eld loss per BIS unit increased with the expected yield and was estima ted to be about 25, 50, and 75 kg ha(-1) for yields of 1000, 2000, and 3000 kg ha(-1), respectively. A consistent relationship could be esta blished between broomrape-infected sunflower yield, crop and broomrape biomass, and BIS parameters: SSYI = 0.2259 x PoBio/(1 + 0.0687 x BIS) (r(2) = 0.7820). The BIS economic threshold was about 1.5 and 3.5 for control treatment cost of $ 40 ha(-1) and potential yields of 2000 an d 1000 kg ha(-1), respectively.