Determination of Critical Period of Crop-Weed Competition in Hybrid Sunflower

Sunflower is gaining importance due to its photoinsensitivity, short duration, low water requirement, drought tolerance and wide range of adaptability to various agro-climatic situations. Despite adoption of good management practices, the productivity of sunflower has been low. Among different production constraints that are limiting the productivity of sunflower, intensive weed competition is one of the major barriers to enhance the productivity. Sunflower, being initially slow growing crop, provides congenial environment for abundant weed growth. The loss in seed yield of sunflower due to full season of weed competition was upto 25% (Singh et al., 1997; Wanjari et al., 2000). Weed competition during entire crop season resulted in yield reduction of 55.8% (Tripathi and Vivek, 2001). Hence, the present study was conducted with an objective to determine the critical period of weed competition in sunflower. A field experiment was conducted during rabi, 2004 at S.V. Agricultural College Farm, Tirupati Campus of Acharya N. G. Ranga Agricultural University, on sandy clay loam soil having pH 6.9, low in available nitrogen and medium in available phosphorus and potassium. The experiment was laid out in a randomized block design with three replications. The treatments (14) consisted of weed free condition for the first 10, 20, 30, 40, 50 and 60 DAS and weedy condition for 10, 20, 30, 40, 50 and 60 DAS alongwith weed free and weedy till harvest. Sunflower cv. MSFH-17 was sown at 5 kg/ha seed at a spacing of 45 x 30 cm during second fortnight of November 2004. The recommended dose of 75 kg N, 90 kg P2O5 and 30 kg K2O/ha was uniformly applied. The entire dose of P and K was applied as basal. N was applied in three equal splits i. e. one third as basal, one third at 30 DAS and the remaining one third at 55 DAS. Density and dry weight of weeds were periodically recorded by using two quadrates (0.5 m2) in each plot randomly. Weed flora of the experimental field consisted of three species of grasses, one species of sedge and six species of broad-leaved weeds. Cyperus rotundus was the only sedge and major weed in the experimental plot. The other predominant weed species found in the field were : Cynodon dactylon, Commelina benghalensis, Euphorbia hirta, Celosia argentia, Leucas aspera and Phyllanthus niruri. Increased initial weed free condition led to decreased total weed density and dry weight (Table 1). Total weed density was high upto 40 DAS, due to initial slow growing habit of sunflower, which provided congenial environment for abundant weed growth and crop canopy did not cover the ground. Most of the weed species were suppressed during later stages, mainly due to smothering of late emerged weeds by crop canopy and some weeds completing their life cycle. However, highest weed density and dry weight were recorded in unweeded control. The results of present investigation are in conformity with the findings of Wanjari et al. (2001). Consistent and significant reduction in the dry matter of weeds recorded with the increase in the duration of weed free condition upto harvest (Table 2). Rapid dry matter accumulation was upto 60 DAS but at harvest weed dry weight decreased. It might be due to suppression of weeds by crop canopy after 60 DAS. However, when weed free conditions were maintained for 10 or 20 days after sowing, most of the broad-leaved weeds reemerged and due to extensive underground system produced substantial dry matter. Sunflower yield and yield attributes viz., head diameter, total and filled seeds per head and 1000-seed weight were significantly influenced due to different weed free and weedy periods. The maximum yield and yield attributes were observed in plots which were kept weed free till harvest. Harvest index was not influenced due to different weedy and weed free treatments. Similarly, oil content of sunflower did not differ significantly due to different weedy and weed free treatments, as the oil content is a genetical character (Table 3). The results are in line with the findings of Kumar et al. (1996) and Wanjari et al. (2000).