Introduction

• Sunflower seedlings are poor competitors to weeds, and young plants are easily damaged by mechanical cultivation.
• For this reason sunflower should not be used as a breaking crop on new land where a large amount of weeds, roots and other debris is normal.
• Where a persistent weed occurs which is difficult to eradicate in sunflower, the previous crop should be one in which this weed can be controlled.
• Conversely, sunflower can be used as a cleaning break for a subsequent crop, `Bathhurst Burr'(Xanthium spinosum) is one cereal weed easily controlled in sunflower.
• As far as practicable, pre-planting operations should leave the seed-bed, weed-free.
• Post-emergence tillage should be as shallow as possible, with implements set to throw soil towards the rows as soon as seedlings are strong enough to with stand this treatment.

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Critical stages of weed competition

• Sunflower is raised during June-July in 'Kharif' October-November 'Rabi' and December-January in summer seasons adopting a wider spacing of 60 cm.
• Thus inter rows are prone for weed infestation.
• The crop weed competition largely depends upon the type of weed flora, their density which in turn depends on soil type and crop season.
• Maintaining weed free conditions up to 45 days after sowing resulted in yield increase in sunflower.
• Period from sowing to 45 days is critical for weed control in sunflower.

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Methods of Weed Control

Mechanical control

Mechanical control of weeds remains the most usual method, but the period of effective machinery use is limited by sunflower's rapid growth, or by unsuitable soil conditions. Over-the-top harrowing with fixed or spring-tined weeders is possible in colder areas, but is not to be recommended in warmer regions where plants tend to be more succulent.

• Spring-tined or rotary weeders are most effective when used between rows of young seedlings and are least damaging to developing roots.
• Sunflower's rapid growth reduces the weeding requirement by shading out annual weeds, although perennial grasses tend to be more tolerant.
• At present there is usually no practical substitute for manual weeding or spot spraying to remove them.
• Harrowing with blade harrows should be done within 20 days after germination, otherwise the plants will break due to their brittleness.
• Harrowing with blade harrow not only control weeds but also looses the soils helps in reducing evaporation and encourages infiltration.
• Light earthing up is advantageous when the plants are at 25-30cm height which avoids lodging.

Chemical control

Pre-emergence herbicides are effective but should be used with care, preferably after small-scale trials to determine their effect on the crop, as results in different countries have varied. Sunflowers are extremely susceptible to hormone herbicides, and these compounds should not be used, nor in their vicinity. As a guide, the following herbicides have been used pre-emergence in commercial sunflower crop either singly or in combination; linuron, chloropropham, dinitramine, eptam, monolinuron, pendimethalin, prometryne simazine,and trifluralin, some of which must be soil-incorporated prior to sowing. Herbicides showing promise are butralin, fluchloralin, nitralin and profluralin, and there are a number of pre-sowing herbicides still in the trial stage, including ethofumesate and ethalfluralin.

• There are presently few recommendations for commercial post-emergence applications, although dalapon has been so used as a directed spray.

Spray recommendations

• Alachlor or pendimethalin at the rate of 1.5 kg a.i./ha in 600 litres of water as pre-emergence spray for the control of weeds on the same evening after sowing.(pre-emergence)There can be differing reactions to chemicals applied to rain-grown and irrigated crops, and also the amount of active ingredient necessary to control weeds.
• In general, irrigated sunflower appears to be more susceptible to herbicide damage at the same active ingredient level than when rain-grown, and this is probably due to leeching of the chemical into the main root zone.
• Cuscuta indecona (large-seed dodder) can be controlled with flourodifen.

Herbicides and time of application

• The above herbicides are applied 2-3 days after sowing, by mixing the herbicides in 800 litres of water as uniform spray on the soil surface.

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