Using cutting regimes for weed control

Cutting and topping weeds will have an impact on the type and weed flora in a field and can be invaluable in preventing return of weed seed to the soil seed bank. Cutting and topping are important for weed management in pasture, grass and leys. Topping can also be used as a remedial measure in vegetable or other crops to prevent weeds from seeding.

Pasture systems: good management involves maintaining the condition of the sward by cultural means. In particular reducing weed intrusions by chain harrowing in spring and topping regularly during the growing season. Mowing during the seeding year must be carefully judged and close cutting avoided. Spring sown stands should be cut no later than mid-August to allow recovery before winter. Summer sowings should be left unmown until November. Undersown lucerne should be left to grow into the winter. Where companion grasses are growing strongly, light winter grazing may be desirable. In grazed pasture weeds that are not eaten by livestock, will need to be topped to prevent seed shed. The established crop may be cut up to four times per year starting in mid-May. The crop is quickly weakened by defoliation, either by grazing or cutting, at too young a stage especially in spring or autumn. Before entering the dormant stage the crop must be allowed to make sufficient growth to replenish the food reserves in the root.

Grassland systems: cutting for hay or silage will have an impact on the weed flora. Silage tends to be cut early in the season when the sward is young and fresh, whilst hay is cut at a later stage. There can be both advantages and disadvantages associated with the timing of cutting depending on the weed flora and the ultimate requirements of the system. Cutting late may allow weeds in the pasture to grow to maturity and set seed. The ripe seeds may contaminate the hay and remain viable when passed through livestock. Dock seeds should not survive low pH silage, however they will survive in a later cut of hay. This mature seed may also shed on the ley surface and find opportunities to germinate in situ or be transported by livestock to other locations. In contrast, cutting early for silage in fields, with for example an infestation of creeping thistle, may encourage the spread and growth of this weed. Hence, there has to be a balance between the requirements of the farming system and weed control implications.

Horticultural and stockless arable systems: ley management will include topping at intervals during the summer to a height of around 10-15 cm. Ideally in fertility building leys the sward should not be allowed to get higher than 40 cm (or knee height). If the vegetation gets higher than this, then topping will create a mat of vegetation that will act like a mulch. This can create dead spots in the ley where clover may be excluded by the more vigorous grasses, or which weeds may colonise. Topping the ley regularly will also ensure that tall weeds that may have germinated will not be able set seed.

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