How to address weed control ahead of reseeding

21 Nov 2022
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Planning ahead is a top priority for weed control

The extremely hot and dry weather experienced during the 2022 season, coupled with the effect of reduced fertiliser inputs on some farms has resulted in lower forage yields than would normally be expected. Weed control in established grassland and in young grass leys should therefore be a top priority for producers in spring, to maximise forage potential.

Controlling weeds in established grassland will easily pay back through increased milk production from forage, 10% ground cover from docks represents a 10% loss in yield, but planning ahead and attention to detail are important to achieve the best results.

It is about getting the basics right and understanding what is required for effective weed control. Perennial weeds must be at the correct stage of growth and in active growth to translocate sufficient chemicals into the root.

Correct application is key

One of the fundamental principles of weed control which is often overlooked is the need to make sure a lethal dose of the product is delivered to the plant. By definition, a sub-lethal dose will not kill the weed. Many perennial weeds of grassland have large root systems, and if the dose of chemical which makes it down to the roots is sub-lethal the weeds will simply regrow after treatment and the whole process will have been an expensive waste of time.

To be successful, the right product needs to be applied at the right rate under appropriate conditions. There needs to be sufficient leaf area on the target weed, for example, docks need to be 25 cm across, roughly the size of a dinner plate. Generally, applications should be made when weeds are healthy and actively growing. In late summer perennial weeds such as docks and creeping thistles will often have flowered and produced seed. If such plants are topped and the regrowth sprayed two weeks later, control can be extremely effective. The plants are moving assimilates into the roots for storage to survive the winter and consequently will draw more chemicals down as well.

Weed control in New leys

Not enough thought is given to planning weed control ahead of time when reseeding. When the conditions are right for grass seeds to germinate, they are also ideal for weed seedlings to emerge. Often the best time for herbicide applications is during establishment when it is possible to achieve 100% control, something which becomes increasingly difficult as weeds become larger.

In a new ley docks and chickweed can be quite damaging and 10 plants/m2 can reduce the population of ryegrass seedlings by 50%. In established grass, every 1% increase in ground cover by weeds will result in a 1% decrease in grass growth and therefore control of weeds such as docks, nettles and thistles is vital. For example, in a grazing sward with 10% weed infestation, grass yields could improve from 10t DM/ha to 11t DM/ha through correct herbicide applications. As a result of increased grazing availability and subsequently increased stocking rates, this could produce a return on investment (ROI) of as much as 14:1.

A range of products are available for use on young grass leys, but in recent years the choice of ‘clover safe’ herbicides has been restricted. The welcome return of the active ingredient tribenuron in 2023 offers effective chickweed control in young clover leys. Another new clover-safe product is expected in spring 2024, which will considerably increase flexibility and options.