Farm Ireland

Thursday 23 November 2017

Spraying will nip the weed problem in the bud

Control plants as seedlings to keep future grass yields high

Tim O'Donovan

In the next few weeks, most of the July and August sown grassland will be at the correct stage for applying a herbicide. Considering the cost so far, it normally pays handsomely to apply a herbicide, especially where there were docks and thistles in the field before you reseeded. A little bit of planning will ensure that you get the most from your investment.

In most cases, applying a herbicide six to eight weeks after reseeding is the cheapest and most effective spray you can apply to a grass field.

If you had docks in the field before reseeding, there can be up to 100,000 dock seeds/ac ready to germinate and reduce your grass yield in future years.

If ever the saying 'a stitch in time saves nine' was appropriate to a situation, then this is it, and the same goes for thistles, buttercups and dandelions.

Control these weeds when they are seedlings and they will not cause you problems in future years.

Annual weeds such as fumitory, fat hen, and dead nettle only live for one year. These weeds should not affect the establishment of the new sward, unless present in big numbers.

They are normally removed by grazing, cutting or leaving them die naturally over winter.

Where high numbers of annual weeds are present, the herbicides in the table (below) will remove them. If left uncontrolled, these weeds will cause gaps, which are an ideal place for docks, etc, to germinate and grow over the winter months.

Also Read

Perennial weeds such as docks, dandelions, creeping buttercup and creeping thistle, etc, can be tolerated, at low numbers, in an intensive grazing situation.

It is rare for any weed to become problematic in a well managed grazing field.

Soil drainage, fertility and grazing management will influence whether a problem occurs in the future. For example, poaching brings on dandelions and buttercups, while high levels of potassium favour the establishment of docks.

Low numbers of perennial weeds should be removed if the field is for silage/hay or has clover in the mix. Silage/hay cutting allows perennial weeds to build up their root reserves to a much greater level than if they were being grazed every three weeks.

Clover-safe herbicides do not control the same range of mature perennial weeds so take them out at the seedling stage if you get an opportunity to do so. Many websites have picture of all the weeds you will see in your fields, or get your adviser or merchant to identify the ones present and recommend a suitable spray.

Weed control trials

A grass reseed trial has been ongoing in Teagasc Kildalton since 2009. As you can see from the picture where herbicides were applied to reseeds (above), there is a very good effect on dock numbers 18 months later. All of the products gave more than 70pc control. There were more than 24,000 seedling docks/ac in the trial field before spraying. In the unsprayed plots, there is now just over 28,000 mature docks/ac, a 16pc increase in 12 months.

The graph (below) shows the differences between the products. Note this field is cut for silage each year and grazed. Also note that the products were applied four months after reseeding due to poor weather in autumn 2009 and all products would be expected to perform better if applied six to eight weeks after re-seeding

Help the spray work

The first point of weed control in new leys is to have an even, vigorously growing sward. Seedbed preparation, soil fertility and seeding rate are key points to get right. There has been much research carried out on the competitive effects of grass (and clover) on weeds.

A competitive sward will enhance the effect of any herbicide applied and fill in the gaps when the weeds die back. Apply the herbicide according to the label instructions. Drought-like conditions before spraying reduce the effectiveness of herbicides so talk to your adviser or merchant if this is the case. Rates of water are normally 220-350L/ha (20-30ga/ac). Remember, all herbicides require records to be kept under cross-compliance rules.

applying herbicides in a new ley

Weeds in new leys are best controlled when they are small (six to eight weeks after reseeding) and actively growing. You can expect very good results from applying any of the herbicides in the table when they are sprayed onto small, growing weeds.

Also follow the product label instructions when applying all plant protection products.

The stage of growth of the clover and grasses also has to be considered. The aim should be to time the spray so that the clover has at least one trifoliate leaf and the grasses have at least two to three leaves.

A list of the main herbicides is shown in the table. The main choice will be made by whether or not you have clover in your sward.

Having clover should not hinder the level of weed control expected as a well-timed clover-safe spray should be very effective.

If clover is not in the sward, then the options available are more comprehensive, especially if you have a heavy weed burden.

If you have weeds emerging from old roots (eg, creeping thistle, docks, etc), it will be difficult to achieve good control with a clover-safe spray and you may have to switch to other options.

There is one major change to the product list this year, the old favourite CMPP is no longer cleared for use on agricultural grassland.

Notes on this change are available from your Teagasc adviser or your agri- chemical merchant.

Indo Farming