Farm Ireland

Saturday 26 May 2018

The holistic approach to weed control

The old saying of "one years seeding is seven years weeding" has never been as relevant to tillage farming as now.

The cost of weed control is not just the herbicide to directly control the weed but also include; pre-harvest round-up, leaving the dirty headland to the end of harvest, stubble cultivation, slower/more accurate ploughing, a stale seed bed, etc.

For effective and sustainable weed control, these measures used in a planned approach will reduce weeds, especially resistant weeds on the farm.

With cost control in mind, the real business of controlling weeds comes down to your knowledge of the field (weed spectrum, crop density and growing conditions), the herbicide choice and the weather.

Increasing your knowledge

Excellent weed control can be achieved by all growers but some basic knowledge is required. In the same way as people memorise a football or rugby team from one to 15 with a few substitutes, knowing the top 15 weeds or so is essential when trying to control weeds.

Arguably the top 15 weeds in arable fields are; 1 Chickweed, 2 Charlock, 3 Knotgrass, 4 Bindweed, 5 Cleavers, 6 Fumitory, 7 Speedwell, 8 Fat hen, 9 Orache, 10 Redshank, 11 Corn Poppy, 12 Red Dead nettle, 13 Hemp nettle, 14 Small nettle, 15 Mayweed.

The reserve players, less common weeds, are Nipplewort, docks, and thistles with wild oats, canary grass and annual meadow grass strong contenders. As with every team there is always the unknown quantity and the same goes for weeds.

Also Read

Good observation and record keeping from year to year builds up a bank of knowledge as to the weeds not controlled or poorly controlled. These weeds should be in the mix when deciding on weed control the following year. Bring a picture of these common weeds at various growth stages (cotyledon, first pair of true leaves and adult plant) with you when walking crops and identify these weeds as you go.

Pictures of these weeds can be found on the Teagasc website or there are many good phone apps (I find the Bayer Weed Spotter very useful).

It's quite easy to identify weeds with patience in the field during inspection. Challenge your advisor to give you a crash course in weeds identification when walking cops with you.

Choosing the herbicide is important but it isn't everything. Crop density, sowing date and general growing conditions all impact on the success of weed control in cereals.

It is fundamental to work with the growing conditions around the spraying time to optimise weed control and reduce costs. The following will improve your weed control and save money:

Apply spring herbicides at 1-2 leaf stage of the weeds (generally coinciding with the four leaf stage of the crop)

Where possible delay application until after three days of good growth weather. This increases the weeds susceptibility to herbicides

A thick crop will ensure good competition after the herbicide application ensuring emerging weeds are starved of light, water and nutrients

Applying a herbicide to an open crop will ensure the herbicide gets to the target weed. This is especially important for wild oat control which should be completed earlier than most people are doing at the moment.

Ensure you are using the correct rate of herbicide for the weed size. Larger weeds need larger rates and vice versa. However timing is as important where trials show better weed control and higher yield were achieved from using half rates (or less) of herbicides applied early compared to full rates at a later timing.

Don't over load the tank with other chemicals. Generally don't apply more than four products together and always check the compatibility of the mix ie it physically mixes together. Also check the mix is biologically compatible ie it may be ok in the tank but it could scorch the plant or one part of the herbicide mix may not function properly (eg don't use Axial with hormone type herbicides - the Axial wont work)

Resistance to herbicides are more common across the country with farms experiences of resistant marigold, chickweed and poppy more commonly seen every year. Other weeds such as sterile brome and canary grass are also more common. Controlling these requires a more holistic approach. This type of approach will result in lower costs over time. l NOTE There is an important change for 2016: Straight MCPA has been reregistered and is no longer cleared for use in cereals. (MCPA is available in other mixes)

Click Here For: Herbicides for Broadleaved Weeds in Cereals 2016

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Reining in wild oats

Controlling wild oats is very important as they can significantly rob yields. Growers are now striving for denser canopies, especially in barley, early in the season. 

These closed canopies can make it difficult for wild oat herbicides to penetrate to the wild oat, especially the smaller tillers beneath the cereal canopy. Earlier application of wild oat herbicides mid-tillering is advisable with these dense canopies. This means controlling broad leaves earlier at the four leaf stage.

Many growers worry that controlling wild oats early will reduce the overall control as late germinating oats may come through. It is not the case as crop competition will smother out any late emerging oats.

Indo Farming