FAQ: Weeds and Herbicides

Is it necessary to spray established lucerne stands for weeds every year?

No, you don’t need to spray a stand every year if you are happy with the stand density.

Spot spraying areas where a weed problem is developing to prevent the rest of the paddock becoming contaminated is an option if the rest of the paddock is relatively weed free. However, we have worked out the cost of spray as being about the equivalent of 500 kg DM/ha. So if you think there will be 500 kg DM/ha of weeds by the end of spring it makes sense to spray them.

The issue is sometimes not what you can see now but what germinates in the spring. Under grazing it may be that the animals eat the seedlings while they are small but if left for silage or hay the newly emerging spring weeds may become a problem- so as always it depends and the final decision needs to take into account the type of weeds present and the final use for the stand (graze or cut).

How can I remove the weeds without damaging autumn sown lucerne in spring?

First, why did you sow in autumn? It is seldom as successful as spring sowing. To remedy graze the paddock to remove lucerne leaf area.

If you have a good plant population from your autumn sowing herbicide may not be necessary if the majority of weeds are spring annuals – they won’t recover from grazing and should be shaded out as the lucerne reforms its canopy after grazing.

If the lucerne population is ok (but not great) it will still come through – but slowly. Wait for the paddock to refresh (2-4 days after grazing) and then spray for broadleaf weeds.

For stands with poor populations following sowing – these can be thickened up by overdrilling with more lucerne. Hard graze, immediately overdrill and herbicide 2-4 days later (non-residual and herbicide application should occur before newly overdrilled lucerne emerges)

BUT….autumn establishment of lucerne should be avoided if possible because it takes at least 18 months to catch up with where you’d be if the stand had been spring sown. An annual forage followed by spring sowing is preferred for successful lucerne establishment.

How do I control dandelion (Taraxacum officinale) in my lucerne?

Dandelions are taprooted weeds that compete with lucerne for water and nutrients. Their numbers increase as the lucerne stand ages, although chemical control is now possible.

In mid/late Autumn after a clean-up graze, dandelions can be sprayed with metsulfuron at a low rate (5-8 g/ha), but this chemical will knock the lucerne. Talk to your chemical representative before using it.

How can I control docks (Rumex obtusifolius) in my lucerne?

Asulox (a.i. = asulam) kills docks – it is able to be used on lucerne in autumn when the docks are still actively growing. Application after a 20+ mm shower of rain that breaks summer dry periods is ideal. The use of lucerne/grass mixes can complicate things but Asulox is also registered for established pasture. For both lucerne and pasture a period of 2-3 weeks of active growth after cutting or grazing is recommended before spraying. Check with agents for product and water rates. Docks have a deep tap root and therefore compete with lucerne so control is important where possible.

How do I control horehound (Murrubium vulgare) in my lucerne?

A grubber in the truck and ever vigilant staff are the most common methods of horehound control. Make sure you clean-out any new patches anywhere on your farm because the seeds will become lodged in the sheep’s wool and be transported into your lucerne paddocks. Where grubbing is impossible some success has been gained using Roundup 490 at 750ml/ha, with 10 grams of metsulfuron/ha as a spot spray. This is definitely off label and is used at the owners risk. In the field comments suggest if you can spray onto wet horehound (i.e. in morning after frost) it is most effective. We at Lincoln University have no experience of this combination to date so are reporting this information without any experience – happy to add comments from anyone who tries it and is successful or unsuccessful to add to the knowledge pool. (Leave us a message by emailing a member of the Dryland Pastures team via the “Contact us” page).

What is the best herbicide to control grass weeds in a lucerne stand?

Paraquat and atrazine are the standard chemicals used but there are lots of options including products containing haloxyfop-P (e.g. Gallant) to control grasses in lucerne monocultures.

How do I control wild plantain (Plantago lanceolata) in my lucerne?

Doug Avery responded that he controls plantain at ‘Bonavaree’ during their normal winter spray programme which is Gramoxone (a.i. 250 g/litre paraquat) and atrazine. Alternatively, after heavy grazing (no lucerne leave surfaces to intercept the spray) use 2,4-DB. The preference would be for this to occur in spring not autumn – the lucerne crop may be suppressed (less feed than expected in the rotation following the application) but spring application will allow time for the crop to grow out of it in the subsequent rotations.

How do I control mallow (Malva spp.) in my lucerne?

Products containing flumetsulam as the active ingredient (e.g. Preside) can control young mallow (< 4 leaves), but weeds must be actively growing to get a good kill. As with most weeds, the herbicide is more effective on young weeds in comparison to older established weeds. For older mallow the best control is with a grubber. However, some farmers report that imazethapyr based products (e.g. Spinnaker) will stop young mallow plants from producing seeds which can contaminate lucerne seed crops.

When should I spray mallow (Malva spp.) and nodding thistles (Carduus nutans) in a lucerne stand?

Mid-summer rainfall provides an excellent opportunity, while temperatures are warm, to use herbicide on difficult perennial weeds that may be affecting older (3 years +) lucerne stands. E.g. Mallow (Preside, a.i. flumetsulam) and Nodding thistles (Velpar DF, a.i. hexazinone).

As always, check the product labels carefully for details and apply as recommended. Spray after grazing when lucerne leaf is minimal.

How can I control scotch thistle (Cirsium vulgare) in my lucerne stand?

In most cases we avoid using expensive herbicides such as Velpar DF ($150 -$200/ha). However, we do use such chemicals were the lucerne establishment is under threat and Velpar is a very effective option for various weeds but it can cause damage on young lucerne stands. I would recommend paddocks be inspected first by experienced agronomists before committing to this option.

People also report success with a well maintained and calibrated weed wiper or wick boom. Directly after grazing (when the lucerne is below thistle height) applying Metsulfuron (Associate/Zeal etc) @ 2-3 g/litre water with a slow double pass (East-West/ North–South) ensuring that the boom is totally saturated with the mix before starting. This option is widely used and is very cost effective. Some farmers have also reported success with using 2L/ha of MCPA on young lucerne (10-15 cm high). This also controls fathen and other broad leaf weeds.

How can I control stinging nettle (Urtica urens) in lucerne?

Stinging nettles will be eaten by sheep if they are topped – the animals will then happily graze them. If a chemical option is required Basagran (a.i. 480 g/litre bentazone) or 2,4-DB should get them as seedlings (not mature established plants). Apply after grazing, when the lucerne canopy has been eaten off (less lucerne surfaces to put chemical on and better access to the weed leaf surfaces). Weed wiping with a stronger chemical, like 2,4-D will also work but only wiped with a calibrated and high quality wiper following a hard graze of the lucerne – it will kill the lucerne if you make contact/drip the chemical on the plants.

What can I spray on my new lucerne to control wireweed (Polygonum aviculare)?

Two suggestions – although we have no experience with either chemical on this weed;

Preside or equivalent (a.i. flumetsulam) is reported to provide reasonable control as a seedling and should be used in warm conditions after summer rainfall.

Equate (a.i. imazethapyr) is reported to control wireweed when applied to lucerne at the two leaf stage or later.