Brassica weed control - timing is everything

Brassica weed control - timing is everything

1 October 2016

Take control of your crop this Spring. Spring-sown brassica crops such as summer turnips, forage rape, kale and swedes all have a critical role to play in maintaining low cost seasonal feed supply throughout the coming summer, autumn and winter.

But, if weeds are not controlled in time, they can very quickly over run a brassica crop.

Forage brassica crops are grown widely both as a supplement and as an alternative to pastures. They are important for their potential to produce high quality and high yields of forage that can be fed ‘in situ’ from early summer through to late winter.

In this context, broadleaf weeds present both a short and a long term problem. In the short term, they can severely compromise brassica crop yield, potentially cutting as much as 30-50% dry matter per ha (DM/ha) off the expected total.

This in turn means less feed is available when it is needed, increasing requirements for imported supplement to fill the deficit.

In the long term, broadleaf weeds effectively contaminate the ground for new pasture sown next autumn or spring.

So how can you ensure this doesn’t happen? It all hinges on the first few weeks after sowing, with a robust programme of post-emergence weed surveillance and control. Your time and money will be well-spent but the window is not a long one – once the weeds get past a certain size and/or the crop canopy starts to close, herbicides become less effective.

And while brassica crops are sprayed for post-emergence weed control more now than they used to be, getting the timing right remains a common challenge, according to Nufarm technical specialist Paul Addison.

“A lot of people still leave it too late,” he says. “They spray when the weeds are too big for the herbicide to do its job properly, so they just end up suppressing them rather than killing them outright.”

Paul’s advice? “When it comes to controlling weed, timing is key. Start looking at your crops closely about three weeks after sowing, to determine what weeds are present and to organise spraying if required. That will help ensure you hit the weed seedlings at the correct stage. The crop will typically have two to four true leaves at this stage.”

Several herbicide options are available, depending on which crop is grown and the species of weeds present. These herbicides give the best results when applied to actively growing small seedling weeds, typically three to six weeks after sowing.

Prestige®, for example, can be used in all fodder brassicas to control a range of broadleaf weeds but is particularly useful against black nightshade and fathen. It should be applied at 350 mL/ha with Bonza® spray adjuvant at 500 mL/100 L of water.

Archer® is another option for broadleaf weed control, especially for yarrow, thistles and Californian thistles. Recommended rates are 0.5 to 1.0 L/ha, after the crop has two true leaves and before canopy closure.

Prestige or Archer can be tank-mixed with SeQuence® for control of both broadleaf and grass weeds where these are also present.

For kale, Kamba® 500 controls a wide range of broadleaf weeds including black nightshade, amaranthus, dandelions, fathen, yarrow and more. Kamba should not be used on rape, turnips, hybrid forage types or swedes.

For more advice on post-emergence control of broadleaf weeds in your fodder brassica crop this season, contact your local Fonterra Farm Source Technical Sales Rep today.

®Prestige and Bonza are registered trademarks of Nufarm Ltd. ®Archer, Kamba and Sequence are registered trademarks of Nufarm Australia Ltd

Article supplied by Nufarm NZ