Integrated weed control: don't feed the buggers

IFYOU haven't done your homework on weed control or if you've got a resistance problem, then applying nitrogen, particularly at the wrong time, could cost you.

Work carried out at the Wagga Wagga Agricultural Institute last year saw yields drop a massive 28 per cent in weedy plots where nitrogen was top-dressed and incorporated at sowing. In fact, all nitrogen treatments reduced yield in the weedy plots with under-crop banding doing least damage with a 10 per cent yield penalty.

NSW Agriculture researcher Eric Koetz, who carried out the work in cooperation with Steve Sutherland (NSW Agriculture) and Incitec's Tony Good, says the message is clear: "If weeds are a problem and you have sufficient mineralised nitrogen at sowing to feed your expected crop, leave the nitrogen in the bag".

Trial results

The trials involved Diamondbird wheat sown at 80 kg/ha in plots with around ISS kg/ha of mineralised nitrogen at sowing. Where weeds weren't a problem, the wheat was sown together with ryegrass (ARG). Urea - 174 kg of it - was applied in a number of different treatments.

It went on either as a top-dressing incorporated during sowing, as top-dressing when the first node was about I cm above ground, banded mid-row, banded under the crop or applied as a tactical application, half broadcast, half top-dressed. No urea was applied to control plots.

But Mr Koetz makes the point that nitrogen can be used as a tool in an integrated weedcontrol strategy.

The right crop feeding schedule is key

"Getting the timing of the application right can stimulate the crop at the right time and can improve the crop's competitiveness. We got our best yield results from a tactical application of nitrogen during the season. Putting it all on at one time is not a smart strategy whether weeds are a problem or not."

How much damage can weeds do? In a straight comparison between weed-free and weedy plots without nitrogen application, the weedy plots dropped 17 per cent in yield. Last year's dry spring took a heavy toll on crops that had produced a big early biomass and high tiller count. So, as expected, the crops that got all their N early paid the price in comparison to those where the application came later in the season.

The trials continued at several locations in southern NSW this year. Results aren't to hand as yet but the strange season, late start and a series of 'just-in-time' rains will probably produce a different set of results.

Contact: Mr Eric Koetz 02 6938 1955