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Crop competition halves weed seed numbers

Wheat and canola crops offer growers some really practical options to improve crop competition against weeds, particularly grasses, and vastly reduce weed seed set.

Researchers at the University of Adelaide, led by Dr Chris Preston and with GRDC investment, conducted an extensive study to identify the agronomic factors that promote strong early crop growth.

They found that simple strategies of growing hybrid canola and sowing wheat early, can couple with pre-emergent herbicides to achieve a very effective double-knock. The result is more yield, less weed seed produced and less selection pressure on the herbicides.

“In the canola trial we used a range of pre-emergent herbicides and compared open pollinated and hybrid canola,” says Chris. “The bottom line of our trial is that if you grow a hybrid canola with pre-emergent herbicides and do nothing else different, you’re going to reduce your grass weed seed set by 50 per cent.”

If you grow a hybrid canola with pre-emergent herbicides (left) and do nothing else different, you’re going to reduce your grass weed seed set by 50 per cent (right, conventional canola and no pre-emergent herbicide).

This level of non-herbicide weed control was also measured in an Australian-first study that looked at the competitive ability of 16 canola genotypes against annual ryegrass and volunteer wheat over two contrasting seasons, led by Professor Deirdre Lemerle at Charles Sturt University.

In a separate trial conducted by Rohan Brill, former research and development agronomist, NSW DPI based in Wagga Wagga, and colleagues at Trangie and Tamworth, a rule of thumb was established that seed size had a greater effect on early biomass production in canola than did cultivar type (hybrid vs OP). This gave rise to the recommendation that all farmer-retained OP canola seed be cleaned and graded to collect planting seed that is 2 mm in diameter or larger.

Their study showed that sowing large canola seed, regardless of the cultivar, is key to strong early crop growth and the crop’s ability to compete with weeds.

Having observed that later planted wheat often hosts more weeds, the Adelaide University team looked at the effect of planting wheat as early as possible.

“Our previous idea for managing weedy paddocks was to delay sowing, apply another knockdown treatment to control more weeds and then put the crop in,” says Chris. “In this trial we found that even in weedy paddocks you can put the wheat in early with a robust pre-emergent herbicide package, and the result is more wheat yield and less ryegrass seed at the end of the season.”

Dr Chris Preston, University of Adelaide, says if you sow the right wheat variety early and apply the right pre-emergent herbicide package, you can halve your grass weed numbers, just from competition in the middle part of the season, and you don’t have to change anything else.

“If you sow the right variety early and apply the right pre-emergent herbicide package, again you can halve your grass weed numbers, just from competition in the middle part of the season and you don’t have to change anything else.”

There are a few practicalities to consider when looking to sow wheat earlier. Firstly, you need to choose a variety that will still flower in the right flowering window for your location. If you are sowing several weeks earlier than normal you need a longer season variety to manage frost and heat risk at the end of the season.

Crop competition trial site at Roseworthy, SA.

Secondly, if you are sowing completely dry, then most of the pre-emergent herbicide options are open to you. If there is some soil moisture, but not enough for crop germination, some of the pre-emergent herbicides will not perform well. You need to give careful consideration to your choice of herbicide to suit the environmental conditions of each season.

These findings underpin WeedSmart’s aim, to promote farming systems that produce ‘more yield and less weeds’.

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