Evidence is mounting that summer crops really can be an effective weapon in the war on herbicide-resistant weeds. With soil moisture at a premium, there is nothing spare to waste on summer-growing weeds.
With four of the most difficult to control summer weeds — feathertop Rhodes grass (FTR), awnless barnyard grass, common sowthistle and flaxleaf fleabane — now with confirmed cases of glyphosate resistance, the pressure is on to find effective non-herbicide control tactics.
These weeds can produce 40000, 42000, 25000 and 110000 seeds per plant respectively.
Various trials in mungbean, soybean and sorghum crops have consistently found that planting these summer crops in competitive configurations can reduce weed biomass and seed production while maintaining or increasing crop yield.
The change in row configuration may involve set-up costs to modify planting equipment but does not require an increase in seeding rate as the effect has been demonstrated using the same established populations for mungbean and soybean. For sorghum, the competitive edge comes through increased plant population rather than narrower rows.
Along with narrowing the row spacing in mungbean and soybean, there are also benefits in early weed control. Keeping crops weed-free for the first three to six weeks seems to be a valuable rule of thumb to give crops the head start required to drive down weed numbers. Weeds that germinate in-crop after the 36-week mark are fewer in number and individual plants also produce less seed.
As a non-herbicide control tactic, crop competition is very important in any integrated weed management program. With investment from the GRDC, researchers from NSW Department of Primary Industries, University of Sydney and Queensland Department of Agriculture and Fisheries are conducting trials to identify ways to increase the competitiveness of sorghum and summer pulses.
In the 2017-18 summer sorghum was sown in trials using three row spacings (50, 75 and 100cm) and two crop densities (five and 10 plants per m2). In a separate trial, mungbean plots were established using three row spacings (25, 50 and 75cm) and two crop densities (20 and 35 plants per m2). Seed of feathertop Rhodes grass and awnless barnyard grass was spread at planting and the plots were irrigated to ensure optimal crop and weed emergence.
With only one year of data from this trial so far it is difficult to make recommendations. Average seed head production in non-crop treatments was 1745 heads/m2 for feathertop Rhodes and 1525 heads/m2 for awnless barnyard grass. Growing either crop more than halved the number of weed seed heads produced, even in the least competitive configuration for the two crops.
Row spacing in mungbean had a clear impact on weed seed head production for both weed species. This effect has been demonstrated numerous times in a range of agronomic trials with mungbean. Narrowing row spacing to 25cm reduced feathertop Rhodes grass seed heads to 32 per m2. For awnless barnyard grass, narrowing row spacing to 50cm or less reduced seed heads to less than 10 per m2.
For sorghum it seems increased plant population had the best effect, significantly reducing weed seed head numbers for both weed species. Unlike mungbeans and soybeans, sorghum yields were reduced in plots with narrower row spacing configurations. At each row spacing, the higher plant density (10 plants/m2) treatment yielded more than the lower density plant population.
In a separate trial, University of Queensland researchers confirmed that row spacing, not plant population, is the key driver to reducing weed growth in soybean crops. In soybeans, weed biomass was reduced by 89 per cent under narrow rows (25cm) and 75 per cent under wider rows (75cm) when the crop was kept weed-free for the first three weeks after planting. If weeds were controlled for the first six weeks, then weed biomass was reduced by 98 per cent under narrow rows and 88 per cent under wider rows.
Summer crops are an integral part of many farming systems and play an important role in an integrated weed management program. Many summer weeds only remain viable on the soil surface for a period of around 12 months so if a competitive summer crop is followed with a competitive winter crop and harvest weed seed control, it is possible to have a real and lasting impact on the weed seed bank.
■For more information about managing weeds in summer crops, visit the WeedSmart website: www.weedsmart.org.au