WILD turnip steals crop yield and requires additional control measures that together cost more than $10 million every year across Australia.
The University of Queensland weed science professor Bhagirath Chauhan said recent ecological studies have shed light on tactics that growers could use to rein in this invasive weed.
"Wild turnip is considered a winter weed, but with sufficient soil moisture and mild temperatures it can also establish and set seed over summer," Dr Chauhan said.
"Some biotypes of Brassica tournefortii have evolved resistance to chlorsulfuron and other group 2 herbicides, identified in South Australia in 1996."
With investment from the Grains Research and Development Corporation, Dr Gulshan Mahajan conducted studies on four biotypes collected in the northern region and investigated the differences in seed dormancy, drought tolerance, effect of competition by wheat and chickpea crops and seed persistence on the surface and at various burial depths.
"These experiments clearly demonstrated the invasive capability of wild turnip," Dr Chauhan said.
"This species can produce vast quantities of seed with variable dormancy, meaning there can be multiple germination cohorts, mainly associated with rainfall events.
"However, it is a poor competitor when faced with a crop, such as wheat, that achieves canopy closure quickly."
Growing competitive crops is one of the WeedSmart Big 6 tactics, providing season-long weed suppression and maximising the value of early weed control efforts.
Wild turnip seeds can persist for up to 18 months on the soil surface and five per cent of seed was still viable after being buried at a depth of two centimetres for 30 months.
Fresh seeds initially have high dormancy when placed on the soil surface, as the seed coat extends dormancy of fresh wild turnip seed and light inhibits germination.
"Once the seed coat has degraded somewhat, seedlings readily emerge from the surface after rainfall events, generating multiple cohorts between February and October, while emergence peaks from March to May, potentially challenging crops sown from the end of April to June," Dr Chauhan said.
"Keeping the weed seed on the surface in a no-till system and minimising soil disturbance at planting, coupled with pre-planting knockdown, pre-emergent herbicide and vigorous early crop growth can reduce germination and weed seed production in-crop.
"Germination rates of up to 14pc occurred when seed was buried at a depth of 2cm in soil with sufficient moisture and alternating day/night temperature of 25/15 degrees, however wild turnip seedlings did not emerge from a depth of 5cm.
A one-off deep tillage event could be an effective control tactic to bury the existing seed bank, provided there was no seed bank replenishment or subsequent tillage.
A wild turnip plant growing in a fallow or fence line can produce about 10,000 seeds, with early emerging cohorts achieving greater plant height and shoot biomass, resulting in greater seed production than later emerged cohorts.
The early emerged plants also enjoyed a longer growing season, reaching flowering stage after 87 days, while later emerged plants reached flowering after only 70 days.
Dr Chauhan said although water stress reduced the seed production to 3000 seeds per plant, this was still ample seed to establish an infestation capable of reducing crop yield.
"Wide-row and slow growing crops such as chickpea do not inhibit wild turnip growth or seed production," he said.
"On the other hand, a fast growing, dense wheat crop suppressed weed growth and seed production by 78pc for the early sown crop (May 15), 96pc for the crop sown on June 5, and 65pc for the late sown crop (June 25).
"This reduction in seed production was achieved without the application of herbicide and the vigorous growth of the wheat crops sown on the latter two planting dates prevented wild turnip plants from producing enough seeds for re-infestation."
When it comes to the best strategy to drive down wild turnip, numbers include pre-plant knockdown, delayed sowing, pre-emergent herbicide and a fast-growing, competitive crop.
Wild turnip can become a problematic weed in no-till systems because emergence of seeds in the surface layer is greater than for buried seeds.
The retention of stubble supports higher soil moisture at the soil surface, creating a favourable environment for germination over a long period.
"However, if emerged plants are prevented from setting seed, it is possible to rapidly deplete the seed bank in a no-till system during a six to 12-month fallow," Dr Chauhan said.
"Shallow tillage may result in the buried seeds remaining viable for more than two years, with the potential for seed to be brought to the surface during subsequent planting operations, triggering the re-infestation of the paddock.
"Avoid slower growing and wide-spaced crops such as chickpea in paddocks with a large wild turnip seed bank."