Wheat with a head s tart can suppress ryegrass

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A three-year trial in South Australia shows early sowing has merit in medium and low-rainfall areas as a tactic to suppress late-germinating grass weeds

Dr Chris Preston, from the University of Adelaide, says early sowing, when soil temperatures are warmer, results in more rapid wheat growth and faster canopy closure. He says this provides an opportunity to increase the amount of competition against weeds without having to significantly change other
aspects of the farming system.

PHOTO: Nicole Baxter

Key points

  • The behaviour of annual ryegrass is changing, with greater levels of dormancy and later germination occurring under selection pressure from continuous cropping
  • Delayed sowing has become a less useful option for control of grass weeds in continuously cropped paddocks
  • Crop competition is an excellent partner with pre-emergent herbicides for the control of grass weeds
  • Trials showed sowing on time to maximise wheat competition with annual ryegrass helps reduce weed seed-set and increases crop yield

Dr Chris Preston, associate professor of weed management at the University of Adelaide, is encouraging growers to consider early sowing as a tactic to better manage grass weeds in crops.

He says early sowing when soil temperatures are warmer allows crops to grow rapidly and close the canopy faster, increasing crop competition against weeds.

“Obtaining early ground cover before the pre-emergent herbicide has dissipated will reduce the competitiveness and seed production of later-emerging weeds,” Dr Preston says.

“When sowing late, wheat takes longer to bulk up and that puts a lot of pressure on pre-emergent herbicides to take out ryegrass.”

Dr Preston says there are several ways to increase the competitiveness of cereal crops. These include increasing the sowing rate or reducing the row spacings of the seeder.

However, Dr Preston and his colleagues at the University of Adelaide, have also investigated an alternative approach – early sowing – to increase crop competitiveness against weeds.

During the past three years, the researchers ran trials in South Australia, at Hart and Roseworthy, with the Hart Field Site Group, to investigate the competition from early sowing in assisting pre-emergent weed control.

In the trials, several pre-emergent herbicide options were used and wheat was sown about one month apart. The researchers collected data on weed numbers, weed seed production and grain yield.

Trial results 2014

In 2014 at Hart, the researchers sowed LongReach Scout wheat on 4 May and 2 June. The pre-emergent herbicide treatments used at each time of sowing were nil, Sakura® and Sakura® mixed with Avadex® Xtra (Table 1). There was an additional knockdown herbicide application between the two sowing times.

Dr Preston says in 2014, the delay in sowing to apply another knockdown herbicide application did not result in fewer ryegrass plants in the crop (Table 1).

“Where no pre-emergent herbicide was used, there were more annual ryegrass seed heads produced from the early time of sowing,” he says.

“In contrast, where effective pre-emergent herbicides were applied there was no difference in the number of seed heads produced.”

Dr Preston says the wheat yield from the first time of sowing (TOS1) was 4.15 tonnes per hectare and from the second time of sowing (TOS2) was 2.93t/ha.

Table 1 In-crop annual ryegrass and seed heads at maturity for two sowing times (TOS) at Hart, SA, in 2014*.
Pre-emergent herbicide Plant counts (per m2) 8 August Head counts (per m2) 10 October
  TOS 1 (4 May) TOS 2 (2 June) TOS 1 (4 May) TOS 2 (2 June)
Nil 59 a 77 a 350 a 164 b
Sakura® at 118g/ha 8 b 8 b 39 c 41 c
Sakura® at 118g/ha mixed with Avaxed® Xtra at 2L/ha# 3 b 3 b 32 c 9 c

* For each result, different letters indicate significant differences. # When used for non-research purposes please always adhere to rates listed on label, which states ‘incorporated by sowing (IBS) rate of 3L/ha or 3.2L/ha depending on weed species and rate of 1.6 to 2.4L/ha is when tank-mixing with TriflurX®’.

SOURCE: University of Adelaide, 2017

Trial results 2015

The trial was repeated in 2015 using nil pre-emergent herbicides, Boxer Gold®, Sakura® and Sakura® mixed with Avadex® Xtra incorporated by sowing wheat on 30 April and 27 May (Table 2). Again, there was an additional knockdown application of herbicide between the two sowing times.

Dr Preston says conditions in 2015 were much drier than in 2014, particularly in spring, leading to lower annual ryegrass seed head numbers than in 2014.

He says although the 2015 yields were lower than the 2014 yields, early sowing still produced a significantly higher yield (2.2t/ha) compared with the later time of sowing (1.5t/ha).

“Despite the difference in seasons, delaying the time of sowing and including an additional knockdown application did not reduce the number of annual ryegrass plants from emerging in the crop,” Dr Preston says.

“Our results confirm other data that indicates the behaviour of annual ryegrass is changing with greater levels of dormancy and later germination under selection pressure from continuous cropping. This makes delayed sowing a less useful option to control grass weeds in continuously cropped paddocks.”

Table 2 In-crop annual ryegrass and seed heads at maturity for two sowing times (TOS) at Hart, SA, in 2015*.
Pre-emergent herbicidePlant counts (per m2) 10 JulyHead counts (per m2) 16 October
 TOS 1 (30 April)TOS 2 (27 May)TOS 1 (30 April)TOS 2 (27 May)
Nil18 a6 b45 a44 a
Boxer Gold® at 2.5L/ha3 bc1 bc5 bc9 bc
Sakura® at 118g/ha1 bc2 bc3 bc13 b
Sakura® at 118g/ha mixed with Avadex® Xtra at 2L/ha#0 c1 bc0 c15 bc

* For each result, different letters indicate significant differences. # When used for non-research purposes please always adhere to rates listed on label, which states ‘incorporated by sowing (IBS) rate of 3L/ha or 3.2L/ha depending on weed species and rate of 1.6 to 2.4L/ha is when tank-mixing with TriflurX®’.

SOURCE: University of Adelaide, 2017

Trial results 2016

In 2016, the trial was moved to Roseworthy where the ryegrass population was higher.

The early sowing treatment was established on 6 May, while the delayed sowing treatment was sown on 1 June.

Dr Preston says without pre-emergent herbicide, sowing earlier produced higher yields because crop competition in the early-sown trial supressed later-emerging ryegrass and stopped seedset.

“In the early time of sowing, every ryegrass plant produced one head, whereas in the late time of sowing, every ryegrass plant produced two heads,” he says.

“But applying Sakura® mixed with Avadex® Xtra reduced the number of ryegrass seed heads by 99 per cent in a large weed population in a wet year.”

Although the later time of sowing produced a slightly higher yield in 2016, Dr Preston says this is the only year where this has happened.

He puts this down to the wheat variety, Mace, which he thinks might have been better suited to the later sowing conditions. For early sowing, Dr Preston suggests changing to a longer-season variety.

“We have run additional trials at other sites and I don’t see any downsides to earlier sowing, except perhaps in higher-rainfall areas,” he says.

“In the medium and lower-rainfall areas there is no downside of putting the crop in at the right time, choosing the right variety and having the right pre-emergent herbicide package.”

In drier seasons, Dr Preston says the trials showed Boxer Gold® can be effective. However, in wetter seasons, Boxer Gold® does not have sufficient persistence to be used successfully as the only pre-emergent herbicide, he says.

“A better strategy in a wetter season involves more robust and longer-persisting pre-emergent herbicide mixtures or following up a pre-emergent herbicide application with a post-emergent Boxer Gold® application.”

According to Dr Preston, changing weed management tactics every year and rotating herbicide groups will help delay the onset of herbicide resistance.

Table 3 In-crop annual ryegrass and seed heads at maturity for two sowing times (TOS) at Roseworthy, SA, in 2016*.
Pre-emergent herbicide Plant counts (per m2) August Head counts (per m2) October
  TOS 1 (6 May) TOS 2 (1 June) TOS 1 (6 May) TOS 2 (1 June)
Nil 341 a 374 a 347 b 685 a
Sakura® at 118g/ha 77 b 40 b 60 de 71 de
Sakura® at 118g/ha mixed with Avadex® Xtra at 2L/ha# 18 b 13 b 4 e 21 de
Boxer Gold® at 2.5L/ha 116 b 60 b 116 cd 112 cd
Boxer Gold® at 2.5L/ha mixed with Avadex® Xtra at 2L/ha# 94 b 89 b 61 de 167 c

* For each result, different letters indicate significant differences. # When used for non-research purposes please always adhere to rates listed on label, which states ‘incorporated by sowing (IBS) rate of 3L/ha or 3.2L/ha depending on weed species and rate of 1.6 to 2.4L/ha is when tank-mixing with TriflurX®’.

SOURCE: University of Adelaide, 2017

GRDC Research Codes UA00144, UCS00020

More information:

Dr Chris Preston
0488 404 120
christopher.preston@adelaide.edu.au