Herbicide efficacy in no-till farming systems in southern NSW

Herbicide Efficacy in No-till Farming Systems in Southern NSW

Greg Condon and Kirrily Condon, Grassroots Agronomy, Junee, NSW

Variable rainfall patterns over the past ten years in southern NSW have prompted the rapid adoption of no-till, stubble retained farming systems to maximise crop water use efficiency. Despite the many benefits of this system, the efficacy of pre-emergent herbicides remains one of the major concerns for growers and advisers who have adopted, or are considering adoption, of the system.

The problem
Pre-emergent herbicides play an important role in integrated weed management programs as they allow for rotation away from high risk Group A and B herbicides. However, poor efficacy of pre-emergent herbicides can still result in the rapid development of herbicide resistance, as has occurred with trifluralin resistance that is common in South Australia.

The efficacy of many pre-emergent herbicides relies on the ability to incorporate them into the soil at sowing. In no-till systems, however, knife points and to a greater extent, disc openers, provide minimal soil disturbance at sowing to limit moisture loss, but which consequently also limit herbicide incorporation. Retained stubble also acts as a physical barrier to herbicide/soil contact. This issue is further confounded where livestock are included in the system, with stubble laid flat from trampling providing a physical barrier to herbicide uptake. Effective pre-emergent herbicide strategies are required to ensure the future sustainability of both herbicides and no-till farming systems.

Finding the solution
To help develop pre-emergent herbicide strategies relevant to no-till farming systems in southern NSW, a local consortium was funded by GRDC to implement three trial sites in 2010 comparing herbicide efficacy in disc and tyne (knife point) systems (Table 1). The sites were located near Grenfell, Wagga Wagga and Lockhart, with commercial machinery used to sow large scale, replicated plots at typical speeds to ensure soil throw and accurate seed placement. Pre-emergent herbicide treatments (Table 2) were applied by AgriTech using a six-metre boom and incorporated by sowing (IBS).

Table 1 – Trial details

Table 2 - Herbicide treatments:

1 not registered for IBS with disc seeders
2 registration pending (due 2012)
3 not registered for IBS in wheat in NSW

*Caution: Research on unregistered pesticide use. Any research with unregistered products reported in this document does not constitute a recommendation for that particular use by the authors or the author’s organisation. All pesticide applications must accord with the currently registered label for that particular pesticide, crop, pest and region.

Results
Although different outcomes were achieved at each site due to variations in weed density, soil moisture and stubble loads, the tyne seeder produced consistently better establishment and early crop vigour at all sites. A summary of the other major outcomes are as follows:

Wagga Wagga
• Site conditions: Wedgetail wheat was sown early into good moisture and 50% stubble cover. Very high ryegrass numbers (up to 190 plants/m2) with low crop competition due to early locust damage represented an extreme scenario to test pre-emergent herbicide efficacy.
• Emergence: Plant numbers were significantly higher in the tyne compared with the disc (80 vs 50 plants/m2 respectively, P<0.05). Greater early vigour by the tyne allowed better recovery from early locust damage.
• Weed control: There was little difference in ryegrass control between disc and tyne, with Sakura providing the best control for both systems. Boxer Gold, trifluralin at 3L/ha and trifluralin mixes with Logran, Dual Gold, Avadex Xtra and Diuron all provided significantly higher levels of control than trifluralin at 1.5L/ha (P<0.05, Figure 1). Sakura also provided excellent residual control of ryegrass for later germinating populations resulting from the favourable season.
• Crop safety: The impact of certain pre-emergent herbicides on crop safety was far more evident in the disc system than the tyne. Crop damage in the disc treatments was greatest with Boxer Gold + Avadex Xtra, although not significantly worse than Sakura + Avadex Xtra or 3L/ha trifluralin (P>0.05).
• To ensure crop damage is minimised across a range of soils and seasonal conditions, soil treated with residual pre-emergent herbicides needs to be removed from the furrow during seeding by soil throw. The single disc openers used in this trial were unable to fully achieve this objective. The ability to shift soil does vary between disc seeders, with features such as row cleaners ahead of the single disc helping to reduce crop damage. Other factors, including disc size, disc shape, disc angle, closer plates and sowing speed all impact upon soil throw and the quantity of treated soil falling back into the seed furrow.

Figure 1 – Ryegrass control in disc and tyne systems at Wagga Wagga; LSD (0.05) = 9

Lockhart
• Site conditions: Lincoln wheat was sown mid-May into dry conditions and patchy stubble cover from an uneven burn. Very low ryegrass and wild oat numbers meant the trial could be retained through to harvest, allowing a valid comparison of disc and tyne systems in the relative absence of weeds.
• Emergence: Plant numbers were significantly higher in the tyne compared with the disc (120 vs 80 plants/m2 respectively, P<0.05), with disc emergence more affected by the presence of stubble and sowing across the seeding rows from the previous year. Greater early vigour by the tyne became less apparent as the season progressed.
• Crop safety: Conditions remained dry for approximately one month after the trial was established, which reduced damage from residual herbicides on the emerging crop. Crop safety was, however, compromised with 3L/ha trifluralin in the disc treatment, resulting in lower vigour and yield, although not significantly lower than other treatments (P>0.05).
• Yield: There were no significant yield differences between the disc and tyne treatments (P>0.05), a reflection of the drier nature of the site and subsequent improved crop safety with pre-emergent herbicides.



Grenfell
• Site conditions: Livingston wheat was sown into moist soil and a thick residue of stubble retained from previous wheat crops. Following establishment, approximately 50mm of rain fell onto an already full profile, which remained wet for the entire season. 1015mm of rainfall was measured for the area in 2010, with 650mm falling between September and December. The wet conditions had a major bearing upon outcomes from this trial, particularly the disc treatments which suffered from poor establishment and severe herbicide damage.
• Establishment: Plant numbers were significantly higher in the tyne compared with the disc (80 vs 30 plants/m2 respectively, P<0.05). Crop vigour was also better in the tyne treatments, which remained the case through the season. As the soil became wetter during winter, the disc area was unable to recover its vigour due to a range of factors including herbicide damage, yellow leaf spot, nitrogen deficiency and waterlogging.
• Weed control: Low to moderate ryegrass numbers were present at establishment, with similar levels of control between the disc and tyne treatments. 1.5L/ha trifluralin + Logran gave the poorest control for both systems, although it wasn’t significantly worse than some other treatments (P>0.05). With low crop competition, particularly in the disc treatments, and later germinations of ryegrass, the trial became heavily infested and was spray fallowed in October to prevent seed set.
• Crop safety: Herbicide damage in the disc areas was particularly evident at this site, with the majority of herbicide treatments causing significantly more crop damage than 1.5L/ha trifluralin (P<0.05), which recorded the least damage (Figure 2). Yield estimates taken prior to spray fallowing showed the disc treatments to potentially average 0.36t/ha and the tyne treatments 1.0t/ha, with a range across all treatments of 0.15 to 1.37t/ha depending on herbicide, seeding system and weed control.
• Despite the wet season, these yields are well below profitable targets for wheat in this region and highlight the risks associated with using pre-emergent herbicides at robust rates in no-till systems. The majority of pre-emergent herbicides are mobile when used in wet soils, with some causing more crop damage in cereals than others due to their mobility. Crops that are stressed during establishment due to waterlogging, poor nutrition or slower emergence from disc systems are put under greater pressure from pre-emergent herbicides and the industry needs to be aware of the variability that can occur.

Figure 2 – Crop safety in disc and tyne systems at Grenfell; LSD (0.05) = 2.2

Discussion
No-till farming utilising sound agronomy is a proven system of crop management that reduces production risk through improved soil structure, enhanced water use efficiency and ultimately stable profits for growers. Knife point and press wheel seeding systems are now well understood for their ability to accurately place seed at consistent depths across a range of soil types in the presence of pre-emergent herbicides. Awareness of differences achieved through variations in sowing speed, row spacing and soil throw are better understood by growers and consequently there is greater confidence with using pre-emergent herbicides in no-till systems. Label registrations now reflect this level of understanding, with the majority of pre-emergent herbicides now registered for use in no-till seeding systems using a knife point and press wheel.

Developments such as full stubble retention and zero-till seeding with discs are now being investigated for their value to further improve the gains made in no-till farming. Disc seeding allows higher sowing speeds and consistent seed placement into marginal soil moisture in the presence of stubbles, which in drier seasons, has achieved yield advantages over tyne systems.

Before these developments are widely adopted, aspects of the disc system need to be further investigated to ensure standards are maintained with key areas such as weed management and yield. The trials outlined in this report highlight the variability that can occur with disc seeding systems in no-till when pre-emergent herbicides are incorporated by sowing (IBS), ranging from little crop damage at the drier Lockhart site to significantly greater damage in the disc treatments at the waterlogged Grenfell site.

Growers and advisers will continue to improve on the no-till system in relation to crop safety with pre-emergent herbicides, particularly using disc seeding technology. Manufacturers are already making adaptations to suit single disc openers that aim to throw more soil for improved herbicide safety with soil shifting row cleaners (eg Tobin disc drill) , seed boot shields with built in soil deflectors (eg Daybreak) and a reversible and adjustable disc closer (NDF Ag Design). The use of ground driven residue managers (eg Aricks wheels) to displace soil and stubble from the seeding row is also being investigated for soil throw and pre-emergent herbicide use.

Further improvements to the no-till system, particularly in relation to canopy management in disc and tyne systems, is being investigated in another GRDC funded project in 2011.


Acknowledgements:
Brent Alexander, Chug Kennedy - Lockhart
Ben Beck, John & Brendan Pattison - Wagga Wagga
Duncan Lander, Rob Johnson - Grenfell

Peter Hamblin, Tony Single, Nic Amos - AgriTech NSW
Lachlan Caldwell - Lachlan Fertilizers Rural
Heidi Gooden - Delta Agribusiness
Bayer, Syngenta, Nufarm
FarmLink Research

Author’s contact details:
Greg Condon
Grassroots Agronomy
gbcondon@westnet.com.au
0428 477348