Herbicide efficacy in retained stubble systems

ɸExtra technical comment by Protech Consulting Pty Ltd

Introduction

The GRDC project ‘Maintaining profitable farming systems with retained stubble - upper Eyre Peninsula’ aims to improve farm profitability while retaining stubble in farming systems on upper Eyre Peninsula (EP). One of the barriers to retaining stubble is the perceived reduction in pre-emergent herbicide effectiveness (efficacy) in stubbles. This component of the project is testing whether various stubble management activities impact on herbicide efficacy?

Weed control in stubble retained systems can be compromised when stubbles and organic residues intercept the herbicide and prevent it from reaching the desired target, or when the herbicide is tightly bound to the organic matter. Reduced herbicide efficacy in the presence of higher stubble loads is a particular issue for pre-emergenct herbicides. Current farming practices have also changed weed behavior with a shift in dormancy in barley grass genotypes now confirmed in many paddocks of the Minnipa Agricultural Centre (MAC) (B Fleet, EPFS Summary 2011, p 177). As a part of the stubble project this trial was undertaken to assess herbicide efficacy in different stubble management systems.

Background

To understand how herbicides perform it is important to know the properties of the herbicide, the soil type and how the herbicide is broken down in the environment. The availability of a herbicide is an interaction between the solubility of a herbicide, how tightly it is bound to soil particles and organic matter, soil structure, cation exchange capacity and pH, herbicide volatility, soil water content and the rate of herbicide applied (Congreve and Cameron, 2014).

Herbicides intercepted by organic material will be subject to a certain level of binding, depending on the herbicide’s characteristics. Some will be tightly bound and lost to the system in terms of as an agent of weed control, others will be loosely bound and relatively soluble and will be returned to the soil by subsequent rainfall events. However, loosely bound herbicides may also be prone to losses by volatilisation and photodegradation (Congreve and Cameron, 2014). The solubility and soil water movement potential of key herbicides is listed in Table 1.

When a herbicide is incorporated into the soil, a percentage will bind to soil organic carbon and soil particles. The strength of binding is called the soil/water adsorption coefficient (Kd). The binding is highly influenced by the level of organic matter so it is calculated by taking into account the level of organic matter Koc = Kd/soil organic carbon. The higher the Koc value the more tightly the herbicide is bound. A low Koc value means the herbicide is less tightly bound and able to move with the soil water, which happens in sandy soils or soils with low organic matter (Congreve and Cameron, 2014). The Koc values for key herbicides are listed in Table 1. 

Soil moisture is also critical to the performance of herbicides in soils. If soil water is low, plant uptake will be lower and a greater percentage of the herbicide will be bound onto the soil and become unavailable.

Stubble, existing weed cover and crop cover (for post sowing applications) in a zero or minimal till system will intercept some of the herbicide before it reaches the soil. The amount of herbicide intercepted will be proportionate to the percentage of ground cover. Interception can have two negative effects; herbicide can be tied up on the stubble or in the canopy and will not be available for weed control; and it can lead to uneven coverage on the soil surface lowering herbicide effectiveness and increasing potential weed escapes (Congreve and Cameron, 2014).

Table 1: Solubility and soil water movement potential of key herbicides.

Data collated from GRDC Pre-emergent herbicide Manual, M Congreve and J Cameron, 2014, and pers comm from A Bates and B Fleet (2015). MOA: Mode of Action

Methodology

To measure the efficacy of herbicides in different stubble management systems, a small plot trial was established at the MAC in paddock S7. In the year prior to the trial, the paddock was sown to Mace wheat on 10 May 2014 and yielded 3.2t/ha with 9.1 per cent protein. Two different wheat stubble management strategies were then implemented at harvest; traditional spread stubble and harvest windrows. The third stubble management strategy was implemented on 15 April 2015 with total stubble removal by burning and the harvest windrows within the trial area were also burnt on the same day. 

The trial was sown with Mace wheat @ 60kg/ha and DAP @ 60kg/ha on 11-12 May in dry conditions. The trial area received a knockdown of 1.2L/ha of Roundup Attack with IQ inside on 11 May. The chemical treatments listed in Table 3 were individually mixed in small pressure containers and applied on 11 and 12 May using a shrouded boomspray at 100L/ha of water.

Minnipa received 35mm rainfall for April and 6mm in the five days before sowing with 3mm after sowing. Conditions were drier the week after sowing before another 6 \mm fell, with a total of 16 mm for May. The 2015 total rainfall at MAC was 333mm, with 258 mm received in the growing season (April- October). The trial was sown at 3-4cm depth with an Atom-Jet spread row seeding system with press wheels. The trial was also sprayed on 27 July with 5g/ha of Ally and BS1000 at 100ml/100L for control of soursob (Oxalis pes-caprae).

Measurements taken were stubble load pre-seeding, soil moisture, plant emergence count, early and late grass weed count, medic growth score, grain yield and grain quality. Data were analysed using Analysis of Variance in GENSTAT version 16. The least significant differences are based on F prob=0.05.

Results and discussion

Stubble treatments (averaged over all chemical treatments)

Early dry matter and grain yield were lower in the spread stubble system than the burnt stubble and burnt windrow systems and this may have been due to less moisture reaching the seed bed and also the tie up of nitrogen resulting in early nitrogen deficiency (Table 2). There may also have been some yellow leaf spot interactions.

Table 2: Effect of stubble management on crop establishment, dry matter and yield as well as weed and medic populations in 2015.

* Visual medic rating system where 0=no medic, 1=small suppressed medic, 2=small and large medic, 3=mostly large medic plants

Weeds

Barley grass is the main grass week issue at the Minnipa Agricultural Centre. Recent research has shown the in-paddock barley grass is a later germinating genotype than what appears along the fence lines. The weeds present in the trial area and weed pressure are listed in Table 3.

Table 3: Weeds and weed pressure present in MAC trial in 2015.

Chemical treatments

There were no impacts of stubble management on the performance of individual chemical treatments so results presented in this section are averaged over all three stubble management treatments.

Wheat establishment was not affected by any chemical treatment and varied between 95 and 103plants/m2. Grain yield was lowest in the untreated control and most chemical treatments increased yields but only by up to eight per cent which suggests grassy weed pressure was low. This is consistent with the (unusually) low populations of barley grass which developed in 2015. As a consequence, few chemical treatments were more profitable than doing nothing for grassy weed control.

On average, 45 per cent of the barley grass population emerged later in the season, approximately six weeks after sowing, excluding those treatments with Sakura®. Effects of chemical treatments on early barley grass numbers were inconsistent, but by late in the season, any treatments containing Sakura®, or Monza® alone, had lower barley grass numbers than the untreated control.

Medic germination was affected by some chemicals and the residual effect may impact on the future seed bank and germination.

Barley grass numbers at the first sampling were low (less than 10plants/m2) across the whole trial, regardless of chemical treatments (Table 4). Sakura® and mixes containing Sakura® decreased early dry matter of the crop (Table 4).

Trifluralin and Diuron mixes caused some crop damage but the crop recovered better than expected and dry matter production of the crop was as good as in the untreated control by sampling time, this was probably due to less soil water movement of the chemicals. In a dry start Boxer Gold® did not appear as effective on barley grass as ryegrass, but post application gave some suppression activity on all grasses.

Medic germination was very low where trails were treated with Monza® and Metribuzin, so carefully consider the use of these chemicals as some will have more than a one year effect on medic regeneration. Ward’s weed (Carrichtera annua) was not controlled in this trial by Monza^ɸ.
^ɸNot registered for this weed

Table 4: Effect of chemical treatments on crop establishment, dry matter and yield as well as weed and medic populations in 2015.

^(0-3 rating where 0=no medic, 1=small suppressed medic, 3=larger medic plants)
# Wheat price of $252/t used for AUH2 on 1 December 2015 at Port Lincoln, less chemical cost.
Note: some treatments in the trial are for research purposes only. ^ɸalower than label rate. ^ɸbhighest label rate in wheat is 550g/ha (except for soursob control that has a range of 710-980g/ha)

Despite high cereal stubble loads, completely removing stubble by burning did not improve the efficacy of any of the chemical packages tested in this trial. The generally low grass weed pressure observed in this trial however makes it difficult to draw any conclusive recommendations relating to the impact of stubble management based on these results alone.

As outlined in the background information, the differences in a chemical’s ability to bind to organic matter and move through the soil profile with soil water will influence the uptake of the chemical by the target weeds, the crop, and the impact on both. Soil texture and soil chemical properties can affect chemical movement and availability in the soil profile. Some chemicals will have greater activity and mobility and be ‘hotter’ in lighter sandier soils than the MAC loam in this trial. The dry seeding conditions and lack of post sowing rainfall at the start of the 2015 season resulted in less damage to the crop than expected with some chemicals such as the diuron mixes, this was due to lower soil mobility.

When choosing the most appropriate pre-emergent herbicide for use in stubble retained systems, it is important to consider:

• The likely rainfall pattern and soil moisture conditions post application,
• the susceptibility of the crop to the herbicide,
• the position of the weed and crop seeds in the soil profile,
• the mobility of the herbicide in soil water,
• the persistence of the herbicide activity relative to the germination pattern of the target weeds,
• specific tillage/seeding system and level of soil disturbance, and;
• long-term objectives of the weed management program.

References

GRDC Pre-emergent herbicide Manual, M Congreve and J Cameron, 2014.

EPFS Summary, p 177, B Fleet. 2011.

Acknowledgements

 
The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the author would like to thank them for their continued support. 

Thanks also to Sue Budarick for her help with processing samples from this trial. 

Contact details

Amanda Cook
amanda.cook@sa.gov.au