Broadleaf weed control and crop safety in lentils

Take home messages

• Reflex® when used alone did not result in any plant establishment reduction, whereas Terrain® reduced plant establishment at seven out of eight trial sites.
• Recovery from herbicide damage symptoms from Reflex® and Terrain® was highly dependent on seasonal weather conditions, with better recovery in 2022 due to higher spring rainfall than 2021.
• Reflex® herbicide damage symptoms progressed slowly through winter, whilst Terrain® symptoms began earlier and improved towards the end of winter.
• Crop damage with Reflex® and Terrain® on alkaline sands was cumulative when applied in combination with diuron.
• Control of bifora, common sowthistle, Indian hedge mustard, and marshmallow populations were achieved with Reflex® and Terrain® applied in combination with registered Group 2, 5 and 12 herbicides.
• New Group 14 herbicides, Reflex® and Terrain®, had associated risks of crop damage and yield loss, which emphasises the need for careful planning of weed control on acidic, alkaline and sandy soils to achieve satisfactory weed control and adequate crop safety.

Background

The release of imidazolinone (IMI) herbicide tolerant lentils coupled with the availability of this technology in most other broadacre crop species has led to the over-reliance of Group 2 herbicides. Developing IMI herbicide resistance in broadleaf weeds is a major constraint to achieving yield potential in pulse crops. Reflex® (fomesafen 240g/L) Group 14 herbicide was registered in 2021 for use in pulses and vetch and provides more opportunities for rotating modes of action. Lentil is the most sensitive pulse species to Reflex®. Terrain® (flumioxazin 500g/kg) is another Group 14 herbicide for broadleaf weed control in lentil, newly registered in 2022.

Lentils are particularly sensitive to Group 14 herbicides; therefore, label rates are lower, and the use pattern is restricted to incorporated by sowing (IBS) when compared to other pulse crops. SAGIT funded project TC121 is investigating crop safety and weed control of Reflex® and Terrain® and their combinations with Group 2 (previously B), Group 5 (previously C) and Group 12 (previously F) herbicides on a range of soil types varying in soil texture and pH in 2021 and 2022 in northern Yorke Peninsula. Lentil crop safety varied significantly between acidic and alkaline sands in 2021 trials, with the use of Reflex®, diuron, metribuzin and terbuthylazine herbicides, with alkaline sand sites incurring more herbicide damage than acidic sand sites. Care needs to be taken when considering the use of Group 14 on lentils in terms of soil types, seeding system and time of rolling. These studies were continued in 2022 through SAGIT project TC121 and GRDC Project UOA2105-013RTX. The results of 2022 including those of newly registered herbicide Terrain® are presented here.

Method

A total of four trial sites were established at Alford and Bute, SA in 2021, and another four sites at Wards Hill, Paskeville and Bute, SA in 2022 to assess crop herbicide safety and weed control on IMI tolerant lentils. The herbicides used in the trials are described in Table 1. Each year included two alkaline sandy light textured sites, one acidic sandy site and one medium textured site with results in Table 2.

Trial establishment

Trials were sown using knife points and press wheels and were sown in late May or early June. All varieties grown were IMI tolerant lentils. Herbicides were applied using a hand boom delivering 100L/ha water volume at a pressure of 200kPa. One alkaline sand site and acidic sand site were rolled post-emergent in 2021, whilst the other alkaline sand site and medium textured site was rolled post-sowing pre-emergent (PSPE). All the sites were rolled post-emergent in 2022.

Rainfall conditions

A total of 219mm and 295mm growing season rainfall (end May–December) was received at Wards Hill (alkaline loamy sand) and Paskeville (neutral clay soil) sites, respectively in 2022. Out of this, Wards Hill and Bute sites received 39mm and 53mm rainfall, respectively within the first two weeks of sowing. Similarly, two major rainfall events occurred after seeding in 2021, with 28mm and 24mm of rainfall received within the first and second week, respectively at Bute. Wards Hill site received 119mm and Paskeville 160mm rainfall in spring 2022.

Herbicide properties and application details

Table 1: Pre-emergent herbicide properties for products used in the herbicide tolerance trials in 2022 (Source: GRDC pre-emergent herbicide fact sheet).

Soil test results

Table 2: The range of pH (H2O), organic carbon (OC) % and soil texture at 0–20cm for the trial sites in 2021 and 2022.

Table 3: Crop safety data for all sites across 2021 and 2022. Plant establishment presented as percent of control (nil), stunting score (1 = no stunting, 9 = plant death), chlorosis score (1 = no chlorosis, 9 = plant death), spring NDVI as percent of control (nil) and grain yield as percent of control (nil).

*Spring NDVI data only comes from two trial sites for all listed treatments, – indicates no data available.

Results and discussion

Crop safety

Crop damage in lentil was assessed using loss of plant number, chlorosis, necrosis and stunting. Lentil crop safety varied between acidic and alkaline sands with the use of Group 14 herbicides Reflex® and Terrain®, and Group 5 herbicides diuron and metribuzin and is summarised below.

Plant establishment

Reflex® did not result in any plant establishment issues on any soil type when applied alone at the low and high label rate (Table 3). Terrain® reduced plant establishment at all sites regardless of soil type, except for one alkaline sand site in 2021 (individual site data not shown). Terrain® caused greater reduction in plant establishment at the alkaline sand and medium textured sites compared to the acidic sand sites. However, plant establishment was still reduced by 25% on average at the acidic sand sites. The Terrain® label states not to use on lighter soil types (sand) due to high levels of crop damage, however, the reduction in plant establishment on the medium textured sites was greater than 50% on average, which was a greater reduction than the sandy sites. This crop damage might be associated to the washing of pre-emergent herbicide into the crop row due to the large amounts of rainfall received within two weeks of sowing in both years. In terms of rolling timing, it is worth noting that in 2021 one alkaline sand site was rolled post emergent and the Terrain® treatment did not affect crop establishment at this site. However, all sites were rolled post-emergent in 2022 with plant establishment reductions for the Terrain® treatment occurring at all sites.

Diuron applied alone did not impact plant establishment at the alkaline sand sites compared to the control except at one site and did not reduce establishment at the acidic sites. Despite this, when adding diuron to Reflex® and Terrain® on either of these soils, the plant establishment reduction was more than when those products were applied alone.

Stunting

Plant stunting was one of the main Group 14 herbicide symptoms observed. Stunting caused by Reflex® was rate responsive and was generally worse on alkaline sands compared to acidic sands and medium textured soils (Table 3, Figure 1). The stunting symptom was barely present within the first six weeks post-emergence but gradually worsened into late winter and early spring (Figure 1). Recovery from this symptom was highly dependent on the amount of spring rainfall received, which influenced plant stress levels and the length of time for recovery. In the 2021 season, the late winter and spring rainfall was well below average resulting in lack of recovery from earlier herbicide damage. Conversely, the 2022 spring rainfall was average to above average, which allowed for good moisture availability and longer recovery time and resulted in better recovery from herbicide damage.

Figure 1. Stunting scores (1 = no stunting, 9 = plant death) for Reflex® and Terrain® treatments recorded on 21 July and 15 August at all sites in 2022. Error bars show LSD (0.05) for 21 July and 15 August are each respective site.

Similar to Reflex®, stunting severity caused by Terrain® was greater on alkaline sandy soils. Herbicide damage scoring from Terrain® in both years in July were generally consistent. In 2022, two timings of herbicide damage scoring were recorded, late July and mid August. Stunting from Terrain® improved over all soil types as the season progressed in 2022, in contrast to Reflex® where stunting increased on the sands (Figure 1).

Chlorosis

Chlorosis symptoms for Reflex® are generally visualised as “bronzing” and are well correlated with the amount of stunting present. It appears these symptoms go hand in hand; therefore, a combination of both stunting and chlorosis is likely contributing to the yield loss.

Terrain® chlorosis symptoms were independent of stunting symptoms. Chlorosis symptoms were very low at the acidic sand and medium textured sites, however, were present at low–moderate levels on the alkaline sand sites.

Springtime NDVI/biomass

There was no relationship between plant establishment and grain yield, as some herbicide treatments reduced plant establishment that ultimately reduced grain yield, whilst others such as Terrain® reduced plant establishment, but this did not influence yield. Terrain® treatments were able to recover with lower plant densities, suggesting compensation by accumulating more biomass per plant into spring allowed these treatments to match the potential yield of the untreated control. This crop recovery might be associated with the above average rainfall received in spring 2022 compared to 2021.

Previous trial work has shown that on sandy soil types or lower yielding environments, there is a strong relationship between spring NDVI (where NDVI is correlated to biomass) and yield for lentil, which was the case for the 2021 alkaline sand herbicide tolerance trial (Figure 2). Maximising biomass is important on soil types that are particularly sensitive to herbicides, such as alkaline sands.

Figure 2. The relationship for Greenseeker NDVI and grain yield recorded (a) 6 September for the alkaline sand trial at Alford in 2021 (y = –5.2444x2 + 7.3026x – 0.706, R2 = 0.77), and (b)15 September for the alkaline sand trial at Wards Hill in 2022 (y = 3.955x – 0.2994, R2 = 0.56).

Grain yield

Over the two seasons, the grain yield differences caused by the preceding herbicide damage was generally consistent across the two sandy sites (Table 3). Herbicides, diuron and Reflex® applied alone were more damaging at the alkaline sand sites, which aligns with recorded spring NDVI values. Reflex® yield loss is rate responsive with the 500mL/ha rate averaging 6% yield loss compared to the control treatment, whilst the 1000mL/ha rate averaged 21% yield loss across sites and years. When Reflex® was applied with diuron, the herbicide damage and resulting yield loss compared to the control was larger.

Terrain® averaged 95% and 97% yield of the control on average at the alkaline sand sites and acidic sand sites, despite losing 40% and 25% of plants on average, respectively. Over the two seasons and soil types in this project, Terrain® herbicide behaviour appears to be influenced less by the soil pH of sands than some other herbicides.

Rolling timing

Timing of rolling is important to keep separation of the soil that is treated with herbicide out of the crop row, where it may then be washed into the root zone by following rainfall events. In theory, post-emergent rolling of lentils allows time for the herbicide to move into the soil and even experience some level of degradation before potentially levelling some of the furrow back over the row. In contrast, PSPE rolling can potentially move the concentrated herbicide band back over the row before the crop has emerged.

The Reflex® label makes note for caution when rolling on sandy soils as they are more prone to soil movement back into the furrow. The Terrain® label states “for lentils, avoid rolling the paddock prior to crop emergence”. This is to prevent pushing excessive amounts of treated soil back into the furrow and reducing crop emergence.

Broadleaf weed control

IMI herbicide Intercept®, did not provide adequate control of Indian hedge mustard (IHM) at the alkaline loamy sand site (Wards Hill), and was not significantly different to the untreated control (Table 4). Similar results for poor IHM control with Intercept® were also reported at 2021 trial sites in lentil growing areas of Yorke Peninsula (Bruce et al. 2022), that might be due to increase of IHM populations resistant to IMI herbicides. However, IHM was effectively controlled with Intercept® at Paskeville light clay site. These results suggest the strategic use of IMI herbicides is important to ensure longevity of the chemistry. This will require rotating modes of action that is now possible with the availability of new Group 14 herbicides Reflex® and Terrain®. Reflex® applied at 500–1000mL/ha and Terrain® at 120g/ha as incorporated by sowing (IBS) were effective at controlling IMI resistant IHM populations at all the sites except at Wards Hill, where Reflex® proved to be slightly stronger than Terrain® (Tables 4, 5, 6 and 7).

Common sowthistle control improved with increasing Reflex® rates from 500mL/ha to 1000mL/ha (Tables 5 and 7). Terrain® proved as effective as Reflex® applied at 750mL/ha or at higher rates. Reflex® treated plots at alkaline sandy soil at Wards Hill had up to 0.3 surviving common sowthistle plants/m2 compared with up to 4 plants/m2 at neutral light clay soil of Paskeville (data not shown). Higher weed density at Paskeville in Reflex® treated plots coupled with moist conditions in medium textured soil due to spring rainfall resulted in surviving common sowthistle plants producing 428–818 pods/m2 compared to 16–21 pods/m2 at Wards Hill (Tables 4 and 5). Similarly, Terrain® treated plots recorded higher common sowthistle pods at Paskeville site. Most of the surviving plants of both common sowthistle and IHM in Reflex® and Terrain® treated plots were found in the in-row spaces, from where the applied herbicide was likely moved out by the seeding operation. Where Reflex® and Terrain® was applied IBS and were followed by a Group 5 herbicide metribuzin,  as a post-sowing pre-emergence (PSPE) application, the surviving weeds in the in-row area were mostly controlled. Similarly, the combinations of Reflex® + Intercept® and Terrain® + Intercept® provided effective control of common sowthistle control at all sites where it was present. Importantly, the paddocks where common sowthistle is IMI-resistant, will still have this weed surviving in the intra-row spaces even after applying Group 14 IBS herbicide followed by Intercept®.

Further, Reflex® was effective in controlling bifora by reducing its seed set from 6724/m2 in unsprayed control plots to 475/m2 when applied at 500mL/ha, and to <1 seed/m2 at 750 and 1000mL/ha (Table 5). Similarly, Reflex® reduced bifora seed set by 94–98% in 2021 trials (Bruce et al. 2022). Application of Intercept®, on its own or in combination with Reflex®, provided excellent control of bifora, reducing seed set to 0–4 bifora seed/m2 compared to existing pre-emergent herbicide options metribuzin and Terbyne recording 6416 and 8010 bifora seeds/m2, respectively. Terrain® did not prove effective for bifora control (11664 seeds/m2), and a subsequent post-emergent application of Intercept® was needed to achieve improved control with weed seed set reducing to 58/m2.

The Paskeville site had a background population of marshmallow. The level of marshmallow control improved with increasing Reflex® rates from 500mL/ha (807 pods/m2) to 1000mL/ha (238 pods/m2) (Table 5). Terrain® proved as effective (286 pods/m2) as Reflex® applied at 750mL/ha (196 pods/m2) or 1000mL/ha for controlling marshmallow and was better than Reflex® 500mL/ha. Both Group 14 herbicides proved superior to Group 5 herbicides metribuzin (1176 pods/m2) and Terbyne (660 pods/m2) for marshmallow control. A follow up application of Intercept® was needed after Reflex®/Terrain® IBS to achieve effective control of marshmallow (<1 pod/m2). Intercept® also achieved effective control of marshmallow without an upfront herbicide. But the IBS herbicides will be reducing selection pressure on Intercept®. Similarly, Intercept® was the standalone treatment for controlling medic up to 100% in lentil, likewise in 2021 research trials (Bruce et al. 2022), with the next best herbicide treatment Terrain® + diuron reducing weed population by 79% (Table 7).

Conclusion

Group 2 IMI herbicides will continue to be a valuable tool for broadleaf weed control in lentil for weeds that have not evolved resistance to this mode of action, and for weeds such as medic that are not effectively controlled with other herbicides. Rotating with other effective modes of action will reduce resistance selection pressure on this vulnerable herbicide group and sustain its efficacy on important weeds further into the future. However, for some weed species in some locations, IMI resistance is already well developed. The availability of the new Group 14 herbicides Reflex® and Terrain® applied in combination with other registered Group 2, 5 and 12 herbicides has increased the options for broadleaf weed control in lentil, including weeds resistant to IMI herbicides. However, consideration should be given to the associated risks of crop damage and a yield loss with new herbicides when applied alone or with Group 5 herbicides, depending on the soil type and herbicide rates. Background information on likely weed types, their population, and resistance status will be crucial for deciding herbicides and rates to achieve balance between satisfactory weed control and adequate crop safety on high-risk soils such as alkaline sandy textured soils.

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 authors would like to thank them for their continued support. Similarly, the authors thank SAGIT for their continued support (project codes: TC121, TC116, TC119). The help received from SARDI Agronomy Clare team in the field work is greatly appreciated. Authors also thank David Keetch and Jason Sabeeney for making available the new Group 14 herbicides for the current research studies.

References

Bruce J, Aggarwal N, Sherriff S, Trengove S, Roberts P (2022) Crop safety and broadleaf weed control implications for various herbicides and combinations in lentil. Proceedings GRDC Grains Research Update, Adelaide, February 2022, pp. 72–80.

GRDC (2022). Pre-emergent herbicides fact sheet. GRDC.

Additional tables for further reference

Contact details

Jordan Bruce
Trengove Consulting
Bute SA 5560
0408422903
jordanpbruce@gmail.com
@jordanbruce5555