Prickly lettuce ecology and management
Prickly lettuce ecology and management
Author: Hanwen Wu, Adam Shephard and Michael Hopwood Wagga Wagga Agricultural Institute, NSW Department of Primary Industries | Date: 26 Feb 2019
Take home messages
- Prickly lettuce germinates at a wide range of temperatures, from 5 to over 35 °C
- Emergence in a wheat crop mostly occurs between April and August in the first year after seed burial, with research showing 92% emergence in the first year and only 8% emergence in the second year
- Control in winter crops is important in reducing the weed pressure over summer
- Seeds of prickly lettuce are relatively short-lived, indicating prevention of seed set over two years should be effective in running down the seedbank
- Prickly lettuce is a surface germinator but can emerge from beyond 5cm burial depth. Cultivation to bury seeds will be less effective for prickly lettuce control than for sow thistle or fleabane
- Mature plants are often cut off during harvest and re-grow quickly with numerous branches. Prompt action through chemical control or grazing is required to stop seed set
- Control within paddocks is not the only consideration. It is also important to control weeds on fence lines, roadsides and other non-cropping areas due to the wind-dispersal capability of prickly lettuce
- Seedbank management is critical. It requires a combination of chemical and non-chemical options as there is no single “silver bullet”.
Background
Prickly lettuce (Lactuca serriola L.) is of Eurasian origin and is adapted to a summer-dry Mediterranean climate (Weaver and Downs 2003). It is distributed globally in Australia, Europe, North Africa, North America and West Asia (Prince et al. 1978). In Australia, it was first recorded in 1899 in the Upper Hunter, NSW and is now widely distributed across several states, particularly in NSW, VIC and SA (ALA 2018). It is a common weed of fallows, roadsides, waste lands and gardens, and it has recently become an increasing problem in cereals and lucerne pastures in southern NSW.
Prickly lettucecan grow up to 2 m high and is highly competitive with crops or pastures. If left uncontrolled, it consumes soil moisture and nutrients in the summer. L. serriola at a density as low as 0.2–1.2 plants /m2 has been shown to cause soybean yield losses of 10% and by up to 80% at densities of more than 50 plants /m2. However, it did not cause significant yield loss in winter wheat (Weaver et al. 2006). Similarly, no yield losses of cereals or grain legumes due to prickly lettuce were reported in Australia, but grain quality and harvesting efficiency were severely compromised (Amor 1986a). Flowering buds are cut together with grain during harvest, resulting in grain contamination and reductions in value. In addition, cut stems release a milky sap which can clog the harvesting machinery and increase the moisture content of the grain (Amor 1986a).
In Australia, Amor (1986b) estimated that a single prickly lettuce plant growing without competition in crop stubble produced 48,000 seeds/plant while it produced 900 seeds/plant in a wheat crop. The light-weight seed with a pappus is easily dispersed by wind and through surface water run-off. The tall stem also facilitates dispersal (Weaver and Downs 2003). Lu et al. (2007) reported that the seed can travel over distances of up to 43 km. The weed has evolved resistance to ALS inhibitors (Group B) herbicides and most recently to EPSP synthase inhibitors (Group M, glyphosate) in Australia. Plants are difficult to control with herbicides once the plants start to elongate. Mechanical control, such as slashing or mowing, is ineffective as it regrows with competitive branches after cutting or harvesting and progresses to set seed (Amor 1986a).
Information on germination ecology, emergence and effective herbicide options for control of L. serriola is limited in Australia. Effective herbicide control options are limited for mature plants. This study was conducted to compare the field emergence patterns of L. serriola with and without crop competition, and to evaluate a range of herbicide options of different modes of action on mature L. serriola plants.
Materials and methods
Germination factors
A series of experiments were conducted to determine factors affecting germination. A randomized complete block design with three replications was used in all experiments. Mature seeds of prickly lettuce (tall and short biotypes) were collected from a lucerne pasture paddock in Wagga Wagga in February 2018. Seed of willow-leaf lettuce (Lactuca saligna L.) was collected from a private garden and used for comparison. Thirty seeds of each Lactuca population were placed on Whatman No.2 filter paper moistened with 5 mL of distilled water in a 9 cm petri dish. Petri dishes were sealed with parafilm and incubated for 7 days under eight constant temperatures of 5, 10, 15, 20, 25, 30, 35 and 40 °C, and two fluctuating day/night temperatures of 25/20 and 30/20 °C, all with a 12-hr photoperiod. Seed germination was recorded after incubation for 7 days.
Light
The influence of light was tested at two constant temperatures (5 and 20 °C) and one fluctuating temperature (25/20 °C) by wrapping petri dishes in aluminium foil during incubation.
Burial depth
The effect of seed burial depth on seedling emergence was conducted in a glasshouse by placing 100 seeds of each population in 15 cm-diameter plastic pots on 28 March 2018. Seeds were placed on the soil surface (0 cm) or covered to depths of 2, 5 and 10 cm with field soil (brown clay, pH 5.4; and organic carbon, 0.6%). Treatments were replicated four times and emerged seedlings were assessed and removed on a weekly basis until no further emergence was noted.
Prickly lettuce field emergence
Seeds of four populations of prickly lettuce were each collected from low and high rainfall areas in southern NSW in January 2016. On 1 February 2016, 150 seeds of each of the eight populations were placed in seedling emergence trays (34 cm x 28 cm) filled with a mixture of coco peat potting mix and garden soil (1:1). Seeds of each population were evenly spread across the tray surface and then lightly covered by the peat/soil mix. The trays were maintained in the field under natural conditions in Wagga Wagga.
Two populations (one each from the high and the low rainfall zones) were used to determine the emergence pattern in wheat. Glyphosate was used as a pre-sowing knockdown. Wheat cv. Corack was sown at 60 kg/ha on 14 June 2016 and on 8 June 2017, respectively, with a 6-row cone seeder at 23cm row spacing using coulter knife points and press wheels. A basal fertiliser DAP was applied at 135 kg/ha each year at sowing. Prickly lettuce seeds (1000 seeds/m2) were sown in plots (3 x 1.3 m) on 15 April 2016. Any surviving or late emerging plants of prickly lettuce were counted and manually removed to prevent any new seed replenishment. Field emergence of prickly lettuce was recorded at monthly intervals from April 2016 to December 2017.
Herbicide options
Two trials were established after wheat harvest in December 2015, one at Lake Cowal and the other at Temora, NSW. Average prickly lettuce density was 9.5 plants/m2 at Lake Cowal and 10 plants/m2 at Temora. The plot size was 2 x 16 m. A range of herbicide treatments were imposed, respectively at the Lake Cowal and Temora sites, each including an untreated control. After the initial application, each plot was equally sub-divided with one half of the plot receiving an additional application of paraquat as a ‘double-knock’ treatment.
Herbicides were applied using a hand-held boom sprayer, calibrated to deliver 100 L/ha at 2 bar pressure. The first application of herbicides was applied at the Lake Cowal and Temora on 10 and 15 December, respectively, and the ‘double-knock’ paraquat application (600 g.a.i. /ha) at Lake Cowal and Temora on 16 and 21 December 2015, respectively. At the time of herbicide applications, prickly lettuce plants were at the elongation/re-branching stage after being cut during crop harvest and not under moisture stress.
A visual rating (% of control) in the single-knock treatments was conducted on 8 January 2016. The number of surviving prickly lettuce plants was also recorded in a 1 x 6 m strip in each plot on February 2, 2016.
Results and discussion
Constant and alternating temperatures on germination
The Lactuca species have wide germination temperatures, ranging from less than 5°C to more than 35°C, depending on the species and population (Figure 1). Willow-leaf lettuce (L. saligna) had 80% germination even at 5°C. The optimum temperatures for germination were 15 – 25°C for L. serriola and 10 – 30°C for L. saligna. Alternating temperatures did not improve the germination of L. serriola (tall) and L. saligna, but reduced the germination of the short biotype of L. serriola (Figure 2).
Figure 1. Germination temperature requirements for prickly and willow-leaf lettuce
Figure 2. Impact of alternating temperatures on the germination of prickly lettuce and willow-leaf lettuce
Higher temperatures induced secondary dormancy in prickly lettuce (Figure 3) (Secondary dormancy is the dormancy induced in the originally non-dormant seeds due to unfavourable environmental conditions). L. serriola seed (short biotype) underwent induced dormancy at 30 °C with only 49% germination at this temperature. The un-germinated seeds gave an additional 46% germination after being placed under 20/25 night/day temperatures. Both biotypes of L. serriola as well as the L. saligna entered secondary dormancy at 35 and 40 °C.
Figure 3. Dormancy of prickly lettuce and willow-leaf lettuce induced at higher temperatures of 30, 35 and 40 °C followed by (fb) exposure to alternating temperature regimes of 20/25 °C
Light
Light stimulated the germination of L. serriola for tall and short biotypes, while light was not required for the germination of L. saligna (Figure 4).
Figure 4. Impact of light on the germination of prickly lettuce and willow-leaf lettuce
Burial depth
Prickly lettuce had major emergence (40 to 76%) from the soil surface (0 cm), depending on the species and population, with L. saligna still having 19% at 2 cm burial depth (Figure 5). Both Lactuca species can emerge (0.25-0.5%) at deeper depths (5 and 10 cm), which is different to sow thistle, indicating cultivation to bury seeds will be less effective in prickly lettuce than sow thistle.
Figure 5. Impact of burial depth on the emergence of prickly lettuce and willow-leaf lettuce under glasshouse conditions
Field emergence
Prickly lettuce populations differed in their final cumulative emergence, ranging from 39 to 341 plants/m2 (Figure 6). However, they had similar emergence patterns, with 69% emergence in late autumn and early winter, 27% in later winter, 2.4% in spring in the first year after burial and only 1.6% emergence between autumn and winter in the second year (Wu et al. 2018).
Figure 6. Emergence of prickly lettuce in seedling trays without crop competition under field conditions from April 2016 to December 2017. The number of seeds sown onto each tray was expressed as seeds/m2
Figure 7. Emergence of prickly lettuce in a wheat crop grown under field conditions from April 2016 to December 2017 (the populations COOT07 and BARM 01 were collected from high and low rainfall zones, respectively)
The two prickly lettuce populations from different rainfall zones had similar emergence patterns, with major emergence occurring between April and August 2016 in the first year after burial, followed by limited emergence between May and September in the second year after sowing. On average, there was:
- 69% emergence in late autumn and early winter,
- 22% in later winter and 1.4% in spring in the first year after burial, with
- 7.5% emergence between autumn and winter in the second year.
The final cumulative emergence at 18 December 2017 was 32 and 24 plants /m2 for the high and low rainfall populations, respectively, while the final cumulative emergence for the two populations (COOT07 and BARM 01) in the above seeding tray trial in the absence of crop competition was 228 and 341 plants/m2, respectively. These results indicate that the emergence of prickly lettuce in a standing crop is only one tenth of that in a bare ground.
Thus, to achieve effective control and to reduce weed pressure in the summer, prickly lettuce management should focus on the major emergence cohorts in autumn prior to elongation in winter crops. A fallow control soon after harvest is also necessary to control the spring emergence cohort.
Effects of herbicides on prickly lettuce in summer fallow
At the Lake Cowal site, single-knock applications of different herbicides differed significantly in controlling prickly lettuce (Table 1). Five treatments, glyphosate, 2,4-D amine + glyphosate, metsulfuron-methyl + glyphosate, glufosinate ammonium and fluroxypyr achieved good control with more than 90% mortality, while the remaining 11 treatments only controlled 30–88% of prickly lettuce. The follow-up ‘double-knock’ treatment with paraquat at 600 g.a.i. /ha provided 100% control on prickly lettuce, even in the untreated plots which did not have the first knock of herbicide applications (data not shown).
The control of the single-knock herbicide application at Temora was generally less effective than at Lake Cowal. Only four treatments had >80% control efficacy including 2,4-D amine + glyphosate, a prepacked mixture of 2,4-D amine and picloram + glyphosate, glufosinate ammonium and a prepacked mixture of amitrole (250 g ai/L) and ammonium thiocyanate (220 g ai/L). The other 14 treatments resulted in poor control of prickly lettuce (7–72%, Table 2). The follow-up “double-knock” treatment with paraquat also provided near 100% control on prickly lettuce, with less than 0.1 surviving plant m-2.
In general, no single treatment, except paraquat, achieved 100% control of mature prickly lettuce plant after crop harvest. Many plants, even though severely damaged, managed to survive and re-branch. Further research is needed to evaluate other double-knock options for prickly lettuce control.
The major emergence of prickly lettuce occurs between late autumn and winter, management focus should be directed to target the young seedlings in crops, minimising the weed pressure at the end of the season when climatic conditions are often not favourable for spraying. The weed plants are also mature and often under moisture stress, making herbicides less reliable.
Table 1. Herbicide control efficacy as a visual % rating and plant density (plants/m2) on mature prickly lettuce plants at Lake Cowal.
Treatment | Rate (mL or g ha-1) | Adjuvant | Single-knockA | |
|---|---|---|---|---|
Visual ratingA (%) | Density (plants m-2)B | |||
2,4-D amine (700 g ai/L) | 1150 mL | Liase at 2% | 30.0 a | 3.7 ac |
2,4-D amine (700 g ai/L) + glyphosate (540 g ai/L) | 515 mL + 1300 mL | LI700 at 0.3% | 93.3 b | 0.6 b |
Glyphosate (540 g ai/L) | 1300 mL | LI700 at 0.3% | 90.0 bd | 2.0 ab |
Fluroxypyr (333 g ai/L) | 600 mL | Uptake 0.5% | 95.0 b | 0.8 b |
Fluroxypyr (333 g ai/L) + glyphosate (540 g ai/L) | 600 mL + 1300 mL | 88.3 bd | 2.4 ab | |
Metsulfuron-methyl (600 g ai/kg) + glyphosate (540 g ai/L) | 7 g + 1300 mL | 95.0 b | 1.6 b | |
Oxyfluorfen (240 g ai/L) + glyphosate (540 g ai/L) | 75 mL + 1300 mL | BS1000 at 1% | 86.7 bd | 2.1 ab |
Dicamba (500 g ai/L) + glyphosate (540 g ai/L) | 240 mL + 1300 mL | 85.0 bd | 4.5 cd | |
Glufosinate ammonium (200 g ai/L) | 4000 mL | 91.7 bd | 1.8 b | |
Mixture of amitrole (250 gai/L) and ammonium thiocyanate (220 gai/L) | 5600 mL | LI700 at 0.3% | 85.0 bd | 1.6 b |
Mixture of 2,4-D amine (300 gai/L) and picloram (75 g ai/L) | 700 mL | 53.3 cf | 3.9 ac | |
Mixture of MCPA (250 g ai/L) and diflufenican (25 g ai/L) | 1000 mL | 81.7 bd | 3.7 ac | |
Carfentrazone-ethyl (240 g ai/L) + MCPA amine (750 gai/L) | 100 mL + 330 mL | 76.7 de | 2.5 ab | |
Clopyralid (300 gai/L) + LVE MCPA (570 g ai/L) | 150 mL +1000 mL | 63.3 e | 2.4 ab | |
LVE MCPA (570 g ai/L) | 1000 mL | 40.0 af | 4.3 c | |
Mixture of aminopyralid (10 g ai/L) and fluroxypyr (140 g/L) + LVE MCPA (570 g ai/L) | 750 mL +1000 mL | 86.7 bd | 3.5 c | |
Control | 0.0 g | 6.2 d | ||
ATreatments applied on 10 December 2015 and a visual rating was conducted on 8 January 2016.
BThe surviving prickly lettuce plants were recorded on 2 February 2016.
Table 2. Herbicide control efficacy as a visual % rating and plant density (plants/m2) on mature prickly lettuce plants at Temora.
Treatment | Rate (mL or g ha-1) | Adjuvant | Visual ratingA (%) | Density (plants m-2)B | |
|---|---|---|---|---|---|
Single-knockA | Double-knockA | ||||
2,4-D amine (700 g ai/L) | 1150 mL | Liase at 2% | 51.7 a | 4.6 ab | 0.0 |
2,4-D amine (700 g ai/L) + glyphosate (540 g ai/L) | 515 mL + 1300 mL | LI700 at 0.3% | 80.0 bd | 1.2 a | 0.1 |
Glyphosate (540 g ai/L) | 1300 mL | LI700 at 0.3% | 78.3 bd | 4.2 ab | 0.0 |
Metsulfuron-methyl (600 g ai/kg) + glyphosate (540 g ai/L) | 7 g + 1300 mL | 28.3 c | 5.5 bc | 0.0 | |
Oxyfluorfen (240 g ai/L) + glyphosate (540 g ai/L) | 75 mL + 1300 mL | BS1000 at 1% | 60.0 ab | 8.7 c | 0.0 |
Dicamba (500 g ai/L) + glyphosate (540 g ai/L) | 240 mL + 1300 mL | 71.7 abd | 2.6 ab | 0.0 | |
Fluroxypyr (333 g ai/L) | 600 mL | Uptake at 0.5% | 53.3 a | 6.8 c | 0.1 |
Fluroxypyr (333 g ai/L) + glyphosate (540 g ai/L) | 600 mL + 1300 mL | 63.3 abd | 5.7 bc | 0.1 | |
Mixture of 2,4-D amine (300 g ai/L) and picloram (75 g ai/L) + glyphosate (540 g ai/L) | 700 mL + 1300 mL | 81.7 bd | 2.6 ab | 0.1 | |
Glufosinate ammonium (200 g ai/L) | 4000 mL | 83.3 d | 1.3 a | 0.1 | |
Mixture of amitrole (250 g ai/L) and ammonium thiocyanate (220 g ai/L) | 5600 mL | LI 700 at 0.3% | 81.7 bd | 4.9 ac | 0.1 |
Mixture of 2,4-D amine (300 g ai/L) and picloram (75 g ai/L) | 700 mL | 30.0 c | 6.1 bc | 0.1 | |
Carfentrazone-ethyl (240 g ai/L) + MCPA amine (750 g ai/L) | 100 mL + 330mL | 6.7 d | 5.4 bc | 0.1 | |
Clopyralid (300 g ai/L) + LVE MCPA (570 g ai/L) | 150 mL +1000 mL | Uptake at 0.5% | 30.0 c | 6.3 bc | 0.0 |
LVE MCPA (570 g ai/L) | 1000 mL | 28.3 c | 5.7 bc | 0.0 | |
Mixture of aminopyralid (10 g ai/L) and fluroxypyr (140 g ai/L) + LVE MCPA (570 g ai/L) | 750 mL +1000 mL | 58.3 a | 5.8 bc | 0.0 | |
Control | 0.0 e | 7.4 c | 0.0 | ||
ASingle_knock treatments applied on 15 December 2015 and the “Double knock” paraquat on 21 December 2015, followed by a visual rating in the single-know treatments conducted on 08 January 2016.
BThe surviving prickly lettuce plants were recorded on 02 February 2016.
References
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Wu, H.,Shephard, A. and Hopwood, M. (2018). Emergence patterns and herbicide control of prickly lettuce (Lactuca serriola L.). Proceedings of the 21st Australasian Weeds Conference, pp. 290-294. 9 –13 September 2018, Sydney, New South Wales.
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.
Contact details
Hanwen Wu
Wagga Wagga Agricultural Institute
NSW Department of Primary Industries
Pine Gully Road, Wagga Wagga, NSW 2650
Ph: 02 6938 1602
Email: hanwen.wu@dpi.nsw.gov.au
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