Cutting weeds with a CO2 laser T HEISEL, J SCHOU*, S CHRISTENSEN & C ANDREASEN Department of Crop Protection, Danish Institute of Agricultural Sciences, 4200 Slagelse, *Department of Optics and Fluid Dynamics, Risù National Laboratory, 4000 Roskilde, and Department of Agricultural Sciences, The Royal Veterinary & Agricultural University, Thorvaldsensvej 40, 1871 Copenhagen, Denmark Received 11 May 2000 Revised version accepted 7 July 2000
Summary Stems of Chenopodium album. and Sinapis arvensis. and leaves of Lolium perenne. were cut with a CO2 laser or with a pair of scissors. Treatments were carried out on greenhouse-grown pot plants at three dierent growth stages and at two heights. Plant dry matter was measured 2 to 5 weeks after treatment. The relationship between dry weight and laser energy was analysed using a nonlinear dose±response regression model. The regression parameters diered signi®cantly between the weed species. At all growth stages and heights S. arvensis was more dicult to cut with a CO2 laser than C. album. When stems were cut below the meristems, 0.9 and 2.3 J mm)1 of CO2 laser energy dose was sucient to reduce by 90% the biomass of C. album and S. arvensis respectively. Regrowth appeared when dicotyledonous plant stems were cut above meristems, indicating that it is important to cut close to the soil surface to obtain a signi®cant eect. When cutting L. perenne plants with 2-true leaves at a height of 2 cm from the soil surface with a laser, the biomass decreased signi®cantly compared with plants cut by scissors, indicating a delay in regrowth. This delay was not observed for the dicotyledonous plants nor for the other growth stages of L. perenne. Keywords: CO2 laser, physical weed control, Chenopodium album, Sinapis arvensis, Lolium perenne.
Introduction Environmental concerns about side-eects of herbicides increase the interest in other means of weed control. Hoeing and harrowing are the main alternative methods in arable crops, uprooting or covering the weeds by soil. The mechanical actions of hoeing and harrowing are to uproot and/or cover the weeds, thereby either delaying their growth and thus their competitive ability or eventually killing them. However, a disadvantage of hoeing and harrowing is that the disturbance of the soil often initiates new weed seed germination and emergence. Furthermore, harrowing can cause severe crop damage. Jones & Blair (1996) have demonstrated that another eective method is to cut plants at ground level in order to kill dicotyledonous plants or reduce monocotyledonous plants in size and thereby delay their growth compared with the crop. Grass
Correspondence: T Heisel, Department of Crop Protection, Research Centre Flakkebjerg, 4200 Slagelse, Denmark. Tel: (+45) 58 11 33 00; Fax: (+45) 58 11 33 01; E-mail:
[email protected]
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weed species recovered from some of the treatments whereas broad-leaved weed species rarely recovered. There are several advantages of cutting as compared with hoeing and harrowing. No energy is used to drag tools through soil and no soil is physically moved. This may prevent heavy clay soils from compaction. Cutting can be performed with various tools that are quickly moveable such as cutter discs, ¯ail discs and leaf strippers (Nawroth & Estler, 1996). These tools were tested on Chenopodium album L. and Echinochloa crus-galli (L.) Beauv. and showed that cutting with either a cutter disc or a ¯ail disc 2 cm above ground reduced weed dry matter by 94±98% when compared with untreated plants at 20 days after treatment. Lasers are used for cutting industrial materials (von Allmen & Blatter, 1995) and in other research areas, e.g. medical surgery (Majaron et al., 1998), wood-cutting (Grad & Mozina, 1998) and sample preparation in microscopy (Stehr et al., 1998). A laser concentrates a large amount of energy in a narrow laser beam and can be directed precisely and quickly on to targets. Furthermore, the laser beam can be focused into a narrow area to increase energy per area and, on the other hand, avoid danger outside of the focus range. Various types of lasers are available in the ultraviolet regime (UV lasers 200±400 nm), in the infrared regime (IR lasers 700± 1500 nm) or in the far infrared regime (FIR lasers 5±15 lm, e.g. CO2 lasers). UV and IR lasers cut via explosive ejection, i.e. ablation, of plant tissue generated by multiphoton and avalanche electron ionization (Bloembergen, 1974). CO2 lasers cut because of large light absorption in tissue water molecules and with a subsequent strong heating and explosive boiling (Langerholc, 1979). Recently, the authors have compared the ability of UV (355 nm), IR (1064 nm) and CO2 (10.6 lm) lasers to cut young stems of C. album, Sinapis arvensis L. and leaves of Lolium perenne L. The investigations indicated that all laser types were able to cut weed stems with doses above 6 J mm)1. However, the CO2 laser provided the best eect with the lowest energy use (unpubl. obs.). Similarly, seed heads of rye (Secale cereale L.) have been burned with a CO2 laser as a project of weed control (Bayramian et al., 1993). Levels above a threshold of 17.3 J eectively killed developing seed heads. Stem thicknesses were on average 1.1 mm. The objective of this study was to investigate whether the cutting of C. album, S. arvensis and L. perenne with a CO2 laser could be a potential method of weed control. Furthermore, the aim was to test if the response of plant biomass to various levels of laser doses could be described by a simple dose±response relationship. The study included three growth stages and two cutting heights above or below the meristems of the cotyledons. The eect of laser irradiation at an arbitrarily chosen threshold level was compared with cutting by scissors.
Materials and methods Growing conditions
Seeds of three species, C. album, S. arvensis and L. perenne, were sown on 27 October 1998 in 35-cL pots. The species were chosen to represent two types of weeds common in sugar beet (Beta vulgaris L.) production with two dicotyledonous plants (C. album and S. arvensis) and one monocotyledonous plant (L. perenne). A total of 132 pots per species was prepared. In each pot three to four seeds were placed in four holes, 2 cm from the pot edge and 4 cm apart in a square. The plants were grown in a greenhouse at the Research Centre, Flakkebjerg at 14 °C, 75% relative humidity and 16-h light. The pots were placed on a table watered from below. The Ó Blackwell Science Ltd Weed Research 2001 41, 19±29
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density per pot was continuously thinned to three (S. arvensis) or four (L. perenne) plants of equal size. Because of insucient germination, C. album pots were thinned to either four, three or two plants per pot at the growth stages cotyledon, 2-true leaves or 4-true leaves respectively. At every growth stage (cotyledon/1-true leaf, 2-true leaves or 4-true leaves), 44 pots per species were taken to Risù National Laboratory to be irradiated by laser or cut by scissors. Four pots per species were untreated. The thickness of the stems of C. album, S. arvensis and leaves of L. perenne at the cutting height were measured with a calliper on an average of 10 randomly chosen plants per species. After treatment the plants were brought back to the greenhouse. Plants treated on the same date were placed in a completely randomized block design. Above-ground fresh weight of all plants was harvested 13 days after the last treatment to allow the plants to recover from laser treatment and eventually regrow. Dry weight of plants per pot was determined after 24 h at 90 °C and the dry weight per plant (DW) was calculated thereafter. Laser and scissors cutting conditions
A 50-W SYNRAD SH CO2 laser with a 64-mm2 beam at the exit aperture was used. The laser was equipped with a lens focusing the beam to 0.6 mm2 at a distance of 24 cm. A computer was connected to the laser in order to control the power (in W), direction and velocity of the laser beam. The power of the laser was checked prior to every treatment using an OPHIR power meter. We set the power to 4, 10 or 20 W and directed the laser beam 15-cm horizontally with velocities of 1, 5 or 10 mm s)1 on the basis of previous results. Because it is unrealistic to perform horizontal laser cuts close to the soil surface in a ®eld, due to an uneven surface and as the equipment can be damaged from touching the soil, some tilting of the laser beam is required. To simulate inclination of the laser beam, the pots were tilted by 15 degrees with two of the plants in focus as shown in Fig. 1. A metal plate protected the two remaining plants in the pot. After one cut with the laser, the pots were rotated to treat the two remaining plants. The cutting was carried out 1 or 2 cm from the soil surface except for the two early growth stages of C. album and S. arvensis (see Table 1). The intention was to simulate a cut above and below the meristem of the two cotyledons. Scissor-cuts were performed on four pots per species per irradiation day.
Fig. 1 Laser cutting arrangement with computer controlled CO2 laser and pot holding device. Ó Blackwell Science Ltd Weed Research 2001 41, 19±29
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Table 1 Number of pot replicates for the dierent levels of DOSE and weed growth stages DOSE (J mm)1) Weed species Chenopodium album Sinapis arvensis Lolium perenne Chenopodium album Sinapis arvensis
Growth stage
0
0.4
0.8
1
2
4
10
20
4-true leaves 4-true leaves All stages Cotyledon and 2-true leaves Cotyledon and 2-true leaves
2 2 2 4 4
2 2 2 4 4
2 2 2 4 4
2 2 2 4 4
4 4 4 8 8
4 4 4 8 8
2 2 2 4 4
2 2 2 4 4
Statistical analyses
The total laser energy used per travelled distance is introduced because of the functional relationship between power and velocity of the laser beam. The combined factor is called DOSE as it gives a unique measure of the actual energy per travelled distance (DOSE power/ velocity energy/distance). A low velocity requires more power to obtain the same cutting result and vice versa. A relationship between power and velocity can be expected as Bilanski & Ferrez (1991) found similar results for travel speeds from 38 to 76 mm s)1 when cutting potato tubers (Solanum tuberosum L.) with a CO2 laser. The relationship between DW and the DOSE was analysed using the dose±response analyses described by Streibig et al. (1993). The following non-linear model was used for each level of weed species, growth stage and cutting height: DW C
1
DÿC
B r
LOG
DOSE1 LOG
ED50 1
1
The errors are expected to be independent and normally distributed with zero mean and variance r2. The parameter D describes the DW of untreated plants. The parameter C describes the lower asymptote of DW. The parameter ED50 describes the level of DOSE where (D ) C) is reduced to 50%. The exponent B describes the slope and the positioning of the dose±response curve around ED50. DOSE was log-transformed to obtain homogeneity of variance. Unity was added to the dose to avoid negative results of the logarithm (see Eqn 1). A contrast analysis in a generalized linear model was used to investigate the dierence between cutting with the laser and cutting with scissors. In the analysis the levels of the DOSE above or below a certain threshold are tested to see if they are signi®cantly dierent from scissor-cutting. The threshold is determined by the respective ED90 value where the maximum dry weight (D) is reduced to 10%. This level is chosen arbitrarily because it can be compared with common control eects. All analyses were performed with the statistical analysis software SAS (1998).
Results Most of the data points follow a dose±response relationship as described in Eqn 1 (Figs 2±4). The estimates for the regression parameters of Eqn 1 and measurements of the mean thickness are shown in Table 2. Generally ED50 increases with growth stage, indicating that more energy is needed to kill larger plants. Some ED50 and B parameters of S. arvensis have large standard Ó Blackwell Science Ltd Weed Research 2001 41, 19±29
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Fig. 2 Fitted CO2 laser dose±response curves of C. album at growth stage cotyledon (d/no ®t), 2-true leaves (s/Ð) or 4-true leaves at cutting height 1 cm (n/¼.) or 2 cm ( ´ /Ð) above soil surface. Points equal mean values of data points in Table 1.
Fig. 3 Fitted CO2 laser dose±response curves of S. arvensis at growth stage cotyledon (d/Ð), 2-true leaves (s/¼.) or 4-true leaves at cutting height 1 cm (n/Ð) or 2 cm ( ´ /¼.) above soil surface. Points equal mean values of data points in Table 1.
deviations, indicating that the chosen model does not describe the data adequately (Table 2). This was primarily due to scattered data at DOSE values from 4 to 10 J mm)1. At the cotyledon stage S. arvensis was the only species that could be described by the dose±response relationship Ó Blackwell Science Ltd Weed Research 2001 41, 19±29
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Fig. 4 Fitted CO2 laser dose±response curves of L. perenne at growth stage 1-true leaf cut 1 cm (d/no ®t) or 2 cm (s/no ®t), 2-true leaves cut 1 cm (n/Ð) or 2 cm ( ´ /¼.) or 4-true leaves cut 1 cm (+/Ð) or 2 cm (e/¼.) above soil surface. Points equal mean values of data points in Table 1.
because all plants of C. album were cut by the lowest DOSE and only the tips of the L. perenne leaves were cut because the plant height was less than 2 cm. The parameter estimates for ED50 vary amongst the three species. At all growth stages and heights L. perenne seems to be the species easiest to cut (lowest ED50 estimate) probably due to the proportionally thinner grass leaves (Table 2). Sinapis arvensis plants needed a signi®cantly larger DOSE than C. album plants at the 4-true leaves stage. When stems were cut below the meristems, 0.9 and 2.3 J mm)1 of CO2 laser dose was sucient to reduce by 90% the biomass of C. album and S. arvensis, respectively. For a cut above the meristem at a late growth stage S. arvensis required a higher DOSE than the other species (ED50 5.2 J mm)1). The minimum dry weight biomass (C) was generally zero when the cut was carried out below the meristem for the two broad-leaved species. When cut above the meristem, C. album showed the ability to regrow re¯ected in the non-zero parameter estimate. Too low DOSE levels might be the reason why S. arvensis did not converge to a C parameter dierent from zero, as the asymptotic minimum dry weight level is poorly supported (see Fig. 3). Lolium perenne showed the expected regrowth re¯ected in nonzero parameter C estimates. The higher the cut the larger the resulting biomass. The thickness of the stems and leaves increased with growth stage and there seems to be a linear relationship between ED50 and the thickness of the stems and leaves for the three species (Fig. 5). Cutting with scissors provided a better control than laser cutting below the ED90 threshold for the two broad-leaved species (Table 3). The contrast analysis further showed that generally no signi®cant dierence in biomass was found between cutting with the scissors and the laser above the threshold. Only L. perenne at the 2-true leaves stage cut with the laser 2 cm from the ground showed a signi®cant (P < 0.05) reduction in biomass compared with the scissors. Ó Blackwell Science Ltd Weed Research 2001 41, 19±29
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1-true 1-true 2-true 2-true 4-true 4-true
Lolium perenne
DNC, did not converge.
Cotyledon 2-true leaves 4-true leaves 4-true leaves
Sinapis arvensis
leaf leaf leaves leaves leaves leaves
Cotyledon 2-true leaves 4-true leaves 4-true leaves
Chenopodium album
Growth stage at cutting
1 2 1 2 1 2
1 1 1 2
1 1 1 2
Cutting height (cm)
0.7 0.4 1.3 1.1 1.6 1.4
0.5 0.9 1.5 1.2
0.5 0.8 1.2 1.1
Thickness (mm)
(0.17) (0.02) (0.04) (0.34)
DNC DNC 0.25 (0.02) 0.25 (0.01) 0.22 (0.01) 0.23 (0.01)
0.25 0.33 0.34 0.33
DNC 0.23 (0.02) 0.32 (0.01) 0.32 (0.02)
Maximum dry weight production D (SE) (g plant)1)
DNC DNC 0.12 (0.01) 0.14 (0.06) 0.08 (0.01) 0.11 (0.08)
0 0 0 0
DNC 0 0 0.04 (0.02)
Minimum dry weight production C (SE) (g plant)1)
(0.26) (0.01) (0.06) (0.44) DNC DNC 0.38 (0.01) 0.41 (0.01) 0.82 (>99) 0.55 (0.07)
0.62 0.82 1.13 5.17
DNC 0.78 (0.03) 0.82 (0.01) 0.82 (>99)
50% reduction in plant dry weight ED50 (SE) J mm)1
Table 2 Summary of regression parameters from the estimated dose±response curves (Eqn 1) and calculated ED90
DNC DNC 0.5 0.5 0.9 1.5
1.6 0.9 2.3 ±
DNC 0.9 0.9 0.9
90% reduction in plant dry weight ED90 (SE) J mm)1
DNC DNC 29 (0.00) 292 (0.00) 94 (>99) 3.0 (1.30)
4.8 (5.30) 22 (10) 9.4 (6.30) 2.0 (0.90)
DNC 21 (30) 33 (40) 89 (>99)
Slope and positioning of curve at ED50 B (SE)
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Fig. 5 Relationship between thickness of C. album (d) stem, S. arvensis (s) stem or L. perenne (´) leaf and the corresponding ED50value (50% reduction in maximum plant dry weight).
Table 3 F-test for contrast sum of squares between cutting with scissors and cutting with the laser below or above ED90 threshold (from Table 2) Growth stage
Cutting height (cm)
Scissors better than laser below threshold
Scissors worse than laser above threshold
Chenopodium album
2-true leaves 4-true leaves 4-true leaves
1 1 2
14.8*** 71.0*** 71.0***
0.0 0.0 0.1
Sinapis arvensis
Cotyledon 2-true leaves 4-true leaves
1 1 1
5.4* 15.3*** 21.8***
0.2 0.0 3.0
Lolium perenne
2-true 2-true 4-true 4-true
1 2 1 2
1.3 7.9* 4.3 2.5
1.7 5.6* 0.0 0.2
leaves leaves leaves leaves
,*, **, *** signi®cant at P < 0.05, P < 0.01 and P < 0.001 respectively.
Discussion The parameter ED50 equals the energy level sucient to reduce the dry matter weight to onehalf of the value for D for the plants tested. Hence, ED50 gives a comparable DOSE requirement level to cut the speci®c weed stem or leaf. Generally ED50 increased with later growth stages and increasing cutting height for all three species. Corresponding results with increasing ED50 for increased weed size have been reported for ¯aming (Ascard, 1995), UVradiation (Andreasen et al., 1999) and herbicide treatment (Mathiassen et al., 1997). This eect could be expected, as stems become thicker during the growing season. There seems to be a relationship between stem thickness and DOSE requirement in this experiment (Table 2 and Fig. 5), indicating that the DOSE can be optimized if the stem thickness of the plant is known. Unfortunately we only measured the thickness of 10 randomly selected plants per species per growth stage and height. If we had measured the thickness of all the plants it would have been possible to incorporate the information into our model (Eqn 1). This will be a possible expansion of the present work. Ó Blackwell Science Ltd Weed Research 2001 41, 19±29
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Investigations with a stationary CO2 laser beam on rye have been reported previously. Bayramian et al. (1993) found that 262 J cm)2 was sucient to cut through stems of rye equivalent to 17.3 J per stem (with an average stem thickness of 1.1 mm and exposure time of 1.73 s). On Carduus nutans L., with a 16-fold thicker stem, a 10-fold increase in energy requirement was found. The results for the arbitrarily chosen threshold ED90 range from 0.9 to 2.3 J mm)1 on stems from 0.8 to 1.5 mm. This would be equivalent to approximately 0.7±3.5 J per stem. Hence, this comparison might indicate that the eect of cutting with a laser improves when the laser beam is being moved. Most of the resulting parameter estimates for B are large, indicating that the slope around ED50 is steep. Thus, the dose is a sensitive parameter and it becomes important to apply a dose above the chosen threshold level (e.g. ED50 or ED90), as a value under the threshold may lead to inadequate cutting. Hence a simple threshold model with a binary output (i.e. dead/not dead) may be adequate to describe the same eect. A threshold model would be a logical consequence of the physical description of a cut: is the stem or leaf cut or not cut? Nevertheless, the dose± response model describes the biomass relationship well when cutting is performed above the meristems of cotyledons (e.g. Fig. 3). More investigations of the nature of the biomass response are expected to improve the model. Important to note here is also that our results would not necessarily apply to all other species or types of habitat. We would, e.g. expect dierences with dicotyledonous types not having only one single strong stem. With increasing cutting height of L. perenne the minimum dry weight biomass (parameter C) also increases. The response was somewhat dierent when the two broad-leaved species were cut above the meristems at the late growth stage. Chenopodium album did regrow and had a C parameter larger than 0, whereas the D and ED50 parameters did not dier signi®cantly from one height to another at this particular growth stage. The laser irradiation of S. arvensis resulted in a considerably less steep curve (indicated by the B parameter) and larger ED50 parameter. These results stress the importance of cutting weed stems as close to soil surface as possible, supporting the ®ndings of Jones & Blair (1996). As expected, the biomass was reduced when stems and leaves were cut with scissors compared with cutting with the laser below the ED90 threshold level. The similar contrast analysis against the laser DOSE level above the same threshold should reveal if one could expect a dierence between an optimal cut by laser or by scissors. Generally, no signi®cant dierence in biomass was found, except for L. perenne at the 2-true leaves stage cut with the laser 2 cm from the ground. This may indicate that there could be a delay in regrowth of monocotyledonous species after the laser treatment. Further investigations will explore these indications. A linear model has been used to describe the depth of a ¯at cut in high-moisture potato tubers as a function of dose with a CO2 laser (Bilanski & Ferrez, 1991). Their model takes tissue re¯ectance, density, speci®c and latent heat into account. They observed a linear relation between the cutting depth and the reciprocal velocity for velocities from 38 to 76 mm s)1. Furthermore, a depth-response curve of increasing velocity for every power level used was found ± indeed similar to the dose±response curves presented in this paper. In our work, no speci®c measurements of actual cutting depths were performed. Nevertheless, our results are approximately similar to the ®ndings of Bilanski & Ferrez (1991), as the dry weight in our experiments and the cutting depth do have similar relationships with the power and the velocity of the laser beam. The amount of power used to obtain a 10-W laser beam is larger than this value because there is an unavoidable energy loss in the equipment. Usually, this is described by a power conversion eciency rating (per cent energy in the laser beam compared with the total used energy). Under Ó Blackwell Science Ltd Weed Research 2001 41, 19±29
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certain circumstances one might expect to reach an eciency of 22% with a CO2 laser (Verdeyen, 1995). However, this ideal value is far above a realistic maximum level for mobile, robust lasers (5%). Nevertheless, it would be desirable to minimize the energy consumption with a much higher conversion eciency. The laser uses the same energy whenever it is emitting the beam independent of whether it is cutting or not. Consequently there could be a potential saving if it were possible to direct the laser beam precisely at the target stem or leaf and turn it o where not needed. However, commercially available CO2 lasers cannot quickly be turned o completely, because they need stable voltage and power supplies. It would be interesting to carry out more detailed investigations of the total energy consumption in CO2 lasers. Care has to be taken regarding safety when working with lasers. Obviously, the laser beam may cause injuries to human tissue or eye during irradiation in or near the focal plane. Moreover, the laser beam may be re¯ected from shiny objects from the ground, e.g. stones or pieces of metal, which makes a certain degree of laser light protection necessary for the operator. A complicating feature is that the beam is invisible to the human eye and can be detected only by heating or ¯uorescent eects. Furthermore, stray beams can cause ®res, so commercial equipment will require careful shielding and proper beam extinction baes. Infrared beams can be routinely extinguished with anodized aluminum baes, so that safe and ecient ®eld-scale designs should be possible (Bayramian et al., 1993). It is important to irradiate the stems in or close to the focal plane of the exit lens system. The energy per area decreases as the square of the distance to the focal plane (Verdeyen, 1995). Therefore, it is necessary to know the approximate distance to the target stem or leaf, as the cutting will be poor at other distances. Poor cutting was seen as little as 3 cm from the focal plane with the lens used in this experiment (unpubl. obs.). This could be an advantage for a guidance and targeting system, as it might be enough just to locate the crop plant and save that from the laser by applying the knowledge of the location of the focal plane. At the same time it is an advantage for safety reasons, as the energy in a speci®c area decreases with increasing distance to the laser. CO2 lasers have the potential of being used as a cutting device for physical weed control. The tool should target small weeds very close to the crop in-row, as it is in-row weeds that create the demand for hand weeding (Mattson et al., 1990). Hand weeding may take up to 150 h ha)1 in sugar beet (Ascard et al., 1995) indicating the potential economic saving. Another potential use could be to cut roadside or pavement weeds.
Acknowledgements We wish to thank Arne Nordskov, Risù National Laboratory for competent technical assistance with the laser irradiation.
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