1	Bioassays on Glyphosate Residue Toxicity and Photodegradation on Polyethylene Mulch James P. Gilreath Bielinski M. Santos Gulf Coast Research and Education Center, IFAS
2	Introduction Glyphosate is the most used herbicide in the world. This is a non-selective molecule that blocks the synthesis of the aromatic amino acids. This herbicide is used in postemergence: Direct in row-middles, Over-the-top.
3	Introduction Over-the-top applications are widely used in polyethylene-mulched vegetables and ornamentals, to eliminate weeds emerged through the film.
4	Introduction This practice is justified because usually two to three weeks pass from the time the mulch is applied to the transplant. Crops are transplanted within 48 h of glyphosate application.
5	Introduction Glyphosate label indicates that there is no soil activity, being rapidly broken down by microbial processes. Injuries similar to those caused by glyphosate have been observed in grower fields, within 72 h after application.
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7	Introduction How can we explain this? Situation: Herbicide applied in water-based solutions on top of the plastic mulch and followed by solution drying. Hypothesis: Dissolution of molecules on the surface of the mulch occurs causing plant injury. Are humidity and sunlight factors?
8	Objectives Determine the concentration of glyphosate needed for plant injury, and Determine the extent of photodegradation over varying sunlight and humidity exposure periods.
9	Materials and Methods Four bioassays: 1. Dose-response, 2. Photodegradation, 3. Dew exposure, 4. Simulated rainfall.
10	Materials and Methods Dose-response bioassay Two trials: Greenhouse (October 2000 to January 2001). GCREC in Bradenton, Florida. Six glyphosate concentrations: 50, 100, 150, 200, 250 and 500 ppm. A non-glyphosate control.
11	Materials and Methods Dose-response bioassay Randomized complete block design with ten replications. Glyphosate rates: Selected based on recommended rates for preplant situations: 1.14 kg ha^-1 are diluted in 520 L ha^-1, Approximately 2180 ppm.
12	Materials and Methods Dose-response bioassay ‘Solimar’ tomato seedlings in the two-true leaf stage. Indicator plants: Sensibility to glyphosate injury. Foliage was dipped into a 250-mL treatment solution for 10 sec. One DAT, plants were transplanted into 1 L containers filled with a commercial potting medium.
13	Materials and Methods Dose-response bioassay Fresh plant weights: 56 and 29 DAT. Regression analysis and standard errors. Data form two trials were combined, since there was no significant (P>0.05) trial by treatment interaction.
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15	Materials and Methods Photodegradation bioassay Two field trials (October-December 2000). GCREC in Bradenton, Florida. Maximum and minimum temperatures were 23.7 and 7.2C, and no rain events occurred. Black low-density polyethylene film: Mounted on 2.5 m long by 0.3 m wide boards.
16	Materials and Methods Photodegradation bioassay Glyphosate was sprayed at a rate of 1.14 kg ha^-1. Eleven treatments were distributed in a randomized complete block design with ten replications. Sunlight-exposure lengths: 1, 2, 3, 4, 5, 7, 9, 11, 13, and 15 days, plus a check. At every sunset, the boards were removed from the field and placed in a dark shelter until the next day.
17	Materials and Methods Photodegradation bioassay Mulch samples of 900 cm² removed from each board and agitated for 30 min with 250 mL of water to dissolve the residual glyphosate. Foliage of ‘Solimar’ tomato seedlings was dipped into the eluants for 10 sec. Fresh plant weights: 32 DAT. Regression analysis and standard errors. Data form two trials were combined.
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19	Materials and Methods Dew exposure bioassay Eleven treatments in a randomized complete block design with ten replications. Sunlight exposure lengths: 1, 2, 4, 6, and 8 days, Removal or no-removal from the field, A non-treated control. Sampling mulch procedure, solution volume, time of exposure, plant material and other experimental procedures were similar to those for the photodegradation bioassay.
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21	Materials and Methods Simulated rainfall bioassay Eight treatments in a randomized complete block design with ten replications. Rainfall effect: Simulated by sprinkler irrigation and started within 1 h after treatment. Simulated rainfall: 65, 128, 191, 254, 317, and 380 mm, plus a check. Sampling mulch procedure, solution volume, time of exposure, plant material and other experimental procedures were similar to those for the photodegradation bioassay.
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23	Summary Dose-response bioassay With 50 ppm of glyphosate, there was a reduction of 73% in tomato fresh weight per plant. This concentration is 40 times lower than the regular glyphosate concentration applied in the field before crop transplanting. An average 90% fresh weight reduction was obtained with 150 or more ppm.
24	Summary Photodegradation bioassay At 15 DAT, there was still enough glyphosate to reduce biomass accumulation. The average value for the control treatment was 28.6 g per plant in contrast with 8.0 g for 15 days of sunlight exposure. No significant photodegradation occurs for two weeks, if humidity is not a factor.
25	Summary Dew exposure bioassay At 8 DAT, the highest fresh biomass obtained was 24 g, which is 84% of the control biomass. It appears that longer dew exposure periods are needed.
26	Summary Simulated rainfall bioassay There was a sharp increase on tomato fresh weight with only 65 mm of rain, representing around 85% biomass increase. Fresh biomass remained stable beyond 65 mm of rainfall. No differences between the control and the treatments that received sprinkler irrigation.
27	Summary Overview Glyphosate residues could be a potential source for injury to crops transplanted in polyethylene mulch beds. This finding might be important for the industry and commercial applicators: Residues in packing lines, and Residues in tractor-mounted applicators.
28	Summary Overview Rainfall showed to be effective to wash the herbicide away from mulch surfaces. More extensive studies are conducted to determine the minimum concentration needed for glyphosate injury.