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Nagler, M., Buangsuwon, D., Jewers, K., Faungfupong, S., Wong-urai, A., Nagler, C, and Tanboon-ek, P.
SUMMARY
Studies in Thailand involving field-drying trials at two agricultural research stations, and 35 grain drying trials in two provinces, strongly indicate that field-drying for between 2 to 4 weeks may have a significant role in controlling levels of aflatoxin in Thai maize.
The field-drying trials were monitored weekly over a 6-week period (7 treatments) in randomised replicates of 4 using the Suwan 1 variety of maize. Fifty plants were used in each replicate and the trials were carried out in both the dry and rainy seasons. No trend towards an increase in aflatoxin contamination was found as field-drying time increased, and levels of aflatoxin were low at harvest ( < 20 ppb). There was also no significant increase in the occurrence of kernels damaged by mould or insects. In the rainy season, standing maize having a moisture content of 27 to 30% at field maturity, was found to have dried to 18 to 22% after. 2 weeks of field-drying. These results were consistent with those obtained during 35 mechanical drying trials. In these, maize with a field-drying history of 1 to 4 weeks was shelled within 48 hours of harvest, put in a drier within 12 hours of shelling, and dried to a moisture content of 14% or 16% in less than 48 hours. The resulting maize had a total aflatoxin content which averaged less than 5 ppb, at a time when controls averaged over 100 ppb.
The advantages of field-drying were found to be: 1) it reduced moisture content to 18-22% which would make aflatoxin control by subsequent 1 - stage mechanical drying to 14% m.c. more economic, 2) it should reduce shelling damage, 3) it reduced initial onfarm aflatoxin contamination of cobs, and may reduce further contamination during long-term storage of cobs. The disadvantages would be: 1) planting of a second crop would be delayed, 2) there would be an increased risk of losses due to storm or rodent, 3) the farmer stands to lose financially unless he is adequately compensated for weight loss.
Feed-mills, and some exporters, are willing to pay a premium for maize having a low level of aflatoxin. If field-drying is used as part of a system to produce low aflatoxin maize at 14% m.c., then part of the premium can be used to compensate the farmer for weight loss, and to encourage him to cooperate with a new system of maize handling.
INTRODUCTION
Mechanical drying of maize to a moisture content of 14%, commencing within 2.5 days of harvest, has been shown (Nagler, M., et al, 1986) to effectively control the level of aflatoxin in maize produced in Thailand during the rainy season. In order to make the mechanical drying more practical and less costly, as well as to reduce shelling damage, it was found necessary to use maize cobs that had been partially dried to a moisture content of less than 22% by natural "field-drying". This is a process whereby maize is left standing unharvested in the field for a period after field maturity which allows the cob to dry as it attains equilibrium with the average relative humidity in the air. Thirty-five drying trials carried out in 2 provinces of Thailand during the rainy season in 1985 indicated that field-drying for between 1 to 4 weeks was effective in reducing levels of moisture from )26% to the range 18 to 22%, whilst maintaining physical quality and controlling levels of aflatoxin to an average of 3 ppb, see Figure 1.
The present studies were undertaken in order to provide further evidence as to the efficacy and advisability of incorporating field-drying in aflatoxin control procedures.
METHODS AND MATERIALS.
Field trials to evaluate the effect of extended fielddrying on maize quality as determined by level of aflatoxin, moisture content, and physical appearance, were carried out at two locations within Thailand. These were at Kasetsart University's "Farm Suwan" in Nakorn Ratchasima Province, and the Department of Agriculture's "Agronomy Centre", Tak Fah in Nakhon Sawan Province.
A randomised complete block design, with 4 replicates of 7 treatments comprising monitoring at field maturity, t = 0 (110 days), and t = 1, 2, 3, 4, 5, and 6 weeks, was used. Fifty maize plants of the most widely planted variety, "Suwan 1", were used for each replicate. Both the rainy season crop, planted in the third week of May 1986, and the dry season crop, planted on the 24 July at the Agronomy Centre, and 4 August at Farm Suwan were studied at each location. At the harvest time for a treatment, The 4 replicates were harvested on the same day and the number of cobs in each replicate was recorded. The number of cobs damaged by mould, insect, rat, or by other means, was also recorded. The cobs were shelled on the day of harvest and the moisture content of the grain was measured using a "Steinlite" moisture meter.
The replicated samples were then dried to a moisture content of < 12% by use of a forced-air oven at Farm Suwan and by sun-drying at the Agronomy Centre. Dried maize was sealed in a plastic bag and stored temporarily in a cold room at the Department of Agriculture awaiting analysis. The bulk samples were divided to give a representative 2009 or 500 9 sample of kernels for physical and mycological studies. The remaining sample, in the order of 5 to 10 kg, was coarse-ground using a hammer mill (Christy-Hunt Essex Minor powered by a lister LT1 diesel engine) fitted with a 6mm screen. The ground sample was divided to give a 2.5 kg subsample which was fine ground using a 1 mm screen in the hammer mill. A 1.25 kg. sub-sample of the fine-ground material was formed into a slurry with water, ratio meal: water 1: 1.25, using a 4 litre Waring blender for 3 minutes, Fifty gramme aliquotes of this slurry were used to determine the level of aflatoxin using a method employing High Performance Thin Layer Chromatography (HPTLC) developed at the Tropical Development and Research Institute, London. The method uses a novel "BondElut" phenyl-bonded clean-up column (Analytichem International). A Camag Scan 2 densitometer controlled by a Hewlett Packard HP 6201 computer was used for quantification and confirmation was by formation of the hemi-acetal derivative by trifluoroacetic acid on a TLC plate.
RESULTS AND DISCUSSION
Effect of Extended Field-Drying on Level of Aflatoxin Contamination
a. Rainy Season.
No increase in levels of aflatoxin was observed, at either location, with increasing length of field-drying over the 6 week monitoring period, see Table 1. At Farm Suwan only 1 out of the 28 samples contained a detectable level of aflatoxin, but this was at the relatively high level of 75 ppb. This sample was one of the replicates taken after just 1 week of field-drying and resulted in an anomalous mean aflatoxin level of 19 ppb for this treatment. At the Agronomy Centre 5 out of 28 samples contained detectable levels of aflatoxin, but ail were less than or equal to 5 ppb. Hence, not only was there no increase in levels of aflatoxin during fielddrying, but also the degree of aflatoxin contamination was very low, being less than 3 ppb at Farm Suwan and less than 1 ppb at the Agronomy Centre. These results provide further evidence that extended field drying can be safely used as part of an aflatoxin control procedure for Thai maize produced during the rainy season.
b. Dry Season
There was no trend towards an increase in levels of aflatoxin contamination with increasing lengths of fielddrying at either location during the dry season trials, see Table 1. At the Agronomy Centre 50% of the samples contained detectable levels of aflatoxin, but the overall mean was still only 5ppb. Field-drying was studied over a 4 week period, instead of over 6 weeks, at Farm Suwan. No aflatoxin was detected in any of the samples from this location.
Levels of aflatoxin in dry season maize are generally considered to be low in Thailand and problems with aflatoxin prior to storage are rare. This study indicates that field-drying, which is already practised in some growing areas, is unlikely to cause an increase in aflatoxin contamination, but production of other mycotoxins cannot be ruled out.
Effect of Field-Drying on Moisture Content of Cobs
a. Rainy Season
The results are presented graphically in Figure 2. It can be seen that drying to a moisture content of <22% was achieved at both locations within 2 weeks. At the Agronomy Centre maize at field maturity had a moisture content of 29.2%. After 2 weeks of field-drying this had dropped to 21.3%. Moisture content continued to decrease and after 3 weeds of field-drying it was less than 20%, and after 6 weeks it was down to 15.2%. At Farm Suwan the moisture content was below 22% after just 1 week, and after 2 weeks of field-drying it was at 19.4%. The moisture content seemed to stabilise at about 17% after the fourth week.
This data strongly indicates that field-drying can be effective even during the rainy season in Thailand. It is thought that drying is possible because the sheath adequately protects the cob from free water during rain, and the cob can equilibriate with the Average relative humidity of 80 to 85%.
b. Dry Season
Field-drying was very effective during the dry season, as would be expected. At Farm Suwan maize had already dried to a moisture content of 14.4% after 2 weeks, and was down to 13.1% after 4 weeks. At the Agronomy Centre moisture content had reduced from 28.3% at field maturity to 14.8% after field-drying for 3 weeks. Moisture content continued to decrease and reached 10.9% after 6 weeks.
Many Thai farmers already take advantage of fielddrying and this may contribute to the low levels of aflatoxin found in the second crop.
Effect of Field-Drying on Physical Quality of Maize
There was no discernable trend towards an increase in either damaged cobs or damaged grains as length of field-drying was increased up to 6 weeks. At Farm Suwan there appeared to be a dramatic increase in damage during the first week, both in terms of damaged cobs and grain, see Table 2. The percentage of damaged cobs increased from 4.5% to 20.8%, and the percentage of damaged grains increased from 2.8 to 5.9%. As field-drying time increased the percentage of damaged cobs and grains oscillated about the mean and did not trend upwards. There was no corresponding increase in damage during the first week of field-dryintg at the Agronomy Centre. The initial increase in damage at Farm Suwan remains unexplained, but could have been due to some unrecorded environmental factor.
The major form of damage to grain was discolouration which was attributed to mould. There was very little evidence of insect damage to grain during the rainy season. Cobs from the rainy season trial at Farm Suwan were found to be, on average,11.3% mould damaged, 4.6% insect damaged and 1.8% rodent damaged.
These studies indicate that field-drying does not cause an increase in physical damage to grain. In fact the process may well result in better quality grain because damage during mechanical shelling of maize after field-drying, when the moisture content is less than 22%, should be less than that of very wet maize at field maturity. Extended field-drying may not be applicable everywhere. If an area is particularly susceptible to some hazard, such as large-scale bird or rodent attack, or frequent high winds, then it would be advisable to limit field-drying to, say, 2 weeks maximum.
CONCLUSIONS
- The planting of a second crop would be delayed, and may no longer be possible.
- The farmer may lose financially unless he is adequately compensated for weight loss.
- There may be an increased risk of losses due to storm or other natural hazard in some areas.
ACKNOWLEDGEMENTS.
The Authors wish to thank the following people for their valuable contribution to the project: Pipat Weerathaworn and Rungnapa Tangaduiratana at Farm Suwan; Pornsawan Koongpo, Chalyasith Rugmanee, and Jaturong Jod-duangjun at the Seed and PostHarvest Pathology Branch, Division of Plant Pathology and Microbiology, Department of Agriculture; and staff at the Agronomy Centre, Tak Fah.
REFERENCES.
1. Nagler, M.J., Buangsuwon, D., Jewers, K., Meadley, J., Kenniford, S., Morris, T., Lorsuwan, C., Wong-Urai, A., Braun, D., Tanboon-Ek, P., Siriacha, P. 1986. Production and Quality Control of Low Aflatoxin maize in the rainy season. Procedings of the 1986 Department of Agriculture Annual Research Conference, Experts Presentation. April 23-28, Bangkok.
