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Prawat Tanboon-ek(2)
ABSTRACT
Only Aspergillus flavus and A. Parasiticus have been found to be Aflatoxin producing fungi, but many other toxin producing fungi have been found in stored maize in Thailand. It has been established that Aflatoxin content is low at harvest and increases during storage. Field drying and mechanical drying were found most effective for controlling Aflatoxin contamination in maize. A Rapid BGYF test was developed to estimate Aflatoxin levels and is being used by many maize dealers. Chemicals were tested for controlling the causal fungi. Only a solution of ammonium bis proprionate and propionic acid will give temporary control and prevention of the growth of fungi in high moisture maize but will not destroy Aflatoxin - present before treatment of the grain. The grain still must be dried. Maize, highly contaminated with Aflatoxin, can be detoxified efficiently with ammonia. The resulting grain is safe and can be used for feeding cattle and swine but is not suitable for human consumption and does not move in international trade.
Maize production in Thailand
Maize is presently the fourth most important crop produced in Thailand. Maize production is particularly important in the north, north-east and central regions. Since maize cultivation is a low cost input system, farmers tend to increase planting area rather then intensify production. Though increasing, production is still low when compared to other maize growing countries. Only some farmers accept new production technology such as fertilizing, pest control and seed of new high yielding varieties. The Fifth National Development plan (1982-1986) set a target to increase production 4.5% per year. The increased production was intended to come from extension of the cultivated area. Several trials show that more than 2.5% of yield increased when recommended fertilizing and cultural practice were used. The Six National Development Plan (1987-1991) has set a 5% production increase as the goal. The promotion by extension of maize quality improvement will be a priority. Maize is cultivated both in single and double cropping system. Generally, varieties resistant to downy mildews are grown throughout the country. There are other hybrids produced by private seed companies available in the market, but cost is the major limiting factor on use of hybrid seed. The total maize produced by Thailand is equal to only 1% of the world production, and Thailand has only a small part of total international trade. The maize importing countries of Asia and other countries prefer yellow color maize.
Postharvest handling of maize
It is estimated that Thai maize production has increased by about 11% during the period of 1960-1975. After this period, the extension to the cultivated area was limited and effected the total period is from midMay to early June, depending on average was obtained each year verying by farm price and drought conditions. A normal planting period is from mid-May to early June, depending on the arrival of early rains. Planting may start as early as mid-March to mid-April. For single cropping the growing season starts from mid-May, but for double cropping planting must be earlier. The second crop can be planted in August/September and harvested in October/November/December. Regional variations in rainfall affect planting dates and time of harvest. Farmers try to plant early because an early crop gives a higher price. To gain this advantage, farmers may have to replant serveral times because of non-reliable rain in the early raining season. For example, the crop in 1987/1988, in some planting area, had to be replanted four times. In a maize shortage year, farmers sometimes harvest an immature crop because of the high market demand. Normal maturity for the recommended varieties is 110 days after planting, though some early maturing varieties of 90 days are available from the government seed centers or local experiment stations but give lower yields. The single crop or the first crop of double cropping are harvested in the rainy season. Only hand picking and stripping cobs are used in harvesting; no mechanical harvesting is found in Thailand. Stripped cobs are removed from the plants and packed in bags in the field. Harvested maize is transported by a range of hand, animals and engine powered carts to storage cribs or local dealers. Some farmers move the cobs immediately after harvest to a prepared area for shelling and sale. Most local dealers provide shellers for farmer's maize by contract. Shelling cost is paid by farmers. One who is not satisfied with the offered price will dry the cobs and keep them in a crib until the price is more acceptable.
Since harvesting is mainly in the rainy season, maize drying is difficult. Farmers will keep maize on the cob either wet or dry while local dealers, who always face a shortage of storage facilities, will keep maize only as shelled grain. A survey showed that 79 % of farmers sell their maize as soon as it is harvested mostly by contact with local dealers. Only 21% of the farmers store the crop to wait for a better price or until forced to sell. On-farm postharvest drying and storage are seldom done by farmers. These practices are not considered worthwhile because farmers keep the maize only for a short period. For longer storage, typically, farmers dry the harvested maize on the cob using a straw mat on the ground. Then the dried ears will be kept in a storage crib. In some areas the ears are left on the plant before harvesting until the moisture content of the grain becames 18%. Topping, which removes the upper part of the mature plant has been recommended for the double-cropping system. This method provides the availability of the land to grow a second crop without the necessity of harvest the first crop. Ears can be left on the plant in the field for 1 month without aflatoxin formation.
During the harvesting season, local dealers are busy on travelling to and fro to buy maize to be stored in godowns, bins and silos. All the purchase is wet. The dealers must dry the grain before storing it because of the ensuing quality and value. Sun drying on cement floors is the most common practice but the amount of rain limits the utility of the floors. Aflatoxin can became a serious problem, when the wet grain cannot be dried quickly. Research by DOA indicates that if the wet grain cannot be dried within 72 hours, Aflatoxin can be a problem. As shown in Fig. 1, local dealers will move the maize to Bangkok wholesalers for export or domestic use. Some local dealers also are involved in moving directly from producing area to overseas importers. Cooperatives also take a small part in the marketing channel. It was found that Aflatoxin contamination in pre-harvest maize is relatively low but increases rapidly during the period the grain is handled by local dealers. As the problem became serious, local dealers now pay more attention to solutions. Several types of mechanical dryers, storage facilities and siloes with dryers have been placed in many parts of the country for drying and handling the grain. Some mould inhibitors also are used in domestic feeds.
Control of Aflatoxin in Maize
Aflatoxin content has become a major factor affecting the export of maize and most importers have set aflatoxin limits, usually in the range of 20 to 100 ppb. Aflatoxin restrictions and the world surplus of maize have made markets increasingly difficult to find and prices have tended to fall until the recent US drought. Since Thailand exports more than 70 % of the production, a discount of 5% on FOB price because of Aflatoxin would cost more than US$ 25 million per annum. Thailand, therefore, assigned top priority to research on aflatoxin control in maize. This work, coordinated by a national committee, has made rapid progress and many of the aflatoxin control measures that have been devised are being implemented and/or evaluated on a commercial scale. Research in Aflatoxin involves several different fields.
Chemical treatment
The DOA Division of Plant Pathology and Microbiology has recently completed screening seven reagents in the laboratory for effectiveness in preventing or reducing aflatoxin contamination of maize. Only three of the reagents were found to be effective, sodium bisulphite, ammonia, and propionic acid:ammonium bis propionate at a ratio of 9:1. Sodium bisulphite and ammonia treatments both result in grain with a strong residual odor; the ammonia treatment also produces darker grain. The most promising regent is the propionic acid-based fungicide formulation, which has been shown to effectively control both mould growth (A. flavus) and aflatoxin formation, while not adversely affecting the physical quality of the grain. The cost of the fungicide treatment may be offset by higher prices for better quality grain. Future work aims to reduce costs by minimizing the inclusion rate and improving the application method.
Mechanical drying
The UK-Thai Aflatoxin in Maize Project (1) has identified a set of criteria, called the UK-Thai Project (UTP) System, which have been shown to reliably produce low aflatoxin-content maize during the rainy season. With the UTP system, maize is first field dried on the stalk for one to two weeks before harvesting to reduce moisture content to 18 to 22%. It is next shelled within 24 to 48 hours of harvest, and loaded into a drier within 12 hours of shelling. Thus, within 48 hours, it is dried to 14% moisture content, with no part exceeding 15%. Aflatoxin content is monitored rapidly by a special adaptation of the bright greenish-yellow fluorescence (BGYF) test. Maize dried to 14% moisture content by the UTP system can be safely stored for a minimum of two months with no increase in aflatoxin content. Using this system, 25 three-ton batches of maize were successfully processed with a mean total aflatoxin content of 2.5 ppb and a range of 0 to 16 ppb at drying sites in two provinces. The system is now being used commercially for about 50,000 tons of maize.
Improved farm storage
A USAID-funded project conducted by DOA was begun in 1985. The project aims to develop and evaluate improved farm storage and drying methods. In 1985, cribs of three sizes (0.5,1.0 and 2.5 meters) were tested, as was a solar drier developed by the Asian Institute of Technology (AIT), Bangkok.
Mycological studies
A collaborative study, involving the Division of Plant Pathology and Microbiology, the Department of Agriculture and Tropical Agricultural Research Centre (TARC) of Japan, is being conducted on the incidence and occurrence of Aspergillus flavus. A very high incidence of A. flavus has been found in soil samples, especially in soil around drying facilities and warehouses. No A. flavus spores were detected in the atmosphere in maize fields, but high levels of spores were found in the air in warehouses used for maize storage
A mycological study of maize was also done by the UNDP/FAO in collaboration with DOA January and February, 1985. The work confirms low concentrations of A. flavus spores in the air in maize fields during the dry season, as well as the high concentration of spores in warehouses. Aspergillus flavus contamination in stored maize was found to be closely associated with weevil infestation (Sitophilus zeamais); the insects carried extremely high concentrations of A. flavus spores. Virtually no A. flavus was found before harvest in the second, dry-season crop of maize, but concentrations increased slowly during temporary storage of ears and grain. It was also found that only A. flavus and A. parasitica produce aflatoxin in maize.
Quality control methods for aflatoxin
The UK-Thai project has amassed data which strongly indicate that an adaptation of the bright greenish-yellow fluorescence (BGYF) test (2) can be used in Thailand to identify the level of aflatoxin presence in maize. Batches were. classified according to the number of observed BGYF particles (e.g. 0, 3,5,10 counts). Correlation between these BGYF counts and the mean total aflatoxin of all batches within each classification was found to be excellent (correlation coefficient, r = 0.92). Sampling was found to be a critical factor when working toward a 20 or 30 ppb aflatoxin limit. The good correlations were only found when a 10-kg representative sample was coarsely ground with a hammer mill fitted with a 6-mm screen and the sample subdivided into four 125-9 subsamples. Aflatoxin quality-control procedures based on the BGYF test have been devised for monitoring the production of low-aflatoxin content maize, and for assisting grain management at regional and export storage facilities. Monitoring is best done in conjunction with mini-column testing to minimize consumer risk.
Analytical Services
Aflatoxin analysis is routinely done at a number of laboratories in Thailand. Unfortunately, sampling methods, sample preparation and analytical methods vary widely, although efforts are being made to standardize them. Inspection companies offer an aflatoxin analysis service that is predominantly semiquantitative, based on mini-column determination which is sometimes linked to a fluorotoxin meter. Fully quantitative aflatoxin determination is mainly performed in government laboratories, using quantitation by thin-layer chromatography (TLC). Sophisticated techniques, such as high-performance liquid chromatography (HPLC) and high-performance thin-layer chromatography (HPTLC), are gradually being introduced, and should soon enable a faster and more accurate analysis of samples.
Future Research
Future research has been approved by the national committee in the areas of:
Cooperative Research
Much of the aflatoxin research in Thailand can now be considered to be coordinated and cooperative, due to the influence of the national committee. Assistance from other countries to provide funding, training and staff is still needed; such support has played a significant role in aflatoxin research in the past. Various foreign agencies have given support to the Department of Agriculture through bilateral or multilateral assistance.
The United Kingdom has provided training, equipment, staff and volunteers to join in collaborative projects with Thai researchers, at a value of approximately 15 million baht (US$ 600,000). The United States Agency for International Development in Phase 1 of its contract, has approved a soft loan of approximately US$ 200,000 and a grant for research staff and overseas training and study tours for Thai scientists for 1985 and 1986. The United Nations Development Programme (UNDP) has approved funds of US$ 38,500 for 1985 and 1986. In addition, the Tropical Agricultural Research Centre (Japan) has approved a cooperative project with the Division of Plant Pathology and Microbiology of the Department of Agriculture on quality and preservation of maize by preventing aflatoxin contamination. The Tropical Agricultural Research Centre supplies senior researchers, training, analytical equipment and software.
Acknowledgements
Credit for the rapid progress in the battle against aflatoxin must go to the National Committee on Mycotoxin Control in Agricultural Commodities and its constituent organizations. Particular mention should be made of the research carried out by the Department of Agriculture, which through its own research and in collaboration with the United Nations, Japanese teams (TARC and JICA), USAID and British teams.
REFERENCES
Fig 1 Marketing Channel for Maize In Thailand
| 1985/86 | ||||
| Planted Area (rai) | Production (rai) | Yield (ton) | Average Yield (kg./rai) | |
| Northern | 7,566,655 | 7,268,451 | 3,042,553 | 5,925 |
| North Eastern | 3,119,853 | 3,078,534 | 1,202,532 | 2,447 |
| Central | 1,210,814 | 1,172,679 | 537,234 | 2,513 |
| 1986/87 | ||||
| Northern | 7,431,431 | 6,949,942 | 2,687,464 | 5,937 |
| North Eastern | 3,176,232 | 2,866,322 | 1,012,226 | 2,253 |
| Central | 1,197,851 | 1,161,164 | 493,457 | 2,207 |
| 1987/88 | ||||
| Northern | 7,039,457 | 5,385,255 | 1,697,075 | 4,871 |
| North Eastern | 2,708,235 | 2,352,159 | 763,477 | 2,205 |
| Central | 1,128,555 | 969,237 | 383,098 | 1,935 |
MAIZE PRODUCING PROVINCES IN THAILAND
