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 Main subject - nov/dec 2006
continuation: Technologies that add value to soybean seed
In the modern seed industry, the quality control must be employed in all phases of seed production process starting with field selection and until the commercialization of the seed lot.
Seed health tests along with other physiological tests such us: tetrazolium test, germination and vigor tests, among others, may explain the causes of the seed low quality and in addition indicates more precisely the seed treatment to be used.
Seed treatment
Seed treatment with fungicides is a practice employed by high number of soybean farmers. The volume of treated soybean seed, that in Brazil, in the 1991/92 growing season hardly reached 5% of the total area planted to with soybean, increased to 93%, since 2001/02. In addition of controlling important seedborne pathogens, it is an efficient practice to assure the establishment of adequate plant populations, especially when the soil and climatic conditions during seed germination and seedling emergence are marginal. Under these situations, the speed of seed germination and seedling emergence are reduced, leaving the "untreated seed" in the soil exposed to common soil inhabitant fungi, such us Rhizoctonia solani, Fusarium spp., Aspergillus spp. (A. flavus) and Pythium spp (especially in Southern Brazil), that may cause seed deterioration or seedling death, in the soil.
Soybean seed treatment with fungicides was officially recommended for the first time in Brazil in 1981 for most of the states. In 1983, this "recommendation" was covering all Brazilian regions. During the 2004/05 growing season it was estimated that in about 95% of the area planted with soybean seed was treated with fungicides.
Today, after 25 years since its first recommendation, the technology of soybean seed treatment has been greatly improved. Mixtures of contact and systemic fungicides are recommended to assure better protection of the seed against the main soilborne fungi like: Pythium sp., Aspergillus flavus, Fusarium spp., and those that are seedborne: Phomopsis spp., Cercospora kikuchii, Fusarium spp. mainly (F. semitectum) and Colletotrichum truncatum. In addition, recent technologies such as the application of organic based polymers, significantly improved the quality of the treatment, improving the distribution of the chemical products on the seed, thus protecting the health of the operators. Recently, the inclusion of insecticides in the seed treatment process (imidacloprid, dimetoxan, fipronil) to control some insect problems, reinforced the importance of polymers.
Seed treatment: cost / benefit effects
Normaly sowing is not performed under ideal moisture contents and soil temperatures, especially in large farms of Central Brazil ("cerrado" area). Under this environment problems with seed germination and seedling emergence are frequent and most of the time there is need for "replanting". Under such circumstances, seed treatment with a combination of a systemic and a contact fungicide would offer extra assurance for the plant population establishment at low costs (less than 0.5%) of the total production costs.
Seed conditioning unit
How to treat the seed
The contact fungicides (thiram, captan and tolylfluanid) are designed to protect the seed against the soilborne fungi while systemic fungicides (mainly benzimidazoles) are used to control seedborne pathogens. In this way, it is important that the fungicides remain in direct contact with the seed. Seed treatment with fungicides, insecticides, application of micronutrients (cobalt and molybdenum) and the inoculation with Bradyrhizobium can be done in a sequential order, in equipments designed for seed treatment, as long as these machines provide separated tanks (compartments) for these products.
Treatment using seed treating machines
Among several advantages that these equipments provide in relation to the conventional method (revolving drums) some can be pointed out:
>> reduced risk for the operators (since the treatments employed are in liquid formulation instead of dust);
>> better seed coverage and adhesion of fungicides, insecticides, micronutrients and the inoculant to the seed;
>> working capacity of 60 - 70, 50 kg - bags of seed per hour; and,
>> easy operation, since the equipment can be attached to the tractor and operate in the field.
Obs.: Industrial equipments with capacity for treating 12-13 tons of seed/per hour are commercialy available.
Farmers should be careful in selecting fungicides, insecticides, micronutrients and the inoculants, choosing between liquid and dry formulations that would allow them to keep a final volume of the treatment up to 300 mL of liquid per 50 kg of seed.
Grading the seeds by size (width)
Application of micronutrients to the seed
Co and Mo are essential for biological nitrogen fixation efficiency for most of the soils where soybean is cultivated. The technical indications for these nutrients are from 2 to 3 g of Co and from 12 to 30 g of Mo/ha though seeds.
The utilization of soybean seed with higher content of Mo through aerial application increases the biological nitrogen fixation with higher yields. However. Even though with this procedure, it is still necessary to apply Co and Mo to the seed. Some seed producers are already using this technique to have seeds with higher yield potential
Seed quality during storage
The maintenance of the soybean seed quality during storage, in tropical and sub-tropical regions, is one of the main obstacles for the expansion of the crop in those areas. High temperatures and relative humidity of the air cause a rapid loss of viability of the seed. Thus, new technologies such as seed encapsulation with polymers or the utilization of impermeable plastic bags are alternatives to help to solve the problem in those regions.
In August 2000, in collaboration with several seed producers, five experiments were conducted in the region. After drying the seed below 9% m.c., seeds were packed in plastic and Kraft paper bags and stored in the farms. After five months of storage, the differences in seed quality between seeds stored in plastic bags and paper bags started to show up. The highest moisture content of the seed stored in paper bags was 13,37 % in the "Cana Brava" farm where seed stored in plastic bags had only 8.26% m.c.. This difference in the m.c. of the seeds proved once more the advantage of the plastic bags over the paper bags to prevent moisture absorption by the seed from the atmosphere and to preserve seed quality. Seed germination was only 54.8% in those seeds stored in paper bags whereas it was 91.2% in seed stored in the plastic bags. Emergence in sand was 60.1% and 95.9%, respectively for seeds stored in paper and plastic bags.
These results support the technical and agronomic viability of the use of impermeable plastic bags to maintain soybean seed quality in areas (regions) of high ambient temperatures and high relative humidity of the air.
In conclusion
Soybean seeds are aggregating many technologies that improved their performance. Seed quality control program using different tests such as: COMDIA (tetrazolium and seed health tests), the pH of the exsudate, seed classification by size to improve their planting precision and reach specific demands, seed treatment with fungicide, insecticide and micronutrient, the coating with polimers, special package and cooling the seeds are the main advances used by the Brazilian soybean farmers.
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