|
 Main subject - sept/oct 2005
continuation: Seed Quality and Performance
Seed Coating and Pelleting - Seed pelleting is an old treatment originally designed to increase the size and change the shape of some very small, irregular shaped seeds of vegetables and ornamentals to facilitate precision and "singulation" planting. Pelleting has also been used to improve the "ballistic" properties of seeds, i.e., increase weight and aerodynamics, for restoration of large grasslands and reforestation of hillsides by aerial seeding. Seed coatings have been and are being employed for many other purposes than direct improvement of seed performance, e.g., control and protection of seed performance.
Protecting Seed Performance - Seeds can have all the potentialities for high level performance, i.e., high germination and vigor, but succumb to micro-organisms, soil insects, too high or too low temperature, and/or conditions that are too wet or too dry. These common adversities and stresses encountered during the sowing season can be very damaging to all sorts of crop production operations. Protecting seeds against stresses and adversities is the focus of research and development attention of a specialized segment of crop protection called seed treatment.
Chemical Seed Treatments - Seeds have long been treated in various ways and with various materials to disinfest, disinfect and protect them against harmful micro-organisms that can severely limit their performance. The most commonly used and effective means of protecting seed performance against adversities and stresses in the seed bed micro-environment is treatment with a fungicide. The fungicide prevents or controls seed rot and seedling blights that are the usual causes of deterioration and death of planted seeds. Fungi involved in seed rotting and seedling blights are especially damaging when the seeds are under temperature or moisture stress, so it is important that the fungicide protect the seeds until conditions become more favorable for germination and emergence. Many types, chemistries and brands of fungicides are used for seeds. Most are now applied in a seed coating but some are still applied as a planter box treatment at the time of sowing. Some seed treatments combine a fungicide with an insecticide to prevent destruction of the seeds by soil insects. A rather recent innovation is the application of biologicals as a seed treatment. The most common biological seed treatment is a selected strain of a common bacterium, Bacillus subtilis, which has been used on peanut and cotton seed and some vegetable seeds.
Hydrophobic and Hydrophilic Coatings - It has been well established that some kinds of seeds can be seriously injured by too rapid imbibition especially under deficit oxygen and low temperature stress. The resulting traumas are variously termed imbibitional injury, imbibitional chilling injury or just chilling injury. Reducing the rate of imbibition by seeds in overly wet soils should prevent or at least reduce the extent and severity of imbitional injury. Several of my graduate students evaluated simple and inexpensive treatments such as coating seeds with hydrophobic materials such as linseed oil or lanolin to protect susceptible kinds of seeds, e.g., soybean, bean and cowpea seeds against imbibitional injury in cool, wet soils. They found that under some conditions the hydrophobic oil coating slowed the rate of imbitition so that injury was minimal. Furthermore, seeds with a less permeable seed coat such as soybeans with a black or semi-permeable normal color seed coat were protected against imbibitional injury.
Small seeds and seeds at moisture contents above 12% were also less susceptible to imbibitional injury. Some seed companies routinely "moisturize" bean seeds before marketing to minimize mechanical damage in handling and imbibitional injury in the seed bed. Another common situation involving the status of moisture supply in the soil is knowingly planting seeds in soil that is too deficit in moisture for germination and emergence in anticipation of a rain in a few days. When the rain doesn't arrive on time the seeds increase substantially in moisture content even in the relatively dry soil because they have a great affinity for water. They might increase in moisture content to 25% moisture or higher which is much to low for germination of seeds kinds such as those of soybean, cotton, bean, peanut, but high enough to result in "accelerated aging" of the seeds in warm soil. A hydrophobic coating sensitive to soil moisture tension could retard or prevent the absorption of water by the seeds until the moisture deficit was overcome by rain or irrigation. This would protect the seeds against "accelerated aging" and seed rot. On the other hand, a hydrophilic coating also sensitive to soil moisture tension might permit a seed to gather enough moisture in its immediate environment in a relatively dry seedbed for germination and emergence.
The advances in coating technology and polymer chemistry during the last few years provide a great opportunity to extend and improve the protection of seed performance against adversities and stresses in the seed bed beyond that provided by the use of just chemical fungicides or to even reduce reliance on fungicides.
Controlling Seed Performance
The control of seed performance, e.g., the time of germination, is a rather old vision experienced by many farmers, seed technologists and scientists. Wouldn't farming be less risky and less stressful if a crop could be planted during some very favorable weather but not germinate and emerge until the proper time for the crop? About 35 years ago some workers in Canada experimented with controlling germination of spring wheat so that the crop could be planted in the fall before the advent of winter freezing and snow but not germinate until the temperature became favorable the following spring when conditions are often very poor for planting, e.g., too wet. They used a coating that prevented absorption of water by the seeds until the soil warmed to the desired temperature for germination, and they applied for a patent. Apparently, the chemistry and machinery of the times were not adequate for fulfillment of their vision because the procedures were not widely used. More recently, however, new temperature-sensitive polymers and continuous coating machines make possible "early" planting with germination delayed until the soil warms to a favorable level. I am not aware that the early planting concept has been applied to cotton but it would seem to provide an excellent opportunity and model.
Producers of hybrid maize seeds long sought an effective means of controlling germination of the parental line that flowered earliest so that both parents could be planted at the same time with a delay in germination of the earliest parent to synchronize flowering for the best match between the male and female parents. This would be much more efficient and less costly than planting the two parents at different times or use of all sorts of mechanical and heat treatments to slow development of the earliest line. Coatings have been developed that delay germination of seeds of the earliest lines to synchronize flowering of the hybrid parents.
The new and powerful coating chemistry and equipment for applying the coatings continuously at great capacity provides great opportunities for finally realizing many, but probably not most and certainly not all, of the dreams and visions of controlling germination of crop seeds.
Assuring Seed Performance
It is probably not possible to assure or guarantee seed performance regardless of the adversities and stresses encountered. And, it surely would not be good business for a seed company to offer an unlimited warranty or unconditional guarantee on the germination and emergence of the seeds it markets. Micro-environmental conditions in the seedbed can be so stressful that they exceed the maximum inherent potential and added protection for the crop species. The greatly increased value - and cost - of seeds, however, and the importance of timely and undiminished seed performance are or should be powerful incentives on both the supplier and user sides of the seed industry to take advantage of all technologies available and to develop newer, even more innovative technologies, for assuring seed performance under all but the most extraordinary conditions of climate and seedbed. Farmers of all sorts increasingly expect that the seeds they purchase at considerable cost and often under rigorous terms be in as near a"zero defects" condition as feasible so that their desires regarding performance of the seeds are realized except under those "extraordinary conditions" mentioned above. Meeting this challenge will require a multi-avenue but integrative approach to further develop and refine and then integrate the technologies and procedures for improving, protecting and controlling seed performance.
Priorities for Additional and Future Research and Development
Research and development efforts aimed at improving, controlling and protecting seed performance should focus on inherent improvements in the performance qualities of seeds and address the most common and harmful adversities encountered by seeds in field plantings.
Genetic or Inherent Improvements
Plant breeders have had to address and take into account so many crop improvement objectives in their breeding work that for the most part little attention has been and is being given to improvements in qualities or attributes that affect seed performance. In some cases improvements in the product have been made at the expense of seed qualities, e.g., "super sweet maize", green beans. These circumstances in the past and even in the present for elite products are understandable but changes in the times, technologies and opportunities justify and demand a new and full examination of the potentialities for dramatic inherent improvements in seed performance. There is considerable variability in the populations of most of our crops for germination and emergence under a wide range of temperatures and soil moisture tensions, for emergence force under soil crusting conditions, for rate of imbibition, for elongation and growth of seedling structures involved in emergence, and so on. In the view of some specialists, one of the outstanding examples of an inherent improvement in seed performance was the apparently inadvertent selection of maize seeds for performance in cooler climates through the development and widespread use of the "cold test" for seed vigor. Another example that was deliberate rather than inadvertent was the development of the multi-adversity resistant (MAR) cotton varieties which involved selection for germination, seedling growth, and freedom from fungi under rather cool conditions.
Priorities for inherent improvements in seed performance should include:
1. The elimination of inherent weaknesses or defects that cause problems in seed production and in seed performance.
2. Increasing the tolerance of seeds to micro-environmental temperature and moisture stresses, or put another way, expanding the range of conditions under which the seeds can perform satisfactorily.
3. Increasing the inherent vigor and storage potential of seeds.
Progress on these priority research objectives can be achieved by taking advantage of the natural variability available in the crop species populations, closely related populations, and traits in exotic populations that can be engineered into the crop population, i.e., a biotechnological approach.
Physical, Chemical and Physiological Treatments and Procedures
Some of the approaches to improving the performance of seeds by physical, chemical and physiological treatments have been introduced and discussed. Priority needs to be given to refining and improving the procedures in use, extending their application to more kinds of seeds, and developing and evaluating new treatments and procedures. Some seed technologists have pointed out that much more progress could have been made in improving seed performance if it had received even the same amount of attention and resources spent on defining and evaluating seed quality.
Seed coating helps the performance
Producing High Quality Seed -The First Priority
This presentation begin with a statement of one of the basic precepts of seed science and technology, viz.,
High quality seeds perform better than those of lesser quality.
Nothing presented here has lessened or weakened this precept. The very first priority in the seed industry is and should remain the production of the highest quality seeds possible. The performance of seeds is dependent on a high quality base and there is little or no scope for improving performance of low or even average quality seeds. Seed technologists seed scientists and seed producers should increase attention and efforts to produce seeds of the highest quality even as they address more vigorously the matter of improving seed performance.
|
|
............
|
