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Sabry Elias, Adriel Garay, Bill Young and Tom Chastain
Maintenance of seed quality in storage from the time of production until the seed is planted is imperative to assure its planting value. Storing seeds, just like storing any other live organism, has its risks. The risks can be high or low depending on the species, the prevailing weather, the market requirements and the management provided during storage. The best alternative to avoid the risks associated with storing seeds is to avoid storing seeds, and in many regions around the world the seed industry has figured out how to do just that. One such example is the grass seed industry in Oregon where most of the seed is shipped in a few months after harvest, or Bolivia where the wheat seed harvested in the highlands in April is being planted in May in the Lowlands, or Colombia where rice can be produced twice a year, which decreases the storage period. These strategies are becoming highly desirable not only because it reduces storage but especially because it makes possible to market and meet the financial obligations in a shorter time. Despite the best strategies, there are times when seed growers and dealers carry over seed lots from one year to the next due to weak market, to insure an adequate supply the following year, because the production system does not provide choices, and other reasons. Under such circumstances the question is how to manage the seeds to maintain a high viability. This may be the case when there is carryover of grass seeds in Oregon beyond a year. Thus, the purpose of this article is to review briefly the basic principles and practices that need to be observed to reduce the risk of a decline in viability during storage.
Historically, grass seeds in Oregon have maintained their viability (measured by TZ and/or germination) for longer periods than one year if the seeds are stored following some basic principles. This track record in Oregon can be attributed to several factors including the dry weather that prevails during seed maturation and harvest that makes it possible to harvest seeds not only with low moisture but also with high initial viability. This is followed up by placing the seeds in cool and dry warehouses provided by the natural environment. Yet accidents can happen and do happen if the seeds are exposed to prolonged rain before threshing which can result in field weathering and physical damage at harvest (bruising, cracked or even broken), piling up moist seeds in a bin or elsewhere which leads to heating, a leak in the roof, etc. This is why it is important to pay attention to the following factors to lower the risks in storage.
One could ask when storage really begins. This may sound a philosophical curiosity but it is not. It has very practical implications in the management of seed quality in general and the management of seed viability in particular. People have been trained to believe that seed storage is synonym to placing the seeds in a physical structure called storage. This is a fallacy which in some cases led to building complex and costly storage building which could have been avoided while forgetting other factors that have high impact in the maintenance of seed viability. People may think that seed storage is equivalent to placing seeds in storage, but what is more important is how the seed and its internal biological-physiological-biochemical processes function and interact with its surrounding environment. In reality, if we pay attention to the way the seeds function, seed storage actually starts in the field. It starts when the seeds have reached physiological maturity, because after that moment, the seed does not receive the full protection of the mother plant any more. Rather, starting in that physiological point, the seed depends on the external environment in terms of moisture, temperature, even biotic pressures. So the environmental conditions during seed maturation, in the windrows, and during threshing have high impact in the seed viability and the storage potential of the seed. This is why, the location where the seeds are produced have a high impact not only in yield, but also in seed moisture management and overall quality in terms of viability, germinability, seed health, vigor and even plant performance. Those regions where the weather is rain-free, low relative humidity and cool enough during seed maturation and harvest are more suitable for seed production; whereas those that present rainy, high humidity, and excessively high temperature present more problems. This has been documented extensively, through maturity studies, studying the effect of production environment, etc. Most of the studies indicate that storage starts in the field. This is why any seed production and marketing plant needs to understand the effect of the pre-storage factors that influence seed quality and plan accordingly. All the management practices provided during the storage phase will only be able to build on this initial factor. For example if the seed starts with high quality due to the optimum pre-harvest factors, it is possible to follow up successfully during the storage phase; on the other hand if the seed starts with bad quality (field weathering, high moisture at harvest, heating problems, low germination, low vigor, seed health problems, etc) it is difficult to make up even with the best storage practices.
The seed industry around the world constantly looks for the best agro-eco-climatic regions for the species being produced. For example the vegetable seed production moved from the Eastern USA (with hot, humid, rainy summers) to the Western part of the USA where production can be done under irrigation and the seeds can mature under dry weather conditions. Another example is the grass seed production in the Northwest of the USA where the temperature is low-enough to promote flower induction, the moisture is high enough for high yield and the weather is dry enough for harvesting high germinating seeds with low moisture content. These regions have become specialized in supplying consistently high quality seeds to other states and the whole world. A second approach used by the industry, especially in low value bulky seeds is to find the most suitable production site within a country in order to supply seeds within the country and neighboring regions. Some examples are Parana state is a good seed supplier to the rest of the country in Brasil; or Cotton is produced in California and Arizona for other states in the USA; or wheat seed produced in the highlands to supply to the lowlands within Bolivia. A third approach is being used by Bolivia to produce soybean seeds in the winter (to be harvested in August- September) to be planted in October November for commercial oil production. The common denominators of all these cases is to achieve seeds with high quality, low moisture content at harvest, and no or minimal or simpler storage requirement.
1. Kind of seed: First understand your seed. Some species store better than others. For example grass seeds store better than corn and this stores better than soybeans and other oily seeds. Not all grasses are the same. There may be minor differences within grasses due to chemical differences that make them more or less hygroscopic, or physical differences that present resistance to moisture inflow to the seeds (example hard seeds), or simply present a physical resistance to air flow though the seed mass. For example, small seeds have higher compaction in the bag and influences the rate of air movement. So it is good to develop records of moisture and germination or TZ readings for different species through time. Books give us some general principles based on experimental settings, but to have more realistic data under specific storage conditions the best thing is to keep records. For example if you have records of germination/TZ levels of ryegrass seeds that had 12% initial moisture, 90% initial germination, it was stored in burlap bags in your particular storage (where you have records of temperature and relative humidity), and after one, two or three years you evaluated viability by TZ or germination, you have a valuable information that no one else in the world has. This is why, books can only present principles and each particular organization needs to generate data for its particular case.
A study has shown that annual ryegrass has better storability than Chewings fescue even though they are similar in chemical composition. In the late 1970s, a relative storability index was developed for some crops that showed 50% of Kentucky bluegrass, perennial ryegrass and tall fescue seeds are expected to germinate even after 3 to 5 years of storage, whereas 50% of creeping bentgrass seeds are expected to germinate even after 5 years or more. The same study reported that 50% of orchardgrass seeds, are expected to germinate after 1 to 2 years of storage. More studies on the potential storability of different cool season grasses under different storage conditions would be desirable.
2. Seed moisture content: The moisture content in the seed influences many factors, by increasing metabolic activities, higher respiration, fungal problems, heating, weakening and finally ending in the death of the seed. This is why moisture is not only one more factor among others, but it is a causal factor of many other problems. The ease or difficulty in moisture management after harvest depends to a great deal on the climatic conditions during seed maturation and harvest. If the natural field environment does the job of drying the seed most storage problems are minimized. If not, drying has to be done artificially, and in some cases this approach can be complex and expensive. In either case, the most important, urgent, and crucial requirement when the seed is being harvested is to measure the moisture content to see it is at a safe level.
People have made up so many imaginary excuses not to do certain things at certain times like weekends, holidays, etc. The truth is ( this is why if we are in the seed business it is important to think like seeds) that seeds do not understand holidays, when seeds have high moisture it has to be dried. Seeds do not understand if it is nigh time or weekend or holiday, not even if we have the equipment, electricity or fuel or a budget to do it. This simply highlights the importance of moisture. In some crops the urgency is higher than in others and the higher the moisture the greater the urgency.
For practical purposes, a moisture level below 13% is safe for storage of most seeds. If the natural environment does this job you have a masterful production plan. Seeds stored at higher level exhibit increased respiration, which leads to heating and fungal invasion, which leads to poor seed viability and vigor. The higher the moisture content the worse the problem would be if not dried soon. A low moisture content in the seed to be stored, on the other hand, is the best prevention for all moisture derived problems. The lower the moisture content (below 13%), the longer seeds can be stored provided that the moisture level can be controlled throughout the length of storage. It has been reported that seed moisture content of about 6% is optimum for storage of most crop species for maximum longevity. Seed moisture content fluctuates with the changes in relative humidity, which changes from the summer months (low RH) to the winter months (high RH). Given the low RH in the summer, it is possible that the seed moisture content is below 13% and in some cases it is below 10%. It probably increases somewhat in the fall and winter months due to the high relative humidity. The magnitude of these fluctuations can vary with the type of storage, type of bags used, and the kind of seeds, which influence the migration of moisture from the air to the seed and vice-versa. It should be noted that the higher RH in the Fall, Winter and Spring are accompanied by low temperatures, which decreases the negative effect of high RH. To monitor the moisture of seeds in storage measure the moisture content directly, or measure the relative humidity inside the storage (indirectly, see 3). If the moisture content is to be measured in a laboratory the sample should be submitted in a vapor proof container.
3. Initial viability of the seed: Seeds that have high initial viability maintain their quality for longer period than seeds with low viability. The dry and cool conditions that prevail in Oregon during seed maturation and harvest makes possible to start with high viability. For example the OSU Seed Lab records show that the germination of most freshly harvested grass seeds are above 85% and TZ above 90%. For practical purposes these are high viability levels. This high initial viability factor has been one of the key factors that contribute to a successful supply of high quality seeds year after year. Since most grass seeds are being tested for germination or TZ right after harvest, it is easy to know what seed lot has high viability (good candidate for storing) and which have low viability (higher risk in storage).
4. Storage temperature and relative humidity: Seeds are hygroscopic, which means they pick up moisture from the air and release moisture to the air. This is the same property of the salt, which picks up moisture in the rainy season and it dries out in the dry season. If the seeds are full inside sealed containers, there is little air space and the moisture content of the seed determines the relative humidity. So if the seed has 10% moisture you can be assured that it is going to stay the same through time. If the seeds are in bulk storage or in bags that allow air movement, seed moisture is determined by the RH of the air. The seeds adjust their moisture by trying to reach an equilibrium with the relative humidity. This moisture change is not instantaneous and may take several days or weeks depending on the magnitude of the moisture gradient between the seed and the air, and depending on the speed of air movement through the seed mass. This is how, for example, perennial ryegrass tries to reach 11% moisture if the RH is 55%; 12.1% if the RH is 65% and 13.4% if the RH is 75%. Similarly, colonial bentgrass will equilibrate at 9.8, 10.7 and 12.5%, respectively.
Temperature influences how much moisture air can hold. It has been suggested that the sum of the percentage of relative humidity plus the temperature in degrees Fahrenheit should not exceed 100 for safe storage (example: 50% RH and 50°F temperature; or 40% RH and 60°F; or 60%RH and 40°F). This rule can be a useful reference, but should not be taken rigidly. In general, a temperature below 60°F and a RH below 60% is still safe storage for most seeds. The longer the storage time needed, the lower these two factors should be. If the seeds are stored in ambient conditions (a typical seed warehouse) the temperature and RH fluctuates during the seasons and even during the day. It is good to know that in Oregon, the summer days - even though they are warmer than other seasons - are accompanied by very low relative humidity, which is good for storage. The winter, even though the RH is high, the temperature is low. This explains why seeds can be stored well in Oregon beyond one year. One way to monitor these factors, is by having a thermometer and hygrometer inside the storage room, and having a chart of equilibrium moisture content on the side for the species being stored.
5. Length of storage Prolonged storage can lead to a gradual loss of vigor and finally a loss of viability. Obviously, how long is too long depends on all the above factors and what levels of viability is desired at the end of the storage period. It has been reported that the actual age of the seed is of less importance than the environment in which the seed has been stored. With current technologies and moisture management principles it is possible to have older seeds that germinate at high levels. The longevity also varies among species, varieties, seed lots, and even among individual seeds inside the same bag. This is why, within a seed lot, some seeds are alive and others are dead. The relative proportion of these two components (live seeds and dead seeds) can be determined by TZ and/or germination.
6. Protection from storage fungi and insects: Storage fungi have the capacity to grow at very low seed moisture content. Most storage fungi belong to Aspergillus and Penicillium genera. They cause seed deterioration by producing toxic substances that destroy the cells of seeds which creates dead tissue to sustain the saprophytic fungi. Insects such as weevils can cause substantial damage to stored seeds. The best prevention to these problems is by storing seeds with low moisture and maintaining low enough during the duration of the storage period. Another critical practice against insects is by cleaning the warehouse and avoiding any source of infestation from old infested seeds.
The death of seeds in storage is a symptom that indicates there are causal factors such as starting with poor quality seeds, high moisture content, or high relative humidity and/or temperature which accelerates the deterioration process. To prevent problems, the management should focus in the causal factors first. But it is also good to measure the expected results such as viability by TZ or germination. Even though the loss of viability is about the last thing that happens as seeds gradually get weaker it is still a simple and practical way to see what is happening to the seed.
If need be, some vigor tests such as accelerated aging test, cold test, electric conductivity test and seedling vigor classification test may provide better and earlier picture about the physiological changes happening in the seed. Oregon State University’s Seed Laboratory is equipped to perform a wide variety of vigor tests to measure the potential storability of seeds especially if the seeds are of high value and prone to deterioration. The process of loosing vigor first and dying later is no different than the aging process in people, we get weaker first and die later. In the mean time, and for most grass seeds, a standard germination test or a TZ test is very useful to estimate the viability of the seed. How often should this be done? It depends on the interest, but a good rule of thumb is at least once a year under Oregon conditions and more often in hot, humid environments. No matter what, after a prolonged storage always test for germination or TZ before selling or using seeds for planting. It surely can save a lot of headaches later on.
Biologically, seed storage does not end when the seeds move out of the warehouse. Sometimes the problems start once the seeds step outside the doors of the storage. This can happen even if the pre-storage phase and storage phase were managed perfectly.
Cabrera, E., and H. Lansakara. 1995. Open Storage of Soybean Seed. Mississippi Agricultural & Forestry Experiment Station. Technical Bulletin 204.
Copeland, L., and M. McDonald. 1995. Principles of Seed Science and Technology. 3rd ed. Chapman & Hall.
Justice, O.L., and L.N. Bass. 1978. Principles and Practices of Seed Storage. USDA Agricultural Handbook 506.