Seeds & Seed Technology
I. Where Does Seed Come From?
-Good seed comes from selection and breeding.
- Seeds can be classified into two groups:
1) Local varieties, landraces and prehistoric seeds: those seeds which were developed under natural selection, or under selection at the farmer level over a long period of time; and
2) Selected or improved seed, hybrids and high-response varieties: those seeds which are the result of controlled propagation and breeding programs.
Ask farmers where their seed came from.
In Senegal, most farmers have experience with improved seed (SODEVA, SONAGRAIN, SONACOS, SODIFITEX); but where does it originate?
Improved seed is produced by a seed breeder, who produces pre-foundation seed (ISRA). Pre-foundation seed is then provided to PAS (Projet Autonome Semence), where foundation seed is produced- either off season, or in isolation to assure genetically pure seed. Foundation seed is then diffused to cooperatives or groups specializing in the production of certified seed. Certified seed is then passed on to farmers. Farmers use and multiply seed; quality and purity are largely unknown.
II. The three BIG PROBLEMS with seed selection at the local level and some solutions:
1) Poor selection process, most farmers don't select the best possible quality seed.
Prior to harvest, select several plants that exhibit desired characteristics (vigor, disease resistance, maturity, etc.) and mark them. Ideally, these plants should be selected from areas not on the periphery of the field, where out-crossing is likely to occur. At the time of harvest[1], separate these plants from the portion of the crop to be consumed or sold. Perform a visual observation to pull out any plants exhibiting irregularities or infestation. Process the seed and select the larger, heavier grains (they are the one having the highest germination and vigor) to be used as seed.
2) Genetic breakdown, the quality of seed can decrease over successive generations.
Improved seed can be put into two main categories:
a) Hybrids- seeds created by the breeding of different races, which do not produce reliable second generation seeds, and should be replaced with successive generations, if continued vigor and homogeneity are poor; and
b) High yielding or high response varieties- which respond well to specific conditions and may require additional support. The seed is viable and stable but should be well selected and should be replaced or replenished every few years to limit the effects of out-crossing.
Landraces can also change as the result of cross pollination but good selection processes can be used to direct those changes.
3) Poor storage conditions, improper storage of selected seeds can undermine an otherwise strong selection regime.
Seed deterioration is progressive, inexorable and irreversible. However, high quality seeds store better, and observing a few simple rules can slow down the rate of seed deterioration; they are:
a) Separate seed from grains for consumption prior to storage.
b) Maintain low moisture content and avoid excess temperature.
c) Use within a reasonable period.[2]
d) Clean seed of insects and other pests.
e) Do germination tests prior to seed storage, and 1 month prior to seeding. If it is found to be necessary to purchase seed, do so from a reputable source paying attention to the seed selection criteria listed above and germination test prior to seeding. (See back page.)
Germination Test
In order to perform a simple, reliable germination test, fill 3 large, open containers (2kg tomato paste cans work well. With nail punch several holes in the base of the can to allow for drainage) with two inches of clean, sand. Pack them level. Randomly select a representative sample of seeds from throughout the seed lot being tested, Place either 50 (for seed of large size) 100 (for small size seed like millet) seeds on the sand in each container, such that they do not overlap. Cover the seeds with an additional 1/2 inch sand and water. Loosely cover the containers to slow evaporation and prevent pests. Keep them moist and cool until germination occurs. At seven to ten days count the number of sprouts and calculate the germination percentage. For most crops, seventy percent germination or better is desired. If the percentage varies greatly between the repetitions of a given seed lot (>10%) you may consider redoing the test.
III. What to Save
The seeds from almost all garden, vegetable, tree and field crops can be saved and used, but only some of them will have the characteristics of the source from which they were derived.
Field Crops / CommentsCowpeas / 5 year replacement, to limit genetic contamination
Peanuts / " "
Rice / " "
Corn / 2-3 year replacement, with extremely strict selection practices
Millet / " "
Sorghum / " "
Garden Crops / Viability / Comments
Beet / good / not economical
Bissap / excellent / remove from flower at time of processing
Cabbage / poor / out-crosses readily, poor seed quality
Carrot / - / will not set seed in tropics (ECHO exception)
§ / Cucurbit /Melon / OK / need hard seed, will have some breakdown in hybrids
§ / Eggplant / good / 2 generations, 3 year field life
§ / Jaxatu / OK / take seeds from over ripe fruit, dry well
Lettuce / good / no breakdown seen in 3 generations
Manioc/cassava / excellent / select virus free stock
Okra / good / allow to dry on plant
Onion / OK / Low longevity, does not conserve well
Peppers / OK / poor germination, does not store well
Potato / excellent / reproduce from tuber, fungicide advised
Radish / good / not economical
§ / Squash / good / watch for pollination, may need assistance
Sweet Potato / excellent / done vegetatively from cuttings, clean, virus free stock
§ / Tomato / variable / lack of uniformity of fruit
Watermelon / OK / Sugar Baby 1 or 2 generations only; Charleston Gray?
§- allow pulp to ferment in fruit or water 4-5 days before seed extraction.
For a more complete description of the process involved in seed collection see Growing and Saving Vegetable Seeds, and the CDH guide to vegetable seeds for regional and seasonal recommendations, it also provides a list of locations where seed can be purchased.
Seed Saving Techniques:
A few principles:
High quality seed, as determined by its germination rate and vigor (the capacity of a seed lot to germinate faster and more uniformly; for low vigor seed, the germination is spread over a longer time period), tends to store better and deteriorate more slowly than lower quality seed.
Seed moisture content is more detrimental to seed quality than excess temperatures, such that a one-percent decrease of moisture content can double the seed’s longevity, whereas a 10 degree decrease in temperature is needed to obtain the same result.
Sanitation is essential for effective seed storage.
During seed storage, periodic observation is necessary to ensure that there is no damage to the seed.
High temperature observed in a batch of seed is indicative of a serious pest problem.
Hard, airtight containers (metallic drums, plastic oil containers, water bottles, etc.) are useful both for food and seed storage, especially large grains such as peanuts, beans, soybeans, and corn, etc.
A 1:1 volume ratio of seed to sand can prevent attacks from insects and maintain seed viability from one harvest to the next planting.
A 1:1 volume ratio of seed to ash performs similarly to sand, and additionally has the property of desiccating objects, but large amounts of ash can be hard to obtain.
A fumigant, Phostoxin (hydrogen phosphorous – PH3) has better results than the sand or ash mixtures. Dose is one tablet for every 130kg of seed. If properly protected from moisture, seed can be saved for up to two years with this method. This treatment is safe for food storage as well, as the pesticide decomposes after three days.
Sturdy plastic bags (Ziploc, etc.) are useful for storing small amounts of vegetable seed, when clean and dry on the inside. Treatment with an insecticide/fungicide is recommended.
[1] The best seed quality can be obtained: 24 days after flowering from sorghum; 16 days after the beginning of pod formation for cowpeas; 30 days after flowering forrice; 30 days after flowering for corn; and 1 week after harvest for peanuts. All seeds should be removed from the field and processed ASAP.
[2] Seed has a limited life span and should be used before it loses vigor, or viability. Onion and Peanut seed, when saved under ambient conditions, should not be kept for more than one year.