Maintaining Seed Viability and Vigor
At times, it is necessary to store seed for a period of several months to a year or even more. Unfortunately, exposing seed to temperatures of 90°F or higher or to relative humidities of 60 percent or greater can lead to a rapid decline in seed germinability and vigor. As a rule of thumb, stored seed best maintains viability and vigor when the combination of air temperature in degrees Fahrenheit and percent relative humidity of the air is 100 or less. Thus, a seed storage relative humidity of 30 percent or less at a temperature of 70°F or lower would maintain viability and vigor in the seed of most plant types.
Conversely, as is often the case where seed is stored, air temperatures during part of the year may be 80 to 90°F or higher, and the relative humidity of the air may be 70 percent or greater. Such conditions, often found in the southern states, can lead to lower germinability and vigor in only a few months.
Seeds exposed to air gain or lose water according to the relative humidity of the surrounding air. At 50 percent atmospheric relative humidity, the equilibrium moisture content of wheat and rye seeds is about 12 percent; of barley, about 11 percent; and of oats, about 10.5 percent. Small-grain seeds and, in fact, seeds of most plant kinds store reasonably well if the moisture content in the seed is 10 to 12 percent. Seeds with a high fat or oil content, such as soybeans, store better at 9 to 10 percent moisture or lower. Oat seeds should be dried to 10 percent or less moisture content.
At 60 percent or greater atmospheric relative humidity, seeds of many plant kinds rapidly gain water. Thus, the equilibrium moisture content of small-grain seed exposed to 70 percent relative humidity is nearly 15 percent. This is too high for safe seed storage. At 90 percent atmospheric relative humidity, the seed moisture content of the several small-grain crops swells to 20 to 23 percent. Viability and vigor are lost rapidly under these conditions (Table 1.3-1).
Although it usually is not practical to control the temperature and relative humidity of the space where carryover seed is stored, you should pick a place where the temperatures are below 86°F most of the time and where the seed is not ex- posed to humid conditions.
Be sure the seed is dry when stored. Seed of oats, wheat, barley, rye, soybeans, red clover, and timothy often is farm grown. Such seed may be harvested at moisture contents of 16 to 18 percent or higher. This seed should be dried carefully to 10 to 12 percent seed moisture content soon after harvesting. Do not dry such seed at temperatures over 90 to 95°F, and do not maintain elevated temperatures any longer than is required to dry the seed.
Seed of corn, sorghum, alfalfa, and most grasses usually is not farm grown under Pennsylvania conditions. To maintain viability and vigor in purchased carryover seed of such plant kinds, reseal the containers to reduce air interchange, and store where temperatures are moderate and atmospheric relative humidity is as low as possible.
At times, farm-grown red clover or timothy seed is bagged and laid in a bin on top of freshly harvested oats. Moisture migrates upward as the oats dry and may condense on the surface of or in the bags of timothy or red clover. This can lead to a rapid decline in germination and vigor.
Remember that heat and humidity are enemies of seed viability and vigor. If insects are a problem in stored seed, see the following.