IS1364 Cotton - Undestanding and Using Results of cottonseed Germination Tests

Cotton

Understanding and Using Results of Cottonseed Germination Tests

Stand establishment is an important, critical, and sometimes difficult period in cotton production. Often, cold, wet soil conditions result in poor stands and make replanting necessary. Replanting is expensive, is complicated by the use of herbicides and insecticides, and may cause late maturity.

The cool germination test can provide additional information to the standard germination test about the quality of cottonseed. This additional information can be helpful in determining planting time and seeding rates. In order to properly use results of these tests, you should understand both tests and the meaning of the results.

Standard Germination Test

The standard germination test is conducted in the laboratory under temperature and moisture conditions that are favorable for germination and development of seedlings. These conditions are controlled in the laboratory germinator. Seeds are planted on moist towels. Towels are rolled to hold the seeds in place and to reduce drying. (Germination towels are manufactured according to specifications for germinating seeds.) Towels are then placed in a germinator to maintain favorable temperature and moisture during the test period. The temperature may alternate between 20-30 °C (68-86 °F), 16 hours at 20 °C (68 °F) and 8 hours at 30 °C (86 °F), or tests may be conducted at a constant 30 °C (86 °F). The test consists of 4 replications of 100 seeds each (or 4 replications of 50 seeds each under some circumstances) meaning a germination test may be composed of 400 or 200 seeds.

The first evaluation is made four days after planting. The towels are unrolled, the seedlings which are developed enough to be considered normal are counted, removed from the towels, and recorded. The towels are rerolled and placed back in the germinator. A second evaluation is made at eight days after planting. As in the first evaluation, those seedlings that have been developed enough to be considered normal (as prescribed by the rules for testing seed) are counted, removed, and recorded. If at this time it can be determined that no additional normal seedlings will develop, the test may be terminated. If the test is not terminated, towels are rerolled and returned to the germinator, and a final evaluation is made 12 days after planting.

Seeds are given every opportunity to produce a normal seedling under favorable conditions. The number of normal seedlings produced are totaled and a percentage germination is calculated. This is the information on the tag on a bag of seeds designated "percent germination."

Cool Germination Test

The cool germination test is conducted in a much different manner. Seeds are planted on moist germination towels as in the standard germination test. The test consists of 4 replications of 50 seeds each. Rolled towels are placed in a germinator set at a constant 18 °C (64.4 °F). This temperature is a critical breakpoint for cotton seedling development. Very little variation from this temperature is acceptable, i.e., 2 degrees below (16 °F), may result in a greatly reduced number of normal seedlings; while 2 degrees above (20 °C), may result in almost as many normal seedlings as the standard germination test.

The seeds remain at a constant 18 °C (64.4 °F) for 7 days after planting. After 7 days, only one evaluation is made. Only strong, vigorous seedlings that reach a length of 1 1/2 inches (4 cm) long are counted. The combined root-hypocotyle length of the seedlings is measured, i.e., from the tip of the root to the base of the cotyledonous leaves. The percentage of strong or vigorous seedlings is computed and results reported as "percentage cool test germination."

Sampling Procedure

Only seeds that have been delinted, cleaned, and treated with a fungicide are tested by the cool germination test. The research in developing this test is based on the use of delinted, treated seed.

The sample of seeds to be tested must be as representative of the seed lot as possible. If the sample is collected during seed conditioning, systematic sampling by hand or by automatic sampler should provide a representative sample. If seeds are bagged and divided into lots, a specified number of bags must be sampled. (The AOSA Rules for Testing Seeds, Journal of Seed Technology, Vol. 6. No. 2. 1981, gives specific instructions on sampling.)

When a lot contains more than 6 bags, samples should be drawn from 5 bags plus 10 percent of the bags in the lot, up to a maximum of 30 bags. Bags are best sampled using a slotted probe (trier) so that seeds are taken from different parts of the bag. Samples from bags are then thoroughly mixed and a 1- to 2-pound sample is submitted for testing.

Test Results and Their Meaning

The combined results of the standard germination test and the cool germination test will provide a better guide for determining the planting value of seeds. When field conditions are favorable, i.e., soil temperatures are warm (near 86 °F or warmer) and when soil is moist, emergence should approach the percentage standard germination.

If soil conditions are cool (near 65 °F) at planting, or if temperatures drop near 60 to 65 °F after planting for a few days, then emergence should approach the results of the cool germination test.

If the soil becomes excessively wet, and temperatures drop near 50 °F or lower, the resulting emergence might be well below the results of the cool germination test. Under these conditions, emergence could be near zero or zero. Field conditions may possibly be so severe that regardless of how good seeds are, an acceptable stand cannot be obtained. The recommended minimum 65 °F soil temperature for planting cotton should be followed no matter how high the cool germination test results are. This minimum recommended soil temperature is near the temperature at which seeds are germinated in the cool germination test. Performance of seeds at lower temperatures should not be speculated or based upon the cool temperature germination test results.

Excessive moisture has an adverse effect on seed germination in the laboratory germinator or in the field. Weak seeds will not produce normal seedlings under these conditions. Extended wet periods may also affect the performance of strong seeds. Thus, when fields become water logged shortly after planting and this condition lasts for several days, stands may be poor even when soil temperatures are favorable. Stand establishment problems are greatly increased when soils are cold and wet.

What test results are acceptable? Experience has dictated 80 percent or better standard germination test results as the standard. Cool germination test results should be 50 percent or better. We assume that field survival will be similar to the cool test germination percentage if cool soil temperatures occur at, or shortly after, planting. Again, if soil temperatures are considerably lower, this percentage may not survive.

Seeding Rate Determination

Cottonseeds vary in size, both among varieties and from year to year. Therefore, basing seeding rate on seeds per foot of row rather than on pounds per acre will be more accurate and should result in a more predictable plant population when combined with the information from both standard and cool germination test results.

Cotton plant populations can vary from 30,000 to 55,000 plants per acre without seriously affecting yield. Keep this in mind when planting. For example, if the desired population is 45,000 plants per acre and seed germination is 80 percent, and if seedling survival is expected to be high, then 45,000 plants divided by .80 (80%) equals 56,250. This means that 56,250 seeds should be planted per acre. Under favorable conditions, plant population should be about 42,000 plants per acre.

If field conditions are less favorable, then a plant population nearer the cool germination test results would be expected: 56,250 seeds per acre x 50% = 28,125 plants per acre. This population is high enough to produce good yields under good management, assuming good seedling survival after emergence. This plant population, 28,125 plants per acre, is equivalent to 2 plants per foot of row on a 38-inch row and is adequate on good cotton soils.

The supply of seeds from year to year normally varies in quality, i.e., germination may be 80 percent or better, while cool germination may range from less than 50 percent to about 70 percent or higher. Buy seeds early to insure that you will get the best quality seeds. Waiting until just before planting to obtain seeds could mean you must take what is available. Quality may be lower than desirable, especially for cool germination test results. Germination tests cannot be conducted in a few days. Allow 1-3 weeks to get results.

There may be years when there are few or no seeds that germinate 50 percent or better in the cool germination test, or 80 percent or better in a standard germination test. In this situation, the quality of the seeds must be a primary consideration in scheduling planting to minimize stand-establishment problems.

Test Result Variability

Many people have problems understanding test results because there are variations from laboratory to laboratory. There will even be variation from one test to another on the same sample within the same laboratory. A certain amount of variation between tests is due to sampling variation, i.e., every 100 seeds are not exactly the same. Therefore, there will be differences in test results. This has been taken into consideration in seed testing.

Studies have been made to determine how much variation can be expected. This has resulted in the development of tolerances, i.e., how much variation is acceptable when a certain test result is reported. For example, if cotton seeds are labeled 80 percent germination, the tolerance is 7 when 400 seeds are tested. This means that germination test results can vary from 73 to 87 percent and results can still be considered the same. The differences are due to sample variation.

If only 200 seeds are tested, the tolerance is 9, meaning results can vary from 71 to 89 percent and still be considered the same. If the variation is greater than the tolerance, test results are considered to be different from that which is printed on the label, i.e., out of tolerance, and seed labeling is in violation of the state seed law.

Therefore, there can be some variation between laboratories and between tests on the same lot of seed, and the test results will still be correct. The failure to realize that there will be variation in testing has caused great concern for some farmers and seedsmen. We must understand and accept the fact that there is variation in biological systems and we account for this in our testing.

In spite of the identified weaknesses, the use of both standard and cool germination results will give growers a better chance of establishing the desired plant population when information from these tests is understood and properly used.

By William H. McCarty, Extension Agronomist - Cotton, and Dr. Charles Baskin, Agronomist - MAFES

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Information Sheet 1364
Extension Service of Mississippi State University, cooperating with U.S. Department of Agriculture. Published in furtherance of Acts of Congress, May 8 and June 30, 1914. Ronald A. Brown, Director

Copyright by Mississippi State University. All rights reserved.

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