Cotton Seed Quality

Keith L. Edmisten, Cotton Extension Specialist
A. M. Stewart, Graduate Research Assistant
(CCN-98-4a, April 1998)

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Seed quality may be more important than usual for several reasons this year. Results of germination tests are often thought of to be good indicators of seed quality and potential seedling vigor. In cotton there are two common germination tests, standard germination and cool germination. Standard germination results are reported on the seed tag. Standard germination tests are conducted at 86 degrees F for sixteen hours per day and 68 degrees F for 8 hours per day. In North Carolina it is highly unlikely that all of the cotton seed you plant will benefit from this close to ideal conditions. The test that is of more practical value to growers in North Carolina is the cool germination test often referred to as "cool germ".  Cool germ tests are not reported on the seed tag. The seed companies run this test on all seed and the dealer or distributor usually has this information. If not, the value can be obtained by calling the seed company with the lot number of the seed. NCDA can run cool germ tests on your seed if needed. There are several reasons why I am concerned about cool germination values this year:

1. Due to the shortage of roundup-ready seed available for sale companies may respond to pressure to provide seed and release some seed that is lower in quality than they would like.

2. This potential problem may be further exaggerated by the new pricing structure for Roundup-Ready, Bt and stacked genes. Growers may want to plant lower than optimal plant populations to save money.

3. Some growers are purchasing stacked Bt and Roundup-Ready seed in order to have Roundup-Ready seed. The new pricing structure will cause many to look hard at reduced seeding rates. It is very important that growers make seeding rate decisions with as much information as possible. The most important information for a grower to have when making seeding rate decisions is cool germination results. What is considered to be "good" cool germ results? Being aware of the cool germ results is probably more important than what is actually a good or bad cool germ. As long as you are aware of the cool germ values for a given seed lot you can plan accordingly. A somewhat arbitrary division of cool germination values follows in Table 1.

Table 1. Cool germination ratings.

*What is meant by using "special care with this seed?.

There are several things a grower can do to make it likely that this type of seed (cool germ 50-65%) will produce an acceptable stand:

1. Do not plant during cool periods. Cool temperatures can be especially detrimental during the first two days after planting. The DD60 forecast for the five days following planting is the best indicator we have of planting conditions. The table below offers guidelines as to the relationship between DD60's and planting conditions. Table 2. The relationship between DD60's and planting conditions. (Adapted from Deltapine Cotton Management Guide)

  Cotton seedlings are particularly susceptible to cool weather when they first imbibe water and the 2 days following imbibition. Figure 1 shows the sensitivity to chilling injury from with time.

Figure 1. Sensitivity of chilling injury to germinating cotton seed.

2. Do not plant too deep. This is especially critical on our Coastal plain soils that tend to crust.

3. Do not use low-end seeding rates for a given soil type to save money on biotechnology fees.

4. Consider protecting the seed with in-furrow fungicides especially if the field has a history of seedling disease or is wet natured. This is especially true if planting under less than ideal temperatures. Most texts list 50 DD60's as the number required from planting to emergence. We have seen cotton seed germinate in considerably less than 50 DD60's. There are several reasons why this can occur.

1. The DD60 concept is a simple concept that is by no means perfect. For example it is common to see information that states that 350 to 450 DD60's are required to reach first square. Data collected by Dr. Ozzie Abaye of Virginia Tech at the Tidewater Research Station found that DD60's from planting to first square ranged from 276 in 1997 to 363 in 1994. However, in both years cotton reached first square in 45 days. The range of days from planting to first square for the 4 year experiment was 38 to 45 days. This indicates that cotton development is not only influenced by DD60's but that time has an influence as well. One way to look at this is: If you have the DD60' you can make up for some time, and likewise if you have the time you can make up for some DD60's.

2. The soil temperature may be warmer than air temperatures due to radiant energy from the sun. DD60's based on soil temperatures would likely be more accurate than air temperatures during germination. Dark soils will absorb more heat than lighter colored soils. Likewise drier soils absorb more heat than wet natured soils. The worst place to plant seed with questionable cool germ would be on lighter colored, wet natured soils.

3. DD60's based on daily maximum and minimum temperatures do not give much of an idea of duration of temperatures. A more accurate way to determine DD60's is two use hourly maximum and minimum temperatures to determine degree hours and divide the total by 24 to get degree day or DD60.

4. We mentioned above that the DD60 system is not perfect and in fact most people use the simple formula for determining DD60's shown below.

DD-60 calculation for each day: [(ºF Max + ºF Min Temp)/2] - 60 = DD60s

For example, if today's high and low temperatures were 80 ºF and 60ºF, respectively, then the formula would arrive at [(80ºF + 60ºF)/2]-60 = 10 DD60s. If temperatures remained like this for 5 days it would take about 5 days for cotton to emerge. But what if today's high and low temperatures were 70ºF and 50 ºF, respectively, then the formula would arrive at [(70 ºF + 50ºF)/2]-60 = 0 DD60s. If we had a month of this weather we would accumulate no DD60's and theoretically cotton would never emerge. Cotton would still emerge although it probably would take longer than 5 days based on the conditions discussed above. This is an example of how the DD60 formula falls apart when the minimum temperature is less than 60. There is a more complicated formula that takes this in to account. If maximum temp is less than 0ºF then DD60's = 0. If minimum temperature is over 60 then use the simple formula above. All other cases use this formula:

(ºF Max - 60)² ) / ((Max - Min) x 2) = DD60s if we use this formula with the high and low temperatures were 70ºF and 50ºF, respectively, then the formula would
arrive at  (70ºF - 60)² ) / ((70ºF - 50ºF) x 2) = 2.5 DD60s
instead of the 0 DD60's the simple formula would arrive at. The table below provides DD60 values based on this more complicated formula.

  Table 3. DD60's based on a 60 degree minimum temperature threshold.