NC State University|Crop Science|College of Agriculture and Life Science|NC Cooperative Extension
Keith Edmisten, Cotton Extension Specialist
Department of Crop Science
North Carolina State University

I have looked at the long-term forecast for Raleigh (http://nc.weather-forecast.ws/raleigh) which goes through April 15th as of now (April 7). Based on this prediction we do not really start accumulating heat units until the 13th of April.
Heat units are calculated by adding the high and the low, dividing that number by 2 and then subtracting 60
[(°F Max + °F Min Temp)/2] - 60 = DD60s.

Here is an example of heat unit calculation and accumulation for April 13 through 15 based on the predictions given today at the site above.

The accumulated DD60ís for the 3 day period would be: 5.5+6+7.5 = 19 DD60ís

We normally do not recommend planting cotton prior to April 15. If the above prediction holds true at least we will go into the planting period with some heat (DD60 accumulation). We should watch the forecasts as we get closer to April 15 to determine the predicted accumulation of DD60ís for 5 day periods after April 15. Table 2 below relates 5 day DD60 accumulation to planting conditions

The high cost of seed makes it more important that growers consider seed quality. 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.

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.
Cool Germ Value Ratings - Comments
under 50 bad - most companies would not sell this seed
50-65 acceptable - use special care with this seed*
65-80 good
over 80 superior

*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.
DD60's accumulation in the 5 days following planting Planting Conditions
Less than 10 Very Poor
11 to 15 Marginal
16 to 25 Adequate
greater than 25 Very Good

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

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

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, including:

1. 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.

2. 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 to use hourly maximum and minimum temperatures to determine degree hours and divide the total by 24 to get degree day or DD60.

2006 - Carolina Cotton Notes

NCSU Cotton Team


2006 crop science©
last modified April 7, 2006 11:15 AM
page by Gary Little