The single most important issue to recognize at cotton planting time is that cotton seedlings can be damaged by cool, wet soils.
Soil temperatures have been quite cool this spring. Last year, we planted our first trial on April 24 and had experienced warm soil temperatures and an excellent forecast. Every season is different.
Dry soils will warm up faster than moist soils. Since we continue to have roller coaster air temperatures, when we do get rainfall the soil temperatures will be lower. It is a good idea to have your own soil thermometer to check your own specific field situation and wait for soil temperatures appropriate for cotton planting.
BMPs for cotton planting
Best management practices for cotton planting under normal soil moisture conditions would be to delay planting until:
1) The 3-day Oklahoma Mesonet bare ground average soil temperature at the 4-inch depth is at least 65 degrees. See: http://www.mesonet.org/index.php/weather/soil_temperature and http://www.mesonet.org/index.php/weather/category/soil_moisture_temperature
2) The 5-day forecast calls for dry weather and a minimum of 25 to 50 DD60 heat units. The normal calculation for cotton DD60 heat units is: (maximum air temperature + minimum air temperature) / 2) - 60 = DD60 heat units.
Essentially, the average air temperature for the day is determined and the 60-degree developmental threshold for cotton is subtracted. The DD60s for each day are totaled. If you have faith in your local forecast, the projected high and low for the following several days can be used to calculate DD60s.
3) Low temperatures are forecast to remain above 50 degrees for the 5 days following planting.
If equipment constraints and large acreages require producers to plant during less than optimum conditions, they should realize that seed quality and seeding rate become very important. The seeding rate can be adjusted on the planter. However, with expensive transgenic seed, seed treatments and technology fees, increasing seeding rate is not a palatable option for most producers. Therefore, seed quality becomes very important.
Testing can be critical
The Texas Cool Germination test was developed specifically to test cotton seed under cool soil temperature conditions. This germination data is NOT required on the state seed tag, but many seed companies will provide this information. The state seed tag reports Standard Germination data and it is performed in a different manner. It is usually guaranteed on the seed tag at a minimum of 80 percent.
Texas Cool Test data are obtained from a test conducted at 64 degrees Fahrenheit with seedlings counted after 7 days. Higher Cool Test data indicate higher vigor under temperature-stressed conditions. If the Cool Test data for a specific lot of cotton seed is known, then potentially more vigorous seed lots can be identified. This can be used to determine the planting sequence and possible planting date. Producers should begin planting with higher vigor seed under cooler temperatures and finish up with lower vigor seed under warmer temperatures. Planting conditions for rapid germination and emergence include:
- high quality seed with good to excellent Cool Germination Test data (>60 percent)
- a favorable 5-day forecast
- minimum air temperature of at least 50 degrees
- maximum air temperature of at least 80 degrees
- plant into a firm, moist seedbed 1 to 2 knuckles deep
- proper and uniform seeding rate of no more than 4 to 5 seeds per foot in 40-inch rows.
Imbibitional chilling injury occurs when cotton seed is subjected to cold conditions during the first 2 to 3 days after planting, or during the time when the seed is imbibing moisture from the surrounding soil. Cotton seed contains lipids which must be converted to energy during germination. The cell membranes must develop properly. Soil temperatures 50 degrees or below around the seed can damage seedlings during this time. Soil temperatures of 41 degrees or less may kill or severely injure the seedling.
Normal cotton seedlings seeding rate
Stand components consist of both uniformity and density. Uniformity of planting seed in the row is affected by planter type. The newer vacuum planters are extremely effective at controlling vertical distribution of the seed in the seed furrow and horizontal spacing down the row. These modern planters typically provide excellent seed to soil contact capability, which results in an increased likelihood of an individual planted seed being able to germinate. Seeding rate or density is controlled by the producer. The newer vacuum planters, coupled with the generally higher seed quality than what we often encountered in the past, have allowed most producers to reduce seeding rates successfully. However, because of the cost of transgenic varieties, in addition to cost of upgraded insecticide and fungicide seed treatments, many producers are pushing the agronomic minimum and living on the edge with little margin margin for error, so to speak.
Many seeding rate trials have been conducted in southwestern Oklahoma and the Rolling and High Plains regions of Texas over the last several years. Results all indicate that seeding rates can be pushed to a lower level than was generally accepted 10 to 15 years ago; however, the producer must have extreme faith in his planter and its adjustment, field-specific planting situation, seed quality, and environmental conditions after planting. It is difficult, agronomically, to justify less than 2 seed per row-foot as a best management practice in dryland cotton production.
Cotton has a remarkable capacity to compensate yield across a fairly wide range of plant populations. Recent seeding rate studies have indicated that within the final plant stand range of 1.5 to 4.5 plants per row-foot in 40-inch rows, lint yield can remain reasonably unaffected.
How a producer gets from a seed drop rate to a final plant stand can be a treacherous journey. Assuming that good soil conditions are present, and an excellent vacuum planter is used to control seed distribution both down the row and in planting depth, a range of 2 to 4 seed per row-foot in 40-inch rows is probably acceptable. Under dryland conditions, the low end may be targeted.
In poor planting conditions (such as low seed quality, marginal soil moisture in the seeding zone, a large amount of crop residue that may affect seed to soil contact, lack of precision planting equipment, or poor forecast conditions) it may be more important to increase the seeding rate. If a low seeding rate is used, the producer must have high confidence in the seed quality and planter precision.
Randy Boman is research director, Oklahoma State University Southwest Research and Extension Center in Altus and cotton Extension program leader.