Recent articles in Delta Farm Press> have established the fact that cotton has had a tough time so far this year. Cotton irrigation is always challenging and it looks like this season is going to hold to that pattern.

This article is being written on a rainy day following a week to 10 days of much needed dry weather. Hopefully, this rain system will be followed by some days of dry, sunny, cotton-growing weather.

Given the problems that the excess moisture has caused, it probably sounds crazy to be talking about irrigation. However, we also realize how quickly it can go from being wet to getting dry. The soil can only hold so much moisture regardless of how much rain has fallen on the field. It is pretty well accepted that we are always only seven to 10 days from a drought once we get into the season.

Root system development this year has probably been slowed and is limited enough that the crop will not be able to take advantage of any deeper soil moisture that might exist. When (not if — I'm optimistic) good growing conditions do come, it is very possible that the cotton will quickly use up the available soil moisture.

Even though most of the cotton is late, it will be very important that the first irrigation not be delayed to a point that the crop experiences additional stress. Cotton irrigation studies clearly show that delaying the initial irrigation can reduce yields and result in lower returns.

A two-year study conducted by Dr. Earl Vories and others showed that delaying the first irrigation resulted in a $25-per-acre loss in returns in 2001 and a $106 per acre loss in 2002.

Cotton researchers Drs. Phil Tugwell and Tina Teague, have also documented yield losses associated with delaying the initial irrigation.

Previous crop years have shown that cotton can make up for much of the initial late development if it is managed well when good growing conditions do finally come.

The current University of Arkansas cotton irrigation recommendation is to begin monitoring the crop moisture status at emergence and maintain well-watered conditions until bolls begin to open.

Flow estimates for holes in irrigation tubing
Pressure in feet
Hole size: inches
7/16 1/2 9/16 5/8 11/16 3/4 13/16 7/8 15/16 1
Flow in gallons per minute
1.0 2.4 3.2 4.0 5.0 6.0 7.2 8.4 9.8 11.2 12.8
1.5 3.0 3.9 4.9 6.1 7.4 8.8 10.3 12.0 13.7 15.6
2.0 3.5 4.5 5.7 7.1 8.5 10.2 11.9 13.8 15.9 18.1

The irrigation scheduling computer program referred to in the May 2, 2003, issue of Delta Farm Press can be helpful in staying on schedule with irrigations (www.aragriculture.org/computer/schedule/default.asp).

The reality is that getting the first irrigation started on time is often delayed until after lay-by, especially on furrow-irrigated fields. Some growers can lay the 9- to 10-mil irrigation tubing out before lay-by and get by with running over it while others have either tried and had problems or don't want to risk tearing up the tubing.

The shortage of labor on many farms is also a major factor in getting the irrigation water started when desired.

If watering every other middle fits well with your furrow irrigation layout, it can also help avoid potential stress from excess moisture if rain occurs during or soon after the irrigation. Irrigations with every other middle will have to be more frequent than when watering every middle unless water soaks across the dry middle.

Most growers lay parallel tubing if two irrigation sets are needed in a field to avoid plugging and unplugging holes in tubing.

The table (adapted from NRCS information) included should be helpful in determining the approximate flow from different size holes in tubing.

The tubing pressure near the well or riser will usually be in the range of 1.5 to 2 feet and 1 to 1.5 feet toward the end of the tubing if the turn row is relatively flat.

Starting irrigation on time should be easier with center pivot irrigation. This is good since pivot irrigations will typically only affect the soil moisture to a depth of about 8 to 10 inches. Starting on time with pivots is critical to avoid getting behind when rainfall doesn't occur during the crop's peak water use period (bloom to boll fill).

Even though it has been a rough start, there is still a lot of growing season left. Don't add to the problems caused by weather by being late with irrigation.


If you have questions or suggestions on topics please contact me: Phil Tacker, 501-671-2267 (office), 501-671-2303 (fax), 501-944-0708 (cell), or ptacker@uaex.edu (e-mail).

Phil Tacker is an Arkansas Extension ag engineer.