Irrigation timing can affect everything from weed control to yield and fiber quality, according to an on-farm study by Tom Barber, Extension cotton specialist for Arkansas, Phil Tacker, former irrigation specialist at the University of Arkansas and Paul Francis, agriculture professor at the University of Arkansas at Monticello.

The study, funded by Cotton Incorporated, looked at three initiation timings, two weeks prior to bloom, a week prior to bloom and at bloom. Several termination timings were also included — 300 heat units after cutout, 450 heat units after cutout and 600 heat units after cutout.

The study is based on research that once cotton starts squaring, demand for water begins to increase, climbing as the plant reaches bloom and peak boll fill.

Barber noted that irrigation research in the Mid-South is more difficult because rainfall can be so scattered, even in a single field. “We did have weather stations on the fields, but it was impossible to record every inch of rainfall that falls on a farm.”

The research was conducted on a 160-acre field belonging to producer Steve Stevens, in Tillar, Ark., and included both silt loam and silty clay loam soils. The variety planted on all the plots was ST 5458B2RF. Rainfall was variable during the course of the research.

The irrigation method was furrow, and researchers used the PHAUCET (Pipe Hole and Universal Crown Evaluation Tool) which helps farmers improve timeliness on fields with different row lengths. “It’s a great tool for saving water and preventing runoff,” Barber said.

Watermark soil moisture sensors were also used in the three year study which concluded in 2010.

The research indicated that much more plant structure was built by bloom with an early, (two weeks prior to bloom) initiation of irrigation versus late initiation. In terms of plant structure at bloom, there was no difference between plots where irrigation was initiated a week prior to bloom and two weeks prior to bloom. However, by the end of the year, the earliest initiated irrigation was ranker than the later timings.

An early initiation did hasten canopy closure, noted Barber. “This is important for us because we’re not only watering the crop, we are fighting resistant pigweed. If we don’t get that canopy closed early, and the sunlight hits the furrow, we’re not in a good position to control them.”

Barber noted that the crop did not canopy in any of the treatments in which irrigation was delayed to bloom or later.

Less small boll shed

Barber noted that there was less small boll shed when irrigation was initiated early. “When we wait until bloom to irrigate, many times the cotton is under severe water stress. When we start watering under these conditions, especially with high nitrogen levels, the cotton will shed and it’s going to grow. That’s what we saw.”

Across the three years, cotton yields varied in the research, noted Barber. In some cases where rainfall was plentiful, like in 2008, there were fewer differences. “In hot and dry years, like 2010, we gained 120 pounds with our mid- initiation (a week prior to bloom) over the late irrigation.”

Yield data also indicated that if irrigation is started late, it doesn’t necessarily equate to the ability to terminate early. Research showed that later initiations of irrigation simply shifted the fruiting pattern up one or two nodes. This was especially true on silt loam soils.

The researchers concluded that a pre-bloom irrigation is the optimum timing, especially on sand and silt loam soils. “It’s hard for growers, as many acres as they’re farming, to start much earlier than one week prior to bloom,” Barber said. “But that looks like that is the critical time. Starting on time can retain fruit and set yield early on silt loam soils and protect fiber quality, primarily micronaire.”

The researchers also concluded that starting early often meant irrigation could conclude early. “If something happens, like we get a heavy infestation of plant bugs and we remove some fruit, that may change. We still have a lot to learn on our clay loam soils. I think we’re still over-irrigating some of them.

“But there is a fine line there. If you let it get too dry, the cracks open up and water goes in them and goes down and never goes to the end of the field.”

Barber advises cotton producers to maintain soil moisture “until the cotton starts to open, which is equal to about 450 heat units. Maintaining a two-inch deficit works well on our silt loams. We may need to be closer to 3 inches on our clays.

“With center pivots, it’s going to be a little different because of the amount of water you can put on. We also have to think about putting water on bolls that are starting to crack and we don’t want to carry that too far.”

A 5-year study from 1999 to 2003 by Phil Tacker indicated an 85 pound yield advantage when cotton irrigation is initiated a week prior to bloom versus at bloom, except in years when rainfall coincided with irrigation initiation.

Barber says be ready to turn on irrigation a week prior to bloom, maintain a 2-inch deficit on silt loam soils and 3-inches on clays. At termination, maintain moisture until an open boll up to 5 percent open or 450 heat units after NAWF-5 (nodes above white flower is equal to 5).