Knowing “when” is vital to terminating cotton irrigation in Oklahoma

Deciding when to stop irrigation is one of the key decisions in cotton production. In this article we explain the effects of terminating irrigation too early or too late, discuss the findings of recent research projects from Oklahoma and the region, and explain different factors that must be considered when deciding to terminate cotton irrigation.

Stopping irrigation too early can result in yield loss, especially if soil moisture is not adequate to support boll filling after irrigation termination. In a research project that was conducted from 2015 to 2017 at OSU’s Southwest Research and Extension Center near Altus, the effects of three irrigation termination dates on cotton yield and fiber quality were studied.

The termination dates were: 1) end of August (usual termination date), 2) one week earlier, and 3) two weeks earlier (mid-August).

Terminating mid-August resulted in more water conservation. On average, seasonal water use was 28% smaller compared to end-of-August termination. This may be beneficial for growers within the irrigation district as more water can be stored in the reservoir for the next irrigation season.

Terminating irrigation one week later than mid-August resulted in 16% water savings. 28% saving in irrigation application would translate to about 14,000 acre-feet of water in an average year in the district. Despite water savings, cotton lint yield was negatively impacted by early termination.

When averaged over the three years (2015-2017), lint yield for end-of-August termination was 1,448 lb/ac, larger than the average yield for terminating one week earlier (1,221 lb/ac) and two weeks earlier in mid-August (1,138 lb/ac).

Average micronaire decreased from 3.89 to 3.65 and average uniformity declined from 82.8% to 81.7% when terminated 2 weeks earlier than the end of August One major observation was that the impact of earlier termination was highly variable from year to year depending on the late-season rainfall and soil moisture availability.

In years that were characterized with dry late season, ending irrigations even one week earlier had a substantial effect on cotton performance. In contrast, irrigation could be terminated two weeks early without any impact if timely rainfalls replenished soil moisture towards the end of the growing season.

The results found in the OSU study was similar to a study by Texas A&M AgriLife that reported the best time of irrigation termination to be the first week of September in normal years and the last week of August in wet years for the Texas Panhandle region.

Stopping irrigation too late can have negative effects on yield and fiber quality by increasing pest and disease pressure and leading to hardlock or boll rot. This is especially the case for pivot irrigation as canopy gets wet during water application and may stay wet for several hours depending on weather conditions.

Another issue caused by terminating too late, especially this year that we are still behind in terms of crop progress or maturity, is fueling excessive late-season vegetative growth. Finally, irrigating too late can reduce the efficacy of defoliation.

Crop development must be considered in deciding when to terminate irrigations. A general recommendation for furrow irrigation is to stop irrigation at first crack boll if root zone moisture is adequate. Center pivot and drip systems may need to go longer than first crack boll (for example 10% open bolls) as a smaller amount of water is applied by these systems in each event compared to deep, heavy irrigations of furrow systems.

Some previous studies have recommended irrigation termination to be decided based on growing degree days (heat units) accumulated after cutout (NAWF=5). For cotton, degree days are typically estimated based on 60˚F as the threshold temperature and hence degree days are referred to as DD60’s.

Crop development is directly related to DD60’s. A general recommendation is to terminate irrigation at 350 DD60’s past cutout. In a study in west Texas, no benefit in yield and fiber quality

was observed for irrigation termination later than 400 DD60’s past cutout under subsurface drip irrigation.

Many factors play a role in determining the best time to terminate cotton irrigation, making this decision highly variable from field to field and year to year. Here we have summarized some of the main factors that must be considered.

● Crop development: some previous studies in the region have recommended termination based on boll opening (first crack boll for furrow irrigation and 10% open boll for pivot). Other studies have used 350-450 DD60’s after reaching cutout.
● Root zone soil moisture: late-season water availability in the root zone has a significant impact on cotton yield and fiber quality. If all other factors indicate irrigation should be terminated but the soil moisture is not adequate and no rain is in forecast, irrigation should continue a bit longer to avoid yield losses.
● Soil type: deep clay soils can hold more water and thus take more advantage of late-season rains, especially if the root system is deep and healthy. Sandy soils or soils with restricting layers (for example shallow bedrock) will hold less water and may need additional irrigation.
● Irrigation type: Furrow systems apply large amounts of water and can soak the entire root zone across the field. Irrigation can be terminated earlier for these systems. Subsurface drip systems apply the smallest amounts of water but at much higher frequency. In addition, they typically do not wet the entire field and may have crop roots that are concentrated near drip tapes. They may need to run later especially under drier conditions. Overhead sprinklers wet the canopy and can cause boll rot if they are terminated late.

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