What’s the best time to shut down late-season irrigation?
“Depends,” says Texas A&M AgriLife Engineer Dana Porter in what she admits is typical engineer-speak.
Porter explains that farmers will want to consider numerous factors before they pull the plug on the season’s irrigation schedule.
“Irrigation water capacity is a major determining factor. Do you have water available? Can you speed up your center pivot system or shorten your irrigation sets to adjust irrigation applications to crop water requirements?”
(Photo by Shelley E. Huguley)
In addition to water availability and irrigation system capacity, Porter says producers should look at other relevant factors, such as crop condition, crop growth stage, irrigation system capability, and available soil moisture, which will vary with soil type.
“Look at where you are,” Porter says. “What’s your soil water status? If you’ve kept up with irrigation pretty well, it may be in good enough shape to start backing off irrigation applications.”
First open boll
She recommends cotton producers watch for the first open boll. “After that, water demand begins to decline. Don’t let the crop completely run out of water, but after first open boll, it can be okay to allow the soil water to dry down some.
“We don’t want to allow the soil to dry all the way to wilting point, since that will limit our crop yield,” Porter adds. “We use a managed allowable depletion (MAD) threshold to avoid drying the soil too much, hence avoiding excess drought stress.”
She explains that 50% of plant available water is a general MAD level for many commodity crops. “Drought sensitive crops would have a lower depletion threshold.”
Watch the forecast. “Weather affects the soil water storage, as well as crop water demand. Is cool, rainy weather predicted in the next few weeks? Or are hot, dry conditions expected?”
Soil texture also makes a difference. “How much can soil in a particular field hold?” Well, that depends, too, Porter says.
“Plant-available water is moisture held between field capacity and wilting point. Soil water holding capacity is largely a function of soil texture. Clay and loam soils can retain more water than coarse (sandy) soils.
“Clay loam or silty clay loam soils hold about 2 inches of water per foot of soil in the root zone. If you have a 3-foot root zone, you should have about 6 inches of moisture stored with clay loam.”
She says in most cotton fields, the rootzone extends to 3 feet depth, assuming there isn’t a shallow caliche or other impeding layer.
(Photo by Shelley E. Huguley)
Other soils hold less moisture. A sandy loam soil holds about 1½ inch. Sandier soils will hold less, from about ½ inch to 1 3/4 inch per foot of soil.
“It all depends on the soil type,” Porter says. “A shallow soil, or a soil with a caliche layer or other impeding layer, limits root development (root zone depth), as well as water infiltration.”
She says with cotton, the key is to watch for that first open boll, soil moisture capacity, and weather forecasts.
Rootzone also varies with different crops. Cotton and corn could range from 2.6 to 5.6 feet. Alfalfa and sorghum roots go from 3.3 to 6.6 feet or more. Peanuts go to about 3.3 feet. Porter says most vegetable crop rootzones extend from 1 to 3 feet.
Peanut water demand is a bit different.
“After kernels begin to fill in late August to early September, irrigation applications can be slightly reduced, depending on crop maturity and rainfall,” Porter explains. “Changing from a twice-a-week irrigation schedule to once-a-week helps stop blooming.
(Photo by Shelley E. Huguley)
“Lower relative humidity in the canopy moves the crop into a maturation phase and reduces susceptibility to pod rot organisms. (Excess moisture can promote pod rots.) During the maturation period, the plants will mobilize nutrients and food reserves to developing kernels, and plant water use moderates compared to the more critical bloom, peg and pod development periods.”
She says avoiding large fluctuations in pod zone moisture can help prevent hull splitting, which can lead to increased loose shelled kernels, and greater risk of aflatoxin development.
In situations where producers are watering cotton and peanuts under the same pivot, peanut demand may last a bit longer.
“Peanuts need moisture in the pod zone (near the soil surface) to mature the pods (but not excessive moisture that can encourage pod rot),” Porter says.
She says cultivar selections for cotton and peanuts as well as planting dates also affect irrigation management and timing. “Full season cultivars are more likely to be irrigated later and longer.
“Also, planting and harvest dates vary by region. Cotton farmers in South Texas plant and harvest much earlier than in the High Plains. Peanuts have a narrower window in Texas because the geographic coverage is narrower.
“High Plains expected first frost date also affects planting and late-season management decisions.”
She adds that soil types affect irrigation management with each crop. “Peanuts are grown primarily in sandy soils (easier to harvest). Cotton is grown on more soil types and is more likely to be grown in more limited irrigation conditions (dryland, deficit irrigation).
“Peanuts in arid and semi-arid areas need to be irrigated, as they are much more drought sensitive than cotton. In hot, dry conditions, peanuts are more likely to be contaminated by aflatoxins, so irrigation management can be essential to crop quality.
“Finally, price/market factors probably outweigh other factors in irrigation decisions.”
Porter adds that deciding when to shut down irrigation systems should not be the last in-season decision producers make. “They need to winterize them,” she says. “Late in the season, during harvest, producers get busy, so they may need a reminder before freezing weather to winterize their systems.”
But it’s an important function of irrigation maintenance. “First, drain water out of the lines. Drain the pivots and the micro-irrigation lines. The lines underground are less at risk, but if we don’t drain the pipes above ground at the filter stations, they could freeze and burst.”
Taking care of these late-season chores will improve resource management and save time when producers start the system for the next crop.