Making more cotton with the water you have
By Jennifer M. Latzke
A farmer can carefully plan his fields. He can select his seed varieties, line up his crop protection inputs and even plan for labor and equipment outlays.
But all that doesn't matter if he doesn't manage his water.
And, if he is a farmer who happens to have irrigated fields in the High Plains region above the Ogallala Aquifer, he is probably looking at any way to become more efficient in using this valuable resource. An ongoing drought that stretches across the region has many farmers looking for alternative cotton production methods that will help them conserve the water they have available.
Bob Glodt, owner of Agri-Search, a crop consulting and contract research firm out of Plainview, Texas, said growers should start this path to increasing water use efficiency (WUE) by better understanding the needs of their cotton plants. Glodt also spoke at the Deltapine 2013 Weed Resistance and Water Management Summit in Lubbock, Texas, at the end of January. He challenged growers to start considering deficit irrigation, or strategic irrigation in combination with selecting varieties for their WUE characteristics as a way to make more cotton with the water they have available.
How thirsty is the plant?
"Growers have to aggressively begin to better understand plant physiology and how much water that plant is demanding at any one time in the production cycle," Glodt said. "How thirsty is it?" He explained that growers can do so by monitoring a specific percentage of potential evapotranspiration, or PET, for their fields at a given point in the plant's life cycle.
"Evaporation is the amount of water that comes off the leaves, or the amount that evaporates off of the soil," he said. "Transpiration is the plant's cooling function, so the amount of water coming out of its stomates. Together, that's the potential evapotranspiration.
"Study your crop," he added. "In my experience it isn't about the amount of water you have, it's about when that water goes on. You have to start thinking in terms of managing your water in the critical events along the cycle of the cotton plant." One helpful tool for farmers is the Texas Alliance for Water Conservation's website, www.tawcsolutions.org. Here, growers can find charts that consider their location and help estimate how much water their cotton plants are using on any given day.
Matching the water supply to the crop demand is the target, Glodt said. Each crop has a bell-shaped curve of water use for its life cycle, he said. "When cotton comes up, it doesn't need as much as when it's at early to mid-bloom," he said. "As it matures, it tapers off. Generally cotton uses .05 to .35 inches per day. Most of the time it needs .3 inches or less per day."
Therefore, farmers can consider their available water, their target yields, and choose to apply their irrigation at the critical stages of the cotton's growth, rather than over-applying water that will just be lost to evaporation. This percentage of ET regime means growers won't be applying 100 percent of the water needs of the cotton plant all the time, but rather, applying 30, 60, or 90 percent of its needs at each pivot pass. But it only works if there is a moisture reserve in the soil that the plant can tap into.
Building a reserve
Glodt said that to begin watering on a percentage ET, or "deficit irrigation" or "strategic irrigation," as some call the system, you have to have a period built in to charge the soil profile, or pre-water to account for percolation needs of the field. "If you don't have a reserve in the soil, there is no water to draw from if you are watering at less than 100 percent," he said. "You have to charge the soil profile with water early in the season so the plant can draw it out."
Now, some farmers try to pre-water all in March or April, Glodt added, but by the time the plant needs it, the water is gone. He recommends shifting from pre-watering at squaring to early bloom. "The crop demand at that period is very low, about .1 inch per day," he said. "So, if you put on one inch of irrigation, you'll have .9 inches left that the crop isn't using and can be stored."
Glodt said that growers should consider the top three feet of their soils as the reservoir for the cotton plant's root system. Soils around Plainview will hold 1 to 1.25 inches per foot, so that's 3 to 3.75 inches available to the plant for the top three feet, if the grower has built his reservoir.
Another factor that growers must consider is whether they use a broadcast or Low Energy Precision Applied irrigation system.
"LEPA means that water is concentrated in the furrow, rather than broadcast that goes the full span of the nozzle spacing," Glodt said. Broadcast is inefficient, especially if a farmer is trying to bank water in the soil, he said. LEPA will push the water deeper into the soil and have less evaporation. So, if a farmer applies 16 inches of water via broadcast, and only half is available to the plant, that means 8 inches of water were lost. "But, if you apply the same 16 inches in a LEPA system, and it's 80 percent efficient, that means 12.8 inches are available to the plant," Glodt said. And, that's water the plant can use to make lint.
How much can be applied?
Once farmers know the plant's water needs for its point in the life cycle, they can then work back and figure out how much of their available water they can use on that particular field in their strategic irrigation plan. Glodt said growers should manage their water applications considering a targeted percentage of evapotranspiration, or ET.
"If you can only put on 200 gallons per minute and you're trying to grow 60 acres of cotton, your percentage ET is different than the guy with a well at 400 gallons per minute," he said. "So, you tailor your irrigation to what you can handle with your irrigation system. You could still have that 60 acres on a 200 gpm well, but maybe you plant just 30 acres of cotton and you water twice as often.
"Think about how much water you apply in each irrigation pass," Glodt said. "If you are applying less than three-quarter of an inch of water per irrigation, slow the pivot down and put on more water, even if you have to water less frequently." If the field is at the bloom stage, where cotton uses up to .2 inches of water per day, and the farmer is only applying one-half of an inch of water and he's losing half of that water, that leaves .25 inches of water available to the cotton plant, Glodt explained. "That's not even meeting the demand for that day's ET," he said.
He gave the example a cotton field at July 4, where a farmer has finished his pre-watering and the soil profile is filled. "If that cotton plant is using .2 inch per day for 6 days, at July 10 we'll have used up 1.2 inches of water," Glodt said. "If we irrigate at 60 percent ET, that's 60 percent times 1.2, which gives us .96 inches. So we'll put on one inch.
"Irrigating like this, tracking your water based on ET demand is getting the most yield from the cotton," Glodt said. "As you start, the crop will use .2 to .25 inches of water, and as the cotton gets more flowers the demand goes up .3 inch per day, times 6 days, that's 1.8 inches."
Of course, if that grower has selected a cotton variety that has a high WUE, and he can bank a little bit of rain, then he'll get a bump in production.
Glodt said seven years of studies into WUE and variety selection has changed the way he thinks about cotton production.
Glodt said that growers all too often start out their cotton selection process by looking over variety trial results that usually offer the highest yields according to their maturity characteristics on the best water possible.
"I ask you, though, would that highest yield variety be the highest if it was irrigated at a reduced level?" Glodt asked. "Some will and some won't. Some are terribly ineffective. Some respond poorly to water." He said it's just as important to look at WUE, or pounds of cotton produced for the amount of water used on the field.
"Some varieties are just more water efficient than others, and the top variety may not be the top performer when irrigation levels are reduced," Glodt said. Many cotton companies are starting to look to WUE as a standard in their variety results now, he added. Growers who are considering a deficit irrigation regime should really select those varieties with the potential to perform at their peak under limited irrigation.
"Lots of people think WUE will mean the cotton will make more with less," Glodt said. "But WUE varieties make more with what you have, what's free like rainfall." It's a switch in mindset that irrigated cotton doesn't mean throwing all the water a farmer has available to him on the crop.
"Learning how to bank water into the soil, then applying only so much over time at a percentage of ET, if a farmer can learn that then he's going to yield more product," Glodt said.
Jennifer M. Latzke can be reached at 620-227-1807 or firstname.lastname@example.org.