Interest in converting furrow and sprinkler-irrigated cotton to drip systems has increased substantially over the past few years as water availability has become more of an issue.
Additionally, the shift away from cotton in favor of permanent crops, vegetables and forage crops has relegated some of the remaining cotton acreage to fields with less stable water supplies as well as soils that are less than ideal.
Researchers have made tremendous strides in understanding the nuances of drip irrigation in cotton production schemes over the years, and how the system can best be managed to maximize the advantages it offers. Switching to a drip irrigation system changes the way water and nutrients are delivered to cotton plants and can have significant impacts on various plant growth processes, according to Bob Hutmacher, UC Extension agronomist/state cotton specialist.
At the Central Coast Cotton Conference, Hutmacher outlined some of the factors to consider when putting cotton into a drip system.
“A change to a drip irrigation system can relieve problems such as water stress associated with variable soils, sand streaks and compacted areas,” he pointed out. “It allows more frequent application of nutrients directly to the active root zone. Additionally, in some soils, drip irrigation helps avoid aeration problems by using small, frequent applications of water versus larger volumes of water applied by furrow or flood.”
The physiological response of the cotton plant is not necessarily changed when irrigation is switched over to drip, but the method of delivery of water and nutrients brings new considerations into the management mix.
“Part of the potential with these types of systems is to be able to avoid water and nutrient stresses to have a more sustained type of growth and more manageable plant,” he says. “Some of the situations where these types of systems can be beneficial are in soils where physical or chemical characteristics make water infiltration difficult. It also certainly has the potential to reduce runoff and evaporation losses if managed correctly.”
There are two concepts about how stress avoidance due to higher frequency irrigation can impact shoot and root development, according to Hutmacher.
“If you avoid deep cycles of severe soil water deficits occurring with long irrigation delays, you will maintain more uniform and continuous new shoot and root growth in late season,” he says. “That results in the potential to extend the fruiting cycle and improve yields.”
However, yield improvements are generally only realized when fertility levels are also maintained, and factors impacting fruit retention are under control such that photosynthate and nutrients are also directed to fruit.
In the second scenario, stress avoidance can be beneficial due to higher frequency irrigation during more severe high temperature periods which can mitigate some temperature effects on fruit retention and yield potential.
There are several physiological processes that are important to consider with a drip system due to the way plants respond to changes in how water and nutrients are delivered. Root function is particularly critical.
“You want the tissue to remain hydrated and functioning so you will avoid severe water stress that will have an impact on the plant's growth,” Hutmacher says. “Roots also have a function as a method for evaporative cooling. Also, some soluble nutrients such as nitrate can be moved up into the plant along with the water.”
There are many factors that can impact where roots grow, how fast they grow and how long they will be retained and active. “Temperature can have a big impact,” Hutmacher says. “That really comes into play early season. Water availability is another — where it's available and replenished. The aeration status of the soil — the relative balance of how favorable those conditions are for root growth are things that you can impact with a drip system.”
Root distribution under a drip system can be altered fairly dramatically and is one factor in particular that growers should be aware of when switching from furrow or sprinklers. It's important to realize that cotton has a taproot and does not tend to develop a lot of secondary roots that “look” for water and nutrients in response to irrigation. Knowing where the water is being delivered in relation to where the roots are is critical in a drip irrigation scheme.
“Studies have shown over the years that while you can have a fairly widely distributed root system with drip, you tend to have a fairly concentrated root volume with typical drip irrigation systems and management,” Hutmacher says. “It's important to keep in mind what happens with less soluble nutrients that don't move as readily with water. You need to have roots to intercept those nutrients. You can rapidly mine what's in the soil if you're not replacing it.”
High frequency irrigation with less amounts of water can have potential benefits in drought situations. Cotton plants that are “used” to that situation often perform better than cotton plants that are programmed to receive larger amounts of water at one time. However, interruptions in water can result in quicker, more severe stress on plants that have become used to frequent, short irrigations.
“Plants can tolerate a moderate amount of water stress,” Hutmacher says. “A grower may want to consider going for slightly reduced yields when water gets tight.
Across a number of years of studies we've looked at total evapotranspiration — including total water use — versus lint yield response. There are opportunities to back off some on total evapotranspiration under drip irrigation systems and maybe look for a little bit better balance between total water use and yield.”
Water savings from a drip irrigation system vary tremendously depending on soil types. Hutmacher says the savings have been most pronounced at the Shafter Research Station where sandy loam soils are prevalent and cotton is typically more shallow-rooted.
“We've had savings up to 30 percent in water on those types of non-uniform soils that have terrible late season infiltration,” he says. “We've seen tremendous improvements in both yield and water savings with drip as compared to furrow irrigation. Some of those fields under typical furrow irrigation would be getting in excess of 40 inches of water to try to keep them reasonably low on stress.”
At the Westside Research Center, Five Points, Calif., where soils are clay loam soils, water savings from drip irrigation have generally been much lower — in the neighborhood of 5 percent to 10 percent. Knowing where drip could potentially provide the greatest benefit and understanding how drip alters the delivery of water and nutrients to the plant are both critical to success.
“There certainly is a physiology behind the type of responses plants have to the ability to change where you deliver the water and how frequently you deliver it,” Hutmacher says. “Some of those are not necessarily unique to drip, but they definitely need to be taken into consideration when switching to a drip system.”