Declining irrigation capacity, high energy costs, varieties with biotechnology traits, new irrigation systems, and other changes require that cotton farmers use the most efficient water management techniques available to produce acceptable yields of high quality fiber.
“Adequate soil moisture at planting is an important component of water management as it enables cotton plants to establish a more effective root zone early in the growing season,” says Jim Bordovsky, irrigation engineer with Texas AgriLife Research at Halfway.
“Significant yield reductions can occur when the soil profile is dry and preplant irrigations are not applied. On the other hand, our research has shown heavy preplant irrigations fail to increase yield significantly over treatments where adequate in-season irrigation was initiated early in the crop year.
“Preplant irrigation represents an attempt to ‘bank’ water in the soil profile prior to planting for later use in fields where in-season irrigation capacity cannot meet the evaporative demand of the crop,” Bordovsky says. “However, high wind speeds and low relative humidity can cause severe water losses during preplant periods.”
Bordovsky studied preplant soil water losses over a three-year period. “We found the combined losses of irrigation and rainfall were 67 percent, 60 percent, and 47 percent for LESA (low elevation spray application), LEPA (low energy precision application), and SDI (subsurface drip irrigation) treatments, respectively, over a 45 day period from mid-March to the first of May. These losses were from evaporation and from water movement below the root zone,” Bordovsky says.
“Based on this limited study, our recommendations for preplant irrigation with different irrigation systems are as follows:”
Unless the tape is relatively close to the surface, applying sufficient irrigation to ensure seed germination in dry years may be very difficult, if not be impossible, with some SDI systems. “Based on our study, attempting to fill the soil profile resulted in water losses below the root zone, especially when significant rainfall occurred late in the preplant period. And these efforts generally failed to increase lint yield compared to treatments receiving less preplant irrigation.”
“The reason filling the soil profile with enough water to wet the seed zone is difficult is because, in some soils, water does not ‘sub-up’ readily from the irrigation tape,” Bordovsky says. “However, the odds of getting enough timely rainfall to establish a stand are very high. Therefore, if irrigation capacity is less than 0.2 inches per day, applying water to fill approximately 75 percent of the root zone below the tape might be the best approach.”
“At low irrigation capacities, apply sufficient irrigation to fill approximately 75 percent of the effective root zone just prior to planting is a good approach. This should be done with multiple pivot passes in a wedge of the field in quantities that do not cause runoff. Furrow diking will significantly increase the amount of water that can be applied without runoff.”
The effective root zone, or the effective water holding volume of both LEPA and alternate row SDI soil profiles, is comparatively small because water is applied in alternate furrows and tends to move down instead of laterally. As a result the effective root zone of a LEPA-irrigated field may be only half that of a LESA-irrigated field.
“For example, if the water holding capacity of a 4-foot-deep soil is 2 inches per foot of depth, the effective root zone of a spray-irrigated field could hold 8 inches of water, whereas a LEPA-irrigated field could hold only 4 inches,” Bordovsky says. “Irrigations in excess of the effective water holding capacity may be lost below the root zone.
“Our test results showed with irrigation capacity of less than 0.2 inches per day, failing to fill the effective root zone of LEPA treatments prior to planting resulted in significant yield losses over the three-year test period.”
In general, a field with LESA irrigation should be furrow diked and preplant irrigations started after April 1.
“In low irrigation capacity areas, adequately irrigate (bring to 75 percent of field capacity) one-quarter to one-third of the pivot area by ‘windshield wiping’ the area, applying the largest irrigation possible per pass without causing runoff,” Bordovsky says. “When this area is adequately watered, irrigate the next pivot quadrant using the same application plan.
“As soon as an area is reasonably dry, mulch it with a rotary hoe or sand fighter. This will minimize soil moisture loss and will significantly lower the probability of wind erosion.”
Additional information on Irrigation/Water is available at website http://lubbock.tamu.edu/
Results of the annual soil moisture survey made by the High Plains Underground Water Conservation District 1 may be helpful in making preplant irrigation decisions. The report may be requested from Carmon McCain at mailto:[email protected]/, or by phone: 806-762-0181.