It's mid-July and your cotton is thirsty for a drink, especially in the afternoon. But applying spray irrigation water during what should be a critical watering period might actually reduce pollen viability and ultimately reduce yields.
When hit with water of any kind, pollen from flowering cotton plants is made sterile in less than a minute according to lab and research field data from John Burke, USDA plant pathologist.
“Our research shows that yields are reduced by 25-50% when even small amounts of water reach inside an open flower on a cotton plant,” says Burke, who's located at the USDA Plant Stress and Water Conservation Laboratory in Lubbock, TX. In lab tests using a microscope, even a single drop of water causes pollen to explode into uselessness in just over 30 seconds.
And it doesn't matter if it's irrigation spray or rainwater; the same results occur when water strikes open flowers. The damage is done. “More than 90% of flowers that were open during sprinkler irrigation were shed within one week of water exposure,” says Burke.
In his studies, funded by Cotton Inc., Burke and his associates documented yield losses throughout the day as a result of sprinkler applications during cotton flowering.
Between 8 and 10 a.m., there is zero yield loss because flowers are usually not yet open. But between 10 a.m. and noon, there is a 29% yield loss; 27% between noon and 2 p.m.; and 36% between 2 and 4 p.m.
As a result, Burke recommends either watering at night or in early morning, or converting center pivots to LEPA (low energy precision application) drop socks.
LEPA prevents pollen damage. “From 10 a.m. to 4 or later, virtually any flower open when the sprinkler comes around is pretty much going to be lost,” says Burke. “But with the LEPA drag socks, the plants still receive water without getting the flowers wet. The socks apply water onto the furrow, below the plants.”
LEPA was developed by Texas A&M ag engineers in the 1970s and perfected in the '80s and '90s. It has consistently provided better irrigation efficiency, offering less evaporation, especially in hot and windy southwestern producing areas.
Texas A&M data show that a LEPA pivot equipped with drag socks running in a field with furrow dikes achieves 95-98% irrigation efficiency. Only 2-3% of the water pumped is lost to evaporation.
Yields have also been higher with LEPA socks. In drought-plagued '98, Texas cotton producers with LEPA systems on average produced ½ bale/acre more than pivots equipped with low-profile spray drop lines, says Randy Boman, Texas A&M extension agronomist in Lubbock.
He says the Burke data is good information that should certainly be studied when determining how yields are impacted by LEPA and other types of irrigation.
He notes that cotton fields normally see a substantial number of flowers naturally fail to perform. “We usually see a large percentage of the flowers that don't make it into a productive boll,” he says. “Incomplete pollination may result in a reduction in the number of fertilized seed, thus resulting in a ‘parrot-beaked’ boll.
“We believe the benefits of LEPA are not just in irrigation efficiency, but that yields are good because this method of watering prevents flowers from getting wet,” Boman concludes. “It's definitely a consideration growers should make in setting up or refitting their cotton irrigation program.”