A little wilting early in the season isn’t the worst thing for cotton. In fact, it’ll probably make for a more robust plant later on.
“Based on our experience, cotton that is stressed early in the season actually does a little bit better later in the season,” says Glen Ritchie with the University of Georgia Soil and Crop Science Department. “You don’t have to use as many PGR’s, it’s going to be less prone to late water stress because, it’ll have a more extensive root system and less above-ground vegetative growth, and it’ll have a more robust vascular system as well.”
Ritchie talked about cotton water use during the recent Beltwide Cotton Conferences held in Atlanta.
As water efficiency becomes increasingly important, cotton producers need to know when a plant needs water the most, and when it doesn’t, says Ritchie.
“When it comes down to it, even in Georgia, which has a pretty humid climate, we’re using much more water for agriculture than for home use. So anything we can do to be as efficient as we can with water, but continue to have good crop production and good quality, is important,” he says.
In Georgia, about 1 inch of water is going to cost between $12 and $20 to pump, and different amounts of water are used at different times, says Ritchie.
“There also are decisions to be made about different types of irrigations systems to be installed or the type of systems that you already have installed,” he adds.
When there is inadequate water going to the cotton plant, the plant is going to invest more energy into producing fruit because that’s where the plant will find water, explains Ritchie.
Shooting for a balance
“Therefore, the shoots and the leaves will decrease their growth. And, depending on drought stress and the time of year, you can have increased or decreased retention in specific nodes on the cotton plant. If we have adequate water, we have much better balance between the roots and the top of the plant, and our fruit production tends to be limited by the next limiting factor, whether it’s fertility or something else,” he says.
But if there is excessive water, other problems arise, says Ritchie. “Our root system doesn’t develop as well because the plant doesn’t sense there’s any real need to invest in the root system. We tend to have taller plants, a higher leaf area index, and we tend to have decreased fruit retention earlier in the season.”
If there is too much water, then there will be excess vegetative growth, says Ritchie.
“If you have adequate water, the plant still will be actively growing during the production season, and you’ll have a much easier time controlling the growth chemically.
“You can skip some of that chemical control if you supply inadequate water. The plants generally will be shorter with less leaf area. They’re also going to have less photosynthesis, and you can run into fruit shedding. Unless you really have something that’ll make up for that period of drought, overall, you’ll have less yield potential if you’re unable to get adequate water to the plant.”
Leaf water content is going to affect several functions of the leaves, says Ritchie, primarily orientation, expansion, and the structure within the leaf. It’s also going to affect the opening of stomates and transpiration.
“If you decrease transpiration, that’s great from a water-saving standpoint for the plant, but from the standpoint of plant temperature, it’s bad, and from the standpoint of incoming carbon dioxide, it’s bad. The plant can’t really do much when it is water stressed, and it’ll cause a lot of other problems.”
With water stress, leaf expansion immediately stops and photosynthesis will eventually slow down and stop, says Ritchie. Early in the season, plants are going to be more prone to leaf wilting and in showing water stress than later in the season.
“The nice thing about having these stress signals is that now we have a way of knowing what the plant is doing during the season and hopefully be able to fix it in a timely manner.”
Once a grower knows when to water, then he needs to focus on making it as efficient as possible with irrigation, he says.
Water lost several ways
“Even with pivot irrigation, you have a lot of sources of water loss. You can lose water through evaporation, drift, runoff and percolation. All of these factors will affect how much water you end up with in the soil profile. The other challenge with pivots is that you’re going to have a part of your field that’ll be difficult to water. If you don’t have an end gun, you’re looking at having only about 79 percent of your field irrigated. When your irrigation covers the whole field, you might run into the hassle of the hose.”
With drip irrigation, there are a different set of challenges, says Ritchie. “The nice thing with drip irrigation is that you can put the water right into the rooting zone so you have less water lost through evaporation. The challenge is putting the drip tape right on or near the surface. But you can essentially fit drip irrigation to your field, and then you have the potential to irrigate the entire field.”
Another technology is using precision irrigation with variable rate technology so that water can be cut off in some areas and increased in others, he says.
But just because a plant is water stressed during a certain point in the season doesn’t mean that it can’t recover, says Ritchie. That’s especially applicable to Georgia, with a normally humid climate and sandy soils in some locations.
Several irrigation strategies have evolved from University of Georgia research, he says.
“First of all, early wilt, early in the season prior to first square, is going to occur in wet soils. Mild to moderate wilt, in the seven years of the Georgia study, have not decreased the yield when it shows up in the early part of the growing season. If you get heavy wilting, you might want to consider putting on some water. But in the time we’ve been here, that has been rare.”
Peak water use, says Ritchie, is going to be plus or minus two weeks from peak bloom. “If you’re shooting for three bales of cotton, and if you assume that for every 400 pounds of dry plant matter, you’re going to have 400 pounds of water, it runs at about 18.5 inches, which is a pretty good ballpark estimate. Our Extension recommendations generally have been that 18 to 20 inches of rain is going to be the minimum amount of rain/irrigation you can work with.
A more important thing is the timing, soil type, wind, rainfall — all of these things will affect how much water the plant uses. Our Extension recommendations are for early on, about 6/10-inch per week; prior to first flower, about 75/100-inch; and then during the middle of the season, you’ll be as high as 2 inches per week. That’ll probably be higher in drier climates, but in Georgia that follows pretty well.”