What is in this article?:
• Cotton that is stressed early in the season actually does a little bit better later in the season,”
• As water efficiency becomes increasingly important, cotton producers need to know when a plant needs water the most, and when it doesn’t
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.”