Irrigation timing: Success or failure

Too-late first application can lead to early cutout of cotton crop The timing of your first irrigation can set you up for success or failure, according to Tom Kerby, vice president, technical services, Delta and Pine Land Co.

Kerby, speaking at the 2000 Bootheel Irrigation Conference and Trade Show in Portageville, Mo., said a timely irrigation can provide cotton with the manufacturing plant (leaf structure) needed to support and feed the boll load, while a too-late irrigation can lead to early cutout.

Late-maturing varieties can handle more water stress early on, according to Kerby. But there's always the possibility that if you water those varieties too early and often, you could end up with too much plant and not enough fiber.

"In cotton, you have a conflict," Kerby explained. "You have light capture and dry matter production, then as the boll load comes on, it's almost like it cannibalizes the capacity of the canopy to produce. So you're always walking a tightrope."

One question growers struggle with is how much they should stimulate the development of the leaf area. A lot of that depends on geography, noted Kerby. "In the northern part of the Cotton Belt, where you have a short-growing season, there's no reason to stimulate that leaf area beyond some point because it would produce bolls you can't pick.

"The object is not how many bolls you have at the end of the season, it's how many you have open," Kerby said. "You have to moderate your thinking based on planting date, early-season temperatures, growth rate and fruit retention."

Kerby noted that growers who plant early plant types need to pay a lot of attention to establishing leaf area, because "an early plant type has the tendency to put on a very heavy boll load and not produce a lot of leaf area."

Kerby said an incomplete understanding of how to manage these early-season varieties could be behind the so-called flattening-out of yield over the past few years.

"They can produce a lot real soon, but if you have a long-enough season, they can't take advantage of it. On the opposite end of the spectrum, we have plant types that can tolerate some stress and maintain some leaf development because they don't produce a lot of early fruit."

No doubt, the size of their manufacturing plant is crucial to maximizing a variety's yield potential, according to Kerby. "The early growth of the leaf area is going to determine how much of a boll load you can carry before the plant starts shutting down."

Producers should also be aware of when cotton is most sensitive to water stress.

"If you have any water at all, you're going to get a stand," Kerby said. "From emergence to square, it's very rare for there not to be adequate water and in cases where the plant has been stressed at this stage, there's practically no long-term effect.

"Part of the reason for that is the leaves in the lower part of the canopy not next to fruiting branches are not the leaves that are going to be feeding the bolls. The sensitivity of cotton to stress really begins when you have fruiting branches and you're dealing with the leaves that are adjacent to those fruiting branches."

That means square to peak bloom is the most sensitive period for the impact of water stress on yield in cotton. "It's that stage that it's determining the leaf area and the activity of that leaf area to intercept light and produce the photosynthesis that's going to carry the boll load."

Kerby has narrowed that period down to between the seven- to eight-leaf stage and around 17- to 18-leaf stage. "Those are the leaves that will produce the energy that feed the bolls. That is the period of time when cotton is most sensitive to stresses that will either shut down additional leaf development or will accelerate the aging of the individual leaves in the canopy of the plant."

In addition, "Once the bloom starts, new allocations of energy to roots are very low. So if you don't have a root system established by first flower, you're not going to establish much of one."

Kerby noted that from peak bloom to cutout (five nodes above white flower), "the plant can tolerate some late-season, late-afternoon wilting. From four nodes above cracked boll, it can tolerate a lot of stress. In fact, as you get closer to the end of the season, you need to have more and more stress on the plant or you'll have regrowth."

While water is an important component of plant growth, it doesn't influence cell division in a cotton plant, except under extremes, according to Kerby. However, it does promote cell elongation and sets the plant up for photosynthesis.

"Photosynthesis in cotton is temperature-dependent," Kerby explained. "We talk about cotton as being this rugged plant that can handle hot temperatures. In reality, it isn't. Cotton's optimum temperature for producing the most photosynthesis in the leaf is somewhere between 80 and 90 degrees. Irrigation will influence the temperature-effect on the leaf. If the leaf is not cool to the touch, then that's a problem."

Kerby noted that advances in monitoring cotton development have greatly enhanced the capability for timely irrigation.

"In the late 1980s, we came to understand these principles and now we know how to go out and size up a cotton plant, decide whether to put our foot on the gas, coast, put our foot on the brake or stop. Monitoring allows us to do that."

Some key considerations in plant monitoring include: "how much height we have for degree days; the boll development rate; and the ratio of height to nodes, which is a very good reflection of the amount of stress the plant is accumulating."

"We've also come up with another tool, the internode distance of the most recently expanded node. It's a great signal as to the current growth rate of the cotton plant. NAWF is also important. The rate at which NAWF declines says how fast the node is moving up and is an indictor of stress.

"Another thing we need to look at is the maximum internode distance. You count four nodes down from the terminal node and it's the distance between the fourth and fifth node from the top of the plant. That is the node that is just completing elongation and it represents the history of the field during the last four to five days. If it's compressed and short, it's indicative of stress. If it's large, it's indicative of very strong growth.

"If you maintain an internode distance of about 2.5 inches, you would be on target to produce about a 35-inch plant by the end of the season. If it was 3.75 inches, you would produce a 45-inch plant at the end of day. Over four, it would be 55 inches."

Kerby also pointed out the major differences between two common irrigation methods used in the Mid-South, furrow and center pivot irrigation.

"Furrow irrigation has a higher application rate and works best under a long drought. It's not so good if you just irrigate a field and you get a heavy rainfall, or during years with high rainfall during flowering.

"Center pivot irrigation has a lower application rate and works best in relatively wet summer and you're using this system to augment or supplement your irrigation.-mail: [email protected]

A NEW regional Pest Management Center, one of four in the nation, has been established at the University of Florida to strengthen connections between agricultural producers and research and education programs in 13 southern states, Puerto Rico and the U.S. Virgin Islands.

"The UF center will improve the link between pest management researchers and farmers and ranchers in the southern U.S.," said Norm Nesheim, professor and pesticide information coordinator with UF's Institute of Food and Agricultural Sciences in Gainesville.

"Our major objective will be to assist the U.S. Department of Agriculture and the Environmental Protection Agency in implementing the Food Quality Protection Act passed by Congress in 1966," he said. "We will focus on a full range of agricultural pests - from insects to rodents."

Three other regional centers will be headquartered at Michigan State University and the University of Illinois for the north central region; Pennsylvania State University and Cornell University of the northeast, and the University of California-Davis for the western region.

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