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Research Highlights

Research Highlights
Southern Soybean Research Program

By Dr. Ole Wendroth

Soil and Crop Irrigation Management Kick-off Meeting held for a 4-State Initiative in the Southern United States

Avoiding risks in crop production has become relevant with the needs for high yields and high quality products for grain and especially soybean growers in the southern United States. One of the major risks is sufficient water supply. This risk even exists in regions where the total annual amount of rainfall seems sufficient to grow a high yielding crop due to the unpredictability of rainfall patterns. Therefore, farmers in humid regions increasingly rely on irrigation.

Irrigation decisions require knowledge of when and how much water to apply based on crop and soil information in order to stay economically feasible and environmentally sustainable. In general, farmers or farm managers can only spend a very limited amount of time – often less than a minute per day and per pivot – in order to make the appropriate decisions on whether or not to turn it on or wait another day, and how much they are going to apply.

Under the existing soil conditions in the southern U.S., farmers know that the soil underneath a single pivot unit can be extremely spatially variable. This fact adds another layer of complexity to irrigation decisions: “where” to irrigate in addition to “when” and “how much” because the right amount of water in one spot of the field at a particular time can be the wrong or non-optimum amount or timing in another zone. For instance, water applied to avoid water stress in a coarse-textured soil location with good water infiltration and vertical redistribution in the root zone can cause surface runoff or even temporary air stress for the roots growing in a part of the field where the surface soil is more clay­ey. In order to address these situations, variable rate irrigation technology has been developed to site-specifically manage water applications similar to variable rate fertilizer application which is more familiar to producers. In other words, the amount of water delivered by a pivot can be varied by speeding up or slowing down a system and by pulsing the sprinklers on and off down the length of a pivot.

The day-to-day decision on site-specific water application in the field is certainly not a routine inasmuch as natural rainfall during the growing season can change the spatial distribution of water in the field, and therefore the initial condition at the time when irrigation water is applied. Hence, if the pivot system is designed with variable-rate or site-specific water application technology, the decision on rate and timing of irrigation water supply is even more complex and requires more in-depth management.

Variable-rate irrigation now faces the same phenomenon ex­perienced in variable-rate nitrogen application: The technology is available to apply water site-specifically, but the concepts helping the farmer or land manager to make a quick decision need to be developed. A lot of research is needed to develop recommenda­tions that assist the farmer to make these decisions. These deci­sion schemes will be based on detailed knowledge on the spatial variability of functional soil properties and the current status of the crop growth and the inherent spatial distribution of soil mois­ture in the field.

A group of scientists – George Vellidis and Wesley Porter from the University of Georgia, Brian Leib with two graduate students and Dave Verbree, University of Tennessee, and Ole Wendroth, Chad Lee, Carrie Knott, Lloyd Murdock, University of Kentucky (Missouri will join this group) met at the University of Kentucky College of Agriculture in July to review the state of existing knowl­edge and current practical state-of-the-art of irrigation technology and to derive research needs. The purpose of this meeting was to discuss the general background and details for a four-state-pro­posal that will be developed and then submitted to the Southern Soybean Research Program (SSRP). The SSRP provided support that allowed this group of scientists to meet and have two days of getting to know each other, fruitful discussions, and develop a plan for the proposal submission. SSRP Vice-President Jonathan Miller from Island, and SSRP Manager Becky Kinder of the Ken­tucky Soybean Board attended this meeting on the first day and gave a helpful overview on the expectations from SSRP.

The participants at this meeting come from different climatic regions within the southern U.S. which also represent a variety of different soil conditions and which will be a great advantage in this joint research effort. The other big plus of this group is the fact that a broad range of expertise in the fields of crop science, crop physiology, agricultural and environmental engineering and soil science will join together. Within the participating states, the level of experience in irrigation differs currently due to the fact that for example Kentucky soybean growers have recently begun to establish the option of irrigating their fields mainly in the past few years while growers in the other states have already a longer experience with irrigation technology.