Sunflower Water Use Relationships
How much water does sunflower use for its vegetative growth and seed development? And how does that use level affect final yields? Those are bottom-line questions for any sunflower producer - and especially for those in areas which are chronically short on moisture, such as the Central Great Plains.
Around the northeastern Colorado community of Akron, rainfall during the sunflower growing season averages about nine inches. Yet the amount of water required for sunflower to grow without moisture stress is about 34 inches. Some of that 25-inch shortfall is made up for by stored soil water. Still, dryland 'flowers in this area are almost always growing
under a water-deficit condition - which, in turn, lower final yields.
Research reports of a sunflower crop's water use have varied widely - all the way from eight up to 38 inches per year. The higher values come from irrigated studies or high rainfall environments. Sunflower grown where the soil water profile is full also will use greater amounts of water.
Sunflower has, as well, the ability to extract water from deeper in the soil profile than other crops and likewise can extract more water from each soil layer than other crops. As Figure 1 illustrates, proso millet in the Central Great Plains will extract about four inches of water from the top three feet of soil, whereas sunflower extracts about 7.5 inches from the six-foot profile.
Of course, one's crop rotation sequence will make a difference as to how much soil water is available for use by sunflower (Figure 2). For example, sunflower following millet in a sunflower/fallow/wheat/millet rotation uses more water than does sunflower in a sunflower/fallow/wheat rotation. Why? Because more water is typically available to sunflower following millet than to 'flowers after wheat. At Akron, for instance, sunflower soil water use ranges from three to nine inches, depending on the cropping system and year.
What is the relationship between water use and sunflower yield? Figure 3 shows that about 150 pounds of seed are produced, on average, for every inch of water use after the first seven inches.
Depending upon available water, dryland sunflower yields at the USDA-ARS station near Akron have varied greatly over the years: from a mere 100 pounds per acre up to 2,000. Sunflower at Akron is very responsive to rainfall received during August and into September - starting just prior to flowering and continuing through seed fill. The relationship for yield versus rainfall (i.e., 158 pounds per acre per inch of water) is almost identical to that of yield versus total water use (151 pounds).
Figure 4 illustrates predicted dryland sunflower yield distribution for the Akron area over a 33-year period, based on the water use/yield relationship (Figure 3) and the Akron rainfall record for 1965-1997. It assumes a conservative 6.75 inches of soil water use. The result is a predicted yield distribution range from 570 to 2,090 pounds per acre, with an average yield of 1,360 pounds. Nearly half the years of record would suggest yields in the range of 1,000 to 1,500 pounds per acre.
The typically very dry condition of the soil profile following sunflower in the Central Great Plains calls for significant water recharge prior to the planting of the rotation's next crop. That's where standing sunflower stalks play a vital role.
Common sense says that tall standing sunflower stalks will trap more snow than short or flattened stalks. Several years of USDA-ARS research at Akron have documented this contribution by quantifying the value of what's termed the "silhouette factor."
The silhouette factor is the value resulting from sunflower stalk height times stalk diameter times population. Overwinter soil water recharge is closely related to the silhouette factor due to the increase in snow trap that occurs with taller stalks at higher populations.
Figure 5 illustrates this silhouette factor effect. It uses a population of 14,000 stalks per acre (3.57 stalks per 1,600 square inches), averaging one inch in diameter, with an average height of 16.7 inches. This stalk population would have a silhouette factor of 60 and would be expected to recharge the soil water profile by about four inches, given average winter conditions at Akron.
Sunflower stalks averaging 30 inches in height can, depending upon the population and stalk diameter, trap enough snow to add up to nine inches of soil water.
The other major benefit of tall standing stalks is soil erosion control- especially during March through May, which is the most erosive time of year in the Central Great Plains due to high winds and lack of snow or crop cover. For instance, in a field with 16,000 sunflower stalks per acre at an average diameter of two-thirds inch (measured at about two inches below the top of the stalk), a stalk height of 30 inches can triple the silhouette factor and end up reducing wind speeds (measured at eight inches above the soil surface) by more than 25 percent.
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