Irrigated Sunflowers in Northwest Kansas: Productivity and Irrigated Sunflowers in Northwest Kansas: Productivity and Canopy Formation Canopy Formation

Summary Sunflower was grown in a three-year study (2009, 2010, and 2012) at the Kansas State University Northwest Research-Extension Center at Colby, KS, under a lateral move sprinkler irrigation system. Irrigation capacities were limited to no more than 1 inch every 4, 8, or 12 days but were scheduled only as needed as determined with a weather-based water budget. Achene (sunflower seed) yields and oil yield generally plateaued at the medium irrigation level. Dormant preseason irrigation increased achene yield and oil yield by 2% with most of this increase occurring in the extreme drought year, 2012. The optimum harvest plant population for sunflower in this study in terms of achene yield and oil yield was approximately 19,000 to 20,000 plants/a.


Introduction
Sunflower is a crop of interest in the Ogallala Aquifer region because of its shorter growing season and thus lower overall irrigation needs. Sunflowers are thought to better withstand short periods of crop water stress than corn and soybeans, and the timing of critical sunflower water needs is also displaced from those of corn and soybeans. Thus, sunflowers might be a good choice for marginal sprinkler systems and for situations where the crop types are split within the center pivot sprinkler land area.
Center pivot sprinkler irrigation (CP), the predominant irrigation method in the Ogallala region, presents unique challenges when used for deficit irrigation. Center pivot sprinkler irrigation cannot be effectively used to apply large amounts of water timed to a critical growth stage as can be done with surface irrigation methods. The CP systems also cannot efficiently use small, frequent events to alleviate water stress as is the case with subsurface drip irrigation (SDI). Thus, with CP systems, it is important that available soil water in storage be correctly managed temporally in terms of additions and withdrawals so that best crop production can be achieved both economically and water-wise. Three easy ways to control irrigation water additions are irrigation capacity, preseason management, and the season initiation date. Withdrawals can be partially managed by plant population. This study examined sunflower production using the three methods of controlling irrigation additions for three different targeted plant populations.

Procedures
The study was conducted from 2009 through 2012 at the Kansas State University Northwest Research-Extension Center (NWREC) at Colby, KS, under a lateral move sprinkler irrigation system. However, data from 2011 are excluded due to a devastating hail storm that destroyed the crop. Key agronomic characteristics of the annual tests are shown in Table 1.
Whole-plot treatments were sprinkler irrigation capacities of 1 inch every 4, 8, or 12 days as limited by evapotranspiration (ET)-based water budget irrigation scheduling. An additional whole-plot irrigation factor was the addition, or no addition, of dormant preseason irrigation, resulting in a total of 6 different irrigation treatments. The target preseason irrigation amount for those plots receiving this treatment was 5 inches, but in 2012 a total of 9.2 inches of preseason irrigation was applied due to an application error. Three targeted plant populations 18,000, 23,000, or 28,000 plants/a were superimposed on the whole plots for a grand total of 108 subplots. Irrigation amounts were 1 inch applied as needed, but limited by the imposed capacity and the water budget irrigation schedule. The whole plots (6 repetitions) were in a randomized complete block design.
Soil water was measured periodically in each plot each crop season with a neutron probe to a depth of 8 feet in one foot increments. Crop water use was calculated as the sum of changes in soil water between emergence and physiological maturity, precipitation and irrigation amount. Crop water productivity (WP, also known as water use efficiency) was calculated as the achene yield in lb/a divided by the total crop water use in inches.
At R6 development stage and to maturity (R9 development stage), sunflower achene moisture content, dry mass, and oil content were measured by collecting six achenes from each of five representative plants, semi-weekly. At maturity, sunflower heads were hand harvested from a representative sample area and threshed for yield and yield component.
Leaf area index (LAI) was quantified, approximately bi-weekly, by a non-destructive light transmission technique (Welles, 1991;LAI-2000 Plant Canopy Analyzer;Li-Cor, Lincoln, NE). Three sets of four below-canopy measurements were each referenced to an above-canopy measurement, minimizing sensor exposure to direct (beam) irradiance. Readings were screened against apparent transmittance ratios exceeding 1 using the manufacturer's software, FV2000 (Li-Cor, Lincoln, NE). An inverse solution to a model of light transmission through a vegetative canopy, provided by the manufacturer, was used to quantify apparent LAI.
Growing degree days (GDD) were calculated from daily temperature extremes (Equation 1) recorded at the NWREC weather station, using a mercury thermometer.

GDD = T max -T min -T b Equation 1 2
Upper and lower limits to temperature extremes were 34°C and 4°C (93°F and 39°F), respectively. Cumulative GDD (cGDD) was computed by summation of GDD, commencing from planting date.
Statistical analysis used analysis of variance (ANOVA) and analysis of covariance (AN-COVA). Repeated measure of LAI and maximum LAI observed in a year were analyzed by ANOVA, using Proc GLM from SAS 9.4 (SAS Institute Inc., Cary, NC). Seasonal trends in LAI were analyzed by ANCOVA using third-order linear terms of cGDD or days after planting (DAP) as covariates.

Weather Conditions
The crop year 2009 was very cool and wet and irrigation needs were low. In-season irrigation amounts for the 1 inch every 4, 8, and 12 days' treatments were 7.68, 6.72, and 4.80 inches, respectively. During the period April through October, every month had above-normal precipitation and between crop emergence and crop maturity the total precipitation was 9.89 inches.
The early portion of the crop year 2010 was wet, and irrigation needs were lower than normal. However, later in season, it was extremely dry, with only 1.08 inches of precipitation occurring between August 4 and crop maturity on October 11. Precipitation during the sunflower growing period totaled 7.32 inches. In-season irrigation amounts were 11.52, 6.72, and 4.8 inches for the irrigation capacities limited to 1 in./4 d, 1 in./8 d, and 1 in./12 d, respectively. The 2010 sunflower irrigation amounts appear to be approximately 1 inch less than normal as estimated from long term  irrigation scheduling simulations conducted at Colby, KS.
Extreme drought conditions existed for all of 2012, and only 5.25 inches of precipitation occurred during the sunflower growing period. Additionally, temperatures of 100°F or greater occurred on 20 days between June 26 and August 15. Crop establishment may have been negatively affected by excessively hot temperatures (99 to 104°F) that occurred for the entire period between planting and emergence even though small amounts of irrigation kept sufficient amounts of water in the seed zone. Sunflower plant populations at harvest in 2012 averaged approximately 75% of levels that occurred in 2009 and 2010. In-season irrigation amounts were 13.94, 8.18, and 6.26 inches for the irrigation capacities limited to 1 in./4 d, 1 in./8 d, and 1 in./12 d, respectively.
Summarizing the weather conditions, the crop year 2009 was cooler and wetter than normal, the crop year 2010 was approximately normal, though a severe drought began in early August, and the crop year 2012 was extremely hot and dry.

Crop Yields and Yield Components
The addition of dormant preseason irrigation did not significantly increase yields in any of the three years (Tables 2, 3, and 4), but it did increase achene yield and oil yield by 2%, when all years were analyzed together. Most of the increase in yield for preseason irrigation occurred in the extreme drought year, 2012. Preseason irrigation did significantly increase heads per plant in 2009 and harvest plant population in 2010, but these differences were only about 3% greater. There were no statistically significant differences in yield attributable to irrigation capacity in 2009 and 2012, but increased irrigation capacity did increase achene yield in 2010. Increased irrigation capacity tended to nu-merically increase achene and oil yield in all three years up through the 1 in./8 d irrigation capacity but tended to have less or no response above that level (Figure 1). Achene yields were lower in 2010 than in 2009 and 2012, but still were towards the upper range of yields for the region.
There were no plant population effects on achene yield in 2009, but increased plant population decreased achene yield in 2010 and increased achene yield in 2012 (Tables 2, 3 (Tables 2, 3, and 4). Increasing plant population significantly decreased achene mass and significantly increased achene oil content (percentage) in all three years. Within a given year average differences in oil content ranged from 1 to 2% as affected by plant population. Harvest plant populations above 19,000 to 20,000 plants/a resulted in reduced achene yields and oil yields, but oil content was greatest at the greatest plant population in all three years ( Figure 2).

Crop Water Use and Water Productivity
In-season crop water use was significantly greater due to increased irrigation in all three years (Tables 2, 3, and 4, and Figure 1). However, crop water productivity (WP) was significantly reduced by increased irrigation in all three years. Irrigation amounts ranged from 4.80 to 7.68 inches in 2009, 4.80 to 11.52 inches in 2010, and 6.26 to 13.94 inches in 2012. Soil water depletion decreased with irrigation capacity (data not shown).

Canopy Formation
Seasonal changes in sunflower canopy are shown in Figure 3. Preseason irrigation amounts of 9 inches resulted in greater leaf area from mid-vegetative growth through mid-seed fill in 2012. Canopy formation and senescence occurred relatively earlier in 2010 than 2009 and 2012, which were similar. Canopy formation was greatest in 2010 and least in 2012.

Yield Formation
Achene water content, oil content, and dry mass changes during the season are shown in Figure 4. Achene water contents were greatest for the initial sampling dates and declined throughout the seed fill period. In 2010, achene water content was slightly greater for the largest irrigation capacity. Oil content of achenes increased from the R6 to R8 development stage, remaining consistent through maturity; slightly greater oil contents were observed for the smallest irrigation capacity in 2010. Oil contents from late-season samples appeared similar, though the harvest samples from a larger sampling area (Tables 2, 3 ing differences in achenes per head. Cumulative growing degree days appear to provide an inconsistent measure of time relative to onset and completion of the yield formation periods, as indicated by the staggered onset and duration of sampling intervals over the three growing seasons (Figure 4).

Conclusion
Sunflower was grown under sprinkler irrigation in Colby, KS, for three very different crop years (2009, cool and wet year; 2010, near normal overall but very dry after flowering; and 2012, severe drought year with high temperatures). Irrigation capacities were limited to not more than 1 inch every 4, 8, or 12 days, but irrigation events were scheduled only as needed as determined with a weather-based water budget. Seasonal trends indicated earlier canopy formation, greatest canopy extent, and earliest senescence in 2010; least canopy extent developed in 2012. Seasonal trends were similar for achene water content (decreasing through maturity), oil content, and achene mass (increasing through R8 development stage). Achene yield was only statistically increased by irrigation in 2010, but tended to increase numerically up through the medium irrigation level (1 in./8 d) in all three years. Similarly, oil yield plateaued at the medium irrigation level. Dormant preseason irrigation increased achene yield and oil yield by 2%. The optimum harvest plant population for sunflower in this study in terms of achene yield and oil yield was approximately 19,000 to 20,000 plants/a.  Shaded items within a column are significantly different at P < 0.05 when followed by a different lower-cased letter.