Relationship Between Water Stress and Seed Yield of Two Drip-Irrigated Alfalfa Varieties

Malheur Experiment Station

Information for Sustainable Agriculture

Relationship Between Water Stress and Seed Yield of Two Drip-Irrigated Alfalfa Varieties

Clinton C. Shock, Erik B.G. Feibert, Grant Tchida,

and Lamont D. Saunders

Malheur Experiment Station, Oregon State University

Jim Klauzer

Clearwater Supply

Ontario, OR 2002

Summary

Two alfalfa varieties ('Tango' and 'Accord') were grown for seed using subsurface drip irrigation with four evapotranspiration (Et_c) replacement levels: 80, 60, 40, and 20 percent of the accumulated water needs. After the start of flowering the alfalfa was irrigated every 3-4 days at the corresponding Et_c replacement level. In the 2002 season, 'Tango' seed yield was optimized at 51 percent of Et_c replacement or 18.9 inches of applied water and 'Accord' seed yield was optimized at 50 percent of Et_c replacement or 18.5 inches of applied water.

Purpose

Past work at the Malheur Experiment Station in the 1980's demonstrated that water stress was associated with high alfalfa seed yields. There is a strategic balance between the amount of water needed to sustain growth and productivity and water stress sufficient for the alfalfa plant to remain reproductive rather than vegetative. Achieving uniform water stress down the length of the field with furrow irrigation is problematic because water application is not uniform. Alfalfa in areas of the field where more water soaks into the soil remains vegetative, while alfalfa in dry areas can become excessively dry. Subsurface drip irrigation applies water more uniformly, allowing for uniform water stress. Subsurface drip irrigation also has environmental benefits compared to furrow irrigation, due to 1) more efficient water use, 2) elimination of deep percolation of water, and 3) elimination of runoff losses of water and nutrients. The purpose of this experiment was to determine the level of deficit irrigation that optimizes seed yield of two alfalfa varieties.

Methods

Alfalfa was grown for seed on a Nyssa silt loam of modest fertility and productivity. The site was chosen to be representative of fields used for alfalfa seed production. The field was previously planted to wheat. Two varieties of alfalfa were planted on April 6, 2000 at 2 lb/acre in 30-inch rows. 'Tango', with a dormancy rating of six, was planted in the upper half of the field and 'Accord', with a dormancy of four, was planted in the lower half of the field. The alfalfa was irrigated with drip tape (T-Tape TSX 515-16-340) buried at 12-inch depth between two alfalfa rows. The drip tape was buried on alternating inter-row spaces (5 ft apart). The flow rate for the drip tape was 0.34 gal/min/100 ft at 8 psi with emitters spaced 16 inches apart, resulting in a water application rate of 0.066 inch/hour. In 2000 the field was irrigated uniformly the whole season. The seed was harvested with a commercial combine.

Alfalfa Irrigation

The alfalfa was flailed on May 3 to delay flowering. Flower bud break started June 10. Approximately 2 inches of water were applied to all plots on May 17, May 30, June 6, and June 15. After June 15, the alfalfa was irrigated at four levels of alfalfa crop evapotranspiration (ET_c) replacement (20, 40, 60, and 80 percent) with five replicates of each treatment (Table 1). Each treatment was irrigated every 3-4 days to replace the percentage of the Et_c deficit that had accumulated since the last irrigation. Irrigations were terminated on August 20.

Each plot consisted of eight alfalfa rows, 480 ft long, and had two subplots corresponding to the two alfalfa varieties. Each plot was irrigated separately by its own pressure regulator, electronic solenoid valve, and water meter. Water meters were read before and after each irrigation.

Alfalfa evapotranspiration was calculated with a modified Penman equation (Wright 1982) and peak alfalfa crop coefficients using data collected at the Malheur Experiment Station by an AgriMet weather station (U.S. Bureau of Reclamation, Boise, ID) adjacent to the field. The Et_c was estimated and recorded from dormancy break on March 26 until the final irrigation on August 20. After the alfalfa was flailed, the Et_c was adjusted using crop coefficients. The crop coefficients were derived from weekly measurements of the percent ground cover until full cover was achieved.

Determination of Soil Water Content

Volumetric soil water content was determined by one Gro-Point soil moisture sensor (Environmental Sensors Inc., Escondido, CA) installed at 12-inch depth and one at 20-inch depth in each plot. The Gro-Point sensors were installed horizontally halfway between the drip tape and the alfalfa row in the plot center. Sensors were located 70 ft from the center of the field in the 'Tango' subplots. Sensors were connected by buried cables to electronic communication boards housed in two locations in the field. The electronic communication boards were connected by a cable to a personal computer allowing the soil water content to be read and logged every hour.

Volumetric soil water content was also measured with a neutron probe. One access tube was installed in each plot halfway between the drip tape and the alfalfa row in the plot center. The neutron probe access tubes were located 15 ft from the center of the field in the 'Tango' subplots. Neutron probe readings were made twice weekly at the same depths as the Gro-Point sensors and at 32-inch depth. The neutron probe was calibrated by taking soil samples and probe readings at 12-inch, 20-inch, and 32-inch depth during installation of the access tubes. The soil water content was determined gravimetrically from the soil samples and regressed against the neutron probe readings, separately for each soil depth. The regression equations were then used to transform the neutron probe readings during the season into volumetric soil water content.

Alfalfa Seed Yields

On August 22, biomass samples were taken in each subplot by cutting the plants at ground level in 3.3 ft of one row. The samples were weighed, oven dried, and weighed again. The dried samples were separated into stems, leaves, and seed pods.

The alfalfa was desiccated with Boa (Paraquat dichloride) at 0.63 lb ai/acre and Reglone (Diquat) at 0.5 lb ai/acre on August 29. On September 9, 66 ft of each subplot was harvested with a small plot combine (52-inch width). The harvested seed was cleaned to separate the plant debris from the seed. The seed and the debris were weighed.

Lygus Bug Monitoring and Control

Lygus bugs were monitored twice weekly by taking three 180° sweeps with an insect net in each plot. The total number of early and late instars and adults was counted at each location. When the total number of insects (early and late instars, and adults) reached four per sweep, insecticides were applied (Table 1).

Table 1. Aerial insecticide applications for lygus bug control. Malheur Experiment Station, Oregon State University, Ontario, OR 2002.

Date Product Rate

lb ai/acre

June 9 Capture 0.1

June 25 Metasystox-R 0.5

July 6 Capture 0.032

July 16 Metasystox-R 0.5

July 26 Capture 0.032

August 14 Metasystox-R 0.5

Results and Discussion

Differential Irrigation

The total Et_c from dormancy break to the start of flowering (March 25 to June 6) was 10.1 inches, substantially higher than the approximately 5.8 inches applied uniformly to all plots (Fig. 1a). After the start of flowering, the treatments were clearly differentiated in terms of cumulative amount of water applied over time (Fig. 1b). The total amount of water applied after the start of flowering was 19.8, 14.9, 10.0, and 5.0 acre-inches per acre for treatments one through four, respectively. The total Et_c from the start of flowering until the last irrigation was 25.2 acre-inches/acre. The total Et_c for the season was 35.3 inches.

Soil moisture was closely related to the irrigation treatments (Fig. 2). The average soil moisture content at 12-inch depth from June 6 through August 25 was 29, 27, 23, and 19 percent for treatments one through four, respectively. Soil moisture content at 12-inch depth for treatments one through three was similar during irrigations, but became lower between irrigations in accordance with the irrigation treatments. Soil moisture content at 12-inch depth for treatment four (irrigated at 20 percent Et_c), remained lower than for the other treatments during and after irrigations. Soil moisture content at 20-inch depth was lower than at 12-inch depth for all treatments (Fig. 3). Soil moisture content at 20-inch depth for treatments one through three was similar during and between irrigations. Soil moisture content at 20-inch depth for treatment four was not responsive to irrigations.

Soil moisture measured by neutron probe was close to soil moisture measured by Gro-Point sensors (Fig. 4).

Alfalfa Seed Yields

Alfalfa seed yield increased with increasing Et_c replacement (Fig. 5) and applied water (Fig. 6), reached an optimum, and then decreased. 'Tango' seed yield was optimized at 51 percent of Et_c replacement or 18.9 inches of applied water (total water applied from the start of the season) and 'Accord' seed yield was optimized at 50 percent of Et_c replacement or 18.5 inches of applied water.

Whole plant, stem, and leaf dry matter yields increased with increasing Et_c replacement (Fig. 7), and applied water (Fig. 8). Seed pod dry matter yield increased with increasing Et_c replacement and applied water, reached a maximum, and then decreased (Fig. 7 and 8). 'Tango' seed pod yield was optimized at 57 percent of Et_c replacement or 20.5 inches of applied water and 'Accord' seed yield was optimized at 55 percent of Et_c replacement or 19.9 inches of applied water

Lygus bug insecticide applications were effective in maintaining the population below the economic threshold (four lygus bugs per 180° sweep) except for a brief period in late June and during the last 2 weeks of July (Fig. 9).

References

Wright, J.L. 1982. New evapotranspiration crop coefficients. J. Irrig. Drain. Div., ASCE 108:57-74.

Figure 1a. Cumulative water applied from dormancy break to flowering compared to Et for alfalfa seed. Malheur Experiment Station, Oregon State University, Ontario, OR.

Figure 1b. Cumulative water applied after flowering compared to Et for alfalfa seed submitted to four drip-irrigation treatments. Malheur Experiment Station, Oregon State University, Ontario, OR.

Figure 2. Soil moisture response to irrigation treatment in a drip-irrigated alfalfa seed field. Malheur Experiment Station, Oregon State University, Ontario, OR 2002.

Figure 3. Soil moisture response to irrigation treatment in a drip-irrigated alfalfa seed field. Malheur Experiment Station, Oregon State University, Ontario, OR 2002.

Figure 4. Volumetric soil water content measured by Gro-Point sensors and by neutron probe in a drip-irrigated alfalfa seed field submitted to four levels of Et replacement. Malheur Experiment Station, Oregon State University, Ontario, OR 2002.

Figure 5. Alfalfa seed yield response to Et replacement. Malheur Experiment Station, Oregon State University, Ontario, OR 2002.

Figure 6. Alfalfa seed yield response to total water applied from the start of the season. Malheur Experiment Station, Oregon State University, Ontario, OR 2002.

Figure 7. Response of alfalfa dry matter yield fraction to Et replacement. Malheur Experiment Station, Oregon State University, Ontario, OR 2002.

Figure 8. Response of alfalfa dry matter yield fraction to total water applied from the start of the season. Malheur Experiment Station, Oregon State University, Ontario, OR 2002.

Figure 9. Alfalfa seed lygus bug population levels. Arrows denote insecticide applications. A pre-bloom application was made on June 9 (day 160). The last insecticide application was on August 14 (day 226). Malheur Experiment Station, Oregon State University, Ontario, OR 2002.