WATER MANAGEMENT FOR DRIP-IRRIGATED SPRING WHEAT

Clinton C. Shock, Erik B. G. Feibert, and Lamont D. Saunders

Malheur Experiment Station

Oregon State University

Ontario, OR

Andrew Ross

Department of Crop and Soil Science

Oregon State University

Corvallis, OR

Introduction

Initial interest in the use of drip irrigation for row crops in the Treasure Valley was motivated by concerns about groundwater contamination. Furrow irrigation necessary for row crops, especially onions, can lead to nitrate and herbicide byproducts leaching to the groundwater. Drip irrigation can reduce leaching. Drip irrigation was also found to increase onion yield and grade in fields that were difficult to furrow irrigate. Drip irrigation has become more popular for onion production. Other crops grown in rotation with onions could benefit from drip irrigation. This trial tested drip irrigation for spring wheat using three irrigation rates.

Materials and Methods

The trial was conducted on a Nyssa silt loam previously planted to field corn. In the spring of 2005, the corn stubble was shredded and the field disked. Spring wheat (cv. 'Penewawa') was drilled in 30-inch beds on April 1, 2005. Emergence started on April 10. The field was irrigated using drip tape (T-Tape TSX 515-16-340) buried at 12-inch depth 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. The field was divided into plots of 0.2 acre, 8 beds wide by 400 ft long. Each plot was irrigated by four drip tapes. The irrigation duration for each plot was programmed using a controller and solenoid valve. A water meter for each plot was read before and after each irrigation to calculate water applied.

On May 27, the spring wheat was irrigated with 2 inches of water. Starting June 8, the wheat was irrigated weekly with three evapotranspiration (ET_c) replacement treatments: 100 percent, 80 percent, and 60 percent of the accumulated ET_c (corresponding to 20, 15, and 10 inches of water). The treatments were arranged in a randomized complete block design with four replicates. The amount of water to apply at each weekly irrigation was calculated as the difference between the accumulated ET_c and the accumulated water applied plus precipitation (Table 1). The ET_c and water applied plus precipitation were accumulated since the start of emergence. Wheat ET_c was calculated with a modified Penman equation (Wright 1982) using data collected at the Malheur Experiment Station by an AgriMet weather station (U.S. Bureau of Reclamation, Boise, ID). The last irrigation was on July 28.

Soil water content was measured with a neutron probe. Two access tubes were installed in each plot. Tubes were located 160 ft and 320 ft from the top of the field. The access tubes were placed 15 inches to the side of the second drip tape in each plot. Soil water content was measured twice weekly at 8-inch, 20-inch, and 32-inch depths.

The neutron probe was calibrated by taking soil samples and probe readings at each 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.

Soil water depletion for the season was calculated by the difference in the amount of water in the 3-ft profile between the first and last neutron probe reading. Water use efficiency was calculated as the bushels of grain per inches of irrigation water plus precipitation plus depleted water.

Bronate^® at 2 lb ai/acre was sprayed on May 16. On May 18, 100 lb nitrogen (N)/acre as urea ammonium nitrate solution was injected through the drip tape.

On August 11, four samples were harvested from each plot using a small plot combine (52-inch width). Each sample was 60 ft long. One pair of samples was taken 160 ft from the field top and the second pair 320 ft from the field top. The samples in each pair were taken adjacent to each other and centered on the second drip tape of each plot. The samples were weighed, and one sub-sample was taken from each. The sub-samples were cleaned and bushel weights determined by weighing a volume of 1 qt. The sub-samples were shipped to Oregon State University in Corvallis, Oregon for seed analysis. Each sub-sample had 300 kernels analyzed automatically using a Single Kernel Characterization System. The sub-samples were analyzed for kernel hardness, weight, moisture, and size.

An additional set of samples was collected from each plot to determine how much yield was depressed for plants growing farthest from the drip tape. Four quadrant samples adjacent to each other were collected 160 ft from the field top and another four collected 320 ft from the field top. The quadrant samples in each set of four measured 10 inches wide by 39.4 inches long with the long side oriented parallel to the drip tape. The first sample was centered on the second drip tape in the plot. The other samples were adjacent to the first and located between the second and third drip tape (Fig. 1). Sample 1 was centered on drip tape, sample 2 was centered 10 inches from drip tape, sample 3 was centered 20 inches from drip tape, and sample 4 was centered 30 inches from drip tape. The samples were harvested by manually cutting the grain at ground level. The grain was put in a forced air oven to dry. After drying, the samples were weighed and the total number of heads was determined. The kernels were removed from the heads, dried, weighed, and counted.

Data were subjected to analysis of variance (ANOVA) using the general linear model ANOVA procedure (Number Cruncher Statistical Systems, Kaysville, UT). Yield from the combined samples was calculated from the sum of the four sub-samples in each plot. Treatment means with differences greater than the corresponding protected LSD (0.05) were considered to be different.

Results and Discussion

The treatments were clearly differentiated in the actual percent ET_c replaced (100, 80, and 60 percent) and total amount of water applied (20, 15, and 10 inches, respectively) (Table 2). Evapotranspiration from emergence to the last irrigation totaled 25.3 inches. Precipitation from emergence to the last irrigation totaled 4.7 inches. Soil volumetric water content averaged over the 3 depths was significantly lower for the 60 percent ET_c treatment than for the 100 percent or 80 percent ET_c treatments (Table 2). Soil volumetric water content over time at the 3 depths was similar for the 100 percent or 80 percent ET_c treatments (Fig. 2). There was a significant trend for an increase in the amount of soil water depleted by the crop during the season with the reduction in ET_c replaced (Table 2). There was an increase in water use efficiency with the reduction in percent ET_c replaced from 100 percent to 80 percent. Water use efficiency for the 60 percent ET_c treatment was similar to the 80 percent ET_c treatment.

Grain yield based on the combine samples was only reduced by the 60 percent ET_c treatment (Table 3). Grain yield for the 100 percent or 80 percent ET_c treatments was similar. Grain yield increased and then decreased as the percent ET_c replaced increased between 80 percent and 100 percent (Fig. 3). Calculated from the regression equation, the maximum yield would be achieved with an ET_c replacement of 91.3 percent. Kernel hardness, kernel weight, and kernel size were not affected by the irrigation treatments. There was a small reduction in kernel moisture with the 60 percent ET_c treatment.

Based on the quadrant sampling, biomass dry weight, number of heads/ft², seed weight, and grain yield were all reduced by the 60 percent ET_c treatment (Table 4). Based on the quadrant sampling, biomass dry weight and grain yield were affected by distance from the drip tape (Table 4). Biomass dry yield and grain yield were reduced starting at 25 inches from the drip tape, representing the central 10 inches between drip tapes. Consequently, only 16.7 percent of the area between drip tapes would have reduced yield due to incomplete lateral movement of the irrigation water.

References

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

Table 1. Irrigation water applied to drip-irrigated spring wheat with three ET_c replacement strategies, Malheur Experiment Station, Oregon State University, Ontario, OR, 2005.

ETc replacement	27-May	8-June	17-June	23-June	30-June	6-July	15-July	20-July	28-July	Total
%	------------------------------------------------- inches ---------------------------------------------------
100	2.23	3.05	2.58	1.33	1.86	2.56	3.82	0.95	1.90	20.27
80	1.99	1.12	2.27	0.98	1.92	1.57	2.75	1.08	1.63	15.30
60	1.97	0.01	0.99	0.69	1.12	1.40	2.18	0.67	1.18	10.20

Table 2. Water applied, water use efficiency, and average season-long soil volumetric water content from 0 to 3 ft for spring wheat submitted to 3 ET_c replacement strategies. Malheur Experiment Station, Oregon State University, Ontario, OR, 2005.

ETc replacement	Actual ETc replacement	Water applied plus precipitation	Soil water depletion	Water use efficiency*	Average season-long soil volumetric water content
-------- % --------		--------- inches ---------		bu/inch	%
100	99.1	25.0	0.03	4.55	30.9
80	79.4	20.0	0.34	5.63	29.9
60	58.2	14.7	0.77	6.09	25.2
LSD (0.05)	11.0	2.3	0.39	0.82	2.2

*bu/inch of irrigation water plus precipitation plus depleted water.

Table 3. Kernel characteristics and grain yield for spring wheat submitted to three ET_c replacement strategies. Malheur Experiment Station, Oregon State University, Ontario, OR, 2005.

ETc replacement	Kernel hardness	Kernel weight	Kernel moisture	Kernel size	Grain moisture	Test weight	Grain Yield
%		mg	%	mm	%	lb/bu	bu/acre
100	28.6	41.85	8.17	2.93	5.8	63.0	113.6
80	28.24	42.66	8.25	2.96	5.8	63.0	114.3
60	27.23	41.75	7.92	2.88	5.5	62.9	94.2
LSD (0.05)	NS	NS	0.29	NS	NS	NS	13.1

Table 4. Total biomass dry yield, head number, grain yield, seed weight, and test weight of four quadrant samples for spring wheat submitted to three ET_c replacement strategies, Malheur Experiment Station, Oregon State University, Ontario, OR, 2005.

Variable units	Etc replacement%	Quadrant sample location					LSD (0.05)
		1	2	3	4	Average	Treatment	Sample location
Biomass dry yield	100	7.1	7.4	7.6	6.7	7.2
	80	7.9	7.5	7.5	6.8	7.4
tons/acre	60	6.3	6.7	6.3	5.4	6.2
	average	7.1	7.2	7.1	6.3	6.9	0.4	0.7
Number of heads	100	63.3	66.8	70.9	66.2	66.8
	80	68.3	62.9	68.9	69.2	67.3
number/ft2	60	56.9	61.3	64.2	56.6	59.7
	average	62.9	63.7	68.0	64.0	64.6	5.8	NS
grain yield	100	98.1	105.8	115.1	102.4	105.3
	80	118.0	110.6	116.1	104.1	112.2
bu/acre	60	94.0	97.5	94.7	78.6	91.2
	average	103.4	104.6	108.6	95.0	102.9	11.5	9.3*
Seed weight	100	11,383	11,216	11,151	11,250	11,250
	80	11,277	11,251	11,051	11,043	11,156
seeds/lb	60	10,459	10,598	11,125	11,134	10,829
	average	11,040	11,022	11,109	11,142	11,078	300	NS
Test weight	100	61.1	61.0	61.4	61.0	61.1
	80	60.6	60.8	61.2	61.4	61.0
lb/bu	60	61.6	61.2	61.2	60.1	61.0
	average	61.1	61.0	61.3	60.8	61.1	NS	NS

*significant at the 0.10 level.

Figure 1. Diagram of quadrant sampling, Malheur Experiment Station, Oregon State University, Ontario, OR, 2005.

Figure 2. Soil volumetric water content response to three evapotranspiration replacement treatments in spring wheat, Malheur Experiment Station, Oregon State University, Ontario, OR, 2005.

Figure 3. Wheat yield response to ET_c replacement strategy, Malheur Experiment Station, Oregon State University, Ontario, OR, 2005.