AND GROWER RETURNS OF NATIVE SPEARMINT AND HOPS IN WASHINGTON STATE

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1 EFFECT OF DEFICIT IRRIGATION ON YIELD, QUALITY AND GROWER RETURNS OF NATIVE SPEARMINT AND HOPS IN WASHINGTON STATE By PROSSIE NAKAWUKA A dissertation submitted in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY WASHINGTON STATE UNIVERSITY Department of Biological Systems Engineering MAY 2013

2 To the Faculty of Washington State University: The members of the Committee appointed to examine the dissertation of PROSSIE NAKAWUKA find it satisfactory and recommend that it be accepted. R. Troy Peters, Ph.D., PE, Chair Claudio Stockle, Ph.D. Joan Davenport, Ph.D. Steven Kenny, Ph.D. ii

3 ACKNOWLEDGEMENT First, I would like to express my sincere gratitude to my advisor R. Troy Peters for his continuous support during my Ph.D. study and research. I acknowledge his time, patience, ideas and guidance that have made my Ph. D. work manageable and productive. I also acknowledge his immense knowledge in irrigation engineering that he shares abundantly plus his insightful career advice that has helped me to clearly put my future career goals in perspective. I would like to also thank the rest of my committee members: Dr. Claudio Stockle, Dr. Joan Davenport and Dr. Steven Kenny, for their time and their insightful and helpful comments and questions that critically stimulated my thinking during my research. Furthermore, I am very grateful to Dr. Nicholas Kiggundu who has supported me since my undergraduate days until now. His interest in my progress has been one of the things that have always kept me going. It s under his mentorship and guidance that I discovered my interest in water resources management and irrigation science. I thank my friends and colleagues: Dr. Kefyalew Desta, Dr. Romulus Okwany, Hossein Sadhegi, and Anthony Mwengi, for their help with field work, stimulating discussions and encouragement. I am thankful to my roommate Madhu Kappangantu for making our living conditions conducive for my study. I thank my friend Pheonah Nabukalu of Louisiana State University whose support and encouragement have sailed me through the common challenges that we both went through during our doctoral studies. My stay in Prosser is enjoyable largely because of my Prosser Community Church friends. I am very grateful for their love, encouragement, and hospitality. I am specifically grateful to Sheila and Rod Stutzman, Connie Peacock, Sherry Whitaker, Diane Torres and Jenni iii

4 McKlure that always opened up their homes to me and have always been handy with support and insightful advice to me. I am also grateful to God for giving me wisdom, strength and guidance throughout my life. Lastly, I would like to thank my family that has always been there for me and whose love, support and encouragement have always been the anchor of my life. For my Mum Immaculate and Dad William who raised me, provided for me and have always supported me in all my pursuits. For my sisters: Jane, Agnes, Mariah, Stellah and Immy and my brother William Jr. who are never so far away or too busy when I need someone to talk to. Thank you. Prossie Nakawuka iv

5 EFFECT OF DEFICIT IRRIGATION ON YIELD, QUALITY AND GROWER RETURNS OF NATIVE SPEARMINT AND HOPS IN WASHINGTON STATE Abstract By Prossie Nakawuka, Ph.D. Washington State University May 2013 Chair: R. Troy Peters This study analyzed the effects of water stress levels on the yield, quality, water use efficiency and grower returns of drip irrigated native spearmint and four subsurface drip irrigated hop cultivars: Mt. Hood, Columbus, Chinook and Willamette. For native spearmint, stress levels were applied at different times during the growing periods. For hops, each stress level was applied throughout the growing season. The field experiments were carried out for two years at the Washington State University Irrigated Agriculture Research and Extension Center near Prosser WA. Mint results showed that biomass production decreased with increasing water stress as is expected. However, mint oil yield and quality were not significantly different across treatments. This suggests that deficit irrigating native spearmint might enhance oil accumulation. This is further supported by the observed increase in oil concentration as water stress increased. A number of mint production costs were affected by the reduction in applied v

6 water thus reducing the total mint production costs. Since oil yields were similar across treatments, revenues were not significantly affected. This resulted into increase in net farm income. Also, water stress improved the water use efficiency in native spearmint. Hop results showed that the dried hop cone yield decreased with increasing water stress in all the cultivars. The hop cone quality however did not change significantly across treatments. In the full year of production, water stress improved water use efficiency in hop production across all the cultivars. In the Yakima Valley, reduction in amount of water applied only affects the cost of pumping of the irrigation water which is minimal when compared to the total cost of producing hops. The yield reductions due to applying less water to hops caused reductions in revenues that were higher than the savings in the cost of producing hops due to pumping less water. This decreased the net farm income. The study observed how the subsurface drip irrigation system coupled with scientific irrigation scheduling can substantially reduce hop water use. This study also provides production functions for irrigation optimization of native spearmint and four hop cultivars. vi

7 TABLE OF CONTENTS ACKNOWLEDGEMENT... iii Abstract... v LIST OF ABBREVIATIONS... xi LIST OF TABLES... xii LIST OF FIGURES... xiii CHAPTER ONE... 1 GENERAL INTRODUCTION... 1 References... 7 CHAPTER TWO... 9 EFFECT OF DEFICIT IRRIGATION ON YIELD, QUALITY AND GROWER RETURNS OF NATIVE SPEARMINT Abstract Introduction Materials and methods Site description Treatments and irrigation scheduling Harvesting and distillation Oil component analysis vii

8 Crop water use Statistical analysis Results and discussion Irrigation and crop water use Fresh hay yields Oil yields Water use efficiency Oil concentration Oil component analysis Economic analysis Conclusions Acknowledgements References CHAPTER THREE EFFECT OF DEFICIT IRRIGATION ON YIELD, WATER USE EFFICIENCY AND ECONOMIC RETURNS OF FOUR DIFFERENT HOP CULTIVARS IN THE YAKIMA VALLEY, WASHINGTON STATE Abstract Introduction Materials and methods viii

9 Site description Treatments and irrigation scheduling Field practices and harvesting Crop water use (ETc) Statistical analysis Results and discussion Irrigation applied and Crop water use Dry weight hop cone yield Water use efficiency Yield quality Economic analysis Conclusions Acknowledgements References CHAPTER FOUR WATER STRESS FOR IMPROVED PROFITS IN NATIVE SPEARMINT WASHINGTON STATE UNIVERSITY EXTENSION FACT SHEET Introduction Deficit irrigation ix

10 4.3. Study objectives Estimated annual expenses Production functions Economic analysis assumptions Costs of establishing and producing native spearmint under drip irrigation Affectation of variable costs by changes in amounts of water applied Summary Acknowledgements References CHAPTER FIVE GENERAL CONCLUSIONS CHAPTER SIX SUGGESTIONS FOR FUTURE STUDIES x

11 LIST OF ABBREVIATIONS RH SMC FC MAD PWP ETc WUE P I D R S SDI T1 T2 T3 T4 W V F LB UB relative humidity soil moisture content field capacity management allowable depletion permanent wilting point crop evapotranspiration water use efficiency precipitation irrigation deep percolation runoff change in soil moisture storage in the soil profile during the growth period subsurface drip irrigation stress levels applied throughout the crop s growing period stress levels only applied 21 days before harvest stress levels only applied 14 days before harvest stress levels only applied 7 days before harvest irrigation amount applied variable cost fixed cost lower bound of the confidence interval upper bound of the confidence interval xi

12 LIST OF TABLES Table 2.1 Effect of irrigation level and timing on irrigation applied, hay yields, oil yields, consumptive use of water, oil concentration and water use efficiency in native spearmint Table 2.2 Mean oil component analysis values in µg/ml for native spearmint oil Table 2.3 Cost* per hectare of Establishing and Producing Native Spearmint under Drip Irrigation Table 2.4 Cost savings per hectare with respect to the full irrigation scenario Table 3.1 Effect of irrigation level on applied irrigation, hop cone yield, consumptive water use, water use efficiency, and alpha and beta acids concentration for the four hop cultivars Table 4.1 Cost per Acre of Establishing and Producing Native Spearmint under Drip Irrigation under full irrigation scenario Table 4.2 Irrigation applied and hay yield under 40% irrigation level Table 4.3 Irrigation applied and hay yield under 54% irrigation level Table 4.4 Irrigation applied and hay yield under 80% irrigation level Table 4.5 Costs per acre affectation under 40% irrigation level Table 4.6 Costs per acre affectation under 54% irrigation level Table 4.7 Costs per acre affectation under 80% irrigation level xii

13 LIST OF FIGURES Fig. 2.1 Daily weather parameters during the periods of experimentation Fig. 2.2 Penman reference evapotranspiration for the site during the periods of experimentation Fig. 2.3 Growing degree days during the period of experimentation (base temperature = 4 C) 19 Fig. 2.4 Experimental plot design of the native spearmint field. The dotted line is the uniform 100% drip line used during the beginning of each growing period and after each harvest before switching to the spliced lines (solid line). S = Water source Fig. 2.5 Soil moisture content in the mint active root zone for the 2012 growing seasons for plots receiving 100% throughout their growing seasons. SMC = Soil Moisture Content, FC = Field Capacity, PWP = Permanent Wilting Point, MAD = Management Allowable Depletion Fig. 2.6 Changes in fresh hay yields due to changes in irrigation amounts applied Fig. 2.7 Mean oil yield per cutting for the irrigation levels across the various timings for both years Fig. 2.8 Variation of water use efficiency with applied water Fig. 2.9 Oil concentration variation with amounts of irrigation applied per cutting per year Fig.3.1 Soil moisture content in the Columbus root zone during the 2012 growing season under the 100% irrigation level. SMC = Soil Moisture Content, FC = Field Capacity, MAD... = Management Allowable Depletion, and PWP = Permanent Wilting Point 43 Fig.3.2 Hop cone yield versus irrigation level for both years for the four cultivars Fig.3.3 Relationships between yield and evapotranspiration for the four cultivars. Values are averages of 2011 and 2012 ETc and yields xiii

14 Fig.3.4 Water use efficiencies at the three irrigation levels for the four cultivars for 2011 and Fig. 4.1 Proportional savings with respect to the full irrigation scenario xiv

15 Dedication I dedicate this dissertation to my wonderful parents; to my dad, William Kiku Sr. and my mum, Immaculate N. Kiku. My siblings and I are what we are today because of the enormous love, support and encouragement that you have so unselfishly given to us. xv

16 CHAPTER ONE GENERAL INTRODUCTION Water scarcity is a global problem (Fereres et. al., 2007, WHO, 2009). As cities grow and populations increase, the problem worsens since needs for water increase in households, industry and agriculture. Climate change has also contributed significantly to the water scarcity problem (WHO, 2009). Rising temperatures increase the rate of evaporation from land and surface water resources; this has caused reductions in river run-off in several areas. The rise in temperatures has also greatly affected areas that rely on snowmelt and mountain glaciers as a water source. Water scarcity does not only occur in arid and semi-arid areas but also occurs in areas that receive ample rainfall and/or have abundant fresh water resources. How the available water is used, managed and conserved, determines if there is enough to meet household, agricultural, industrial and environmental demands (WHO, 2009). Water is an essential resource to sustain life. It is a principal factor in agricultural production: proper development of every plant needs an optimum water supply that meets its physiological needs (Mannocchi and Mecarelli, 1994). Plants are diverse and differ in the minimum amounts of water each plant needs for survival and optimum production. During dry growing periods when there is not enough rainfall to compensate for soil moisture losses through evapotranspiration, application of irrigation water by artificial means is required to maintain proper crop growth and productivity. Nearly 40% of food and agricultural commodities are produced through irrigated agriculture on about 17% of agricultural land (FAO, 2002). Over the last 50 years, irrigated areas have almost doubled and this has been one of the 1

17 factors that have contributed to growth in agricultural productivity over these years (Rosegrant and Ringler, 2000). Irrigated agriculture is the primary user of fresh water resources (FAO, 2002; Fereres and Soriano, 2007; Kenny et al., 2009). Irrigation uses take almost 60% of all the world s freshwater withdrawals. It is therefore not surprising that irrigated agriculture, especially in arid and semi-arid areas, is facing pressures to reduce its water use in order to also cater for other water users like power and water needs for growing urban and industrial areas, and the ample water that is needed to provide in-stream flows to preserve native fish populations in various regions. In some countries like China and Chile, irrigated agriculture is already facing stress as water is already being transferred out of irrigation into urban and industrial uses (Rosegrant and Ringler, 2000). Irrigated agriculture is therefore forced to operate under conditions of water scarcity. Irrigation therefore needs to be managed more efficiently and sustainably, aiming at saving water, maximizing its productivity, and reducing non-point sources of pollution of the environment. Conventionally, irrigation water is applied to avoid reduction in crop production due to water deficits (English, 2002). For commercial farms, water is supplied to allow production of cultivated crops that will produce a satisfactory economic yield (Mannocchi and Mecarelli, 1994; English and Raja, 1996; Rodrigues and Pereira, 2009). Crops are thus supplied with sufficient water so that the crops can transpire and meet their full ET requirements throughout their growing seasons. Under conditions of water scarcity, the water available to farmers is normally below the maximum ET needs of the plants. Farmers are then forced to make decisions on whether to concentrate the limited water over a smaller land area or to irrigate 2

18 the total area with levels below the full ET requirement. Irrigation application below the full ET requirement is termed as deficit irrigation. In the context of improving water use efficiency (WUE), there is growing interest in deficit irrigation which involves applying stress levels to plants that have minimal effects to yields in order to maximize yields per unit of water used by the crop and maximize returns (Rodrigues and Pereira, 2009). Therefore, there is need to recognize the potential for deficit irrigation not only to conserve scarce water resources but to also increase grower profitability and reduce environmental pollution. As mentioned earlier, deficit irrigation consists of deliberately applying irrigation water in amounts below the plant s water requirements. Deficit irrigation can be applied at certain periods during the crop s growing season or throughout its growing season. When irrigation is applied below the full crop ET requirement, the crop extracts water from the soil profile to compensate for the deficit. If there is sufficient water stored in the soil profile (normally from seasonal precipitation), transpiration of the crop is not affected and therefore growth and crop yield are not affected (Fereres and Soriano, 2007). On the other hand, if the moisture stored in the soil profile is not sufficient, transpiration is reduced and this affects the crop s growth and hence productivity. Different crops have different sensitivities to water stress at their various stages of development. The crop s response to deficit irrigation also varies with location, stress patterns and other agronomic factors that affect that crop. A deficit irrigation program should be designed to manage stress such that yield reduction is minimized. However, in areas where water is scarce and/or expensive, maximizing yields may not be the main objective of farmers. Farmers major aims are to keep a positive return from the irrigated crop and to ensure sustainability of irrigation (English and Raja, 1996; English, 2002; Fereres and Soriano, 2007; 3

19 Rodrigues and Pereira, 2009). Deficit irrigation is profitable when the revenue lost due to yield reduction is less than the savings in costs of production due to applying less than the required water. The impact of water stress on yields and economic returns depend upon the irrigation system, the performance of that system, production costs, and the type of crop. Knowledge of the crop s response to deficit irrigation therefore needs to be known in order to achieve the desired objectives. Yield responses to ET deficits in various crops have been studied by several researchers over the years. Alfalfa is grown for its biomass: studies have shown that reduction in its water consumption decreases its biomass production (Fereres and Soriano, 2007; Hanson and Putnam, 2000). The response to deficit irrigation of a number of field crops like corn, wheat, sunflower, cotton and sorghum has been reported to be reduction in biomass production but the fraction of biomass that is harvested stays constant to between 40 to 60% of the maximum biomass (Fereres and Soriano, 2007). Deficit irrigation of potatoes causes reduction in yields and might also cause significant changes in tuber shape (English and Raja, 1996). Deficit irrigation in native spearmint has been shown to reduce hay production but maintain oil yields similar to those from fully irrigated plants (Okwany et al., 2012). Water stress has been noted to enhance quality in some crops. Water deficits can increase grain protein content in wheat and other grains (Pierre et al., 2007). Regulated deficit irrigation (RDI) has also been noted to increase sugar percentages in grapes and sugar beets (English and Raja, 1996). Several researchers have analyzed deficit irrigation practices and have found that the technique can increase net farm income (Hargreaves and Samani, 1984; English and Raja, 1996; English, 2002; Rodrigues and Pereira, 2009). In water limiting scenarios, deficits ranging from 28% to 59% are 4

20 associated with gains in total farm income between 44% and 68% depending on the crop (English and Raja, 1996). Despite the potential benefits of deficit irrigation mentioned above, there are a number of issues and risks that need to be understood and taken into consideration when optimizing irrigation through deficit irrigation. Deficit irrigation field studies normally derive production functions that can be used to predict yield depending on the amount of irrigation applied or the amount of water used by the crop (ETc). Other than water, yield is affected by several other factors some of which are unpredictable such as climate, incidences of pests and diseases and several agronomic factors (English and Raja, 1996). Therefore the production functions will only be an estimate of the true relationships. Salinity is the other risk that is associated with deficit irrigation (Hargreaves and Samani, 1984; English, 2002). Deficit irrigation reduces water applied to volumes which do not meet the leaching requirements. Irrigation water contains varying amounts of salts and since crops utilize only pure water, salts may concentrate in the soil solution leading to increased salinity levels in the soil (Fereres and Soriano, 2007). Depending on the type of crop, soil salinity and irrigation water salinity, enough water may need to be applied so that excess salts will not accumulate in the crops root zone and affect growth and yields. Skilled monitoring and managing of soil salinity levels is thus required in order for crop production not to be affected. Deficit irrigation can produce significant benefits under favorable circumstances. Benefits of deficit irrigation are summarized as: (i) reduction in applied water; (ii) reduction in water losses due to deep percolation and runoff leading to reduction in non-point sources of pollution and increase in irrigation efficiency; (iii) reduction in costs of production; (iv) increase 5

21 in net farm income. Risks associated with deficit irrigation such as increase in soil salinity, crop growth hindrances and reduction in yields should be mitigated through proper management practices. This project was conducted to quantify the effect of various water stress levels applied at different times during the growing period on two major specialty crops that are grown in Washington State: mint and hops. Effect of water stress was evaluated on the crop yield, quality, water use efficiency, production costs and, finally, net farm income. This study presents yield functions for irrigation optimization and economic analysis tools that growers can use to estimate how reduction in the amount of water applied could affect production costs and thus net farm income. 6

22 References English, M., Raja, N.S., Perspectives on deficit irrigation. Agric. Water Manage. 32, I 14. English, M., Irrigation Advisory Services for Optimum Use of Limited Water. 18 th ICID Congress, Montreal, Canada. Available at: (accessed on ). FAO, Deficit irrigation practices. Water Reports 22. Available at: ftp://ftp.fao.org/agl/aglw/docs/wr22e.pdf (accessed on ) Fereres, E., Soriano, A.M., Deficit Irrigation for reducing Agricultural Water Use. Journal of Experimental Botany, 58(2), Geerts, S., Raes, D., Deficit irrigation as an on-farm strategy to maximize crop water productivity in dry areas. Agric. Water Manage. 96(9), Hanson, B., Putnam, D Can alfalfa be produced with less water? Available at: alfalfa.ucdavis.edu/+symposium/proceedings/2000/ pdf. Accessed on 01 March Hargreaves, G., Samani, Z., Economic Considerations of Deficit Irrigation. J. Irrig. Drain Eng. 110(4), Kenny, F. J., Barber, L.N., Huston S.S., Linsey S.K., Lovelace K.J., Maupin A.M., Estimated Use of Water in the United States in Available at: Accessed on 15 February Mannochi F., Mecarelli P., Optimization analysis of deficit irrigation systems. J. Irrig. Drain Eng. 120(3),

23 Okwany, R., Peters, T.R., Ringer, K.L., Walsh, D., Rubio, M., Impact of Sustained Deficit Irrigation on Spearmint (Mentha spicata L.) biomass production, oil yield, and oil quality. Irrigation Science, 30(3), Pierre, S.C., Peterson, J.C., Ross, S.A., Ohm, J., Verhoeven, C.M., Larson, M., Hoefer, B., Grain protein content and composition of winter wheat cultivars under different levels of N and water stress. Western Nutrient Management Conference, vol. 7. Salt Lake City, UT. Rodrigues, C.G., Pereira, S.L., Assessing economic impacts of deficit irrigation as related to water productivity and water costs. Biosystems Eng. 103, Rosegrant, M. W., Ringler, C Impact on food security and rural development of transferring water out of agriculture. Water Policy 1(6): WHO, facts about water scarcity. Available at: Accessed on 28 Jan

24 CHAPTER TWO EFFECT OF DEFICIT IRRIGATION ON YIELD, QUALITY AND GROWER RETURNS OF NATIVE SPEARMINT Abstract Irrigated agriculture is facing increasing competition for dwindling water resources especially in the arid and semi-arid areas. Potential water saving strategies that limit water losses and increase water use efficiency need to be applied in agriculture today in order to ensure sustainable and efficient use of the available water. One of the strategies is deficit irrigation. Successful application of deficit irrigation requires knowing how the crop will respond to the various water stress levels. This paper presents results of a two year study done at the Washington State University s Irrigated Agriculture Research and Extension Center (IAREC) to determine the effect of various water stress levels applied at various times during the growth period to the yield quantity, quality and production costs of drip irrigated native spearmint. The field experiment included four water stress levels (40%, 54%, 80% and 100%) and four stress timings; T1(the water stress levels were applied throughout the growing season), T2, T3, and T4 where the stress levels were applied 21, 14, and 7 days before harvest respectively. Results revealed that water stress caused reductions in hay production. Oil yields, on the other hand, were not significantly different across the various treatments. Water stress also did not significantly affect oil quality. The oil concentration (kg of oil per kg of hay) and water use efficiency (oil yield per unit of water consumed) were significantly higher in the drier plots than 1 Submitted to Agricultural Water Management on 2/18/2013 9

25 in the fully irrigated plots. Irrigation water pumping costs, harvesting and distillation costs, residual disposal costs, and machinery fueling and lubrication costs decreased with increasing levels of water stress. Total mint production costs decreased by up to 10% per hectare for the lowest level of water stress (40% of full irrigation). Deficit irrigation of native spearmint can save water, improve on water use efficiency and reduce costs of production while maintaining oil yields and quality similar to those from fully irrigated plants Introduction Native Spearmint (Mentha spicata L.) is a perennial crop that produces for about six years. Its oil is used as a flavoring ingredient in chewing gum, confectionery products, perfumes, dental products and pharmaceutical products. Native spearmint oil gets its characteristic smell from carvone, an oxygenated terpene, whose composition is about 70 80% of the oil (Kim and Hong, 1999). The United States and specifically the Pacific Northwest is the largest producer of native spearmint in the world (Fuchs and Hirnyck, 2000). Washington State is the largest producer of native spearmint oil producing over 70% of the United States native spearmint (USDA, 2012). Most of these mint growing areas are either arid or semi-arid, and crop production requires supplemental irrigation (Mitchell, 1997). Mint is shallow rooted and heavily depends on irrigation (Ley et al., 2003). In Eastern Washington, it receives 760 to 870 mm (30 to 34 inches) of irrigation water during the growing season (Ley et al., 2003). This high irrigation water requirement suffers competition from increasing municipal, industrial and other agricultural demands for water. Therefore, innovations are needed to more efficiently use the available water. 10

26 Deficit irrigation, the application of water below full crop water demand (Hargreaves and Samani, 1984; Mannocchi and Mecarelli, 1994; English and Raja, 1996; English, 2002; FAO, 2002; Fereres and Soriano, 2007; Geerts and Raes, 2009; Rodrigues and Pereira, 2009), is an important tool in reducing irrigation water use thus increasing irrigation efficiency. Reduction in irrigation water used can result in reduced costs of irrigation water, pumping costs and total cost of crop production (English, 2002). The water saved can then be used to irrigate additional land thereby increasing total farm profit (English and Raja, 1996; Fereres and Soriano, 2007). Although reduction in yield is expected when plants are subjected to water stress, the level and the timing of the stress could be such that the resulting yield reduction is small when compared to the benefits gained. In order for deficit irrigation to be successfully implemented, a crop s response to the different levels of stress at various times during its growth period needs to be known. Increasing levels of water stress in spearmint was found to progressively reduce biomass and increase the monoterpene compounds of the essential oil (Delfine et. al., 2005). The reduction of plant biomass was thought to reduce the total essential oil yield per hectare. However, a study previously done at the Washington State University Prosser Irrigated Agriculture and Extension Center (IAREC) showed that water stress on sprinkler irrigated native spearmint decreased mint hay yields but the total essential oil yields were maintained (Okwany et. al., 2012). However, constant severe water stress on the mint plants was seen to cause thinning of the stands which die out, eventually leading to low oil yields. Severe stress early in the season was therefore not advisable. One concern from the results of this study was that water droplets falling on the leaves during sprinkler irrigation washed off more oil from the 11

27 plants in the fully irrigated plots (Croteau, 1977). To address this concern, a surface drip irrigation system was used. Also, none of the previous studies looked at how applying the different stress levels at different times during the growth period affected the yield quantity and quality. Therefore, research was needed to manage and time water stress such that leaf production and oil accumulation can be balanced to maximize oil yields per hectare (Loomis, 1978). The objectives of this study were to (i) quantify the response of different levels of water stress applied at different times during the growing season on native spearmint s oil quantity and quality grown under drip irrigation, (ii) evaluate whether deficit irrigation had the potential to reduce costs of mint production in order to increase grower returns Materials and methods Site description The field experiment was conducted at IAREC, Prosser, WA (46.26 N, W, and 265 m above sea level) during the 2010, 2011 and 2012 growing seasons. The mint field was planted and uniformly irrigated to establish a healthy stand during the 2010 growing season. Water stress treatments were only applied in the 2011 and 2012 growing seasons. The site s soil was Warden Silt loam, 2 to 5% slopes, with average sand, silt and clay of 21, 68 and 8% respectively (Web Soil Survey). Early March just before the 2011 growing season, the mint field was irrigated. Soil moisture readings were then taken from each plot three days after the irrigation to a depth of 1.5 m (60 inches) at 6 inch increments to determine the field capacity. The volumetric water content at field capacity found to be 27.5%. It is estimated that about half of the water held at field capacity is held too tightly to be accessible to plants. The Permanent 12

28 Wilting Point was estimated to be 13.78% volumetric water content (Saxton and Rawls, 2006). Daily weather data on average temperatures, relative humidity and precipitation received during the growing period of 2011 and 2012 are shown in Fig Treatments and irrigation scheduling The experiment was a randomized block design with four replications of each treatment. The experimental field was irrigated to field capacity at the beginning of the every growing season and after each cutting. Treatments for this study involved applying various irrigation levels at different timings. Four irrigation levels were considered; 40, 54, 80 and 100% of full ET. The four irrigation levels were applied using four different drip tubings with emitters at various flow rates and spacings in order to apply proportional amounts of water. The drip tubes were then spliced together; for the same application duration therefore, each of the specified plots in the plot plan (Fig. 2.4) received the appropriate amount of water. The 100% drip tubing had emitters of 4.01 liters/hour flow rate at 61 cm emitter spacing, 80% tubing had emitters with flow rate of 4.01 liters/hour at 76.2 cm spacing, 54% emitters had a flow rate of 1.89 liters/hour with 53.3 cm emitter spacing and, 40% irrigation level tubing had emitters with 1.59 liters/hour flow rate and 61 cm emitter spacing. Four timings were considered: T1, T2, T3, and T4. For T1, the irrigation level specified per plot was applied throughout the growing period. For T2, T3 and T4, the plants were fully irrigated using the uniform drip line (dotted line in the Figure 2.4) and stress levels were only applied 21, 14, and 7 days before harvest, respectively, using the spliced drip line (solid line in Figure 2.4). A treatment of 54% at T1 means that plants in that experimental plot were subjected to 54% of the crop water requirement throughout its growing period. A treatment of 13

29 80% at T3 means that plants in that experimental plot were fully irrigated (no stress applied) until at 14 days to harvest and then the plants only received 80% of the crop water requirement until harvest. The controls were 100% at timings T1, T2, T3 and T4; all these experimental plots were fully irrigated throughout the growing period and were the reference plots used to schedule for irrigations. Soil moisture was measured in these plots twice every week throughout the growing season using an on-site-calibrated neutron-probe (503DR hydroprobe, Campbell Pacific Nuclear, Concord, CA, USA).Irrigations were scheduled in these plots such that the available soil water in the active root zone (active root zone for mint was found to be 76.2 cm)never exceeded field capacity and never went below 65% of the total available soil water (total available soil water is the difference between soil moisture at field capacity and at wilting point). Figure 2.5 shows changes in soil water content before the first and second cutting of 2012 for the control plots. The experimental plots were 4.6 m by 6.1 m each. Each plot was planted with six rows of plants and the drip lines ran along the rows of plants. As shown in Figure 2.4, plots with timing T1 had one drip line; the spliced line (solid line) running along the plants rows. Plots with timings T2, T3, and T4 had two (2) drip lines running along the plant rows. The first drip line (dotted line) was a uniform 100% line that was to fully irrigate the plants until the time when stress levels were to be applied and then that line was disconnected and the second drip line which was the spliced line (solid line) was connected to apply the stress levels. All experimental plots in the mint field were always irrigated proportionately on the same day. 14

30 Harvesting and distillation Native spearmint in the U.S. Pacific Northwest is harvested twice during the growing season. The first harvest (or cutting) is normally during the first week of July and the second is by mid-september. During harvest, a cutter 99 cm wide was used to cut a sample area of 0.99 m by 4.88 m of swath from each experimental plot. The hay per sample area was weighed and the weight of fresh hay yield per treatment per hectare determined. A 9.6 kg subsample of the hay cut from each plot was divided into three equal parts and packed in 0.3 kg burlap sacks. These 3.2 kg subsamples were air dried for seven days, packed and stomped into individual cooker pots and distilled. During distillation, a wet steam boiler was used as a steam source and steam was delivered to the mint distillation room via an over ground steam line, which goes into a steam regulator set at 414 kpa. The wet steam was passed through the mint samples and it carried the oil up into the condensers through a series of tubes and baffles specially designed. The condensed water and mint oil then flowed from the bottom of the oil condensers into an oil separator where oil being lighter than water floated on the top in each separator. The mint oil from each 3.2 kg sample was collected and measured. The average of the oil yield from the three 3.2 kg sub samples was used to determine the oil yield per hectare per treatment. Samples of the mint oil for treatment were then sent to a laboratory for oil component analysis Oil component analysis Volatile detection of hydrodistilled native spearmint oil was accomplished by a modification of Cramer et al. (2005) using Solid-Phase Micro Extraction (SPME) with GC/MS. The analysis of freshly distilled mint oil was carried out similar to Rohloff (1999,) however an aliquot of 4.0 µl was diluted in 3.0 ml ddh2o, plus 1.0 ml of absolute ethanol, including 0.65 g 15

31 NaCl. The sample was allowed to equilibrate 2-3 minutes with stirring in a 25.0 ml SPME flask, and exposed to a PDMS/DVB fiber for exactly 1.0 min. at room temperature. The volatiles adsorbed onto the SPME fiber were thermally desorbed into the injection port of a HP5890/5970 GC/MS system (HP now Agilent, Avondale PA) equipped with a DB-1 column (60m x 0.32mm i.d., 0.25µm film thickness, J&W Scientific, Folsom, CA). Helium was used as the carrier gas. The injector and detector temperatures were 200 C and 250 C, respectively. The column temperature was initially maintained at 33 C for 5 min before increasing to 50 C at a rate of 2 C/min, and then to 225 C at a rate of 5 C/min. The sample was desorbed for 2-5 min using a SPME liner (Yang and Peppard, 1994) and set in the split-less mode. The retention index (RI) was calculated based on Kovat s retention indices using a series of straight-chain alkanes (C4 - C14) under the chromatographic conditions described above. The quantitative data were determined by running known standards then developing response factors based on water matrices Crop water use Soil moisture content was monitored twice a week using a neutron probe (503DR hydroprobe, Campbell Pacific Nuclear, Concord, CA, USA) and taking measurements at 15 cm increments. PVC access tubes were installed to a depth of 1.5 m. Crop evapotranspiration (ETc) was calculated using the following soil water balance equation (Mitchell and Yang, 1998; Oweis et al., 2011): ETc = P + I ± D R + S (1) Where P is precipitation, I is irrigation, D is deep percolation below the root zone (or upward flow from a shallow water table), R is runoff, and S is the change in the soil profile storage (i.e. 16

32 start of growing period soil moisture reading minus end of growing period reading). There are no shallow water tables in the study site (Web Soil Survey) and hence upward flow of water to the root zone is not of concern. The access tubes were of sufficient depth to provide for detection of potential deep percolation. Irrigations were scheduled only up to 76 cm but ETc for mint was calculated for the entire 1.5 m soil profile for the period between plant emergence to harvest. Deep percolation was negligible. Precipitation received throughout the growing season for the two years is shown in Fig Irrigation amounts applied for each treatment are for both years are given in Table 2.1. The drip irrigation system produced no runoff, and hence R in Equation (1) was zero for the analysis. The four replications of a given treatment gave similar ETc values and were therefore averaged to obtain a single seasonal ETc value for each treatment for each year Statistical analysis Statistical analysis of the data included analysis of variance (ANOVA), using the Minitab Statistical software (2012), Minitab Inc. State College, Pennsylvania, USA, to test for interactions between irrigation levels and timings, and effects of irrigation level and/or timing on the yield and quality parameters, irrigation applied, and crop water use (ETc). Significance level of 0.05 was used. 17

33 Fig. 2.1 Daily weather parameters during the periods of experimentation. 18

34 Fig. 2.2 Penman reference evapotranspiration for the site during the periods of experimentation Fig. 2.3 Growing degree days during the period of experimentation (base temperature = 4 C) 19

35 S T4 T2 T1 T3 T4 T1 T2 T Fig. 2.4 Experimental plot design of the native spearmint field. The dotted line is the uniform 100% drip line used during the beginning of each growing period and after each harvest before switching to the spliced lines (solid line). S = Water source. 20

36 Fig. 2.5 Soil moisture content in the mint active root zone for the 2012 growing seasons for plots receiving 100% throughout their growing seasons. SMC = Soil Moisture Content, FC = Field Capacity, PWP = Permanent Wilting Point, MAD = Management Allowable Depletion Results and discussion Irrigation and crop water use There was significant interaction between irrigation level and timing for the irrigation applied for both years (p < 0.001), and significant differences between irrigation amounts among irrigation levels and timings for the two years (Table 2.1). The various irrigation amounts applied per treatment were effective in providing a wide range of soil moisture conditions needed for this study. Interaction between irrigation level and timing for ETc for both years was also significant (p = 0.016). There were significant differences in crop water use values among irrigation levels and timings for the two years as shown in Table 2.1. ETc values were consistently higher than the applied depth of irrigation for all the treatments which shows mining of the soil profile for water by the mint crop, with the greatest extent of this water mining occurring in the drier treatments Fresh hay yields Mean hay yields are summarized in Tables 2.1. Interaction between irrigation level and timing for fresh hay yields for both years was significant (p = 0.024). This implies that both the level of water stress and the timing collectively affected hay production. The response of fresh hay yields to the amounts of irrigation applied per cutting for both years is as shown in Fig As expected, the lowest hay yields corresponded to the lowest amounts of water applied. This is in agreement with earlier trials carried out on native spearmint that showed that water stress 21

37 reduces biomass production (Ben and James, 1993; Delfine et al., 2005; Ram et al., 2005; Corell et al., 2009; Okwany et al., 2012). The first cutting of the 2011 growing season gave the highest mean hay yields across all treatments. This may be due to the initial water stored in the deep soil which was relatively higher in the first year of the experiment. Hay yield was highest for the first cuttings than the second cuttings for both years. This is because of the temperature differences between the two growing period. The growing period prior to the first harvest for both years is cooler than that prior the second harvest (Fig 2.1). Cooler temperatures facilitate leaf production in mint (Loomis, 1978). Fig. 2.6 Changes in fresh hay yields due to changes in irrigation amounts applied Oil yields Mean oil yields are summarized in Table 2.1. There was significant interaction between irrigation level and timing for oil yields (p = 0.014). For each year, there were no significant differences in oil yields among irrigation levels and timings (Table 2.1). Note that although the hay yields decreased significantly with increasing water stress as previously seen, the oil yields 22

38 did not change significantly with increasing water stress. This implies that considerable water can be saved when water stress is managed properly in native spearmint without significantly affecting oil yields. Similar amounts of oil with less water also suggest that water stress may have encouraged essential oil accumulation. The results show that mean oil yields across all treatments were highest for the growing season of This may be due to the higher initial water stored in the lower layers of the root zone in the first year of the experiment. Figure 2.7 shows the timing effect on oil yields. The figures show that the timing differences in oil yields are more pronounced for the 40% and 54% irrigation levels than the 80% irrigation level. For the 40% irrigation level; the driest timings T1 or T2 gave higher oil yields whereas the wettest timing T4 yielded the least oil. For the 54% irrigation level, the wettest timing T4 yielded the most oil for both years while the rest of the timings gave oil yields that were either similar or slightly lower than those for fully irrigated plants. Some relatively drier treatments gave better or similar oil yields in both 2011 and 2012 when compared to fully irrigated plants, suggesting that there is potential to optimize oil yields by manipulating the timing of water stress. With these results, farmers are able to make informed decisions on how to manage deficit irrigation on their farms in a way that will not significantly reduce yields. When a farmer decides on when to apply a particular water stress level to his/her crop, s/he will be able to approximately predict how much oil yield to expect. 23

39 Fig. 2.7 Mean oil yield per cutting for the irrigation levels across the various timings for both years Water use efficiency Water use efficiency was determined as the mass (kg) of mint oil produced per cubic meter of water used (ETc). Table 2.1 presents mean water use efficiencies for both years for each cutting, irrigation levels and timings. There was significant interaction between irrigation level and timing for water use efficiency (p = 0.001), meaning that both irrigation level and timing jointly impacted water use efficiency. Figure 2.8 shows that water use efficiency increased with 24

40 WUE (kg of oil/m^3 of ETc) WUE (kg of oil/m^3 of ETc) increasing water stress. Increasing the amounts of irrigation applied showed decreasing values of water use efficiency, confirming that deficit irrigation can improve water use efficiency in native spearmint R² = st cut st cut 2nd cut R² = R² = 0.91 R² = Irrigation applied (mm) Irrigation applied (mm) Fig. 2.8 Variation of water use efficiency with applied water Oil concentration Oil concentration is the percentage mass (kg) of mint oil extracted per mass (kg) of hay. Table 2.1 presents the mean oil concentration values for both years for each cutting, irrigation level and timing. Both irrigation level and timing jointly affected the oil concentration. This is because the interaction between irrigation level and timing was found to be significant (p-value = 0.001). Oil concentration generally increased linearly with increasing water stress for all the cuttings for both years as shown in Figure 2.9. This confirms that water stress may encourage essential oil accumulation in native spearmint (Charles et al., 1990). Oil concentrations were lower for the first cutting when compared to the second cutting for both years. This is due to temperature differences of the growing periods prior to the respective cuttings (Fig. 2.1). The 25

41 second cutting always has higher growing degree days than the first cutting (Fig. 2.3). Temperature is said to play an important role in regulating flowering of mint, the timing of which is particularly important since oil composition and yield are at optimum levels at flowering (Biggs and Leopold, 1955; Burbott and Loomis, 1967). Fig. 2.9 Oil concentration variation with amounts of irrigation applied per cutting per year. Table 2.1 Effect of irrigation level and timing on irrigation applied, hay yields, oil yields, consumptive use of water, oil concentration and water use efficiency in native spearmint Irrigation Hay yield Oil yield ETc Oil WUE* (mm) (kg/ha) (kg/ha) (mm) Conc² (%) (kg/m 3 ) 2011 Irrigation level (%) a¹ 32,213 a 75.7 a 361 a 0.27 a ab b 34,971 b 80.8 a 362 a 0.27 a a c 34,320 b 76.3 a 394 b 0.24 a b d 36,943 c 79.1 a 415 c 0.24 a b p-value < <0.001 < Timing T1 225 a 31,204 a 75.9 a 359 a 0.28 ab a T2 283 b 34,993 b 80.4 a 386 ab 0.25 b a T3 294 bc 34,948 b 77.5 a 391b 0.24 b a T4 306 c 37,302 c 78.0 a 396 b 0.22 c a p-value < <0.001 < Cutting a 44,498 a 76.4 a 375 a 0.17 a a 26

42 2 290 b 24,726 b 79.5 a 391 a 0.33 b a p-value <0.001 < <0.001 < Irrigation level (%) a 21,295 a 46.9 a 384 a 0.25 a a b 23,776 b 48.0 a 410 ab 0.24 a a c 23,836 b 50.8 a 434 b 0.26 a ab d 24,184 b 47.4 a 499 c 0.23 a b p-value <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 Timing T1 307 a 22,831 a 51.9 a 357 a 0.27 a a T2 380 b 22,806 a 47.6 a 434 b 0.25 ab b T3 402 c 22,506 a 44.7 a 463 bc 0.24 ab b T4 412 c 24,949 b 48.9 a 473 c 0.23 b b p-value <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 Cutting a 31,135 a 44.4 a 432 a 0.14 a a b 14,990 b 52.2 b 432 a 0.35 b b p-value <0.001 <0.001 < <0.001 <0.001 ¹ Means that share a letter down the column are not significantly different at a significance level of ² Oil Conc = Oil Concentration * WUE = Water Use Efficiency Oil component analysis The main components of native spearmint oil are Carvone, Limonene and Pulegone. Native spearmint oil contains large quantities of terpenes; 70 80% of the oil is composed of Carvone and limonene (Maffei et al., 1986; Kim and Hong, 1999; Delfine et al., 2005). Carvone is an oxygenated terpene and it gives the characteristic odor of native spearmint oil. Limonene on the other hand, is a terpene hydrocarbon and is not a favorable component since it s readily oxidized to produce off-flavor (Filipsson et al., 1998). For an improved quality of spearmint oil, a reduction of limonene content is highly desired. Pulegone is the other off-flavor whose content should be low for high quality spearmint oil. Values for various components analyzed for in mint oil are shown in Table 2.2. There was no significant interaction between irrigation level and timing for all the compounds. There were also no significant differences among irrigation levels 27