Cuphea Nitrogen Uptake and Seed Yield Response ... - PubAg - USDA

Cuphea Nitrogen Uptake and Seed Yield Response to Nitrogen Fertilization Marisol T. Berti,* Burton L. Johnson, Russ W. Gesch, and Frank Forcella ABSTRACT

New Crops

Cuphea (Cuphea viscosissima Jacq. × C. lanceolata W.T. Aiton, PSR23) is an oilseed crop that is a rich source of medium-chain fatty acids. Progress has been made on improving cuphea agronomically, but little is known about N fertility requirements for optimum cuphea production. The objective of this study was to determine the N necessary for maximizing seed yield and oil content. Experiment 1 was conducted at Casselton, ND, in 2005 and at Glyndon, MN, in 2005 and 2006 in which fertility treatments (soil + fertilizer N) were 44, 60, 80, 100, 150, and 200 kg N ha–1. Experiment 2 was conducted at Morris, MN, in 2005 and 2006, in which fertility treatments (soil + fertilizer N) were 51, 93, 140, and 185 kg N ha–1. As N fertility increased, plant tissue NO3 –N increased, but as developmental stage advanced, plant NO3 –N was diluted and decreased. According to the regression model, maximum total N uptake at harvest occurred at 139 kg N ha–1, which includes N from the fertilizer and from the soil. Th is could classify cuphea as a medium-N-requirement crop. Seed yield was enhanced with N fertility only at Morris, where maximum seed yield occurred at 185 kg N ha–1. However, seed yield increase obtained with added N fertilizer (134 kg N ha–1) was only 71 kg ha–1. Nitrogen fertilizer cost would be greater than the profit obtained with the incremental seed yield; thus N fertilizer application was not economical in this study.

he genus Cuphea (Family Lythraceae) is native to North, Central, and South America (Graham, 1989). Cuphea is a promising oilseed crop to replace imports of coconut (Cocos nucifera L.) and palm and palm kernel (Elaeis guineensis L.) oils. Cuphea seed oil is rich in medium-chain fatty acids (MCFA) such as caprylic acid (C8:0), capric acid (C10:0), lauric acid (C12:0), and myristic acid (C14:0). Medium-chain fatty acids are used in large volumes in soap and detergent manufacturing and in other personal care and cosmetic products. Cuphea oil has been reported to have promising applications for biodiesel and jet fuel (Geller et al., 1999) also. Of approximately 260 cuphea species that have been identified, Cuphea viscosissima and C. lanceolata hold the most promise for commercial cultivation. A hybrid of these two species, PSR23 (Knapp and Crane, 2000), is being grown commercially in the Midwest mainly in west central Minnesota and the eastern half of North Dakota, with a contracted area of about 300 ha in 2006 (Tony Rosing, Technology Crops International, personal communication, 2006). The PSR23 cuphea (henceforth, simply “cuphea”) is still only semidomesticated and has several agronomic limita-

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M.T. Berti, Facultad de Agronomía, Univ. de Concepción, Chillán, Chile; B.L. Johnson, Dep. of Plant Sci., North Dakota State Univ., Fargo, ND 58105-5051; R. Gesch and F. Forcella, USDA-ARS, North Central Soil Conserv. Res. Lab., 803 Iowa Ave. Morris, MN 56267. Received 4 Aug. 2007. *Corresponding author ([email protected]). Published in Agron. J. 100:628–634 (2008). doi:10.2134/agronj2007.0266 Copyright © 2008 by the American Society of Agronomy, 677 South Segoe Road, Madison, WI 53711. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.

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tions as an industrial oilseed crop. Poor stands are common in commercial fields due to low seed germination and seed vigor, low soil temperatures at seeding, and planting too deep (>13 mm). Cuphea has an indeterminate growth habit which leads to immature seed at harvest, and also seed shattering which is a serious limitation to yield and quality. Little is known about cuphea’s fertility requirements. A cuphea grower’s guide was created to provide guidelines for the farmers in Minnesota who planted cuphea for the fi rst time. Th is guide recommended using band application of fertilizer 50 mm to the side and 50 mm below seed placement at planting (Gesch et al, 2003b). For most soils, the recommendation was to apply 45 kg ha–1 of potassium sulfate (0–0–20–7) along with 224 kg ha–1 of diammonium phosphate (18–46–0), and 112 kg ha–1 of urea (46–0–0) (Gesch et al., 2003b). The fi rst commercialization efforts with cuphea started in Morris, MN, in 2004. Producers planted between 2 and 4 ha of cuphea and fertilized their fields with N, P, and K, prior to planting, at 56, 56, and 22 kg ha–1, of each nutrient respectively (Gesch et al., 2006) based on soil test results. Seed yields fluctuated between 78 and 744 kg ha–1 with an average of 444 kg ha–1 for the 17 ha harvested during 2004. Estimates in the Grower’s Guide were based upon fertilizer needs of other crops because information is lacking for cuphea requirements. Information and references on other nutrient requirements are nonexistent, except for a study that indicated that high concentrations of vanadium interfered with cuphea plant P uptake (Olness et al., 2005). Cuphea producers still have no experimentally-based published information to make N fertilizer decisions. Consequently, the objective of this study was to determine the optimum N fertility for maximum seed yield and oil content in cuphea. Abbreviations: MCFA, medium-chain fatty acids; GDD, growing degree days.

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MATERIALS AND METHODS Field Establishment and Experimental Design This research was conducted at the North Dakota State University (NDSU) Agronomy Seed Farm at Casselton, ND, (46°53’ N, 97°18’ W, elevation 280 m) on a Bearden silty-clay loam (fine-silty, mixed, superactive, frigid Aeric Calciaquoll) and in a producer’s field near Glyndon, MN, (46° 48’ N, 96° 35’ W, elevation 282 m) on a soil with two soil types, Glyndon loam (coarse-silty, mixed, superactive, frigid Aeric, Calciaquoll) in 2005, and Borup loam (coarse-silty, mixed, superactive, frigid Typic Calciaquoll) in 2006. These three environments were analyzed as Exp. 1. Experiment 2 was conducted, at the West Central Research and Outreach Center (WCROC), at Morris, MN, (45°59’ N, 95°91’ W, elevation 344 m) in 2005 and 2006. In 2005, the soil at WCROC was mostly Hamerly clay loam (fine-loamy, mixed, superactive, frigid Aeric Calciaquoll,) grading into a depression of Parnell clay loam (fine, smectitic, frigid Vertic Argiaquoll,). At the Exp. 2 site, in 2006, the soil was mostly Parnell grading into Hamerly. Rainfall amounts were recorded automatically at all environments. Soil samples for analysis were taken at all locations the spring when the crop was planted. Soil determinations made were pH, organic matter, N–NO3, P, and K. Previous crops were sugarbeet (Beta vulgaris var. saccharifera L.) at Glyndon in 2005 and 2006, hard red spring wheat (Triticum aestivum L.) at Casselton in 2005, and soybean [Glycine max (L.) Merr.] at Morris in 2005, and sudangrass [Sorghum bicolor (L.) Moench] at Morris in 2006. Cuphea was sown at all locations with a cone plot planter. Planting dates at Glyndon in 2005 and 2006, and Casselton in 2005 were 23 May, 22 May, and 6 June, respectively. At Morris, planting dates were 17 May in 2005 and 20 May in 2006. Seeding rates were 21 kg ha–1 in Exp. 1 and 11 kg ha–1 in Exp. 2. Differences in seeding rate were not expected to cause differences in seed yield, since previous research showed that cuphea seed yield was similar in populations ranging from 60 to 228 plants m–2 (Gesch et al., 2003a). Nitrogen fertility treatments consisted of a nonfertilized check treatment, wherein the soil contained about 44 kg N ha–1 (upper 0.62 m of soil), as well as N treatments where N was brought up to 60, 80, 100, 150, and 200 kg N ha–1 (soil N + N fertilizer) for Exp. 1 environments. Check fertility of 44 kg N ha–1 was calculated averaging initial soil nitrate for the three environments. The source of N was urea [CO(NH2)2]. For Exp. 2, N fertility treatments were a check which averaged initial soil nitrate for both Morris 2005 and Morris 2006 (51.5 kg N ha–1), as well as 93, 140, and 185 kg N ha–1 (soil N + N fertilizer). For both experiments, urea was hand-broadcast immediately before planting in each individual plot, and then incorporated with a plot field cultivator. The experimental design at all sites was a randomized complete block with four replicates. Experimental units were 2.0-m wide by 7.6-m long with six rows separated at a 0.31-m row spacing for Exp. 1 environments. For Exp. 2, experimental units were 3.6-m wide and 6.1-m long with six rows separated at a 0.61-m row spacing. Weed control for Exp. 1 plots included preplant incorporation of trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropylp-toluidine) (0.5 kg a.i. ha–1) followed by hand-weeding Agronomy Journal

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as needed. In Exp. 2, in both years, plots were treated with isoxaflutole [4-(2-ethylsulfonyl-4-trifluoromethyl-benzoyl)-5cyclopropyl isoxazole] (80 g a.i. ha–1) immediately after planting. Cuphea PSR23 is tolerant to both herbicides (Forcella et al., 2005a). Plant Sampling and Evaluations Dependent variables evaluated were plant NO3 –N content; plant and seed total N content; biomass and seed yield; harvest index; test weight, oil content; and soil NO3 –N at 0 to 0.3, 0.3 to 0.6, 0.6 to 0.9, and 0.9 to 1.2-m depths. Aboveground portions of whole plants were collected at three developmental stages: vegetative (V6), bloom (R2), and harvest (R5) for plant NO3 –N content determination (Berti and Johnson, 2008). Determination of nitrate in plant tissue was by the nitration of salicylic acid colorimetric method using 0.1 g of dried and ground plant tissue (Cataldo et al., 1975). Immediately prior to harvest, seed and biomass samples were collected from plants in each plot. Then 0.1 g of dried, ground plant tissue was analyzed by the Kjeldahl procedure to determine total plant N content (includes N in proteins) in the biomass and seed. Nitrogen uptake in the biomass was determined by multiplying biomass and seed N content by biomass and seed yield. Biomass samples were taken from a 1-m 2 area within each plot where plants were cut at the stem base. Plant height was measured for five randomly selected plants from the interior of each plot. Thereafter, the interior four rows of each six-row plot where harvested with a self-propelled plot combine (Hege 125B, Wintersteiger, Salt Lake City, UT)1 to estimate seed yield. Harvest index was calculated as the percentage of dry seed weight divided by the total dry above-ground biomass from the 1-m 2 harvested area within each plot. Test weight was calculated by determining the weight of 40 mL of seed from the harvested seed yield from each plot. Seed oil content was determined with a Newport 4000 Nuclear Magnetic Resonance (NMR) Analyzer, Oxford Institute Ltd. Samples were dried in an oven at 110°C for 3 h and then cooled to room temperature before the oil determination. Soil samples from 0 to 0.15 m were taken for the analysis of organic matter, pH, P, and K and 0 to 0.60 m for NO3 –N 2 wk prior to fertilization, according to the recommended procedures for soil analysis (Franzen and Cihacek, 1996). Soil samples were collected from 0 to 0.3, 0.3 to 0.6, 0.6 to 0.9, and 0.9 to 1.2 m depths from all plots for Exp.1, approximately 2 wk after harvesting. Soil samples were collected from 0 to 0.3 and 0.3 to 0.6 m depths from all plots for Exp. 2 in the fall, approximately 2 wk after harvesting. The soil samples were analyzed for NO3 –N using the transnitration of salicylic acid method (Vendrell and Zupancic, 1990) by the Soil and Plant Analysis Laboratory, North Dakota State University. 1Mention of trade names, proprietary products, or vendors does not constitute

a guarantee or warranty for the product by North Dakota State University and does not imply its approval to the exclusion of other products or vendors that may be suitable.

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Table 1. Growing-season rainfall for Casselton, ND, 2005, and Glyndon and Morris, MN, 2005 and 2006, and the deviation (Dev.) from the longterm average. Casselton† 2005

Glyndon† 2005

2006

Table 2. Soil analyses for two depths, at Casselton, ND, 2005, Glyndon, MN, 2005 and 2006, and Morris, MN 2005 and 2006, before Exp. 1 and 2 were sown. 0–0.15 m

Morris‡ 2005

2006

Environment

pH

Month

Total Dev. Total Dev. Total Dev. Total Dev. Total Dev. mm May 64 –4 52 –5 48 –9 55 –20 75 –1 June 161 70 150 57 24 –69 173 72 47 –55 July 34 –48 50 –54 63 –41 68 –26 19 –75 August 113 45 147 82 35 –30 92 16 35 –41 September 104 50 38 –24 92 30 97 39 119 60 Total 476 437 262 486 294 † NDAWN, 2007. ‡ ARS-USDA, 2007.

Statistical Analysis Statistical analysis was conducted by using standard procedures for a randomized complete-block design (Steel and Torrie, 1980). Each location-year combination was defined as an “environment” and was considered a random effect in the statistical analysis. Nitrogen fertility treatments and phenological stages were considered fi xed effects. Experiment 1 environments were analyzed by analysis of variance separately for each trait. The mean square errors for each trait were considered homogeneous among environments and analysis was performed across environments. Experiment 2 environments were combined together since all trait variances were homogeneous. Regression analysis was considered for trait responses when there was a significant main effect for N fertility. Linear, quadratic, and cubic regression models were tested with the corresponding error. The regression models are presented and all parameter estimates were significant at P ≤ 0.05. The plant NO3–N sample data, for Exp. 1, were analyzed according to a randomized complete-block design with a splitplot arrangement where the main plot was the phenological stage at sampling (vegetative, bloom, and harvest), and the subplot was the N fertility treatments (44, 60, 80, 100, 150, and 200 kg N ha–1). The soil NO3–N sample data for Exp. 1 were analyzed according to a randomized complete-block design with a splitplot in space (Steel and Torrie, 1980), where the main plot was the N fertility treatments (44, 60, 80, 100, 150, and 200 kg N ha–1) and the subplot was the soil depth (0–0.3,0.3–0.6, 0.6–0.9, and 0. 9–1.2 m). The same analysis was used to combine soil NO3–N data from Exp. 2, where only two environments (Morris 2005 and 2006), two depths (0–0.3 and 0.3–0.6 m), and four N fertility treatments (51, 93, 140, and 185 kg N ha–1) were the independent variables. Analysis of variance was conducted with SAS System (SAS Institute, 2005). RESULTS AND DISCUSSION Rainfall and Soil Analysis Growing season rainfall varied considerably across environments (Table 1). Growing season rainfall was greater at Morris and Casselton in 2005 than at Glyndon in 2005. Rainfall for the 2006 growing season was below average at Glyndon and Morris, which may have reduced plant growth and seed yield at these environments. 630

Casselton 2005 Glyndon 2005 Glyndon 2006 Morris 2005 Morris 2006

7.8 8.1 8.0 7.7 7.4

OM† % 4.9 2.2 0.6 5.0 3.4

0–0.60 m

P K mg kg –1 20 395 10 85 7 103 6 133 4 240

NO3 –N kg ha –1 58 45 30 44 59

† OM = Organic matter.

Table 3. Analysis of variance and mean squares for cuphea plant-tissue nitrate content for six N fertility treatments and three developmental stages for Exp. 1 and 2 (environments combined within each experiment). Exp. 1 Sources of variation df NO3 –N Environment (Env) 2 44,462,037 Rep(Env) 9 521,180 Stage 2 136,977,037** 4 9,398,064** Env × stage 18 934,649 Rep × stage(Env) Nitrogen 5 5,642,126* 10 614,957 Env × N 10 416,689 Stage × N 20 495,994 Env × stage × N Residual 135 623,511 CV, % 25

Exp. 2 df 1 6 2 2 12 3 3 6 6 54

NO3 –N 13,007,947 816,458 15,216,205** 6,953,957** 229,655 5,670,479 1,840,519* 463,336 161,198 323,560 28

* Significant at 0.05 probability level. ** Significant at 0.01 probability level.

Low soil P levels at the Glyndon environments were improved by fertilizing the field with 22 kg P2O5 ha–1 as triple superphosphate (0-46-0) (Table 2). Organic matter content was the lowest at the Glyndon environments. Initial NO3–N was similar among environments. Plant Nitrate Nitrogen The stage of sampling and N fertility main effects were significant for the Exp. 1 combined analysis (Table 3). Only the environment by stage interaction was significant, but since environments were considered random, discussion will be based on the significance of main effects. As N fertility increased, the nitrate content in plant tissue also increased (Fig. 1A), a common response for many plant species (Black, 1992; Vogel et al., 2002; Teutsch and Tilson, 2004). As growth stage advanced, plant NO3–N content decreased with highest values in the vegetative stage and lowest at harvest (Fig. 1B). This was probably due to a growth-dilution effect. Nitrate-absorption curves leads dry-matter-production curves until reproductive development in most plant species (Black, 1992); thereafter, dry matter accumulation is much faster than N absorption, which causes the dilution effect. Also, nitrate content diminishes as plants grow because it is reduced to ammonia and N-rich organic compounds in the plant (Lam et al., 1996). For Exp. 2, the environment by stage and environment by N interactions were significant. Main effects, however, were not (Table 3). Plant-tissue nitrate content decreased with advancing developmental stage for the Morris 2005 environment; Agronomy Journal

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Table 4. Sources of variation, degrees of freedom, and mean squares for plant and seed N, and total plant N uptake for six N fertility treatments for Exp. 1, and for four N fertility treatments for Exp. 2 (environments combined within each experiment). Sources of variation Experiment 1 Environment (Env) Rep (Env) Nitrogen (N) Env × N Residual CV, % Experiment 2 Environment Rep(Env) Nitrogen Env × N Residual CV, %

Plant N

Seed N

Total N uptake

2 9 5 10 45

0.89 0.04 0.14 0.12 0.15 21

0.25 0.06 0.06* 0.02 0.03 6

13,003 897 1,588 900 1,244 25

1 6 3 3 18

0.01 0.09 0.07 0.04 0.02 10

0.12 0.02 0.01 0.04 0.06 8

43,244 531 1,797 251 416 16

df

* Significant at 0.05 probability level.

lack of trend that was noted in 2005). The roots may not have been taking up nutrients later in the summer, and plants were small indicating lack of growth. Plant and Seed Total Nitrogen Total plant N content at the end of the season was not influenced by N fertility levels at any environment (Table 4, Exp. 1 and 2). The N main effect was significant for seed N content in Exp. 1. Seed N content increased as N fertility levels increased (Fig. 2). The highest seed N content occurred in the treatment with the highest fertility level [200 kg N ha–1 (29 g kg–1)] (Fig. 2). An increase in N and protein content in the seed in response to higher N rates has been reported in wheat (Hussain et al., 2006) and canola (Brassica napus L.) (Ramsey and Callinan, 1994).

Fig. 1. Regression model for mean nitrate content in cuphea tissue (A) at six N fertility treatments averaged across three phenological stages (vegetative, bloom, and harvest) and three environments (Casselton, ND, 2005, and Glyndon,MN 2005 and 2006) (Env) for Exp. 1, and averaged across four N fertility treatments for Morris, MN 2005 and 2006 for Exp. 2 (P ≤ 0.05) (B) at three phenological stages (vegetative, bloom, and harvest) averaged across six N fertility and three environments for Exp. 1, and averaged across four N fertility for Morris 2005 and 2006 in Exp. 2.

Total Plant Nitrogen Uptake The N main effect and the environment by N interaction were not significant for total N uptake for the combined analysis in Exp. 1 or Exp. 2 (Table 4). Mean N uptake was 139 kg N ha–1 for the three environments (Casselton 2005, Glyndon 2005, and Glyndon 2006) combined and 123 kg N ha–1 for the 2005 and 2006 Morris environments combined. Interestingly, N uptake was 131 and 106 kg N ha–1 for the nonfertilized treatment in Exp. 1 and Exp. 2, respectively. Total N uptake values for the check treatment indicate that there was more nitrate available for the plant in the soil than detected by the initial soil test. Mineralization of N from crop residue and organic matter may explain the greater uptake compared with the initial nitrate soil test. Since cuphea’s rooting is restricted to the upper 0.4 m of the soil profi le where 65 to 85% of the roots are found within the upper 0.20 m of the soil (Sharratt and Gesch, 2004), N uptake from the soil below the sampling depth probably did not occur. According to the total N uptake, cuphea could be classified as having a medium N-fertilizer requirement and would be in the same N requirement group as flax (Linum usitatisimum L.), oat (Avena sativa L.), sunflower (Helianthus annuus L.), and buckwheat (Fagopyrum esculentum Moench) (Schlegel et al.,

Fig. 2. Regression model for seed N content as affected by N fertility for Exp. 1 averaged across three phenological stages and three environments. however, the maximum nitrate content (2079 mg kg–1) was

observed at the bloom stage for the Morris 2006 environment (Fig. 1B). In addition, nitrate content increased with increasing N fertility treatments at Morris in 2005, but this trend was not observed at Morris in 2006. At Morris 2006, the highest tissue nitrate content was observed from the 140 kg N ha–1 rate (Fig. 1A). The severe drought that occurred between June and August of 2006 may have been a factor affecting this trend (or Agronomy Journal

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Table 5. Analysis of variance and mean squares for cuphea plant height, biomass and seed yield, harvest index, oil content, and test weight for six N fertility treatments in Exp. 1, and four treatments in Exp. 2 (environments combined within each experiment). Sources of variation Experiment 1 Environment (Env) Rep (Env) Nitrogen (N) Env × N Residual CV, % Experiment 2 Environment Rep(Env) Nitrogen Env × N Residual CV, %

df 2 9 5 10 45

Plant ht.

Biomass yield

Seed yield

178 123 44 79 50 7

7,708,727 2,922,802 2,169,792 1,907,618 2,209,449 20

11,372 9,238 2,770 4,980 2,861 24

1 4,731 158,030,490 772,574 6 34 2,603,181 14,198 3 37 1,543,612 10,145* 3 20 1,679,142 468 18 27 1,696,945 4,965 8 16 18

Harvest Oil Test index content wt. 1.5 1.6 1.1 1.4 0.8 31

176.0 4.7 4.6 3.4 4.3 7

7.8 1.5 0.5 0.2 0.7 18

26.2 1.3 3.1 1.2 2.0 5

* Significant at 0.05 probability level.

2005). In North Dakota, N fertilization recommendations for flax, oat, sunflower, and buckwheat are 100, 100, 84, and 73 kg N ha–1 for a yield goal of 1881, 2500, 1680, and 1546 kg ha–1, respectively (Franzen and Cihacek, 1996). Plant Height No main effects or interactions were significant from the plant height analysis combined across environments for Exp. 1 and Exp. 2 (Table 5) indicating N did not influence plant height. Mean plant height in Exp. 1 was 0.82 m. The ability of cuphea to branch and use efficiently the space surrounding the plant may have played a role in maintaining a similar height despite different N availability in the soil. Biomass Yield The N main effect and the N by environment interaction were not significant for biomass in Exp. 1 or Exp. 2 (Table 5).The mean biomass yield across six N fertility treatments, three phenological stages, and three environments for Exp. 1 was 7392 kg ha–1 (data not shown). Many crops have a significant increase in biomass yield as N fertility is increased. Cuphea’s current early stage of domestication may have limited its ability to respond to N fertilizer. Initial available N may have been greater than required by the plants, as suggested by

Fig. 3. Regression model for seed yield as affected by N fertility for Exp. 2 averaged across environments.

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380 10 17* 1 7 5

the total N uptake in the check treatments, which was not greatly different from plants that received added N. Also, the slow rate of ground cover of cuphea may be responsible for the lack of response on biomass yield. Also, initial ammonium levels or mineralization of organic matter in the soil were not measured, which may have added to the N pool available for plant uptake to sufficiency levels.

Seed Yield The N main effect and the environment by N interaction were not significant for seed yield for Exp. 1 (Table 5). All three environments in Exp. 1 had limited 23.4 2.7 seed yield (mean seed yield, 206 kg ha–1), which may 0.9 have hindered a response to N. A significant response 0.9 was observed for the N effect on seed yield for Exp. 1.6 2 (Table 5). In Exp. 2, a linear increase was observed 2 for seed yield as N fertilizer was increased (Fig. 3). Maximum seed yield (429 kg ha–1) was reached with 185 kg N ha–1 (Fig. 4). According to the regression model, seed yield with 0 kg N ha–1 in the soil was 325 kg ha–1, which differed from maximum yield due to fertilizer addition by only about 25% (104 kg ha–1). The response was observed only at the Morris environments which were higher yielding environments. Soil at the other environments may have had enough nitrates in the soil to meet plant requirements. Nitrogen fertilizers are expensive. At a price of $1 kg–1 N (based on ammonia fertilizer), a cuphea seed contractor’s price of $1 kg–1 seed (Tony Rosing, personal communication, 2006) and a seed yield to fertilizer response of only 0.6 (Fig. 3), a net economic loss would occur and not be practical or sustainable for N fertilizer application in cuphea. Some other oilseed crops, such as canola and sunflower, have a quadratic response in seed yield to increasing N fertilizer rates (Jackson, 2000). This response was not observed with cuphea in this study. With the current cuphea variety available for use, enough residual N generally exists in upper Midwestern arable soils for cuphea to grow close to its site-specific seed yield potential. Harvest Index The N main effect and the N by environment interaction were not significant for harvest index for Exp. 1 or Exp. 2 (Table 5). The mean value for harvest index was 2.8%, a considerable lower value than those reported by Forcella et al. (2005b)

Fig. 4. Regression model for test weight of cuphea seed as affected by N fertility averaged across three environments for Exp. 1.

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Table 6. Analysis of variance and error mean squares for cuphea soil NO3 –N for Exp. 1 and 2 (environments combined within each experiment). Sources of variation Environment (Env) Rep(Env) Nitrogen Env × N Rep × N (Env) Depth Env × Depth Rep × Depth (Env) Nitrogen × Depth Env × N × Depth Residual CV, %

Exp. 1 df 2 9 5 10 45 3 6 27 15 30 135

Soil NO3 –N 2086 496 3032* 806** 300 7010 6381** 294 1519** 265 170 61

Exp. 2 df 1 6 3 3 18 1 1 6 3 3 18

Soil NO3 –N 109,528 2,169 13,319 6,269 2,842 22,861 14,232 4,949 2,958 3,788 2,950 69

Fig. 5. Regression models for residual soil nitrate concentration at different soil depths as affected by N fertility for Exp. 1, averaged across environments. *P ≤ 0.05.

* Significant at 0.05 probability level. ** Significant at 0.01 probability level.

for cuphea (8–10%) and for other oilseeds grown in North Dakota. For instance, harvest index for canola is approximately 30%, for flax 15 to 25% (Dash, 2005; Kandel and Porter, 2006), for sunflower 36% (De la Vega and Hall, 2002), and for soybean 42% (Prince et al., 2001). For cuphea to become a successful crop, harvest index will need to be increased to values similar to that of other crops.

depths (Fig. 5). Nitrogen fertility did not affect soil NO3–N below 0.6 m, which indicates that the majority of cuphea root mass is located in the upper 0.6 m of the soil profi le, and no leaching of nitrate occurred to deeper soil layers. Other studies indicated that cuphea’s rooting is restricted to the upper 0.4 m of the soil profi le where 65 to 85% of the roots are found within the upper 0.20 m of the soil (Sharratt and Gesch, 2004). Crops absorb nitrate to sufficiency levels, but thereafter, nitrate accumulates in soil and can leach to deeper layers in the soil profi le (Black, 1992). Halvorson et al. (1999) reported a similar linear increase of soil NO3–N as N rates were increased to 102 kg N ha–1 in sunflower. Differences in soil texture and rainfall may have caused a differential nitrate movement within soils at different environments. Accumulation of nitrate in the top soil layers in a silty-clay loam, with below average rainfall, occurs even in a deeper-rooted crop such as switchgrass (Panicum virgatum L.) (Vogel et al., 2002).

Seed Oil Content The N main effect and the environment by N interaction were not significant for seed oil content for Exp. 1 or Exp. 2 (Table 5). Mean seed oil content averaged across N fertility treatments was 297 and 292 g kg–1 for Exp. 1 and Exp. 2, respectively. The negative correlation between seed oil content and N fertilizer reported in canola and sunflower (Steer and Seiler, 1990; Jackson, 2000; Starner et al., 1999; Cheema et al., 2001) was not observed for cuphea. Test Weight The N fertility main effect was significant for test weight for Exp. 1 (Table 5). Test weight increased as N fertility increased (Fig. 4). The highest test weight of 54 kg hL–1 was observed for N fertility rates of 150 and 200 kg N ha–1. The N main effect and the environment by N interaction for test weight were not significant for Exp. 2 environments combined (Table 5). Test weight for canola, sunflower, and soybean are 65, 46, and 78 kg hL–1, respectively (Canadian Grain Commission, 2006).

CONCLUSIONS Plant nitrate levels showed the greatest response to N fertility treatments. As N fertility increased, plant tissue NO3–N increased. As developmental stage advanced, plant NO3–N decreased due to a dilution effect. Maximum total N uptake at harvest occurred at the 139 kg N ha–1 N fertility treatment. Based on the results of average N uptake, a general N fertility recommendation would be 140 kg N ha–1 (soil + fertilizer) to maximize seed yield. This could classify cuphea as medium-N-requirement crop. Seed yield was enhanced with N fertility only at the Morris environments where maximum seed yield was obtained with 185 kg N ha–1. However, seed yield increase obtained with added N fertilizer (134 kg N ha–1) was only 71 kg ha–1. Nitrogen fertilizer cost would be greater than the profit obtained with such an incremental seed yield; thus, N fertilizer application would not be economical in the currently available commercial line of cuphea at this time. Although nitrate-N increased in cuphea plant material, it was not necessarily translated into greater seed yield. This may in large part be due to cuphea’s characteristic low harvest index. Selecting for improved harvest index in cuphea could perhaps increase its seed yield responsiveness to N fertility.

Residual Soil Nitrate The N main effect, the environment by N, environment by depth, and N by depth interactions were significant for residual soil nitrate for Exp. 1 (Table 6), but no significant differences were observed for any of the main effects or interactions for Exp. 2 environments. More residual N may be expected in inherently low-productivity sites, especially at the shallower depths close to where it was applied. Plants at sites with high production potential would be expected to scavenge more N and use it more uniformly across soil depths. Only the main effect is discussed because the interactions with environments are random. Residual soil nitrate increased linearly with added N fertilizer at the 0 to 0.3 and 0.3 to 0.6-m Agronomy Journal

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Agronomy Journal

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Volume 100, Issue 3

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2008