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Production Papers

Slope Position, Nitrogen Fertilizer, and Fungicide Effects on Diseases and Productivity of Wheat on a Hummocky Landscape

Agriculture and Agri-Food Canada, Box 1240, Hwy 6 South, Melfort, SK, Canada S0E 1A0

^* Corresponding author (kutcherr{at}agr.gc.ca)

Received for publication January 15, 2005.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Precision agriculture technology allows growers to selectively apply inputs to different management units within a single field. A 4-yr study consisting of a split-split block experiment was conducted in the northern prairies to determine the effects of foliar fungicide (FU) and N fertilizer application to slope (SL) position based management units across a hummocky landscape on leaf spot and root diseases, biomass and seed yield, and seed quality of wheat (Triticum aestivum L.). Fertilizer rates (0, 40, 80, and 120 kg N ha^–1) were applied in strips across two SL positions (upper and lower) as main plots with FU application (treated and untreated) as subplots. Leaf spot diseases were consistently more severe on the upper than lower SL and reduced by FU treatment but varied inconsistently with changes in N rate. Biomass and seed yield in the two dry years were greater on the lower than upper SL. They were increased by FU treatment in 3 of 4 yr and N rate in 1 yr. Thousand-kernel weight (TKW) and grain test weight (TW) were usually greater on the upper than lower SL, and TKW was increased by FU application in 2 yr. Protein content usually increased with increasing N rate, but the effect of SL and FU varied among years. The paucity of interactions among treatment factors indicated that selective application of FU and N fertilizer to SL position based management units for improvement of yield and seed quality was not warranted.

Abbreviations: FU, fungicide • GS, growth stage • SL, slope • TKW, thousand-kernel weight • TW, grain test weight

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
PRECISION AGRICULTURE is defined as the art and science of utilizing advanced technologies for enhancing crop production while minimizing potential environmental pollution (Koch and Khosla, 2003). It involves targeting of inputs to crop requirements on a localized basis (Stafford, 1996) and can minimize unwanted accumulation or migration of input residues, save money, and avoid interactions that would decrease crop yield (Wallace, 1994). While the intuitive appeal of this approach has created high expectations, the physical evidence supporting the agronomic (Lowenberg-DeBoer and Swinton, 1997; Sawyer, 1994) and environmental (Larson et al., 1997) benefits of precision agriculture is limited.

The amount of soil moisture and fertility depends on the SL position of the landscape unit (Pennock et al., 1987). A general trend of lower > middle > upper SL has been observed for the depth to carbonate layer, A horizon thickness, solum thickness, and organic matter (Manning et al., 2001a), as well as soil moisture, nitrate N, extractable P, extractable K, and sulfate S (Manning et al., 2001b), with the opposite trend for the pH of the Ap horizon (Manning et al., 2001a). The amount of N mineralization and total N usually increases with organic matter and degree of landscape convergence, but an explanation of systematic differences in plant available N as a function of landscape requires integrating the effects of soil properties, N cycling, and previous crop management (Fiez et al., 1994a). Residual nitrate N content in the spring was found to be variable either without spatial pattern (Mulla, 1993) or greater in convergent (Malo and Worcester, 1975) or divergent (Farrell et al., 1996) areas. The landscape effect on soil moisture was absent under the combination of high infiltration rate, reduced runoff, and coarse soil texture (Helvey et al., 1972) and for a fine-textured soil with below-normal precipitation (Miller et al., 1988).

Crop production was strongly influenced by landscape position (Afyuni et al., 1993; Moulin et al., 1994). In a hummocky landscape (a complex pattern of knolls and depressions), water distribution is the fundamental factor controlling nutrient cycling and movement, soil productivity, and crop yield (Pennock et al., 1987; Peterson et al., 1993, Grant and Flaten, 1998; Walley et al., 2001). Normally, lower SL positions (e.g., footslopes) have greater water and nutrient content and produce higher crop yield (Halvorson and Doll, 1991; Fiez et al., 1994b; Stevenson et al., 1995). Increase in wheat yield from upper to lower SL position has been attributed to the redistribution of topsoil (Gregorich and Anderson, 1985; Pennock and de Jong, 1990; McConkey et al., 1997) and moisture (Verity and Anderson, 1990). The spring wheat yield in both fertilized and unfertilized treatments and its increase from fertilization ranked foot SL > back SL > shoulder SL, with a significant effect of SL position in the unfertilized treatment (Nolan et al., 1995). Similarly, wheat and canola (Brassica napus L.) yields were higher on the foot SLs and lower on the shoulder SLs (Nolan et al., 1999). In two studies, the optimum N rate for canola yield was less at the lower SL than for the upper SL (Nolan et al., 1999; Kutcher et al., 2005).

Varying the fertility rate has been suggested as an appropriate technique to optimize the efficiency of inputs and crop production on a hummocky landscape (Beckie et al., 1997). However, the increase in sorghum [Sorghum bicolor (L.) Moench] yield from variable-rate over uniform fertilizer treatments was not economical because of low overall yields (Yang et al., 1999), and there was little economic rationale for using variable N rate due to the highly variable response of wheat seed yield (Walley et al., 2001). Knowledge of the crop response to fertilizer N in specific soil and climatic conditions should help producers to improve crop production and reduce losses of N across landscape units.

Variation in moisture level and soil properties within a field may influence crop stand and microenvironment within the canopy, which may in turn alter the severity of plant diseases (Rotem and Palti, 1978) and affect the yield and crop response to fertilization. Grain yield, soil water content, and soil N availability during the growing season, as well as the severity of leaf and root diseases of wheat, were observed to vary with the landscape position (Stevenson et al., 2001). A state-place model indicated that water content, availability of soil N, and severity of common root rot contributed to the landscape-scale differences of wheat seed yield in a pea (Pisum sativum L.)–wheat–barley (Hordeum vulgare L.) rotation, but only leaf and root disease severity explained landscape-scale yield variation in a wheat–wheat–barley rotation. Wheat yield and severity of common root rot [Cochliobolus sativus (Ito & Kuribayashi) Drechs. ex Dastur] increased from upper to lower SL position, with the opposite trend for leaf spot diseases; but changes in N and P fertilizer rate or seed rate did not influence disease severity (Kutcher et al., 1999). Fernandez et al. (1998) observed an increase in leaf spot severity with an increase in N deficiency on spring wheat in southern Saskatchewan. However, increasing N rate increased leaf spot severity due to leaf rust (Puccinia recondite Rob. ex f. sp. tritici) on winter wheat in Louisiana, and FU application increased seed yield as a result of leaf rust control at 4 of the 12 sites (Mascagni et al., 1997).

Many studies have investigated the potential of precision agriculture to vary fertilizer application across a landscape, but only a few have explored the potential benefits of this technology to guide FU application (Secher et al., 1995; Secher, 1997; West et al., 2003). To manage plant diseases and fertilizer inputs using precision agriculture technology, it is necessary to understand the interaction of SL position, N fertilization, and FU application on crop diseases and productivity. The objective of this study was to determine the effects of SL position, N fertilization, and FU application on wheat diseases, biomass and seed yield, and seed quality.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Site Description
Field experiments were conducted from 1998 to 2001 near Prince Albert, SK, Canada located at 53°0' N, 105°48' W. The site is hummocky, with as much as 9% average SLs and maximum relief of approximately 5 m. The soil is an Orthic Black Chernozem (Udic Haploboroll) with a loam texture and pH of 7.1. The landscape topography was defined using aerial photographs, as described by Walley et al. (2001). Landscape positions were classified as either upper or lower SL due to variation in elevation over short distances, which resulted in little to no distance that could be defined as mid-SL. Monthly and long-term mean precipitation during the growing season is presented in Table 1.

In 1999 to 2001, seed samples were assessed for TKW and TW and were analyzed for total N (AOAC, 1995). Protein content in seed was calculated by multiplying the total N content in seed by 5.7 (Williams et al., 1998). The uptake of total N in seed was calculated from the seed yield and total N content in seed.

Statistical Analysis
Data analysis was performed using PROC MIXED of the Statistical Analysis System (SAS Inst., 1999), with N, SL, and FU as fixed variables and replication a random variable. Treatment means were compared by Fisher's protected LSD, with significance determined at P 0.10 because landscape-scale field experiments inherently have a high degree of variability (Walley et al., 1996). Polynomial regressions were used to estimate optimum N rate and maximum seed yield for different SL positions and FU treatments.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Data analysis indicated few significant interaction effects of N rate, SL position, and FU application (Table 3). Therefore, significance levels for the main and interaction effects are presented in Table 3, and data for only the main effects of N rate, FU, and SL treatments are presented in Table 4. Significant interaction effects and estimated optimum N rates and maximum seed yields are discussed in the text.

Leaf Spot and Common Root Rot Diseases
Leaf spot infections were caused by a number of common pathogens in each year of the study. The principle pathogens were Septoria tritici Roberge in Desmaz., Stagonospora nodorum (Berk.) Castellani & E.G. Germano, Pyrenophora tritici–repentis (Died.) Drechs., and Cochliobolus sativus (Ito & Kuribayashi) Drechs. ex Dastur, with only trace levels of other pathogens. Separate assessment of the upper leaves from the whole plant was considered important because of their significant contribution to yield and because the expected benefit of FU is greater on these leaves than others. Leaf spot severity on the upper leaves (flag and penultimate) varied, with a general trend of 2000 > 1999 > 2001 (Table 4).

Leaf spot severity of upper leaves did not vary with N rate in 1999, varied inconsistently among N rates in 2000 although the trend was of lower severity with increased N rate, and increased with increasing rate of N in 2001 (Tables 3 and 4). The interaction of SL x FU was significant in 2000 and indicated that FU reduced severity of upper leaves by a greater magnitude on the lower SL (from 78.5 to 50.0%) than on the upper SL (from 91.8 to 77.0%) although severity was greater on the upper SL regardless of FU treatment. Consistent with 2000, in 1999 and 2001, leaf spot severity on the upper leaves was greater on the upper SL position than the lower SL position and reduced by FU treatment although there was no interaction of the factors (Table 4). From another study in Saskatchewan, Kutcher et al. (1999) also reported an increase in wheat leaf spot diseases from the lower to the upper SL.

In 1998, a significant N x SL x FU interaction was detected (Table 3) as a result of reduced whole-plant leaf spot severity as N rate increased for both SL positions and FU treatments, except on the upper SL at 120 kg N ha^–1 where whole-plant leaf spot severity was similar regardless of the FU treatment. This may have been due to delayed maturity at the 120 kg N ha^–1 rate in 1998, which increased the length of time between FU application and leaf spot assessment, resulting in a much smaller difference in the amount of infection between the FU treatments. In 1999, the N x SL x FU interaction was also significant, but the interaction was not consistent with the previous year. In 1999, whole-plant leaf spot severity increased with increased N rate on the lower SL, and FU treatment decreased severity the most at 40 kg N ha^–1 but was less effective at other N rates. On the upper SL, disease severity was similar regardless of N rate, and FU treatment reduced disease severity, but only at 0 and 40 kg N ha^–1. The difference between these years may have been due to environment during maturation. In both years, maturity was delayed as N rate increased, but warm, dry conditions prevailed in 1998, restricting disease development while conditions in 1999 were cooler, with above-normal precipitation, promoting disease development. No interaction effects were detected in 2000 for whole-plant leaf spot rating, and severity was greater on the upper SL than the lower and was reduced by FU treatment (Table 4). In 2001, significant interactions for N x SL and N x FU occurred for whole-plant leaf spot severity (Table 3). The N x SL interaction was due to greater severity on the upper SL than the lower SL at 0 and 40 kg N ha^–1, by 3.2 and 2.8 points, respectively, but similar for both SL positions at 80 and 120 kg N ha^–1. This was likely due to the dry conditions in 2001 as the crop at the lower SL position had access to more moisture, and it was therefore less stressed and better able to withstand leaf spot disease infection than plants at the upper SL at 0 and 40 kg N ha^–1. However, treatments with increased N fertility (80 and 120 kg N ha^–1) were delayed in maturity regardless of SL position, resulting in increased leaf spot infection and greater leaf senescence as a result of the dry conditions, which may have confounded leaf spot severity data for these later-maturing treatments. For the N x FU interaction, the reduction in whole-plant leaf spot severity was large at 0 kg N ha^–1 (2.3 points), but differences were not detected at other N rates. The inconsistency of leaf spot disease severity response to changes in N rate has been reported in the literature (Fernandez et al., 1998; Mascagni et al., 1997).

Common root rot severity was affected by N rate in 2000 (Tables 3 and 4) when severity increased by 0.3 points (of a possible score of 3.0) with increasing N rate from 0 to 40 kg N ha^–1 but did not increase at higher N rates. Root rot severity did not appear to change with SL position or foliar application of FU, nor were any of the interactions among treatment factors significant. This differs from the results at another location in Saskatchewan where common root rot was less severe on the upper than on the mid- or lower SLs but did not appear to vary with N or P rate (Kutcher et al., 1999).

Biomass and Seed Yield
Biomass yield increased with FU treatment by 0.96 Mg ha^–1 in 1999, and was greater on the lower than the upper SL position, by 2.19 Mg ha^–1 under the dry conditions of 2001, but was unaffected by N rate, other treatments, or treatment interactions in these years (Tables 3 and 4). In 2000, the N x SL and SL x FU interactions were significant (Table 3). Biomass yield increased on the lower SL from 2.10 Mg ha^–1 at 0 kg N ha^–1 to 3.37 Mg ha^–1 at 40 kg N ha^–1, but there was no further increase with additional N application while on the upper SL, it increased with each addition of N, i.e., 1.14 Mg ha^–1 at 0 kg N ha^–1 increasing to 4.07 Mg ha^–1 at 120 kg N ha^–1. The different biomass yield responses between SL positions most likely reflect differences in the ability of the soil to supply N to the crop and differences in soil moisture content. The increase in biomass yield with each addition of fertilizer N indicated that N was a limiting factor on the upper SL. However, on the lower SL, the lack of additional biomass yield from fertilizer N greater than 40 kg N ha^–1 was not due to lack of N. Precipitation was greater than the long-term mean in 2000 (Table 1), which may have caused prolonged periods of wet soil on the lower SL leading to seedling death and reduced emergence (Table 4) or other conditions detrimental to crop biomass production. However, the fact that this interaction only occurred in 1 of 3 yr and may be related to seasonal precipitation indicated that it was not predictable. The significant SL x FU interaction in 2000 occurred because biomass yields at the lower SL were similar regardless of FU treatment (average of 2.89 Mg ha^–1), whereas biomass yield on the upper SL increased from 2.28 Mg ha^–1 without FU to 3.40 Mg ha^–1 with FU application. This indicated that under good growing conditions, such as those on the upper SL, leaf spot diseases were a limiting factor and biomass yield benefited from the application of FU. On the lower SL, leaf spot diseases did not appear to be a limiting factor for biomass yield, but rather as discussed above, prolonged wet soil was probably a more important limitation.

Seed yield of wheat was increased by the addition of fertilizer N in 2000, with the greatest increase between 0 and 40 kg N ha^–1 and in the dry years of 1998 and 2001 was greater on the lower than the upper SL by 0.77 and 0.76 Mg ha^–1, respectively (Tables 3 and 4). Seed yield was increased by FU application in 1998, 1999, and 2000 by 0.57, 0.20, and 0.20 Mg ha^–1, respectively. Lack of seed and biomass yield response to FU in 2001 was likely due to the dry conditions, which reduced leaf spot severity relative to other years (Table 4). In 1999, a significant N x SL interaction occurred for seed yield. This was a result of decreased seed yield on the lower SL, from 2.54 Mg ha^–1 at 0 kg N ha^–1 to 2.07 Mg ha^–1 at 40 kg N ha^–1, with similar seed yield at 40, 80, and 120 kg N ha^–1 while on the upper SL, seed yield was similar at 0 and 40 kg N ha^–1 but increased from 2.16 to 2.66 Mg ha^–1 between 40 and 80 kg N ha^–1 and then declined to 2.35 Mg ha^–1 at 120 kg N ha^–1. Similar to biomass yield in 2000, this interaction for seed yield suggested differences in the ability of soil to supply N. On the lower SL, addition of 40 kg N ha^–1 or greater resulted in reduced seed yield compared with 0 kg N ha^–1, possibly due to delayed maturity and suggested that N was not limiting. However, on the upper SL, addition of 80 kg N ha^–1 resulted in increased seed yield, indicating that N was a limiting factor.

Variation in precipitation among years had a large impact on biomass and seed yield differences between SL positions and affected the N x SL interactions (Table 4). In 1999 and 2000, precipitation was near the long-term mean in most months and above in July of each year (Table 1). Biomass yield was relatively high and did not vary between SL positions in 1999, suggesting moisture was not limiting on the upper SL although excess soil moisture may have reduced biomass yield on the lower SL. The N x SL interaction for seed yield in 1999 indicated that with adequate moisture and N, the upper SL could produce yield similar to or greater than the lower SL. A similar result was obtained in 2000 for biomass yield as indicated by the N x SL interaction where biomass yield on the upper SL was greater than on the lower SL because moisture and N were not limiting on the upper SL. However, excess moisture may have had a detrimental effect on the lower SL. In the dry years of 1998 and 2001, biomass and seed yields were much higher on the lower SL than the upper SL, which indicated the upper SL suffered much more from reduced precipitation than did the lower SL. Our results in years of below-normal precipitation were similar to those of another study in western Canada where yield of both wheat and canola was greatest on the foot SL and lowest on the shoulder SL (Nolan et al., 1995, 1999). Manning et al. (2001c) reported that in an undulating soil landscape in Manitoba, wheat seed yield showed a trend of lower > middle > upper SL in a year with growing season precipitation 37% below normal, and an opposite trend occurred in a year with growing season precipitation 62% above normal. Reduced crop yields in the footslope or shallow depressions can occur under wet conditions due to water logging, weed infestation, poor root development, and delayed crop maturity (Malo et al., 1974; Colvin et al., 1991; Wibawa et al., 1993).

Regressions of biomass yield as a function of N rate indicated that response to N fertilization was greater at the upper SL than the lower SL position in 2000 (data not shown). However, the estimated seed yield in 2000 was less at the upper SL than at the lower SL at the 0 and 40 kg N ha^–1 rates, and the differences declined with increasing N rate so that seed yield was similar for both SL positions at 80 and 120 kg N ha^–1. In other years, regression analysis for biomass and seed yield did not show a consistent effect of SL position. Therefore, our biomass and seed yield results do not consistently support targeting of N fertilization or FU application based on SL position of the landscape or N fertilizer rate.

Seed Quality
Thousand kernel weight was unaffected by changes in N rate (Tables 3 and 4). Fungicide treatment, which reduced leaf spot severity, resulted in greater TKW in 2 of 3 yr (1999 and 2000) with an average increase of 1.9 g. The TKW was greater on the upper SL than on the lower SL in the same years with differences of 1.6 g in 1999 and 3.5 g in 2000. The reduction in TKW at the lower SL position may have been due to delayed maturity, which resulted in relatively poor seed filling or possibly because of better kernel filling on the upper SL due to the formation of fewer kernels per spike as a result of greater moisture stress as has been reported by Baier and Robertson (1967). No interaction effects among factors were detected.

Grain test weight declined with increasing N rate in 2000 and 2001 but showed no trend in 1999 (Tables 3 and 4). With the change in N rate from 0 to 120 kg N ha^–1 for example, TW was reduced by 4.0 kg hL^–1 in 2000 and 3.6 kg hL^–1 in 2001. In each year, TW was lower on the lower SL than the upper SL, with differences between SL positions ranging from 2.0 to 3.8 kg hL^–1. Delayed maturity as a result of N fertilization on the lower SL position may explain the lower TW observed as a result of these treatment factors. The application of FU had no effect on TW. However, a significant SL x FU interaction occurred in 1999, which showed an increase of 0.9 kg hL^–1 in TW as a result of FU application to the lower SL but no effect of FU application on the upper SL.

In general, protein content in seed increased as N rate increased from 0 to 120 kg N ha^–1 in all years (Tables 3 and 4). However, a significant N x SL x FU interaction for protein content occurred in 1999. Protein content was greater on the lower SL than the upper at all N rates except 120 kg N ha^–1 and was increased by FU application on the upper SL but not the lower, at all N rates, except 120 kg N ha^–1. On the lower SL, protein content increased dramatically between 40 and 80 kg N ha^–1 (from 13.7 to 15.3% averaged over FU treatments) but was similar at 120 kg N ha^–1. On the upper SL without FU treatment, protein content increased gradually from 13.6 to 15.2% as N rate increased from 0 to 120 kg N ha^–1. On the upper SL with FU treatment, protein content increased only slightly from 13.1 to 13.6% between 0 and 80 kg N ha^–1 but to 15.3% at 120 kg N ha^–1. In 2000, the significant N x SL interaction was the result of increasing protein content on both SL positions as the N rate was increased from 40 to 80 or 120 kg N ha^–1, but with the change from 0 to 40 kg N ha^–1, the protein content on the upper SL changed little while it declined on the lower SL. This may have been due to the greater increase in biomass and seed yield between 0 and 40 kg N ha^–1 than between other N rates. Walley et al. (2001) reported that differences in wheat seed yield among variable-rate applications of fertilizer N were not consistent partly due to the dual role of N in determining both yield and protein content. Protein content was greater on the upper than the lower SL in 2001, most likely due to the dry conditions, which reduced kernel filling more on the upper than the lower SL, resulting in a greater relative protein content of seed on upper SL positions.

Uptake of Nitrogen in Seed
The uptake of N in seed did not appear to be affected by any of the treatments in 1999 and only by the addition of N in 2001 (Tables 3 and 4). The magnitude of the increase in N uptake with the change in fertilizer rate from 0 to 120 kg N ha^–1 was 22.4 kg ha^–1 in 2001. Fungicide application resulted in an increase in the uptake of N in seed from 54.2 to 60.5 kg N ha^–1 in 2000, which corresponded to an increase in biomass and seed yield. The N x SL interaction was significant in 2000 (Table 3). Uptake of N in seed increased by a similar amount on both SL positions as fertilizer N increased from 0 to 40 kg N ha^–1 (33.6 to 51.3 kg N uptake in seed ha^–1 averaged over SL positions) but with a greater increase on the lower SL (48.1 to 92.3 kg N uptake in seed ha^–1) than on the upper SL (54.4 to 65.9 kg N uptake in seed ha^–1) between fertilizer rates of 40 and 120 kg N ha^–1. Since the relative differences for seed yield were larger than for protein content, it was the seed yield increase of both FU and the N x SL interaction that was able to increase uptake of N in seed in 2000.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Increasing N application did not consistently increase leaf spot or common root rot severity and biomass or seed yield of wheat although seed protein content and N uptake in seed increased and seed TW decreased in at least 2 of 3 yr. Leaf spot diseases were less severe on the lower SL than the upper SL in all 4 yr, and biomass and seed yield were greater on the lower SL than the upper SL only in 2 yr with below-normal precipitation. The TKW and TW tended to be better on the upper SL position than the lower. However, protein content varied between SL positions and was influenced by N rate and variation in precipitation. Foliar application of FU consistently reduced leaf spot severity and was associated with increased biomass and seed yield in 3 of the 4 yr, with the exception of 2001, which was dry. Fungicide treatment did not have a consistent effect on TW, protein content, or N uptake in seed but did increase TKW in 2 of 3 yr. There were a limited number of interactions among treatment factors for other measurements. Although leaf spot diseases were consistently greater on the upper than the lower SL and could be reduced with FU treatment, there was little benefit for seed yield or quality. Therefore, selective application of foliar FU to manage foliar diseases in different landscape units based on SL position and N fertilizer rates did not show additional benefits in terms of yield or seed quality. Overall, the results of this study do not support targeting of N fertilizer or FU application to particular management units on a hummocky landscape defined by SL position.

	ACKNOWLEDGMENTS

 
The authors thank Colleen Kirkham, Dan Cross, and Darwin Leach for technical assistance and Dr. S. Woods for help with statistical analyses. Appreciation is expressed to Dr. B. McConkey, Dr. B. Gossen, and the anonymous reviewers for their helpful suggestions with regard to the manuscript.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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