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Agric. and Agri-Food Canada, Box 1240, Hwy. 6 South, Melfort, SK, Canada S0E 1A0
* Corresponding author (kutcherr{at}agr.gc.ca)
Received for publication May 1, 2004.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: BBCH, Bayer, BASF, Ciba-Geigy, and Hoechst (growth stage scale) • FU, fungicide • %GS, percentage green seed • SL, slope • TSW, thousand-seed weight
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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On hummocky or rolling terrain such as that occurring over large areas of western Canada, crop productivity may vary greatly between SL positions as a result of differing conditions, particularly moisture and fertility (Miller et al., 1988; Afyuni et al., 1993; Moulin et al., 1994). Nolan et al. (1995)(1999) reported greater yield of wheat (Triticum aestivum L.) at lower than at upper SL positions and that the optimum N rate was greater on the upper SL than the lower. In Manitoba, on an undulating landscape, yield of wheat was reported to be greater on the upper SL when precipitation was above average and greater on the lower SL when precipitation was below average (Manning et al., 2001c). Reduced crop yields on the lower SLs or shallow depressions 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).
The potential for N mineralization, immobilization, denitrification, and leaching varies because of large differences in soil moisture availability and the amount of organic C across landscape units (Malo and Worcester, 1975; Pennock et al., 1987; Manning et al., 2001a, 2001b; Malhi et al., 2004). Consequently, this influences the availability of N to the crop from fertilizer and soil. Therefore, knowledge of crop response to fertilizer N in specific soil and climatic conditions is critical to improve crop production and reduce losses of N across the landscape units. Varying the amount of fertilizer at different landscape positions has been suggested as an appropriate technique to optimize the efficiency of inputs and crop production on a hummocky landscape (Beckie et al., 1997). Yang et al. (1999) found that the yield of sorghum [Sorghum bicolor (L.) Moench] was greater using a variable-rate fertilizer treatment than with uniform fertilizer treatment. However, the increase from the variable-rate treatment did not cover the extra cost of variable-rate application. Walley et al. (2001) observed that wheat seed yield response to variable-rate applications of fertilizer N was highly variable, partly due to the dual role of N in determining both the yield and protein content of wheat seed. They suggested that there was little economic rationale for using variable-rate N application.
Differences in moisture, soil properties, and fertility among SL positions affect crop stand and canopy microenvironment, which influence susceptibility to plant diseases and therefore yield and quality (Rotem and Palti, 1978). Increase in disease severity has been correlated with N fertility in southern Saskatchewan (Fernandez et al., 1998) where increased leaf spot severity was observed with an increase in N deficiency on spring wheat. In contrast, increasing N rate increased leaf rust severity on winter wheat in Louisiana (Mascagni et al., 1997). Stevenson et al. (1995b) observed that incidence of leaf spot diseases and common root rot of wheat as well as seed yield, soil water content, and soil N availability during the growing season varied with landscape position. Water content, soil N availability, and common root rot incidence contributed to the landscape-scale differences in wheat seed yield in a pea (Pisum sativum L.)–wheat–barley (Hordeum vulgare L.) rotation, but only leaf spot and root rot disease levels explained landscape-scale yield variation in a wheat–wheat–barley rotation. Kutcher et al. (1999) reported an increase in common root rot [Cochliobolus sativus (Ito & Kuribayashi) Drechs. ex Dastur] and seed yield of wheat from upper to lower SL positions. However, the opposite was observed for leaf spot diseases of wheat, which appeared to be more severe on upper SL positions. While many studies have been conducted on the potential use of precision agriculture to vary fertility across a landscape, only a few investigations have explored the potential benefits of using this technology to guide FU application (Secher et al., 1995; Secher, 1997; West et al., 2003). Previous research that has implications for the effect of landscape and N fertility on plant diseases has tended to be on cereal crops, and to our knowledge, no studies have included diseases of canola.
Pesticide application, including FUs, is usually done uniformly across SL positions of a landscape. To apply precision agriculture technology to plant disease management, the effect of SL position and N fertilization on plant disease ought to be better understood. The objectives of this study were to determine the effect of N fertilization and FU application on canola diseases, yield, seed quality, and N uptake at different SL positions.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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E)-2-[[6-(2-cyanophenoxy)-4-pyrimidinyl]oxy]--(methoxymethylene)benzeneacetate} (Quadris) was applied at the two- to six-leaf growth stage—12 to 16 of the Bayer, BASF, Ciba-Geigy, and Hoechst (BBCH) growth stage scale (Lancashire et al., 1991)—to control blackleg, and vinclozolin [3-(3,5-dichlorophenyl)-5-ethenyl-5-methyl-2,4-oxazolidinedione] (Ronilan) was applied at the 20 to 30% bloom stage (BBCH 62–63) to control sclerotinia stem rot. Dates of field operations are given in Table 2.
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Seed samples were assessed for thousand-seed weight (TSW) and seed bulk density. In 1999 to 2001, data were collected for the percentage of green seeds (%GS) by visual examination of crushed seed and N content (AOAC, 1995). In 2000 and 2001 seed oil content (AOAC, 1990) was determined. Protein content in seed was calculated by multiplying the N content in seed by 6.25 (Williams et al., 1998). Uptake of total N in seed was calculated from the seed yield and total N concentration in seed. Percentage recovery of fertilizer N in seed was calculated as 100 x [(N uptake in kg N ha–1 in fertilized treatments – N uptake in kg N ha–1 in zero-N treatment)/the rate of applied N in kg N ha–1].
Statistical Analysis
The general linear models procedure of the Statistical Analysis System was used to analyze the data (SAS Inst., 1993). Since landscape-scale field experiments inherently have a high degree of variability (van Kessel et al., 1993; Walley et al., 1996), a lower level of significance (P 
0.10) was used to indicate significant effects. When observations were in the range of 0 to 30% for data expressed as percentages, the square root transformation (observed value + 0.5)0.5 was used to achieve normality before analysis (Little and Hills, 1978). This included the incidence of blackleg (1998 and 2000), sclerotinia stem rot (1998, 1999, and 2001), aster yellows (1999), and green seed (all years). The probability of greater F values for main and interaction effects was determined with transformed data, but detransformed means are presented. Since SL position is a fixed effect (not randomized), the differences between SL positions are discussed in general terms without reference to statistical significance. Interactions of SL with N and FU (N x SL and SL x FU) are discussed based on statistical analysis. The data were also regressed as a function of annual N rates using the RSREG procedure to estimate optimum N rate and maximum seed yield for different SL positions and FU treatments.
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RESULTS |
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Incidence of Diseases
Three diseases of canola were observed in most years during the course of this study: blackleg, sclerotinia stem rot, and aster yellows. Generally, disease incidence was low to moderate for each disease with considerable year-to-year variation. Blackleg incidence increased as N rate increased in 1998, 2000, and 2001 (Tables 3 and 4). Compared with the lower SL position, blackleg incidence on the upper SL position tended to be greater although except for 1999, the differences were small. Azoxystrobin FU application significantly reduced blackleg incidence in all years. In 2000, the N x FU and SL x FU interactions were significant for blackleg incidence (Table 3). For the N x FU interaction, the incidence of blackleg was similar between FU treatments at 0 kg N ha–1, but the difference between FU treatments increased with N rate. For the SL x FU interaction, the difference in blackleg incidence between the FU-treated and untreated plots at the lower SL (3.2%) was less than at the upper SL (6.6%).
Sclerotinia stem rot incidence in 1998 was so low that it was biologically meaningless even though statistical differences were detected between FU treatments (Tables 3 and 4). In 1999 and 2000, incidence was lowest at 0 kg N ha–1 and increased with increasing N rate (Table 4). In 2001, results were opposite: Incidence was greatest at 0 kg N ha–1 and decreased as N rate increased. Incidence tended to be greater on lower SLs than upper although the amount of disease was very low except in 2000. Vinclozolin FU application reduced sclerotinia incidence by 2.5% in 1999 and 11.0% in 2000.
A measurable amount of aster yellows disease was observed in 1999 while only trace incidence of the disease was recorded in other years, and these were omitted from the analysis. In 1999, the incidence of aster yellows was observed to be greater with higher N fertility (Tables 3 and 4).
Biomass and Seed Yield
Biomass yield significantly increased with increasing N rate in all years except 1998 (Tables 3 and 4). In 1999 and 2001, biomass increased up to 80 kg N ha–1, with a smaller increase observed at 120 kg N ha–1. In 2000, biomass continued to increase as N rate increased to 120 kg N ha–1, with relatively greater increases between 0 to 40 kg ha–1 and 80 to 120 kg ha–1. Also in 2000, a significant SL x FU interaction was detected, which was a result of increased biomass (0.81 Mg ha–1) on the lower SL as a result of FU treatment but no effect on the upper SL. In 1998, the N x SL significant interaction indicated that biomass increased as N rate increased to 40 kg N ha–1 at both upper and lower SL positions and increased with further increase in N rate on the lower SL position only.
Analysis of seed yield data indicated no significant interaction effects of N rate, SL position, or FU application in any year (Table 3). Yield increased with increasing N rate in 3 of 4 yr (1999–2001), with the greatest increase in yield from the addition of the first 40 kg N ha–1 (Table 4). Like biomass, seed yield tended to be greater on the lower SL than the upper. Fungicide application did not increase seed yield in any year.
Seed Quality
Percentage of green seed increased with the increase in N rate (Tables 3 and 4). This was largely a result of delayed maturity as indicated by differences in maturity dates among N rate treatments (Table 2). On average across years, 1.3, 1.9, 3.0, and 3.7 %GS were found at 0, 40, 80, and 120 kg N ha–1 rates, respectively. In 2000 and 2001, %GS was consistently greater at the lower than the upper SL. Fungicide application had no effect on %GS. There were significant interaction effects of N x FU in 1999 and N x SL in 2000. In 1999, FU treatment increased %GS at 0 and 120 kg N ha–1 while the opposite occurred at 40 kg N ha–1 and there was no effect at 80 kg N ha–1. In 2000, %GS was similar at both SL positions at 0 kg N ha–1, but higher %GS occurred on the lower than the upper SL at 40, 80, and 120 kg N ha–1 with the greatest difference at 120 kg N ha–1.
The effect of increased N rate was to decrease TSW in 1999 and 2001, but there was little influence in 1998 and 2000 (Tables 3 and 4). The TSW was reduced by FU application in 2000. There was a SL x FU interaction in 1998, when TSW was similar between FU treatments on the lower SL, but it was less in FU-treated plots than untreated plots on the upper SL. There was no consistent effect of SL position on TSW.
In 2000, an increase in the N rate from 0 to 40 kg ha–1 increased seed bulk density of canola, but further N rate increases did not affect bulk density, and higher seed bulk density was observed in untreated than in FU-treated plots (Tables 3 and 4). In 1998, the significant N x FU interaction for bulk density was caused by a large difference between FU-treated (51.9 kg hL–1) and untreated (52.8 kg hL–1) means at 80 kg N ha–1, but little difference at other N rates. In 1999, a significant SL x FU interaction indicated that on the lower SL, bulk density was similar between FU treatments, but on the upper SL, FU treatment resulted in higher bulk density (52.7 kg hL–1) than untreated plots (52.1 kg hL–1).
As expected, protein content in seed significantly increased with the increase in N rate (Tables 3 and 4). Between the N rates of 0 to 120 kg N ha–1, seed protein content increased by 4.4% in 1999, 5.7% in 2000, and 6.4% in 2001. Seed protein content was always greater at the lower than the upper SL, and differences of 0.9 to 1.2% were observed. In 2000, seed protein content was lower with FU application than without. The N x SL interaction was significant in 2000 and 2001, which indicated that the differences in seed protein content between SL positions declined with increasing N rate. For example, seed protein content was greater at the lower SL position than at the upper SL position by approximately 1.9% at 0 kg N ha–1 but declined to almost 0 at 120 kg N ha–1. The interaction of SL x FU in 2001 indicated that FU treatment at the lower SL increased protein content by 0.3%, but at the upper SL, it reduced protein content by 0.6%.
Contrary to seed protein content, seed oil content decreased substantially with increased N rate (Tables 3 and 4). There was 4.5 and 9.4% less oil at 120 kg N ha–1 than at 0 kg N ha–1 in 2000 and 2001, respectively. Oil content was greater on the upper than the lower SL by 0.6% in 2000 and by 1.5% in 2001. There was a significant N x SL interaction in 2001 due to greater oil content in seed on the upper than the lower SL at 0 kg N ha–1 but no difference at any other N rate. The trend was similar in 2000 although the interaction was not significant.
Uptake of Nitrogen and Recovery of Fertilizer Nitrogen in Seed
Uptake of N in seed increased with increasing N rate in 2000 and 2001 (Table 3), with the majority of the increase from the initial addition of 40 kg N ha–1 (Table 4). The N uptake was greater by 6.1 to 9.9 kg ha–1 on the lower than on the upper SL in all 3 yr. Fungicide treatment did not affect N uptake, and no interactions were observed among factors.
Opposite of seed N uptake, recovery of fertilizer N in seed declined with increased N rate in 2000 and 2001 (Tables 3 and 4). Recovery of fertilizer N tended to be higher at the upper than the lower SL. Fungicide treatment reduced recovery of fertilizer N in 2001 but did not appear to affect it in other years. A significant interaction of SL x FU occurred in both 1999 and 2000, which was due to greater N recovery without than with FU treatment at the lower SL but the opposite at the upper SL.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Blackleg usually increased with increasing N rate and tended to be greater at upper than lower SL position. Application of FU to control blackleg only to upper SLs, especially with high N fertility, would result in a reduction in the total area of the field requiring FU. This is opposite to alternaria blackspot (Alternaria spp.) disease of canola where greater severity was observed at the lower SL, resulting in a greater benefit of FU treatment at the lower than the upper SL (Kutcher et al., 1999). The difference is likely due to the environmental conditions required by each pathogen. The pathogen that causes blackleg tends to require a relatively small amount of moisture to infect the plant at early growth stages, i.e., precipitation events are required for spore release, but infection can occur with relatively short periods of leaf surface wetness (McGee, 1977). On the other hand, alternaria blackspot requires longer periods of dampness and high humidity at canopy closure to cause significant infection (Bailey et al., 2003), which are conditions more likely at the lower SL position.
The incidence of sclerotinia stem rot has been reported to increase on soils with high organic matter due to more vigorous plant growth, which provides a favorable crop canopy for infection (Ferraz et al., 1999). In this study, high N fertility increased sclerotinia stem rot in 1999 and 2000, but the reverse was detected in 2001, likely because of differences in environmental conditions in those years. In 1999 and 2000, total precipitation and distribution throughout crop development was close to normal. However, precipitation was much more limited in 2001, with most precipitation events occurring in late June and early July. Since the disease requires moist conditions during flowering for infection, early flowering treatments such as 0 and 40 kg N ha–1 were more at risk of sclerotinia stem rot infection than those that flowered later (80 and 120 kg N ha–1). It is possible that the reduced emergence observed at 80 and 120 kg N ha–1, which was particularly severe in 2001, also contributed to delayed flowering, thinner crop canopy during flowering, and reduced sclerotinia stem rot incidence although by the time of sampling for biomass, the yield was similar among treatments.
Lower SL positions were expected to be at greater risk of infection by sclerotinia stem rot than upper SLs because of greater available moisture and fertility, which would have promoted a thick crop canopy conducive to disease development. This trend was observed but was not borne out more clearly because sclerotinia stem rot incidence was very low on either SL. This may have been because the primary inoculum (transported by air movement) was distributed across the landscape uniformly and because canopy density (as inferred from biomass yield) on both lower and upper SLs was generally similar. Vinclozolin FU application significantly reduced sclerotinia incidence in 3 of 4 yr. However, because the impact of the disease on yield at low disease levels is limited, the benefit of FU treatment for sclerotinia stem rot control was also limited. Thus, FU treatment to control sclerotinia stem rot based on topographical variation was not warranted.
Biomass yield significantly increased with increased N rate in 3 of 4 yr and showed a similar trend in the fourth year. Even though the emergence was reduced at higher N rates, these treatments produced more biomass yield. In the year that N rate was not significant for biomass yield (1998), the N x SL interaction indicated that it was only on the upper SL that biomass did not increase with increasing N rate. This may have been due to the generally dry conditions that prevailed in 1998 even though a number of heavy showers were received in late June and early July. It may have been that the large amounts of precipitation received over short periods of time that year resulted in water running off the upper SLs, and therefore regardless of N rate, biomass was limited by moisture availability on the upper SL. The preceding point was supported by a relatively greater difference in biomass yield at lower and upper SLs in 1998 (1.23 Mg ha–1) compared with other years (0.34 to 0.56 Mg ha–1). The trend for greater biomass yield at the lower than the upper SL was considered to be due to better soil and moisture conditions at the lower SL. Lack of FU treatment effect on biomass yield was due to low incidence of diseases.
Like biomass yield, the seed yield increased with increase in N rate in 1999 to 2001 and tended to increase with N rate in 1998. The greatest impact on seed yield was due to the first addition of 40 kg N ha–1. Seed yield was greater on the lower SL than the upper SL (by 0.11 to 0.18 Mg ha–1). This result is similar to that obtained by Nolan et al. (1999) in southern Alberta, Canada, who found that canola yield was greatest on foot (lower) SLs and lowest on shoulder (upper) SLs. Estimated optimum N rate (rate which resulted in maximum yield) at each SL position from the regression analysis for each year, when averaged for the study years, was lower at the lower SL position (71 kg N ha–1) than at the upper SL position (88 kg N ha–1) (regressions not shown). Nolan et al. (1999) also found that the optimum N rate for canola seed yield on the foot SL was lower (74 kg N ha–1) than on the shoulder SL (107 kg N ha–1). The nature of seed yield response to N fertilization suggests that it may be a good strategy to target greater N fertilization to the upper SL in preference to the lower SL, provided there is enough soil moisture to support crop growth and yield. However, seasonal precipitation is not a factor that can be predicted in advance with any confidence. Fungicide treatment had little impact on seed yield, indicating that although diseases were present, severity of infection was not great enough to cause detectable yield loss.
The higher biomass and seed yield and lower optimum rate of N for seed yield at the lower SL position than at the upper SL position suggested greater availability of N from soil for use by the crop as has been reported previously (Malhi et al., 2004). Nutrient cycling and movement, soil productivity, and crop yield in a hummocky landscape have been reported to be controlled by water distribution (Pennock et al., 1987; Grant and Flaten, 1998). Lower SLs normally had more water, available nutrients, and crop yield (Halvorson and Doll, 1991; Fiez et al., 1994; Stevenson et al., 1995a). Redistribution of topsoil (Gregorich and Anderson, 1985; Pennock and de Jong, 1990; McConkey et al., 1997) and moisture (Verity and Anderson, 1990) was considered to be responsible for an increase in wheat yield from upper to lower SL positions. The general trend of greater organic matter and plant available nutrients in soil at lower SL positions than at upper SL positions was reported earlier (Manning et al., 2001a, 2001b; Malo and Worcester, 1975). The differences in yield between SL positions in various years in the present study did not appear to be due to differences in precipitation between years (Table 1). Our results appear to be consistent with the general trend of relatively better soil productivity at lower than upper SL positions.
The greatest impact on seed quality occurred when treatments delayed maturity of the crop, which resulted in poor seed filling as indicated by lower TSW and greater proportion of green seed. High N rate treatments usually had lower TSW, and both high N rate and lower SL positions were associated with higher proportions of green seed. In other studies, an increase in seed protein concentration with N fertilization has been observed in canola (Malhi and Gill, 2002) and wheat (Malhi et al., 1999). The results were similar in this study except that seed protein content was greater at the lower SL in 2 of 3 yr than at the upper SL at 0 kg N ha–1, but the difference between SL positions declined to almost 0 at 120 kg N ha–1. Opposite to seed protein, seed oil content was greater at the upper than at the lower SL at 0 kg N ha–1 and decreased as N rate increased. The decrease in seed oil content with increasing N rate has been found in other studies (Malhi and Gill, 2002) and is likely due to the dilution effect of increased seed yield with increased N fertilization and the inverse relationship of protein and oil content.
Like seed yield, the response of N uptake in seed to N fertilization was always greatest with the addition of the first 40 kg N ha–1. The greater N uptake at the lower than the upper SL position was probably due to the fact that soil at the lower SL provided more available N to canola plants than the soil at the upper SL (Malhi et al., 2004). Higher moisture and organic matter in soil at lower than at upper SL positions were considered responsible for greater amounts of soil available N. Recovery of fertilizer N in seed declined with the increase in N rate and tended to be higher on the upper SL than on the lower SL. Similarly, the recovery of applied N was slightly greater at the upper SL position than at the lower SL position in heads of wheat in an N15–labeled experiment in the same area (Malhi et al., 2004). Better N-supplying capacity of soil at the lower than the upper SL indicated that the contribution of fertilizer N to canola seed was relatively smaller at the lower than the upper SL. This also suggests that there is more need for fertilizer N on the upper SL than on the lower SL for optimum yield, which supports higher optimum N rate at the upper SL mentioned earlier. As the plots received the same N rates during the 4 yr of the study, there was the potential for cumulative effects of N treatments, but this did not translate into increases in disease severity, biomass or seed yield, or seed quality with time.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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