Effect of nitrogen fertilizer rate, herbicide rate, and soil disturbance at seeding on the productivity of a wheat-pea rotation

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1 Effect of nitrogen fertilizer rate, herbicide rate, and soil disturbance at seeding on the productivity of a wheat-pea rotation R. M. Mohr, D. A. Derksen, C. A. Grant, D. L. McLaren, M. A. Monreal, A. M. Moulin, M. Khakbazan, and R. B. Irvine Agriculture and Agri-Food Canada, Box 1000a, R.R. #3, Brandon, Manitoba, Canada R7A 5Y3 ( rmohr@agr.gc.ca). Received 25 November 2005, accepted 7 November Mohr, R. M., Derksen, D. A., Grant, C. A., McLaren, D. L., Monreal, M. A., Moulin, A. M., Khakbazan, M. and Irvine, R. B Effect of nitrogen fertilizer rate, herbicide rate, and soil disturbance at seeding on the productivity of a wheat-pea rotation. Can. J. Plant Sci. 87: Rotational productivity is a function of the rotational crops and their interactions, as well as the management employed. Understanding the functioning of the overall production system, as well as its component parts, may contribute to improved management. Effects of nitrogen fertilizer rate (25, 50, 75, 100 kg N ha 1 as urea) and herbicide rate (66 or 100% of recommended) applied to wheat, and of the level of soil disturbance at seeding, on the productivity and N status of a 2-yr rotation of spring wheat (Triticum aestivum L.) and field pea (Pisum sativum L.) were assessed over two rotation cycles at two locations in southwestern Manitoba. Management factors typically acted independently to influence the cropping system. In both wheat and pea, high soil disturbance at seeding reduced or tended to reduce plant density in most site-years, resulting in reduced yields in about half of site-years. In 2 site-years where weed pressure was high, wheat yields for high disturbance treatments were less than 60% of low disturbance seeding, demonstrating the importance of adequate plant stands under sub-optimal growing conditions. The herbicide rate applied to wheat had few significant effects on wheat and pea. In most site-years, N fertilization had limited or negative effects on wheat yields due partly to relatively high soil NO 3 -N levels. Soil NO 3 -N levels declined over the years of the study, suggesting that N contributions from peas did not exceed crop N removal and/or N losses from the wheat-pea rotation. The N rate applied to wheat typically did not affect pea yields. Key words: Wheat, pea, rotation, herbicide, nitrogen, soil disturbance Mohr, R. M., Derksen, D. A., Grant, C. A., McLaren, D. L., Monreal, M. A., Moulin, A. M., Khakbazan, M. et Irvine, R. B Incidence du taux d application des engrais azotés et des herbicides ainsi que de la perturbation du sol à l ensemencement sur la productivité d un assolement pois-blé. Can. J. Plant Sci. 87: La productivité d un assolement dépend des cultures en rotation et de leurs interactions, mais aussi des pratiques agricoles. En comprenant mieux le fonctionnement global du système de production et de ses composantes, on pourrait parvenir à une meilleure gestion. Pour cette raison, les auteurs ont évalué l incidence du taux d application des engrais azotés (25, 50, 75 ou 100 kg de N sous forme d urée par hectare), du taux d application des herbicides (66 ou 100 % du taux recommandé) au blé et du degré de perturbation des sols lors des semis sur la productivité et sur le bilan N d un assolement de deux ans de blé (Triticum aestivum L.) et de pois de grande culture (Pisum sativum L.). L expérience a duré deux cycles d assolement et s est déroulée à deux endroits du sud-ouest du Manitoba. En général, les facteurs de gestion agissent indépendamment sur le système agricole. Pour le blé et le pois, une forte perturbation du sol aux semis a diminué ou eu tendance à diminuer la densité du peuplement la plupart des années-sites, ce qui a entraîné une diminution du rendement pour environ la moitié des années-sites. À deux années-sites où les mauvaises herbes étaient nombreuses, le rendement du blé sur les terres très perturbées était de 60 % inférieur à celui observé sur les terres moins perturbées aux semis, signe de l importance d un peuplement adéquat quand les conditions de croissance ne sont pas optimales. La quantité d herbicide appliquée au blé n a eu que peu d effets significatifs sur le blé et le pois. Pour la plupart des années-sites, l engrais N a eu des effets limités voire négatifs sur le rendement du blé, en partie à cause d une concentration relativement élevée de N-NO 3 dans le sol. La concentration de N-NO 3 dans le sol a diminué pendant la durée de l étude, ce qui laisse croire que l azote procuré par le pois ne dépasse pas la quantité utilisée par le blé ou perdue par l assolement. La quantité d engrais azoté appliquée au blé n a généralement aucune incidence sur le rendement du pois. Mots clés: Blé, pois, assolement, herbicide, azote, perturbation du sol 241 In western Canada, field pea is the most common annual legume grown in crop rotations. Field pea may provide both N and non-n benefits to subsequent crops in rotation, although the magnitude of these benefits varies. In one Saskatchewan study, field pea contributed an estimated 45 to 63 kg N ha 1 to the soil depending on landscape position (Stevenson and van Kessel 1997). In other studies in Saskatchewan, the total N benefit of field pea to a succeeding non-legume crop was 25 kg N ha 1 (Beckie et al. 1997), and the N residual effect of field pea to the succeeding crop ranged from 12 to 28 kg N ha 1 (Beckie and Brandt 1997). Because pulse residue typically has a lower C:N ratio than Abbreviations: PWPW, pea-wheat-pea-wheat phase of the rotation; WPWP, wheat-pea-wheat-pea phase of the rotation

2 242 CANADIAN JOURNAL OF PLANT SCIENCE cereal straw, more rapid mineralization or reduced immobilization of N may result in greater N availability following pulse crops (Grant et al. 2002). However, the residual N benefit from field pea may be positive or negative (Armstrong et al. 1994). In mesh bag studies with pea residues in Alberta, an estimated 0.9 kg N ha 1 was immobilized in one season, while 12 kg N ha 1 was mineralized in the next season (Soon and Arshad 2002). In the longer term, including annual legumes in rotation may increase the N-supplying ability of the soil thereby reducing N fertilizer needs (Grant et al. 2002). Non-nitrogen benefits of field pea may also be significant. Stevenson and van Kessel (1996a, b) attributed more than 90% of the yield advantage in a pea-wheat rotation over a wheat-wheat rotation to non-nitrogen benefits, such as reduced disease severity and grassy weed infestation; less than 10% of the yield advantage was attributed to the N benefit from the pea crop. In contrast, Soon et al. (2004) reported that for barley following pea, the N benefit was considerably more than the non-n benefit. While including field pea in rotation may benefit subsequent crops, it may be less competitive and therefore more prone to yield losses due to weed competition than other crops (Harker 2001). Stevenson and Johnson (1999) found higher infestations of some broadleaf weed species where rotations included a high frequency of broadleaf crops under zero-till, likely due to more limited herbicide options for the broadleaf crops. The authors suggested that higher weed populations could have a greater negative effect on rotations with less competitive crops than those with more competitive cereal crops. That said, balancing rotations to include both grass and broadleaf crops may reduce weed populations compared to rotations containing only one crop type (Derksen et al. 2002). Weed removal soon after pea emergence is required to optimize pea yield (Harker et al. 2001) and can increase total N uptake by pea. However, in terms of the N benefit derived from pea, time of weed removal had no effect on the yield or N uptake of barley following pea in rotation (Soon et al. 2004). The overall impact of pulses on the yield of subsequent crops in rotation may differ due to many factors including year, soil type, pulse crop species and residue incorporation (Wright 1990). In Saskatchewan, wheat yield in a pea-wheat rotation was 43 62% greater than in a wheat-wheat rotation (Stevenson and van Kessel 1996a, b). Winter wheat following dry pea and lentil produced slightly higher yields than wheat following mustard in 1 of 2 yr (Guy and Gareau 1998). For barley in Alberta, yield was greater after pea than after barley regardless of N rate (Soon et al. 2004). Much of the research on field pea in rotation systems has focussed on specific impacts of field pea on subsequent crops, such as nitrogen and non-nitrogen benefits. An integrated assessment of the cropping system overall, including interactions among field pea, other rotational crops and management practices, will improve our understanding of the overall functioning of the cropping system and lead to improved management. Field studies were conducted to assess the productivity and N status of a 2-yr rotation consisting of spring wheat, managed with varying rates of N fertilizer and herbicide, in rotation with field pea. Both crops were established under low and high soil disturbance at seeding to assess the effect of seeding practices. MATERIALS AND METHODS A 2-yr crop rotation study consisting of Canadian Western Red Spring wheat (Triticum aestivum L. cv. AC Barrie) and field pea (Pisum sativum L. cv. Carneval) was conducted at two sites in southwestern Manitoba from 1997 through Experiments were located on an Orthic Black Chernozem (Fairland loam; ph 7.4; EC ds m 1 ; NaHCO 3 -extractable P 21 kg ha 1 to 15 cm; NH 4 OAcextractable K 694 kg ha 1 to 15 cm) south of Brandon, MB, and an Orthic Black Chernozem (Ramada clay loam; ph 7.7; EC ds m 1 ; NaHCO 3 -extractable P 60 kg ha 1 to 15 cm; NH 4 OAc-extractable K 718 kg ha 1 to 15 cm) located at the Brandon Research Centre (49 52 N latitude and W longitude). Soil ph and conductance were measured in deionized water (Carter 1993). Experiments were arranged as a split plot design, with crop rotation phase (i.e. crop species) as the main plot treatment. Each phase of the rotation was present in each year, such that both wheat and pea were present at each site in each year. Within each main plot of wheat, treatments consisted of a three-way factorial combination of nitrogen fertilizer rate (25, 50, 75 or 100 kg N ha 1 as urea), herbicide rate (66 or 100% of recommended), and level of soil disturbance at seeding (low or high disturbance). During the pea phase of the rotation, the same level of soil disturbance at seeding was applied to pea as to the previous wheat crop. However, no urea fertilizer was applied and only one rate of herbicide (100% of recommended rate) was applied to all pea treatments. Four replicates of each treatment were established. Plot dimensions were 3.65 m wide by 15 m long. In 1997, the establishment year of the study, all wheat treatments received 74 kg N ha 1 as urea, and 66% of the recommended herbicide rate. The level of soil disturbance for both wheat and pea was as indicated by treatment. Beginning in 1998, treatments were applied as outlined above. To ensure a uniform basis to compare treatment effects on weeds, seeds of the dominant grassy and broadleaf weeds for the region, green foxtail [Setaria viridis (L.) Beauv.] and wild mustard [Brassica kaber (DC.) Wheeler var. pinnatifida (Stokes) Wheeler], were spread on the plot area prior to seeding in 1997 at rates to achieve typical densities (Thomas et al. 1999). The actual rates spread were adjusted for dormancy. Emerged densities of these and other species present in weed communities at both sites were typical of those found in a provincial weed survey conducted by Thomas et al. (1999). In 1998, 2000 and 2001, both wheat and pea were seeded in late April or very early May. In 1997 and 1999, crops were seeded in mid- and late-may, respectively. In all years, 0.44 kg a.i. ha 1 Roundup (glyphosate) was applied to all low disturbance treatments 1 to 2 d prior to seeding. Both crops were seeded into untilled soil using a 3.65 m wide ConservaPak seeder. For low disturbance treatments, the

3 MOHR ET AL. PRODUCTIVITY OF A WHEAT-PEA ROTATION 243 seeder was equipped with hoe openers spaced at 23 cm, and for high disturbance treatments, the same seeder was equipped with sweeps. A modified ConservaPak seed chute, circular in shape with an indent along one side similar to a frock boot, was used with the sweeps to distribute seed and fertilizer together in a wide band which resulted in essentially a solid-seeded stand with no distinct inter-row spaces evident. Seeding depth was approximately 2.5 cm for wheat and 3.5 cm for pea. Seeding rates were 120 kg ha 1 for wheat and 198 kg ha 1 for pea. High disturbance treatments were harrow packed once immediately following seeding. In wheat, N fertilizer was applied at time of seeding in a narrow sideband in the low disturbance treatments, and in a wide band with the seed in high disturbance treatments as described above. In pea, no N fertilizer was applied, but 5 6 kg ha 1 of a granular inoculant (Rhizobium meliloti) was applied with the seed at seeding. Regardless of treatment, 26 kg P 2 O 5 ha 1 was applied to wheat and 20 kg P 2 O 5 ha 1 to pea at seeding in the form of seed-placed monoammonium phosphate. In wheat, a preformulated mixture of MCPA, mecoprop and dicamba (Target ) was tank-mixed with clodinafoppropargyl (Horizon ) and the adjuvant Score. Herbicides were applied at the rates indicated by treatment approximately 4 to 6 wk after seeding. To achieve the recommended 100% rate, 275 g a.i. ha 1 MCPA, 62.5 g a.i. ha 1 mecoprop, 62.5 g a.i. ha 1 dicamba, 55 g a.i. ha 1 clodinafop-propargyl, and 1.0 L ha 1 Score were applied as a tankmix. To achieve 66% of the recommended rate, 66% of the indicated rates were applied. For all pea treatments, 35% imazamox and 35% imazethapyr (Odyssey ) was applied at 42 g ha 1 with an adjuvant (1.25 L ha 1 Merge ). Recommended fungicides and insecticides were applied at recommended rates as required to control disease and insects pests. Crop density was determined prior to tillering. In the low disturbance treatments, crop density was determined by counting the number of plants in a 1-m length of row at four locations within each plot. In the high disturbance plots, the number of plants within a 0.25-m 2 area measuring 50 cm by 50 cm was determined at four locations within each plot. Spike counts were also determined at four locations in each plot by counting the number of spikes in a 0.25-m 2 area within each plot. At crop maturity, the entire plot was harvested. Peas were swathed with a small-scale commercial swather then combined using a small-scale commercial combine. Wheat was straight-combined with a small-scale commercial combine. Grain yield and moisture content were determined, and a subsample of grain retained for further analysis. Wheat samples were cleaned using a Carter-Day dockage tester as outlined in the Official Grain Grading Guide (Canadian Grain Commission 1996). Pea samples were cleaned using a combination of sieves and a Clipper. Reported yield of the cleaned sample is adjusted to 14.0% moisture for wheat and 16.0% moisture for pea. Thousand-kernel weight was determined by counting by hand and weighing a subsample of 200 seeds from a 500-g sample of cleaned seed. Cleaned grain samples were ground, and total N concentration in grain determined. A 0.25-g sample of wheat flour was digested with sulphuric acid (Westerman 1990) and the N concentration in the digest was determined using an autoanalyzer. To check the reproducibility of the analytical procedure, one blank and two standard samples of wheat flour were included in every batch of 40 samples. Soil samples were collected each fall immediately after harvest, to a depth of 120 cm in increments of 0 15, 15 30, 30 60, and cm. In all years, soil samples were airdried, ground and stored for nutrient analysis. Soil NO 3 -N was extracted using 0.5 M NaHCO 3 or 2 M KCl, then the NO 3 concentration was determined using an autoanalyzer (Carter 1993). In order to assess crop and soil parameters over the course of the rotation, data were analyzed separately for each siteyear-crop combination using PROC GLM in SAS (SAS Institute, Inc. 1999). Analysis of variance was performed on all data using a randomized complete block design with a factorial combination of N rate, herbicide rate and soil disturbance included in the model. The effect of N rate was further assessed using orthogonal polynomials to determine if responses to N application were linear or quadratic. Where effects of a given factor and interactions involving that factor were not statistically significant in any site-year, data were re-analyzed without that factor. A P value of 0.05 was considered statistically significant. RESULTS AND DISCUSSION Growing season rainfall and temperature conditions were typical of Manitoba for the majority of site-years (Table 1). Exceptions were 1998 and 1999, when total growing season precipitation from May through August ranged from 160 to 175% of the 30-yr normal reported by Environment Canada (2004). Regardless of site-year, pea yields typically ranged from about 3000 to 4000 kg ha 1. Mean wheat yields were numerically higher on the clay loam than on the loam soil except in While these differences in wheat yield between sites may be due partly to greater water-holding capacity in the finer-textured soil in those years when precipitation was near-normal, very high wild oat pressure at the loam site likely contributed significantly to observed yield differences. Plant Establishment For wheat, plant density for individual site-years typically averaged from 50 to 100% of the provincially recommended plant stand of 250 to 300 plants m 2, whereas plant density for pea typically met or exceeded the provincially recommended plant stand of 50 plants m 2 (Table 2). The high disturbance seeding system usually reduced early-season plant stand for both wheat and field pea (Table 2). High disturbance resulted in lower plant densities for wheat than low disturbance in 1998 and 1999 for the Ramada CL site, with similar trends evident in 2000 (P = 0.08) and 2001 (P = 0.11). Significant declines were also evident for the Fairland L site in 1998 and 1999 (P < 0.05), and a numerical decline was evident in 2001 (P > 0.10), but in 2000, a small increase in plant density was evident with

4 244 CANADIAN JOURNAL OF PLANT SCIENCE Table 1. Monthly precipitation and mean monthly temperature during the growing season measured at Brandon Research Centre, Precipitation (mm) Mean temperature ( C) Month Normal z Normal z April May June July August September October Total (May-August) z 30-yr normals as reported by Environment Canada for Brandon CDA (Environment Canada 2004). Table 2. Effect of soil disturbance at seeding and rate of N fertilizer applied to wheat on plant density of wheat and pea at experimental sites, Soil N fertilizer rate Ramada clay loam Fairland loam Crop disturbance (kg N ha 1 ) 1997 z z plants m 2 Wheat low high SE SE Effect y df P value rep 3 NS NS < NS ds 1 < < NS < < NS fert 3 NS NS NS NS NS NS fert ds 3 NS NS NS NS NS N linear NS NS NS NS NS N quadratic 1 NS NS NS NS NS NS NS NS CV (%) plants m 2 Pea low high SE Effect y df P value rep 3 NS NS NS < ds 1 < < NS NS < CV (%) z In 1997, soil disturbance treatments were applied, but a single rate of N fertilizer and herbicide was applied. Means are presented for reference. y Herbicide rate did not have a significant effect on wheat, and herbicide and fertilizer rate did not have a significant effect on pea; therefore, data were reanalyzed without these variables. higher disturbance. In the 4 site-years where high disturbance significantly reduced plant density, densities in the high disturbance system ranged from about 70 to 80% that of the low disturbance system. Where non-significant (P > 0.05) declines were evident, plant density in the high disturbance system typically ranged from 92 to 96% of the low disturbance system. For pea, high disturbance seeding significantly reduced plant density at both sites in 1998, 1999 and Where significant declines occurred, plant stands in high disturbance treatments were equivalent to between 80 and 90% of low disturbance seeding treatments except for the Fairland loam in 2001, where plant stands were 71% of the low disturbance treatment. In 2000, plant stands were the same in both systems. Despite observed reductions, pea stands in high disturbance treatments often approached or exceeded the recommended plant stand of 50 plants m 2. Johnston et al. (1999) similarly reported that pea emergence was less affected by opener type and packing than was wheat, although differences among opener-packer combinations were found to be of little agronomic or economic significance. Several factors may have contributed to reduced emergence in the high disturbance system, but the specific cause of reduced emergence is unclear. Previous studies suggest that soil moisture in the furrow opener

5 MOHR ET AL. PRODUCTIVITY OF A WHEAT-PEA ROTATION 245 groove and evaporative losses of soil moisture were important factors affecting plant emergence achieved with different openers, in addition to factors such as compactness of the furrow (Chaudhuri 2001). In the current study, the sweeps used in the high disturbance system may have produced a drier and/or less firm seedbed that was less conducive to crop emergence in some years, especially for the smaller and more shallowly seeded wheat. As well, seeding depth in the high disturbance system may have been less uniform, perhaps contributing to reduced emergence. The rate of N fertilizer applied to wheat had no impact on the plant density of field pea in a given rotation, but influenced wheat establishment in 1999 and 2000 at the clay loam site (Table 2). In those 2 yr, the density of wheat plants declined linearly over the range of N rates applied. In both cases, there appeared to be some trend toward proportionately greater declines in plant stand with the high rate of N in the high than in the low disturbance seeding system. In 1999, for example, increasing the N rate from 25 to 100 kg N ha 1 reduced plant stand from 207 to 188 plants m 2 in the low disturbance treatment, and from 175 to 133 plants m 2 in the high disturbance treatment. In 2000, the effect of increasing N rate was less consistent but suggested a similar trend, with increasing increments of N resulting in plants stands of 147, 145, 134 and 141 plants m 2 in the low disturbance system, and 143, 137, 141 and 124 plants m 2 in the high disturbance system. In part, the lack of separation between the fertilizer and seed where sweeps had been used may have resulted in damage to the germinating seed. In contrast, at the loam site in 2000, a significant disturbance fertilizer interaction indicated that plant densities declined with the high N rate only in the low disturbance system. In the high disturbance system, plant stand ranged from 125 to 135 plant m 2 regardless of N rate while, in the low disturbance system, plant stands ranged from 126 to 129 plant m 2 for the lower N rates, and decreased to 114 plants m 2 in the high N treatment. In this case, seedling damage may have resulted from the close proximity of the concentrated fertilizer band to the seed row. The rate of herbicide applied to wheat had no effect on plant establishment of either field pea or wheat. A significant disturbance herbicide rate interaction occurred for wheat in one case only, but the reason for this was unclear (data not presented). Spike Counts In all site-years, the high disturbance seeding system reduced the number of wheat spikes produced per unit area (Table 3). Spike counts in the high disturbance system were typically between 65 and 95% that in the low disturbance system. In most, but not all, cases, the high disturbance system had also reduced plant densities. Plants appeared to compensate somewhat in that proportional differences between low and high disturbance systems were less for spike density (86 94%) than for plant density (69 83%) in the 4 site-years where the level of disturbance affected both plant and spike density. Even where plant density had not been significantly affected by the level of disturbance at seeding (e.g., 2000 and 2001 at the clay loam site), differences in spike density still occurred, suggesting that seeding method may have directly influenced tiller production. Increasing N fertilizer rates resulted in a quadratic increase in spike counts at the Ramada CL site in 2000, and a linear increase in spike counts in 1999 and 2001 (Table 3). In contrast, at the Fairland L site in 1998 and 1999, a trend (P ) toward slight reductions in spike counts with increasing N rates was evident. Positive crop responses to N were observed only at the clay loam site in years in which the soil NO 3 -N content was comparatively low (Table 4). Herbicide rate did not affect spike counts, providing indirect evidence that phytotoxicity of the herbicide was not a factor (data not shown). Grain Yield Wheat yield was generally not highly responsive to N fertilizer application except at the Ramada CL site in 2001 (Fig. 1a). At the Ramada CL site, N fertilizer application resulted in a statistically significant quadratic response in wheat yield in 1998, 2000 and In both 1998 and 2000, yield increased slightly with moderate rates of N fertilizer then levelled off. In 2001, a more pronounced yield increase resulted from increasing rates of N fertilizer, with wheat yields ranging from 2260 kg ha 1 for the lowest N rate to 2948 kg ha 1 for the highest N rate. The initially high soil N levels at the clay loam site apparently led to limited crop N responses (Table 4). Greater positive crop responses to N fertilizer later in the study may have been due to depletion of soil NO 3 -N over the course of the study. In contrast to the Ramada CL site, increasing N fertilizer rate typically reduced wheat yield at the Fairland L site. Wheat yield declined linearly with increasing N fertilizer rate in 1998 and 1999, declined quadratically in 2000, and tended to decrease linearly in 2001 (P = 0.06 for a linear contrast). In treatments receiving the lowest rate of N fertilizer, yields were between 110 and 135% that of the yields in the highest N rate treatment. Because a comparable 0 N fertilizer control treatment had not been included in this study, it is unclear if the lowest N rate also depressed yield. In some years, yield declines may have been a function of increased lodging with higher N fertilizer rates. Although mean soil NO 3 -N content at the Fairland L site declined somewhat over the course of the study (Table 4), mean soil NO 3 -N content remained at moderate to high levels throughout the study in comparison with the Ramada CL site (Table 4). Based on current Manitoba provincial soil test recommendations, given the soil test NO 3 -N levels at the Fairland L site in the fall prior to wheat establishment, between 0 and 6 kg N ha 1 fertilizer N would have been recommended in most years to achieve a 2360 kg ha 1 (35 bu ac 1 ) wheat crop under moist conditions. Average wheat yields at the Fairland L site were typically lower, ranging from approximately 1100 to 2100 kg ha 1 over the course of the study; therefore, optimum yield could likely have been achieved without N fertilizer. The moderate to high soil NO 3 levels that persisted throughout the study may have been partly the result of comparatively less crop N removal at the Fairland L site than at the Ramada CL site (Fig. 3). As well, because the Fairland L site had been used for alfalfa

6 246 CANADIAN JOURNAL OF PLANT SCIENCE Table 3. Effect of soil disturbance at seeding and rate of N fertilizer applied to wheat on spike counts of wheat at experimental sites, Soil N fertilizer rate Ramada clay loam Fairland loam disturbance (kg N ha 1 ) 1997 z z Spikes m 2 low high SE SE Effect y df P value rep NS < < ds 3 < < < < < fert NS NS fert ds 3 NS NS NS NS NS NS N linear NS NS NS NS N quadratic 1 NS NS NS NS NS CV (%) z In 1997, soil disturbance treatments were applied, but a single rate of N fertilizer and herbicide was applied. Means are presented for reference. y Herbicide rate did not have a significant effect on spike density of wheat; therefore, data were re-analyzed without this variables. Table 4. Soil NO 3 -N content in fall following crop harvest Site Crop Year z 0 30 cm 0 60 cm cm kg NO 3 -N ha 1 in soil Ramada clay loam Wheat Pea Fairland loam Wheat Pea z For 1997 through 1999 inclusive, reported values are the mean of all treatments. For 2000 and 2001, reported values are the mean of all treatments receiving the 100% herbicide rate. production 5 yr previous, greater mineralization of soil N may also have occurred. Increases in the rate of N applied to the wheat crop in a given rotation resulted in a significant increase in pea yield only at the Fairland L site in 1999; no other significant effects were evident (Fig. 2a). Because the pea crop had been inoculated with rhizobium at seeding, N fixation was expected to provide sufficient N for pea growth in most cases. Of the factors assessed, soil disturbance at seeding had the most consistent effect on wheat yield (Fig. 1b). High levels of soil disturbance at seeding resulted in significant yield reductions at the Ramada CL site in 1998 and 2001, and at the Fairland L site in 2000 and A similar trend (P = 0.08) was evident for the Fairland L in At the Ramada CL site, yields in the high disturbance system were 86 to 96% that of the low disturbance system while, at the Fairland L site, yields in the high disturbance system were 43 to 96% that of the low disturbance system. In most cases, high disturbance had also reduced or tended to reduce earlyseason plant density and the number of wheat spikes per unit area, suggesting that the wheat crop may have been unable to compensate fully for these effects to achieve optimum yield. At the Fairland L site in 2000 and 2001, where the greatest declines in yield were evident, high wild oat populations likely contributed to yield reductions in the high disturbance system. Soil disturbance at seeding had comparatively less effect on field pea than wheat, with high soil disturbance at seeding resulting in significant declines in yield at the Ramada CL site in 1998 and 1999, and at the Fairland L site in 2001 (Fig. 2b). In all cases where yield declines were observed, the high soil disturbance system also had reduced plant densities, suggesting that observed yield reductions may be a function of reduced plant establishment. However, not all declines in pea plant density resulted in reduced yields (e.g and 1999 at the loam site, and 2001 at the clay loam site). Because plant densities in the pea crop typically approached or exceeded the recommended goal plant density regardless of treatment, observed reductions in plant density may have had minimal impact on yield. In most site-years, herbicide rate either had no effect on wheat yield, or the higher herbicide rate increased or tended to increase wheat yield (Fig. 1c). The 100% rate resulted in

7 MOHR ET AL. PRODUCTIVITY OF A WHEAT-PEA ROTATION 247 Fig. 1. Effect of N fertilizer rate applied to wheat (a), level of soil disturbance at seeding (b), and herbicide rate applied to wheat (c) on the grain yield of wheat in a wheat-pea rotation. The standard error is indicated on the graph. P values for main factor effects based on the analysis of variance are presented. higher wheat yields than the 66% rate at the Ramada CL site in 2001 and at the Fairland L site in 2000, and tended (P = 0.08) to increase wheat yield at the Fairland L site in At the Fairland L site in 2000, the full rate of herbicide may have resulted in better control of a serious wild oat infestation, thereby contributing to a higher yield. In one case only, at the Fairland L site in 2001, the full herbicide rate resulted in a lower yield than the 66% rate, although the reason for this effect is not clear. In 75% of site-years, applying the full herbicide rate in the wheat phase of the rotation resulted in a numerically but not statistically (P > 0.05) higher pea yield (Fig. 2c). Studies by Harker et al. (2001) demonstrated that the critical weedfree period to minimize yield losses in pea was 1 to 2 wk after emergence. These findings suggest that improved weed control in the wheat component of the rotation might have reduced early-season weed pressure in pea thereby improving yield potential. Herbicide rate had a statistically

8 248 CANADIAN JOURNAL OF PLANT SCIENCE Fig. 2. Effect of N fertilizer rate applied to wheat (a), level of soil disturbance at seeding (b), and herbicide rate applied to wheat (c) on the seed yield of pea in a wheat-pea rotation. The standard error is indicated on the graph. P values for main factor effects based on the analysis of variance are presented. significant effect on pea yield only at the Fairland L site in 2000, where the 66% rate resulted in a higher pea yield. Cumulative Grain Yield In order to assess the effect of management treatments on productivity of the rotation overall, cumulative yield of field pea and wheat over two full cycles of the rotation was determined for each rotation phase (PWPW vs. WPWP) (Table 5). Soil disturbance at seeding had no significant effect on cumulative yield at the Ramada CL site, despite some differences in individual years. In contrast, at the Fairland L site, the high disturbance seeding system resulted in a substantially lower (P < ) cumulative yield than the low disturbance seeding system, likely due to substantially lower wheat yields in the high disturbance system in 2000 and Nitrogen fertilizer rate affected cumulative yield, largely through effects on wheat yield, in the PWPW sequence both

9 MOHR ET AL. PRODUCTIVITY OF A WHEAT-PEA ROTATION 249 Table 5. Effect of soil disturbance at seeding, and rates of N fertilizer and herbicide applied to wheat on cumulative grain yield within a rotation at experimental sites, Ramada clay loam Fairland loam Soil N fertilizer rate Herbicide (PWPW) (WPWP) (PWPW) (WPWP) disturbance (kg N ha 1 ) rate (%) LS mean SE LS mean SE LS mean SE LS mean SE kg ha 1 Low High Source df P value rep 3 < < < ds 1 NS NS < < fert NS NS ds fert 3 NS NS NS NS herb NS ds herb 1 NS NS NS NS fert herb 3 NS NS NS NS ds fert herb 3 NS NS NS N linear NS N quadratic 1 NS NS NS at the Ramada CL and Fairland L sites. At the Ramada CL site, increasing N fertilizer rate resulted in a significant linear increase in yield for the PWPW sequence. In contrast, at the Fairland L site, increasing N fertilizer rate linearly reduced cumulative yield. As noted previously, the contrasting effects of N at these two sites is likely due, at least in part, to higher inherent soil N levels at the Fairland L site, which reduced the chance of a positive crop response to N application. The full herbicide rate resulted in a higher cumulative yield than the 66% rate for the PWPW sequence at the Ramada CL site and the WPWP sequence at the Fairland L site, likely due to positive effects on both wheat and pea (Figs. 1c and 2c). No significant interactions among treatments were evident at either site. Nitrogen Supply and Removal Soil NO 3 levels were measured post-harvest in all treatments from 1997 through Because no significant effect of herbicide was evident from 1997 through 1999, only 100% herbicide treatments were analyzed from 2000 to At both sites, soil NO 3 -N levels generally declined over the course of the study, which included two full cycles of a wheat-pea rotation (Tables 4 and 6). Overall, N rate had the most consistent effect on fall soil NO 3 -N level following wheat. Post-harvest soil NO 3 content (to 120 cm) increased or tended to increase linearly with increasing N fertilizer rate at the Ramada CL site in 1999, 2000 and 2001, and at the Fairland L site from 1998 through 2001 inclusive. Increasing the rate of N fertilizer applied to wheat in rotation also resulted in a linear increase in soil NO 3 -N content following pea at the Ramada CL site in 1999 and 2001 and at the Fairland L site in 1998, 2000 and Observed increases in soil NO 3 -N content suggest that the incremental increases in N fertilizer rate exceeded incremental increases in N removal from these treatments, whether through crop removal or through leaching or gaseous losses from the system. High disturbance at seeding decreased residual soil NO 3 following pea at the Ramada CL site in 1999, but had the opposite effect at the Fairland L site in Herbicide rate had no effect on residual soil NO 3 levels (data not shown). Overall, cumulative N removal in the grain of wheat and pea was numerically greater for the Ramada CL than for the Fairland L site (Fig. 3). Cumulative N removal for the PWPW sequence averaged approximately 298 kg N ha 1 and 369 kg N ha 1 at the Fairland L and Ramada CL sites, respectively and, for the WPWP sequence, averaged approximately 281 kg N ha 1 and 372 kg N ha 1 for the Fairland L and Ramada CL site, respectively. Greater N removal by crops at the Ramada CL than at the Fairland L site may have contributed to the more marked declines in mean post-harvest soil NO 3 -N levels observed over the course of the study at the Ramada CL site than at the Fairland L site (Table 4). High soil disturbance at seeding significantly reduced cumulative N removal by crops at the Ramada CL site for the WPWP sequence, and at the Fairland L site for both rotation sequences. Nitrogen fertilizer rate also influenced N removal by the crop at the Ramada CL site, with increasing N fertilizer rate increasing cumulative N removal for both the PWPW sequence (linear effect) and the WPWP sequence (quadratic effect). At the Fairland L site, no sig-

10 250 CANADIAN JOURNAL OF PLANT SCIENCE Table 6. Effect of soil disturbance at seeding and rate of N fertilizer applied to wheat on the soil NO 3 -N content (to 120 cm) following crop harvest at experimental sites, Soil N fertilizer rate Ramada clay loam Fairland loam Crop disturbance (kg N ha 1 ) 1997 z y z 1998 x y 2001 kg NO 3 -N ha 1 to 120 cm Wheat Low High Source df P value rep 3 < < NS < NS ds 1 NS NS NS NS NS NS NS NS fert 3 NS NS NS NS ds fert NS NS NS NS NS NS N linear 1 NS N quadratic 1 NS NS NS NS NS NS NS kg NO 3 -N ha 1 to 120 cm Pea Low High Source df P value rep 3 NS < NS < NS ds NS NS NS NS fert 3 NS NS NS NS NS NS ds fert 3 NS NS NS NS NS NS NS N linear 1 NS NS NS N quadratic 1 NS NS NS NS NS NS NS NS z In 1997, soil disturbance treatments were applied, but a single rate of N fertilizer and herbicide was applied. Means are presented for reference. x A significant (P = ) fertilizer herbicide interaction was evident for wheat only in this site-year. nificant effects of N fertilizer rate were evident in the WPWP sequence, but increasing N rate tended (P=0.07) to reduce cumulative N removal for the PWPW sequence. The higher herbicide rate increased cumulative N removal by the PWPW sequence at the Ramada CL site and by the WPWP sequence at the Fairland L site, but the opposite trend was evident for the PWPW sequence at the Fairland L site (P = 0.07). Observed differences in N removal were largely a function of differences in crop yield. While the lack of a 0 N treatment and the lack of a nonpea rotation limit estimation of the N contribution of pea in this rotation, an attempt was made to estimate net N removal from the system (calculated as the difference between N removal by the wheat and pea crops within a rotation cycle, and the sum of fertilizer N applications within the rotation cycle plus the change in soil NO 3 -N reserves to 120 cm from the beginning to the end of the rotation cycle) (Table 7). Only the period was assessed, once the rotation had been in place for one full cycle. In all cases, crop N removal during the 2-yr rotation cycle exceeded the sum of N fertilizer applied plus changes in soil NO 3 -N over the 2-yr period. Net N removal over the 2-yr rotation ranged from 151 to 156 kg N ha 1 at the Ramada CL site, and from 75 to 125 kg N ha 1 at the Fairland L site in the 25 kg N ha 1 fertilizer treatment. These findings indicate that a significant amount of N, in excess of the amount added to the system as fertilizer, was plant-available. However, the relative contribution of soil N mineralization versus N fixation by peas is not known, nor is the magnitude of N losses from the system known. In all cases, increasing fertilizer rate significantly decreased net N removal during the rotation cycle, suggesting that increases in crop N removal at higher N rates were not equivalent to increases in the amount of N applied as fertilizer. At the Fairland L site only, high disturbance at seeding reduced net N removal compared to low disturbance at seeding, likely due in part to lower yield and thus lower N removal in this treatment. Nitrogen Concentration in Grain In all site-years, the average N concentration in wheat grain was relatively high, ranging from 2.7 to 2.9% at the Ramada CL site and from 2.7 to 3.4% at the Fairland L site. In studies to assess grain protein concentration as an indicator of crop N sufficiency, Fowler (2003) reported that for AC

11 MOHR ET AL. PRODUCTIVITY OF A WHEAT-PEA ROTATION 251 Fig. 3. Effect of level of soil disturbance at seeding (a), N fertilizer rate applied to wheat (b), and rate of herbicide applied to wheat (c) on cumulative N removal in the grain of wheat and pea in a wheat-pea rotation over two rotation cycles. P values for main factor effects based on the analysis of variance are presented. Barrie wheat, 128 to 135 g protein kg 1 (2.2 to 2.4% N) was associated with 90% of maximum yield, while 148 to 159 g protein kg 1 (2.6 to 2.8% N) was associated with maximum yield. Although the use of grain protein concentration as an indicator of N sufficiency has some limitations, grain N concentrations in the current study combined with minimal positive yield responses to N fertilization (Fig. 1a) suggest that the wheat crop likely had sufficient N in most cases. Of the factors assessed, N fertilizer rate had the most consistent effect on the N concentration of wheat grain, resulting in significant increases in grain N concentration at the Ramada CL site in 2000 and 2001 and at the Fairland L site in all years (data not presented). Increases in grain N concentration were more pronounced at the Ramada CL, where positive yield responses to N had also been observed (Fig. 1a). At the Ramada CL site, grain N concentration for the

12 252 CANADIAN JOURNAL OF PLANT SCIENCE Table 7. Effect of soil disturbance at seeding, and rates of N fertilizer applied to wheat on net N removal z from a wheat-pea rotation at experimental sites, Ramada clay loam Fairland loam Soil N fertilizer rate (PW) (WP) (PW) (WP) disturbance (kg N ha 1 ) LS mean SE LS mean SE LS mean SE LS mean SE kg N ha 1 Low High Source df P value rep < < ds 1 NS NS < fert < < ds fert 3 NS NS NS NS N linear < < N quadratic 1 NS NS NS NS CV (%) z Net N removal is calculated as: (cumulative crop N uptake for 2000 and 2001) [(total fertilizer N application during the period of 2000 to 2001, inclusive)+(net change in soil NO 3 -N to 120 cm between fall 1999 and fall 2001). Only 100% herbicide treatments were used for these calculations. lowest and highest N rates was 2.70 and 2.98% in 2000, and 2.61 and 2.79% in At the Fairland L site, where N application typically reduced grain yield (Fig. 1a), lowest and highest N rates resulted in grain N concentrations of 2.66 and 2.72% in 1998, 2.82 and 2.91% in 1999, 2.98 and 3.1% in 2000, and 3.09 and 3.18% in Higher grain protein is typically associated with higher available N levels and lower yield potentials (Grant et al. 2002), as was the case at the Fairland L site, particularly in 2000 and In pea, increasing N fertilizer rate increased the N concentration in pea seed only at the Ramada CL site in 1998, and had inconsistent effect on grain N concentration at the Ramada CL site in 2000 (data not shown). Soil disturbance at seeding and herbicide rate had inconsistent effects on the N concentration of wheat grain and pea seed. CONCLUSION In the current study, the overall productivity of the wheatpea rotation was most strongly and consistently influenced by the rate of N fertilizer applied to the wheat crop and the level of soil disturbance at seeding, and to a lesser degree by herbicide rate. The general lack of significant interactions among these factors suggests that they acted independently. High disturbance seeding systems typically reduced early-season plant establishment for wheat and pea. The specific cause of reduced emergence in the high disturbance system is unclear, although various factors such as a less firm seedbed or less uniform seeding depth may have contributed to reduced emergence. While lower plant densities sometimes produced small decreases in yield, substantial yield reductions occurred in wheat under high weed pressure, emphasizing the importance of adequate plant stands under less-than-ideal growing conditions. These large yield declines resulted in significant declines in the cumulative yield of the rotation overall. Impacts of N fertilizer rate on crop productivity were strongly influenced by the inherent fertility of the field sites. Where the soil N supply met or exceeded crop needs, N fertilizer had no effect or negatively affected wheat yields. However, where the soil N supply did not meet crop needs, N fertilizer significantly increased wheat yield. Despite the presence of pea in rotation every second year, soil NO 3 -N levels at one site were depleted over the course of the study to a level at which positive wheat responses to fertilizer N were obtained. Because the amount of N removed in the harvested seed of pulses is similar to the amount of N symbiotically fixed by the plant, including pulse crops in rotation is not expected to add large amounts of N to the system, although the N fertilizer requirements of crops following pulses in rotation are generally reduced (Grant et al. 2002). In the current study, significant amounts of N were removed from the cropping system in the harvested wheat grain and pea seed. During the second cycle of the rotation, crop removal exceeded fertilizer N applications plus soil NO 3 -N depletions by an average of 43 to 130 kg N ha 1, suggesting that significant amounts of plant-available N may have been provided by N mineralization and/or N fixation. Pea yields were largely unaffected by the rate of N fertilizer applied to the preceding wheat crop. Application of the full herbicide rate to wheat rather than a reduced herbicide rate significantly increased wheat yield in 2 site-years, and decreased yield in 1 site-year. Although effects were not statistically significant, the full herbicide rate resulted in pea yields that were similar to or numerically higher than the reduced rate in 6 of 8 site-years. As a result, the full rate typically produced a cumulative yield for the rotation overall that was similar to or higher than the reduced rate. Reduced weed pressure may enhance yield potential, particularly for pea, which is prone to yield losses from early-season competition (Harker 2001; Harker et al. 2001).