Adults emerge from the previous year's crop stubble in late spring to early summer and, following mating, the adult female seeks out a suitable host plant to oviposit, usually an adjacent wheat field (Criddle, 1922). A healthy female can successfully lay up to 50 eggs; thus, the population and subsequent damage to wheat can increase exponentially in a single generation (Ainslie, 1920). Shortly after an egg is deposited into a stem of wheat, a larva will hatch and begin boring the stem (Criddle, 1923). This activity continues throughout the growing season until the host plant reaches physiological maturity. Chlorosis associated with plant ripening and the reduction of whole plant moisture cues the larva to begin preparation to overwinter (Holmes, 1979). The larva moves to the base of the stem, notches a v-shaped groove around the stem, fills the region with frass, and encases itself in a cocoon below the groove. The groove weakens the stem and causes it to easily lodge or topple over, which proves difficult to recover at harvest (Ainslie, 1929).
The injury caused by stem boring reduces photosynthetic rates (Macedo et al., 2007) and results in grain weight losses ranging from 10 to 17% (Holmes, 1977; Morrill et al., 1992; Seamans et al., 1944). An additional loss in yield potential occurs when toppled stems are not recovered at harvest (Ainslie, 1920; Beres et al., 2007). Therefore, overall yield potential in wheat infested by WSS can be reduced by >25% and can lead to annual losses of approximately US$350 million. It is considered the most important economic insect pest in Montana and among the top insect pest threats in North Dakota, Saskatchewan, and Alberta (Beres et al., 2011c). The use of solid-stemmed cultivars helps mitigate crop losses and can also affect the survivorship of C. cinctus. The mechanical pressure of developing pith in a solid stem can result in mortality of the egg (Holmes and Peterson, 1961), and the boring activity of larvae that do hatch can be restricted, creating negative effects to health, fitness, and survivorship (Cárcamo et al., 2005; O'Keeffe et al., 1960). Therefore, the efficacy of “resistance” is based on the plant's ability to develop pith in the culm of the stem, which is influenced greatly by interactions between the genotype and the environment in which it is grown.
All solid-stemmed spring and winter wheat cultivars developed to date derive resistance from the line S-615 (Kemp, 1934; Platt and Farstad, 1946), but two other sources exist (Clarke et al., 2005). The recessive nature of the genes controlling resistance derived from S-615 leads to inconsistent pith expression in the field (Hayat et al., 1995). This was acknowledged shortly after ‘Rescue’ was released when observations of high susceptibility to stem cutting were noted at Regina, SK (Platt and Farstad, 1949). It was later determined that pith expression is probably influenced by photoperiod. Intense sunlight results in maximum expression and pith development whereas precipitation, shading, or cloudy conditions inhibit pith development (Eckroth and McNeal, 1953; Holmes, 1984).
Solid-stemmed cultivars currently available in the Canada Red Western Spring class are ‘AC Eatonia’ (DePauw et al., 1994), ‘AC Abbey’ (DePauw et al., 2000), and ‘AC Lillian’ (DePauw et al., 2005). Solid-stemmed spring wheat cultivars available in Montana include ‘Fortuna’ and ‘Choteau’. Resistance in winter wheat is also important because Montana has a biotype of WSS that has gradually adapted to become synchronous to winter wheat growth phenology by emerging 10 to 20 d earlier than normal. The adaptation seems to have occurred as a response to a shift away from spring to winter wheat production (Morrill and Kushnak, 1996). Solid-stemmed winter wheat cultivars available to Montana producers include ‘Vanguard’ (Carlson et al., 1997), ‘Rampart’, and ‘Genou’ (Bruckner et al., 1997, 2006).
Wheat row spacing and seeding rates can influence C. cinctus infestation rates, and the response varies between solid- and hollow-stemmed cultivars. Luginbill and McNeal (1958) reported that narrow row spacing and high seeding rates reduced stem cutting in ‘Thatcher’, a hollow-stemmed cultivar, but the same treatments reduced pith expression and led to increased levels of cutting damage in Rescue, a solid-stemmed cultivar.
Crop nutrient management can significantly change crop canopy architecture and influence overall plant health, which in turn could influence WSS infestation rates. Luginbill and McNeal (1954) observed that when a blend of N and P was applied to wheat there was generally an increase in stem cutting. Nitrogen applied separately did not influence cutting whereas P applied alone produced a slight increase in stem cutting. In contrast, a recent Montana greenhouse study reported that P-deficient wheat plants were most susceptible to sawfly damage (Delaney et al., 2010). In a Saskatchewan study, no effects of N or P could be detected due to the strong influence of other environmental factors (DePauw and Read, 1982), which is similar to a North Dakota study that reported significantly more sawfly cutting in fertilized plots in only one of eight experiments (O'Keeffe et al., 1960). The disagreement among these studies underscores the stochastic nature of site-specific, soil-plant fertility dynamics. Moreover, the studies did not report any detailed agronomic assessments and no information on the effects of micronutrient blends on pith expression in solid-stemmed wheat has been reported.
The inconsistency in pith development should not dissuade producers from growing solid-stemmed wheat in areas prone to attack because cultivar selection is critical to a successful integrated pest management system for WSS (Beres et al., 2011a). Beres et al. (2009, 2007) demonstrated that solid-stemmed wheat can be agronomically superior to hollow-stemmed wheat in the presence of sawfly pressure, and newer cultivars with the solid stem trait have further improved yield and quality even in the absence of sawflies (DePauw et al., 2005). This research has resulted in a dramatic increase in planted hectares of solid-stemmed wheat (CWB, 2011). AC Lillian (DePauw et al., 2005), the latest solid-stemmed cultivar to be released, occupies almost one-third of the wheat hectares in Saskatchewan and 17% of the prairie-wide wheat hectares (CWB, 2011). Based on the vast area now sown to solid-stemmed wheat, our objective was to develop an integrated nutrient and planting strategy specific to solid-stemmed spring wheat using modern farming techniques. This paper tests the following hypotheses: (i) physiological crop changes may reduce pith in the culm of wheat with increasing planting densities and N rates, (ii) tolerance to WSS infestation may be reduced with increased planting density, and (iii) postemergent applications of micronutrient blends may increase pith expression in solid-stemmed wheat.
MATERIALS AND METHODS
Experimental Design and Management
The WSS is difficult to chemically manage because the adults emerge and lay eggs over a 3-wk period and the larvae feed within the wheat stem beyond the reach of contact or stomach poisons. There are no registered insecticides for application on the WSS. Therefore, innovations from field experiments evolve from strategies that integrate multiple factors such as cultural methods, biological control, and resistant plant varieties. Two study locations in the traditional distribution area of the WSS were selected near Lethbridge (49°41′ N, 112°45′ W), AB, Canada, and near Bow Island (49°44′ N, 111°20′ W), AB, Canada. The Lethbridge site is an Orthic Dark Brown Chernozem clay loam soil (Typic Boroll) with 3.0% organic matter content and a pH of 7.5. The Bow Island site is a Brown Chernozem loam soil (Aridic Boroll) with 2.0% organic matter content and a pH of 6.0. A new area at these locations was selected each year of the study (Table 1). At Lethbridge, the experiment was planted into a continuously cropped system into a field previously cropped to oats (Avena sativa L.). Two experiments were planted at Bow Island: (i) wheat-fallow cropping system and (ii) continuous-cropping system direct seeded into spring durum wheat stubble. Three site–years of data were collected for each year during the period 2007 to 2009 for a total of nine site–years.
|Location||Lethbridge, AB, Canada
||Bow Island, AB, Canada
|Latitude and longitude||49°41′ N, 112°45′ W||49°44′ N, 111°20′ W|
|Soil zone, series, and texture||Dark Brown Chernozemic Lethbridge series clay loam||Brown Chernozemic Chin seriessilty loam|
|Sowing date||2 May||13 May||11 May||7 May||7 May||7 May|
|Harvest date||13 Aug.||16 Sept.||28 Aug.||8 Aug.||5 Sept.||3 Aug.|
|Soil NO3−N 0–60 cm kg N ha−1||40.7||35.7||20.2||13.7s†||29.9f||3.4s‡||18.9f||4.7s||42.4f|
|1 May to 15 September(Bow Island: long-term average = 210)(Lethbridge: long-term average = 251)||164||380||241||141||270||175|
|Annual(Bow Island: long-term average = 358)(Lethbridge: long-term average = 398)||342||525||417||254||398||303|
A 3 × 5 factorial combination of sowing density and N rate was arranged in a randomized complete block experimental design with four replicates each year. To study effects of planting density, three levels of seeding rate were selected: (i) low density (100 seeds m−2), (ii) moderate density (300 seeds m−2), and (iii) high density (500 seed m−2). Five levels (0, 30, 60, 90, and 120 kg ha−1) of urea [CO(NH2)2] N fertilizer (46–0–0) were banded midrow at the time of planting. In 2008, a postemergence application of a water-soluble micronutrient fertilizer was added to an additional 90 kg N ha−1 treatment of basal N. The micronutrient blend was commercially available as Yield Max (18–20–20; Nexus Ag Business Inc.) and derived from ammonium phosphate, potassium nitrate, urea, ammonium sulfate, sodium borate, Cu-chelated ethylenediamine tetra-acetate (EDTA), Fe EDTA, Mn EDTA, Zn EDTA, and sodium molybdate. The blend was foliar applied at both the three- to four-leaf stage and again at the flag leaf stage at five times the recommended rate. An excessive rate was selected to ensure all nutrients would be elevated to levels that would facilitate plant uptake and elicit a notable response if any were to occur because some micronutrients such as Fe, Mn, and Zn are only present in the blend at 0.1% of net weight.
A zero tillage drill manufactured by Fabro Manufacturing and configured with single shoot openers (Atom-Jet Industries) spaced 20.3 cm apart was used to plant the experiment in all locations. Experimental plot unit dimensions were 2 m wide by 7 m long. Each study area was treated with glyphosate (RoundUp; Monsanto Company) a few days before seeding applied at a rate of 900 g a.i. ha−1 using a motorized sprayer calibrated to deliver a carrier volume of 45 L water ha−1 at 275 kPa pressure. In-crop herbicides were chosen based on the weed spectrum present at each site–year and applied in early June at label rates. No insecticides were used at any site during the study period.
Three handheld instruments were used to discriminate differential plant responses to the study treatments by assessing chlorophyll levels or the amount of vegetation in the study plots. The Greenseeker active lighting optical sensor (NTech Industries Inc.) consists of two diodes that emit energy in 671 and 780 nm wavelengths. The light reflected back from the crop is measured by a photodiode and the normalized difference vegetation index (NDVI) is computed ([R780 – R671]/[R780 + R671]). The principle is that NDVI relates to greenness and canopy size (i.e., chlorophyll levels) and therefore N management. Readings were collected from plots at all locations in 2008 and 2009 at Zadoks growth stages 31 to 37 (Zadoks et al., 1974). The Field Scout CM1000 chlorophyll meter (Spectrum Technologies) was used in 2009 at all locations when the plant growth stage was at approximately Zadoks 31 to 37. The chlorophyll meter senses light at wavelengths of 700 and 840 nm, which are then used to estimate the quantity of chlorophyll in leaves. The ambient and reflected light at each wavelength is measured. Chlorophyll a absorbs 700 nm light and, as a result, the reflection of that wavelength from the leaf is reduced compared to the reflected 840 nm light. Light having a wavelength of 840 nm is unaffected by leaf chlorophyll content and serves as an indication of how much light is reflected due to leaf physical characteristics such as the presence of a waxy or hairy leaf surface. The LP-80 AccuPAR Ceptometer (Decagon Devices), which measures light in the 400 to 700 nm (photosynthetically active radiation) waveband, was used to determine leaf area index (Decagon Devices, 2006), which is defined as the one-sided green leaf area of the crop canopy per unit of ground area. This was achieved by first measuring above the canopy on a leveled tripod in a location with an unobstructed view of the sky and below the canopy, placing the ceptometer level and linearly between rows. All measurements were taken within 2 h of solar noon.
Temperature and light intensity data were collected at each site using Hobo Pendant temperature and light loggers (Onset Computer Corporation; part no. UA-002-XX). The data loggers were attached near the top of 1-m fiberglass “whisker stakes” (Imagine That Signs and Designs) and positioned at the center of each of the three ranges.
Plant counts were performed in mid to late May by staking a 1-m section of row in two randomly selected areas of the plot. The staked sections were counted again in mid to late July to assess spike density. To ensure an adequate estimate of stem solidness (Cárcamo et al., 2007), a 0.50-m section of row was collected in late July or early August in two random locations in each plot to determine stem diameter and pith expression or degree of stem solidness in the culm of the main stem. Mean stem diameter was determined by measuring the outside diameter of the first three internodes using a digital caliper. Each stem was then split lengthwise from crown to neck and, starting from the crown, each internode was assessed visually for pith development. Ratings were as follows: 1, hollow stem (no pith development); 2, some degree of pith development (may appear “cotton like”; 3, large hollow tunnel in the stem or a huge cavity at a particular point in the internode; 4, size of hollow equivalent to a pencil lead or some cavitation has occurred at a particular point in the internode; and 5, solid stem (DePauw and Read, 1982). Stem cutting data (recorded as percent stems cut) were collected by visually estimating the percentage of stems that had been cut by WSS in each plot.
Plots were harvested at crop maturity using a Wintersteiger Expert (Wintersteiger AG) plot combine equipped with a straight cut header, pickup reel, and crop lifters. Grain yield was calculated from the entire plot area and retained postharvest to characterize seed weight (g 1000 −1) and grain bulk density (kg hL−1). Grain protein concentration was determined from whole grain using near infrared reflectance spectroscopy technology (Foss Decater GrainSpec, Foss Food Technology Inc.).
Data were analyzed with the MIXED procedure of SAS (Littell et al., 2006). Homogeneity of error variances was tested using the UNIVARIATE procedure of SAS, and any outlier observations were removed before a combined analysis over years and environments was performed. Normality assumptions were also tested on the categorical data “pith expression” and observational “stem cutting (%)” data as multiple categories were used for rating pith expression and percentages for stem cutting were generally not extreme (Cochran, 1954). For analyses by environments, replicate was considered random and treatment effects were considered fixed and significant if p ≤ 0.05. Results by environment indicated similar treatment response patterns among environments; therefore, a combined analysis was performed with replicate, years, environments, and their interactions considered random effects and treatment effects treated as fixed effects and significant if p ≤ 0.05. Response variable least square means generated for each site–year were used to create a Pearson correlation coefficient matrix of stem diameter, stem solidness, WSS damage, and yield components, whole grain protein, and canopy reflectance data using the CORR procedure of SAS.
A grouping methodology previously described by Francis and Kannenberg (1978) and later adapted to agronomy studies (Beres et al., 2010a; Gan et al., 2009; May et al., 2010) was used to further explore treatment responses. The mean and CV were estimated for each level of the treatment. Means were plotted against CV for each level of the treatment. The overall mean of the treatment means and CVs was included in the plot to categorize the biplot ordination area into four quadrats per categories: group I: high mean, low variability (optimal); group II: high mean, high variability; group III: low mean, high variability (poor); and group IV: low mean, low variability.
An economic analysis was conducted to determine the net return associated with each fertilizer rate alone or when combined with seed input costs, based on seed and N rates used, yield response, and grain protein observed. Crop prices were based on the final year of the study (2009) and obtained from the Canadian Wheat Board historical payments for #1 Grade Canada Western Red Spring for each protein level observed for the mean of the seed rate by N treatment (CWB, 2010). Seed input costs were based on the average selling price for AC Lillian during the period of the study (C$13 per bushel; Wes Woods, personal communication, 2011). Fertilizer prices were based on the average price of urea 46–0–0 fertilizer for 2009 (Agriculture and Agri-Food Canada, 2010). The net return was calculated as N = (YP) – [(C × S) + (F × R)] for seed and fertilizer inputs or as N = (YP) – (F × R) for a net return for fertilizer only, in which N is the net return in Canadian dollars per hectare, Y is crop yield (t ha−1), P is crop price in Canadian dollars t−1, C is the cost of seed, S is the seeding rate (100, 300 or 500 seeds m−2 converted to t ha−1), F is the cost of fertilizer, and R is the N rate (0, 30, 60, 90, or 120 kg N ha−1 converted to t ha−1). This equation was adapted from Mason et al. (2007) and O'Donovan et al. (2001).
Average annual precipitation at Lethbridge and Bow Island during the study period was close to normal but below average rainfall occurred in 2007 at both sites and in 2009 at Bow Island (Table 1). Growing season precipitation was 150% of normal at Lethbridge in 2008. Mean temperature and light intensity were also unique in 2007. Trends in light intensity and temperature were similar at all sites with the notable exception of Bow Island fallow in 2007, where higher light intensity was recorded in August (Fig. 1). Light intensity levels were similar for June, July, and August at the other sites in 2007; however, light intensity peaked in June for 2008 and 2009 at all sites. Temperature also peaked in 2008 and 2009 at all sites but was highest in July in 2007, which was the hottest month of the entire study period (Fig. 1). Background levels of available soil NO3−N were generally low at all sites and lowest in the continuously cropped system at Bow Island (Table 1).
Reduced pith expression in solid-stemmed wheat concomitant with increased stem cutting caused by WSS was observed as seeding rates increased over the low rate of 100 seeds m−2 (Table 2; Fig. 2). Single degree of freedom contrast results indicate that the best fit of the response was linear but the significant quadratic response for pith expression may indicate that the downward trend for pith expression was not strictly linear. The average pith rating of main stems was reduced by 13% when planting density was increased to 300 seeds m−2 and an additional 5% when increased to 500 seeds m−2. Similar results were observed for stem diameter. The magnitude of change was greater in stem cutting as visual estimates increased by 38 and 50%, respectively, for moderate and high seeding rates (Table 2). Across all site years, N management practices had no effect on stem solidness but did influence stem diameter and visual estimates for stem cutting. Higher N rates resulted in greater stem cutting and increased stem diameter from 2.17 to 2.44 mm at the highest rate of N (Table 2; Fig. 2). In site–years where N did influence pith expression when averaged over seeding rate (Bow Island stubble 2008 and Bow Island fallow 2007), the response generally resulted in greater pith at 60 to 90 kg N ha−1 vs. the control or 30 kg N ha−1 (data not shown).
|Factor||Treatment||Stem solidness (pith expression)†||Stem diameter (mm)||Sawfly damage (% stems cut)|
|Seed rate, seeds m−2||100||3.03||2.53||24|
|p > F||<0.0001||<0.0001||0.0111|
|Nitrogen rate, kg N ha−1||0||2.66||2.17||22|
|p > F||0.4513||0.0035||0.0061|
|p > F||Quadratic||0.8323||0.0923||0.4759|
|Seeding rate × fertilizer||p > F||0.9408||0.2236||0.7325|
Yield components and grain quality parameter results are summarized in Table 3 and Fig. 3 and 4. Seeding and N rates affected all traits except grain bulk density (Table 3). With the exception of seed mass and grain bulk density, single degree of freedom contrasts indicate that the response to varying rates of both main effects was primarily linear (Table 3; Fig. 3 and 4). Grain yield and spike density responses were greater from low to moderate seeding rates but diminished from moderate to high seeding rates. Plant establishment decreased from 87% at the lowest plant density to 62% at 300 seeds m−2; however, plant stands increased proportionally at each seed rate level (Table 3). All yield components were optimized at the 60 to 90 kg N ha−1 N rate, but grain protein concentration continued to respond through the 120 kg N ha−1 rate. However, grain protein diminished with increased plant density (Table 3; Fig. 4). According to the biplot summaries, grain yield and protein associated with the lower rates of N were either too low or too variable. Increasing plant density to 300 seeds m−2 produced high and stable grain yield with 60 to 120 kg N ha−1. The same plant density combined with 90 or 120 kg N ha−1 also produced high protein. The low plant density produced high and stable protein at the higher N rates, and the same combinations also produced consistent grain yield, but the results were average or just below average (Fig. 5).
|Factor||Treatment||Yield at 14% m.c.†||Stand establishment||Spike density||Seed mass||Grain bulk density||Grain protein|
|Mg ha−1||plants m−2||heads m−2||g 1000−1||kg hL−1||%|
|Seed rate, seeds m−2||100||2.74||86||293||31.2||70.3||14.2|
|p > F||0.0425||<0.0001||<0.0001||<0.0001||0.6089||0.0046|
|Contrasts, p > F||Linear||0.0323||<0.0001||<0.0001||<0.0001||0.3268||0.0013|
|Nitrogen rate, kg N ha−1||0||2.47||194||345||30.8||71.0||12.6|
|p > F||0.0006||0.0551||<0.0001||0.0329||0.7606||<0.0001|
|Contrasts, p > F||Linear||0.0013||0.0085||<0.0001||0.1030||0.6801||0.0002|
|Seeding rate × fertilizer||p > F||0.3900||0.1940||0.6579||0.7431||0.6773||0.9237|
The superior pith expression observed in the low planting density is apparent in the biplot (Fig. 5). The lowest planting population produced higher pith and low variability when combined with all fertilized treatments. The moderate planting density also produced above average pith expression and low variability when combined with the 60 kg N ha−1 fertilizer rate. However, the visual stem cutting estimate for this treatment (33%) was just above the average of 31%. Lower and stable stem cutting was observed when planting at 100 seeds m−2 with 0, 60, or 90 kg N ha−1 (Fig. 5). Adding a micronutrient blend to a basal rate of 90 kg N ha−1 as previously described had no effect on pith expression or cutting susceptibility compared to the same basal rate of N without added micronutrients (Table 4).
|Factor||Treatment||Yield at 14% m.c.†(Mg ha−1)||Stem solidness(pith expression)‡||Sawfly damage(% stems cut)||Grain protein (%)|
|Basal fertilizer, 90 kg N ha−1||Without micronutrient||3.66||2.85||36||13.6|
|Basal fertilizer, 90 kg N ha−1||With micronutrient||3.45||2.78||35||13.9|
|p > F||0.1542||0.4464||0.8536||0.3787|
Correlation coefficients were generated to further explore relationships between sawfly related parameters and crop production parameters (Table 5). Positive relationships exist between stem diameter and grain yield, protein, NDVI, and plant chlorophyll, but an inverse relationship exists with plant stands. There was also a positive relationship with stem solidness and stem diameter indicating larger stems generally produce more pith than smaller diameter stems. Negative correlations were observed between stem solidness and stem cutting, stand establishment. and spike density.
|Pearson correlation coefficients
|Stem solidness||Sawfly damage||Stand establishment||Spike density||Grain yield||Grain protein||NDVI†||Plant chlorophyll|
A similar pattern for leaf area index was observed for N applications and sowing density (Fig. 6). Leaf area index peaked at Zadoks 58 (Zadoks et al., 1974) with the greatest accumulation observed using the higher rates of N or sowing density. There was no apparent benefit to leaf area index when in-crop applications of the micronutrient blend was applied to the 90 kg N ha−1 basal rate of N (Fig. 6).
Two economic net return scenarios were considered, based exclusively on N input costs (Fig. 7a and 7b) or the combined input costs of seed and fertilizers (Fig. 7c and 7d). In the first scenario, pricing was based on the 2009 input costs per tonne of urea N. In the second scenario, the input cost for N was increased by 50%, which is similar to a pricing scenario for fertilizer in 2008. In both cases, the seed input cost was based on the 2009 price of approximately C$13 per bushel, which was a typical price for AC Lillian in the first 3 yr of commercial production (2007 to 2009). In both cost scenarios, net returns were maximized at the 90 kg N ha−1 rate with either the 300 or 500 seeds m−2 sowing density (Fig. 7a and 7b). However, when seed input costs were added to the model, the highest and lowest sowing densities provided similarly low net returns compared to 300 seeds m−2 (Fig. 7c and 7d).
Our findings do not correspond with a previous study on planting density that reported an increase in stem cutting of Rescue solid-stemmed wheat with lower plant populations (Luginbill and McNeal, 1958). Rescue planted at a rate of approximately 100 seeds m−2 sustained 57% cutting whereas plots sown at 600 seeds m−2 had 50% less cutting. The N results of our study agree with Luginbill and McNeal (1954) as the authors reported no effect when N was applied alone at rates of 60 and 120 kg N ha−1. A study comparing unfertilized spring wheat plots to those with basal rates of approximately 40 kg N ha−1 also report little effect from fertilization (O'Keeffe et al., 1960), and DePauw and Read (1982) reported that environmental factors were more important for pith expression than fertility management practices. Environmental factors that would affect pith expression are those related to precipitation. Cloudy conditions and reduced photoperiod negatively effect pith expression (Holmes, 1984). Shading effects within the canopy could have a similar effect. Leaf area index was greatest for the highest levels of N and seeding rates (Fig. 6), which corresponds to the responses observed in the same treatments for pith expression and sawfly damage. Therefore, the management of crop canopy architecture through appropriate seeding and N rates will probably influence plant tolerance or cutting susceptibility.
There are two important considerations for plant density and N management recommendations for solid-stemmed wheat. First, any recommendation based on this study or previous findings must have agronomic merit. The second consideration is for a system that enhances the plant's ability to produce maximum pith in the culm of the stem. Wallace et al. (1973) reported that a mean pith expression of 3.75 would be required to achieve consistent high tolerance to WSS infestations. We did not observe this degree of stem solidness in AC Lillian in any of the treatments, which indicates that the weather parameters or the genetic potential of AC Lillian prevented maximum resistance to WSS infestation. Low plant populations maximized pith expression in our study and all combinations with fertilized treatments at this level displayed acceptable stability. However, low plant populations may not always optimize grain yield. A number of studies report that higher plant populations enhance weed competitive ability and herbicide efficacy and produce higher grain yield (Beres et al., 2010b; O'Donovan et al., 2006). A study of variety selection and seeding rates ranging from 150 to 450 seeds m−2 reports that the ideal sowing density for solid-stemmed wheat is in the range of 250 to 350 seeds m−2 (Beres et al., 2011b). Therefore, the compromise may be to combine moderate seed rates with moderate levels of N to maximize pith expression and grain yield and to meet the minimum protein standards (13.5%) required by the Canadian Wheat Board for marketing purposes. This strategy may improve net economic returns as herbicide management and weed competitive ability is enhanced over the low seeding rate.
Luginbill and McNeal (1954) reported increased susceptibility to cutting in Rescue wheat when N was combined with P or P alone. Other studies report a reduction in yield and cutting damage when plants are P deficient (Delaney et al., 2010). Our N treatments generally did not affect pith expression but we did observe reduced pith in two site–years for the control and 30 kg N ha−1 N treatments (data not shown). This suggests that once N or P sufficiency is achieved, additional application of macronutrients elicits a physiological response (vegetative growth leads to shading) in spring wheat that inhibits pith expression. Therefore, N rates beyond 60 kg N ha−1 may begin to reduce the efficacy of a solid-stemmed wheat variety due to shading effects unless a very low sowing density is used (Fig. 5).
In our study, we were also interested in expanding these findings to test if a blend of both macronutrients and micronutrients would create a similar result. Adding a micronutrient blend to a basal rate of 90 kg N ha−1 as previously described had no effect on pith expression or cutting susceptibility compared to the same basal rate of N without added micronutrients (Table 4). Furthermore, there were no differences observed in grain yield or grain protein concentration. Therefore, the hypothesis that micronutrient blends would have a positive effect on pith expression, grain yield, or quality parameters in solid-stemmed wheat was not supported. Nitrogen management and the use of micronutrient blends will alter canopy architecture in a similar fashion as seeding rates. However, there was no direct effect on pith expression observed in solid-stemmed wheat that was attributed to anything other than shading effects; fertilization did not influence pith expression but N did influence cutting susceptibility. Nutrient management should focus on plant health and therefore standard amendments are recommended: that is, 60 kg N ha−1. Most agronomists would recommend N amendments in accordance to soil available N results from soil sampling and lab analysis to confirm N requirements.
The inverse relation observed between chlorophyll and stem cutting indicates that prolonged “greenness” or delayed crop maturity could reduce stem cutting susceptibility. This could simply be a case of asynchrony during the WSS flight period as later maturing treatments were at a crop phenology stage unattractive to an ovipositing female. Ovipositing females require a host that has entered the reproductive phase and has reached at least Zadoks 31 growth stage (Holmes, 1979; Zadoks et al., 1974). The positive relation between chlorophyll content and yield components suggests that the chlorophyll meter could be a very effective tool for predicting both yield potential and the threat to WSS cutting in solid-stemmed spring wheat. Further research into the relationship between chlorophyll, sawfly infestation, and subsequent damage from stem cutting is warranted.
The results of this study indicate that low plant populations were often most effective at maximizing pith expression in solid-stemmed wheat and reducing sawfly cutting damage. However, this required one of the three the highest rates of N fertilizer (Fig. 5), and a system of low seeding rates and high N may not be economical based on current fertilizer input costs and the generally lower grain yield response. In our economic analysis, the lowest seeding rate produced the lowest economic returns at every rate of applied N (Fig. 7). Therefore, a decision to adopt a system for the exclusive purpose of optimizing pith would be cost prohibitive. The biplot for pith indicates that moderate seeding and N rates produced high and stable pith in AC Lillian. The economic returns for this system appear greatly improved over a system with a lower density. However, the economic returns may improve further with N rates over 60 kg N ha−1 at the moderate sowing density (+C$80 to C$100 ha−1).
A producer is therefore faced with a decision to risk less than desirable pith and possible greater stem cutting vs. potentially higher net returns. The problem with the former scenario is that the higher net returns would probably erode in an environment of heavy WSS pressure due to greater harvest losses (Beres et al., 2007). A recommendation to hold N applications to moderate levels may appear conservative but is likely to be more stable over time, particularly in environments with heavy WSS pressure. In summary, an integrated planting and nutrient management plan when using a solid-stemmed spring wheat cultivar consists of seeding rates no greater than 300 seeds m−2 and 30 to 60 kg N ha−1.