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a North Dakota State Univ., Dickinson Res. Ext. Ctr., 1089 State Ave., Dickinson, ND 58601
b Dep. Plant Sci., North Dakota State Univ., Fargo, ND 58105
* Corresponding author (pcarr{at}ndsuext.nodak.edu)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: HRSW, hard red spring wheat • CONT, conventional-till • RT, reduced-till • NT, no-till
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Soil N availability generally declines when tillage first is reduced. The reduction in soil N availability may result from several factors, including accumulations of crop residue at the soil surface and subsequently cool spring soil temperatures, both of which contribute to immobilization of soil N (Westfall et al., 1996). However, following an adjustment period, greater amounts of soil N may occur in RT systems, as occurred during the final 14 yr of a 36-yr study (Johnston et al., 1995).
Suboptimal stands may result, when tillage is reduced, because of inadequate penetration by seed delivery systems and hair-pinning of crop residue into the bottom of seed furrows. Lower yields may result when suboptimal stands occur. Fewer plants were established when winter wheat was seeded in a seedbed with no preplant tillage compared with a tilled seedbed in the Pacific Northwest (Wilkins et al., 1989). Grain yield was not measured but there was a trend for dry matter production to be elevated when preplant tillage was used. However, wheat was seeded at different depths in the two tillage systems, making it impossible to separate the effect of tillage from seeding depth on plant stand.
The impact of plant population differences on hard red spring wheat (HRSW) yield has been studied in both tilled and untilled environments. A positive asymptotic response in yield occurred as seeding rates increased from 34 to 202 kg ha-1 in seedbeds where crop residue was maintained on the surface (Lafond, 1994), and in tilled seedbeds in a subsequent study in Saskatchewan (Lafond and Derksen, 1996). Yield was less responsive to adjustments at low rates in untilled seedbeds, suggesting that seeding rate adjustments may be needed to reflect changes in tillage.
Seeding rate adjustments produced a different yield response when HRSW was seeded into barley (Hordeum vulgare L.) stubble versus fallow during a 3-yr study in Saskatchewan (Wright et al., 1987). Yield response was greater as the seeding rate increased from 22 to 124 kg ha-1 in bare fallow than in stubble. The researchers suggested that more favorable soil N and soil water levels following fallow may explain why the yield response was greater in fallow plots. Research is needed to determine if similar differences exist when both tilled and untilled seedbeds are preceded with fallow before seeding HRSW.
Yield response of spring wheat to reductions in tillage is inconsistent in annual cropping systems. Yield increased in some studies because more soil water became available as tillage was reduced (Brandt, 1992; Peterson et al., 1996). In other instances, yield reductions occurred when tillage was eliminated, even though available soil water was increased (Bauer and Kucera, 1978). Cultivar differences across and even within studies may have contributed to the inconsistency in yield response to changes in tillage.
A tillage x cultivar interaction for spring wheat yield has occurred in annual cropping systems. The relative ranking of four soft white spring wheat cultivars across three tillage systems changed for grain yield and test weight in a 2-yr study in the Pacific Northwest, prompting the recommendation that cultivars must be developed for specific tillage environments (Ciha, 1982). A similar recommendation resulted when 18 cultivars were compared in tilled and untilled environments in South Dakota over a 3-yr period (Hall and Cholick, 1989). Wheat was established in fields where wheat or barley were seeded the previous year in the study by Ciha (1982) and the study by Hall and Cholick (1989).
A tillage x cultivar interaction has not been considered in a wheat-fallow monoculture, even though preceding wheat with a 14- to 21-mo fallow period remains a common practice in the Great Plains (Johnston et al., 1995). Therefore, the objective of this study was to determine if a tillage x cultivar interaction occurred for grain yield, protein concentration, kernel weight, and test weight of HRSW in a wheat–fallow system. Since a seeding rate x cultivar interaction has occurred for yield in a Mediterranean climate (Anderson and Barclay, 1991), an additional objective was to determine if seeding rate x cultivar and tillage x seeding rate x cultivar interactions occurred. We hypothesized that the tillage x cultivar interaction would occur for grain yield, test weight, and possibly other traits in a wheat–fallow system, on the basis of the occurrence of the tillage x cultivar interaction in annual cropping systems. Further, we hypothesized that both seeding rate x cultivar and tillage x seeding rate x cultivar interactions would occur for yield in the Great Plains, on the basis of results of previous research in different environments.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Conventional-till, reduced-till, and no-till treatments were established and maintained in 27- by 12-m plots in wheat and fallow phases. Conventional-till plots were cultivated to a 10-cm depth with 15-cm sweeps mounted on shanks attached to a tool bar at approximately 45-d intervals from May through August during the 20-mo fallow phase, and lightly disced to a 8-cm depth and leveled before seeding HRSW during the crop phase. Reduced-tillage plots were lightly disced in September or October before seeding HRSW the following spring. No other tillage occurred during crop or fallow phases. No soil disturbance except by a low-disturbance seeder at seeding occurred in NT plots.
The study began in 1995. The HRSW cultivars AC Minto, Amidon, Bergen, Grandin, and Norm each were seeded at 123, 247, and 371 live kernels m-2 in 1.8- by 12-m plots during the crop phase of the wheat-fallow monoculture. These five cultivars were widely grown or recently released when the study began and varied in maturity and other characteristics (Table 1).
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Fenoxaprop-P ((+)-ethyl 2-[4-[(6-chloro-2-benzoxazolyl)oxy]phenoxy] proponoate; MCPA isocytyl ester of 2-methyl-4-chlorophenoxyaceticacid) was used to control grass weeds in 1995 and 1997 during the HRSW phase, while diclofop (methyl 2-[4-(2,4-dichlorophenoxy) phenoxy]propanoate) was used in 1996 and 1998. Dicamba (3,6-dichloro-o-anisic acid), tribenuron methyl (methyl 2-(([[[(4-methoxy-6-methyl-1,3,5-triazin-2-YL)methylamino] carbonyl]amino]-sulfonyl) benzoate), 2,4-D LV ester (ethylhexyl ester of 2,4-dichlorophenoxyacetic acid), and MCPA ester (isooctyl ester of 2 methyl 4 chlorophenoxyacetic acid), alone or in various combinations, were used to control broadleaf weeds. Glyphosate (N-(phosphonomethyl glycine, in the form of its isopropylamine salt) and paraquat (1,1'-dimethyl-4,4' bipyridnium dichloride) were used from two to four times during the fallow phase to control weeds under RT and NT management. Weeds were controlled mechanically during the fallow phase under CONT management. Excellent weed control was achieved all four years during both wheat and fallow phases across all tillage treatments.
Soil water content was determined to a 106-cm depth with a soil moisture probe in early April before seeding HRSW as described by Brown et al. (1985). Daily precipitation and temperature were recorded at a weather service station 50 m from the test site. Wheat plant stand, plant tiller production, and spike-bearing tiller density were determined (Carr et al., 2003). Grain was harvested at maturity (Zadoks 92; Zadoks et al., 1974) from the center seven rows of each nine-row HRSW subplot using a small-plot combine (Kincade Equip., Haven, KS). Wheat plants lodged each year, so grain was harvested by cutting plants close to the soil surface. Erect wheat stubble approximately 5 cm in height remained after harvesting grain.
Grain yields were reported on a 120 g kg-1 moisture basis. Grain kernel weight and test weight were determined from subsamples. Grain protein concentration was determined by near-infrared spectroscopy (Infratec grain analyzer, UAS Service Corp., Hawley, MN).
The experimental design was a randomized complete block with a split split-plot arrangement with treatments replicated four times. Tillage treatments comprised whole plots, seeding rates comprised subplots, and HRSW cultivars comprised sub-subplots. Subplots and sub-subplots were randomized each year, but the initial whole-plot randomization was maintained throughout the study.
Data were analyzed across all four years by means of the GLM procedure available from SAS (SAS Institute, 1985). Tillage treatments, seeding rates, and HRSW cultivars were considered fixed effects. Blocks and years were considered random effects. Mean comparisons using an F-protected LSD were made to separate tillage treatments, seeding rates, and HRSW cultivars where F-tests indicated that significant differences existed (P < 0.05). Polynomial equations were determined with seeding rate as the independent variable for grain yield, grain protein concentration, kernel weight, and test weight where tillage x seeding rate, seeding rate x cultivar, or tillage x seeding rate x cultivar interactions did not occur.
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RESULTS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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A tillage x seeding rate interaction did not occur for protein concentration (Table 2). Protein concentration also was unaffected by seeding rate adjustments. These results indicate that seeding rate changes did not impact protein concentration of HRSW within the seeding rates evaluated in this study.
Seeding rate x cultivar and tillage x seeding rate x cultivar interactions did not occur for protein concentration of HRSW grain (Table 2). Protein concentration was highest for AC Minto and averaged 154 g kg-1 (Table 3). Grain produced by Bergen and Norm was relatively low in protein, averaging only 141 and 142 g kg-1, respectively. The low protein concentration of grain produced by Bergen may have resulted from a yield induced N-deficiency. Both AC Minto and Bergen typically produce grain with a similar protein concentration (Table 1), but Bergen was the highest-yielding cultivar while AC Minto was the lowest in this study (Table 3). These data indicate the inverse relationship between grain yield and grain protein concentration that may occur for HRSW when N fertilizer is applied for a yield goal that underestimates actual yield, as occurred in this study.
Kernel Weight
A tillage x cultivar interaction did not occur for kernel weight (P = 0.86, Table 2). Lack of the tillage x cultivar interaction indicates that ranking of the HRSW cultivars for kernel weight was unchanged as tillage was reduced. For example, kernel weight was 2.4 g (1000 kernels)-1 heavier for Bergen than AC Minto under CONT, 2.6 g (1000 kernels)-1 heavier under RT, and 2.5 g (1000 kernels)-1 heavier under NT. Ranking of other cultivars for kernel weight was consistent across the three tillage systems (data not presented).
Kernel weight was unaffected by seeding rate adjustments, nor did tillage x seeding rate, seeding rate x cultivar, and tillage x seeding rate x cultivar interactions occur (Table 2). Kernel weight did vary by as much as 2.2 g (1000 kernels)-1 among the five HRSW cultivars (Table 3). Bergen, Grandin, and Norm produced heavier kernels than AC Minto and Amidon.
Test Weight
A tillage x cultivar interaction did not occur for test weight of HRSW grain (P = 0.48, Table 2). Lack of the tillage x cultivar interaction indicates that ranking of the HRSW cultivars for test weight was unaffected by changing tillage systems. For example, the test weight of grain produced by Bergen was 14 kg m-3 heavier than the test weight of grain produced by AC Minto under CONT, and 10 kg m-3 heavier under both RT and NT. Ranking of other cultivars for test weight was consistent in CONT, RT, and NT systems (data not presented).
Grain test weight was affected by seeding rate and a tillage x seeding rate interaction occurred (Table 2). Test weight increased by 9 kg m-3 as the seeding rate increased from 123 to 371 kernels m-2 under CONT, and by 14 kg m-3 under NT (Fig. 3). Test weight increased by 5 kg m-3 as the seeding rate increased from 123 to 247 kernels m-2 but not when the seeding rate was increased to 371 kernels m-2 under RT, for reasons that are unclear.
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DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Previous research suggested that soil water benefits occur when tillage is reduced in a wheat–fallow monoculture. Results of several studies summarized by Peterson et al. (1996) indicated that reducing tillage increased water storage from 102 to 183 mm during a 14-mo fallow period in the central Great Plains. The benefits in soil water conservation from reducing tillage partially explained increases in winter wheat yield from 1068 to 2688 kg ha-1 over a 74-yr period. Similar though less dramatic improvements in water storage and yield resulted when CONT was replaced by NT in the northern Great Plains and HRSW was grown (Deibert et al., 1986).
A tillage x cultivar interaction did not occur for yield in a wheat–fallow monoculture in our study, perhaps because soil water content was similar between tillage systems. Moist soil was measured to a depth of 106 cm at seeding, and no differences in soil water content between CONT, RT, and NT systems were detected (data not presented). If soil water was conserved at depths >106 cm, however, then a yield advantage could have resulted since HRSW wheat roots can extend to soil depths of about 130 cm when HRSW follows fallow in a wheat–fallow monoculture (Merrill et al., 2002).
Soil texture affects the impact of tillage on soil water in a wheat–fallow monoculture. Tillage did not affect soil water in a sandy loam soil in southwestern Saskatchewan, presumably because the additional water conserved by NT exceeded the water storage capacity of the soil (McConkey et al., 1996). Differences in grain yield did not occur since soil water was unaffected by tillage. Our study was located on a sandy loam soil and differences in yield did not occur between CONT and NT systems, possibly because the water conservation benefits under NT exceeded the water storage capacity of the soil. Results of our study and previous research by McConkey et al. (1996) support the hypothesis that soil water benefits may not result when tillage is eliminated in a wheat–fallow monoculture on coarse textured soils.
Riveland et al. (1979) suggested a seeding rate of 247 live kernels m-2 for optimum yield of HRSW in a wheat–fallow monoculture under CONT in North Dakota. A common practice among commercial HRSW growers is to increase seeding rates following adoption of RT or NT because of the belief that crop residue at the surface reduces the effectiveness of seed delivery systems. However, results of our research suggest that seeding rate adjustments are not needed when replacing CONT with RT or NT systems in a wheat–fallow monoculture. Our results also support the conclusion of Riveland et al. (1979) that optimum yield of HRSW in a wheat–fallow monoculture results at a seeding rate of around 250 kernels m-2.
Grain Protein Concentration
We hypothesized that a tillage x cultivar interaction would occur for grain protein concentration of HRSW in a wheat–fallow monoculture, in part because we believed that plant-available soil N would be less under RT and NT than CONT. Johnston et al. (1995) found that protein concentration of HRSW was reduced because available soil N was depressed under NT compared with RT in a wheat–fallow monoculture. The depression in available soil N was attributed to the conservation of soil organic matter that may result when tillage is reduced. Reduced tillage lowers soil temperatures and minimizes mixing of crop residue with surface soil layers, thereby slowing the microbial conversion of organic N to plant available forms (Lamb et al., 1985).
Soil N was unaffected by tillage systems in our study (data not presented), nor did tillage systems affect grain protein concentration. Likewise, reducing or eliminating tillage generally had no effect on grain protein concentration when growing HRSW in southwestern Saskatchewan (McConkey et al., 1996). These researchers observed a trend for protein concentration to be less under NT in wet years, possibly because denitrification early in the growing season reduced available soil N. Results of the Canadian research supports the hypothesis that protein concentration of HRSW grain may be less under NT than CONT when early-season wet conditions occur in a wheat–fallow monoculture.
A tillage x cultivar interaction did occur for protein concentration among 14 winter wheat cultivars in a continuous small-grain system in eastern North Dakota (Cox and Shelton, 1992). However, the interaction indicated a change in the magnitude of response by cultivars to tillage reductions and not a change in rank among cultivars in contrasting tillage systems. The tillage x cultivar interaction did not occur for grain protein of 10 HRSW cultivars in a continuous small-grain system in north central North Dakota (Thompson and Hoag, 1987). Our study indicates that the tillage x cultivar interaction does not occur for HRSW grain in a wheat–fallow monoculture. Results of our study and research by others (Thompson and Hoag, 1987; Cox and Shelton, 1992) do not support the hypothesis that the tillage x cultivar interaction occurs for protein concentration of wheat grain.
Kernel Weight
We hypothesized that a tillage x cultivar interaction would occur for kernel weight in a wheat–fallow system, based on previous research indicating that tillage systems affected kernel weight in wheat. For example, kernel weight was reduced when tillage was eliminated in Mediterranean climates (Ciha, 1982; López-Bellido et al., 1998), perhaps because soil water or soil N were conserved as tillage was reduced. There was no evidence that tillage affected soil water or soil N content in our study (data not presented), nor did the tillage x cultivar interaction occur.
Hall and Cholick (1989) observed that the tillage x cultivar interaction did not occur for kernel weight when HRSW was seeded under CONT and NT in a continuous small-grain system in South Dakota. Likewise, Ciha (1982) observed that the tillage x cultivar interaction did not occur in two of three continuous small-grain environments when soft white spring wheat was seeded under CONT, RT, and NT in the Pacific Northwest. Results of these two studies in continuous small-grain environments concur with our results in a wheat–fallow environment suggesting that the tillage x cultivar interaction does not occur for kernel weight in HRSW.
Test Weight
A tillage x cultivar interaction occurred for grain test weight when soft white spring wheat was seeded in a continuous small-grain system in the Pacific Northwest (Ciha, 1982). For this reason, we hypothesized that a tillage x cultivar interaction would occur for HRSW in a wheat–fallow monoculture. However, the ranking of five HRSW cultivars did not change for test weight under CONT, RT, and NT in our study.
A tillage x cultivar interaction did not occur for grain test weight when HRSW was grown in continuous small-grain systems in the Great Plains (Thompson and Hoag, 1987; Hall and Cholick, 1989). Our study suggests that the tillage x cultivar interaction does not occur in a wheat–fallow monoculture. Results of these three studies do not support the hypothesis that a tillage x cultivar interaction occurs for test weight of HRSW in the Great Plains.
Test weight of HRSW grain sometimes increases as tillage is reduced (López-Bellido et al., 1998), possibly because soil water and soil N are conserved. Comparison of soil water and soil N content under CONT, RT, and NT did not indicate any differences in our study (data not presented), suggesting that soil environments were similar among tillage systems. Other research indicates that soil water and soil N can be affected by tillage systems (McConkey et al., 1996; Peterson et al., 1996). For example, soil N tended to increase 5-yr after NT first was adopted in a wheat-fallow monoculture compared with RT at one of three locations in a 12-yr study (McConkey et al., 1996). Soil N levels tended to be less at the other two locations under NT compared with RT.
The height and orientation of erect wheat stubble in RT and NT systems may explain why soil water and soil N are increased under NT in some environments but not others, because of the impact of wheat stubble on soil conservation. Soil erosion declines as the amount of erect wheat stubble above the soil surface increases (Unger, 1994). Most organic soil N occurs near the soil surface, so more soil N is retained when soil erosion is reduced. Erect wheat stubble also catches and retains winter precipitation more effectively as stubble height increases (Bauer and Black, 1990), thereby increasing production of crop biomass and organic N by spring seeded crops in semiarid regions.
Erect wheat stubble about 5 cm in height remained following harvest under NT in this study, since HRSW plants lodged and grain could be harvested only but cutting mature plants close to the soil surface. Wheat residue no longer was recognizable when HRSW was seeded following the 20-mo fallow phase. Few differences were observed in the amount of precipitation retained under NT, RT, and CONT systems during the fallow phase. The short height of erect wheat stubble may explain why tillage did not affect soil water and soil N, and subsequently why test weight and protein concentration were unaffected by tillage systems. Adjusting cutting height so that taller wheat stubble occurred after harvest might affect soil N under contrasting tillage systems because of the effect on soil erosion and water storage.
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Wheat–fallow monoculture is being replaced by annual cropping systems in the Great Plains and other dryland regions. Previous research has demonstrated that a tillage x cultivar interaction can occur for grain yield and other traits when wheat is grown after wheat and barley. Canola (Brassica napus L.), peas (Pisum sativum L. subsp. sativum), and other dicotyledonous crops are being rotated with HRSW and other small grain crops to diversify cropping systems in the Great Plains. Research is needed to determine if the tillage x cultivar interaction occurs when HRSW is rotated with other grass and nongrass crops in diverse rotations.
Research indicates that soil water or soil N is conserved when tillage is reduced in some environments, but not in others. The apparent inconsistency between studies suggests that complex interactions between climatic and edaphic factors affect the impact of tillage on soil water and soil N content in a wheat–fallow monoculture. More research is needed to elucidate why soil water and soil N status are not affected consistently across environments. Understanding the factors that determine how tillage reductions impact soil properties may allow better prediction of a tillage x cultivar interaction when HRSW is included in diverse cropping systems.
A common practice when reducing tillage in a wheat–fallow monoculture is to increase the seeding rate because of the belief that crop residue remaining on the surface interferes with optimum placement of seed in the seedbed. Previous research indicated that yield of HRSW was maximized at a seeding rate of 247 kernels m-2 in CONT systems in North Dakota. Results of our study suggest that a seeding rate of 247 kernels m-2 for optimum yield can be extended to RT and NT systems.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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1 Mention of a proprietary product name is for identification purposes only and does not imply warranty or endorsement to the exclusion of other products. 
Received for publication January 22, 2002.
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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from the analysis of variance for grain yield, crude protein, kernel weight, and test weight of five hard red spring wheat cultivars grown in wheat–fallow monoculture at three seeding rates across three tillage systems during 1995–1998 at Dickinson, ND.
