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Agronomic Traits Improvement and Associations in Hard Red Spring Wheat Cultivars Released in North Dakota from 1968 to 2006

^a Dep. of Plant Sciences, North Dakota State Univ., Fargo, ND 58105
^b NDSU Carrington Research Extension Center, 633 Hwy. 281 North, Carrington, ND 58421

^* Corresponding author (Mohamed.Mergoum{at}ndsu.edu).

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Periodic evaluation of cultivars allows researchers to evaluate genetic variation and progress made in various traits. Determining genetic gain or lack can lead researchers to develop new strategies for trait improvements. A two-year study was initiated in 2004 to examine the changes in agronomic performance of hard red spring wheat (HRSW) (Triticum aestivum L.) cultivars released by North Dakota State University (NDSU) over the past 40 years. The experiment was conducted in North Dakota at three sites in 2004 and two sites in 2005. The study included 33 HRSW genotypes laid out in a randomized complete-block design. Cultivars developed since 1968, three advanced lines developed by NDSU, and three cultivars released by other breeding programs were included in the study. The Canadian cultivar Marquis (released in 1911) was included for comparison purposes. Linear regression of cultivar means on year of release showed an annual increase in grain yield of 1.3% yr^–1, grain-volume weight of 0.2% yr^–1, and thousand-kernel weight of 0.3% yr^–1 since 1968. There were also significant gains in lodging and disease resistance. Resistance to leaf rust (Puccinia recondita Roberge ex Desmaz. f. sp. tritici) and Fusarium head blight (Fusarium graminearum Schwabe [teleomorph Gibberella zeae (Schweinitz) Petch]) was substantially improved in genotypes released since 2002 and 2000, respectively. Therefore, we can conclude from this study that no evidence of a decline has occurred in the improvement of most agronomic traits and that breeders should be able to continue improving these traits by introgressing favorable alleles.

Abbreviations: FHB, Fusarium head blight • GVW, grain-volume weight • HRSW, hard red spring wheat • MR, moderate resistant • MS, moderate susceptible • NDSU, North Dakota State University • R, resistant • S, susceptible • SDSU, South Dakota State University • TKW, thousand kernel weight

	ACKNOWLEDGMENTS

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

Received for publication January 12, 2007.

Agronomic Traits Improvement and Associations in Hard Red Spring Wheat Cultivars Released in North Dakota from 1968 to 2006

^a Dep. of Plant Sciences, North Dakota State Univ., Fargo, ND 58105
^b NDSU Carrington Research Extension Center, 633 Hwy. 281 North, Carrington, ND 58421

^* Corresponding author (Mohamed.Mergoum{at}ndsu.edu).

Periodic evaluation of cultivars allows researchers to evaluate genetic variation and progress made in various traits. Determining genetic gain or lack can lead researchers to develop new strategies for trait improvements. A two-year study was initiated in 2004 to examine the changes in agronomic performance of hard red spring wheat (HRSW) (Triticum aestivum L.) cultivars released by North Dakota State University (NDSU) over the past 40 years. The experiment was conducted in North Dakota at three sites in 2004 and two sites in 2005. The study included 33 HRSW genotypes laid out in a randomized complete-block design. Cultivars developed since 1968, three advanced lines developed by NDSU, and three cultivars released by other breeding programs were included in the study. The Canadian cultivar Marquis (released in 1911) was included for comparison purposes. Linear regression of cultivar means on year of release showed an annual increase in grain yield of 1.3% yr^–1, grain-volume weight of 0.2% yr^–1, and thousand-kernel weight of 0.3% yr^–1 since 1968. There were also significant gains in lodging and disease resistance. Resistance to leaf rust (Puccinia recondita Roberge ex Desmaz. f. sp. tritici) and Fusarium head blight (Fusarium graminearum Schwabe [teleomorph Gibberella zeae (Schweinitz) Petch]) was substantially improved in genotypes released since 2002 and 2000, respectively. Therefore, we can conclude from this study that no evidence of a decline has occurred in the improvement of most agronomic traits and that breeders should be able to continue improving these traits by introgressing favorable alleles.

Abbreviations: FHB, Fusarium head blight • GVW, grain-volume weight • HRSW, hard red spring wheat • MR, moderate resistant • MS, moderate susceptible • NDSU, North Dakota State University • R, resistant • S, susceptible • SDSU, South Dakota State University • TKW, thousand kernel weight

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
HARD RED SPRING WHEAT (Triticum aestivum L.) is an important crop on farms in the northern Great Plains. North Dakota is the second-largest wheat-producing state and the largest spring wheat–producing state in the United States (North Dakota Wheat Commission, 2007). North Dakota harvests 4.0 million ha of wheat each year worth more than $1 billion directly and another $2.5 billion in indirect economic contributions. Spring wheat harvested in North Dakota consists of almost half of the entire nation's spring wheat crop and accounts for nearly 80% of the total wheat harvested in North Dakota.

Wheat provides more than one-fourth of the world's cereal output and constitutes the main source of calories for more than 1.5 billion people (Reynolds et al., 1999). Global demand for wheat, however, is growing faster than gains in genetic yield potential. Therefore, periodic evaluation in hard red spring wheat (HRSW) is essential for understanding yield-limiting factors for major economic traits and for identifying traits or targeting environments that might require additional amounts of research (Cox et al., 1988). Evaluation of cultivars released over a given period of time can be made by growing the cultivars in common environments.

Previous genetic improvement studies on spring wheat from 1900 to 1983 showed that average wheat grain yields in North Dakota increased steadily since 1930 (Deckard et al., 1985). Grain yields for the period of 1930 to 1983 increased by an average of 740 to 2020 kg ha^–1. Similar studies conducted in Mexico (Waddington et al., 1986) and Argentina (Calderini et al., 1995) showed a 1.5% mean annual genetic gain in grain yield for spring wheat from 1950 to 1982 and a 1.0% gain in grain yield for winter wheat from 1920 to 1989 for the studies, respectively. Schmidt and Worrall (1984) found that genetic progress for winter wheat yield potential in the southern Great Plains reached a plateau in the late 1970s. Cox et al. (1988), however, found no evidence of a yield plateau in winter wheat under a range of environmental conditions in Kansas. Similarly, Donmez et al. (2001) contradicted the concept of a "barrier" to yield increase in the future by showing that the mean annual rate of genetic gain for grain yield in winter wheat was actually increasing over time in the Great Plains growing region. The research of Feyerherm et al. (1984), with different wheat classes and cultivars grown in the Midwest, showed significant genetic improvement from 1920 to 1979, and they stated "that no slowing of the rate of yield improvement is evident."

Genetic improvement in the North Dakota State University (NDSU) HRSW breeding program can have a major impact on the cultivars grown in the spring wheat growing region. The specific breeding objectives of the wheat improvement programs over the past century include improving or maintaining disease resistance, bread-making quality, and grain yield potential (Deckard et al., 1987). Increasing grain yield and grain yield stability are of primary importance for wheat breeders due to the growing demand for this product. Genetic improvement and correlations that have been associated with higher grain yields include early maturity, decreased height, reduced lodging, improved disease resistance, and increased kernel number (Donmez et al., 2001).

Over the last 40 yr, HRSW grain yield and other agronomic traits have increased dramatically in North Dakota and the neighboring wheat-growing regions. However, since the relationship between grain yield and grain quality among cultivars is generally negative, maintenance of adequate bread-making quality places some constraints on the improvement of yield potential (Deckard et al., 1987). Therefore, this study was conducted to assess the genetic variation in productivity traits of HRSW cultivars released by NDSU since 1968 that have been grown in North Dakota and neighboring states. The specific objectives of this study were (i) to determine if agronomic traits improved during the past 40 yr, (ii) to examine the gains achieved in agronomic traits resulting from the NDSU HRSW breeding efforts, and (iii) to observe the correlations between the major agronomic traits studied.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Field Trials
Twenty-six HRSW cultivars released over the past 40 years from the NDSU HRSW breeding program, three NDSU advanced lines with the potential for future release, and three widely grown cultivars in North Dakota from other breeding programs were included in this study (Table 1 ). The cultivar 2375 was released by Pioneer HiBred in 1990, Briggs was released by South Dakota State University (SDSU) in 2002, and Hanna was released by Agripro in 2002. The Canadian cultivar Marquis (released in 1911) was included for comparison.

The entries were arranged in a randomized complete-block design with four replicates. The experimental units at the Casselton, Prosper, and Carrington locations were 2.4 m long by 1.2 m wide and consisted of seven rows 15.2 cm apart. At Lisbon, plots were 3.6 m long by 1.2 m wide and consisted of seven rows 15.2 cm apart.

Data Collection for Agronomic Traits
Grain samples were cleaned by using a clipper grain cleaner before grain yields, grain-volume weights, and 1000-kernel weights were measured. Grain yield (Mg ha^–1) was determined by weighing the cleaned grain harvested from each plot. Grain-volume weight (GVW) (kg m^–3) was measured according to AACC standard method 55-10 (AACC, 2000). Thousand- kernel weight (TKW) (g) was calculated on the basis of 250 kernels obtained using an electronic seed counter (Seedburo Equipment Co., Chicago, IL). Kernel number per spike for each plot was calculated by averaging the kernels from 10 individual spikes selected at random from each plot. The kernels were counted by the electronic seed counter after being mechanically threshed from the spike. Plant, stem, and spike counts were determined by counting the number of seedlings, stems, and spikes in two 61-cm-long rows per plot. Counts were then converted to determine plants, stems, and spikes per square meter.

Plant height (cm) was measured from the soil surface to the top of the spike, excluding the awns. An average plant height was recorded from the plot. Heading date was recorded when the inflorescence fully emerged in at least 50% of the plants. Days to heading was calculated as the number of days from planting to heading.

Lodging (%) was estimated by the degree and amount of plants that lodged per plot when lodging occurred at an environment. Disease notes were taken on leaf rust and Fusarium head blight (FHB). Leaf rust was scored on adult plants based on disease severity (0 to 100%) and response type (R = resistant; MR = moderate resistant; MS = moderate susceptible; and S = susceptible). The severity was estimated according to the modified Cobb scale (Peterson et al., 1948), while host response was determined using the modified system described by Mains and Jackson (1926). Based on these two scales, genotypes were grouped as R (no disease symptoms), MR (moderate resistant and 10–40% severity), MS (moderate susceptible with 40–80% severity), or S (severity above 80%). Similarly, FHB disease severity (%) was visually estimated by the amount (%) of scab infected heads within an experimental unit (Stack et al., 1997).

Data Analysis
Data were subject to an analysis of variance, using the GLM subprogram of the Statistical Analysis System (SAS Institute, 1999). Within the model, genotypes were considered fixed effects and environments and blocks were considered random effects. The genotype-by-environment mean square was used as the denominator when testing the level of significance for genotypes. A 5% probability level was used on all statistical analyses. Because error variances were homogeneous among the five environments, a combined analysis of variance was conducted.

Means were separated using an F-protected LSD value. Correlations between the different agronomic traits were calculated using combined data by calculating the correlation coefficients from homogeneous environments. Regression analysis using year as the x variable on individual environment means and overall combined means was done to determine the amount, if any, of the annual progress made for a particular trait over the past 40 yr.

The cultivars Marquis and Glupro were omitted from the statistical analyses and were not included in the overall mean, LSD, regression, or correlation analysis, although their means across environments for each trait are shown in Table 1. Marquis was omitted because it was released 57 yr before the next-oldest cultivar in this study. Glupro (a cultivar derived from a cross with T. dicoccoides) was omitted because of its poor agronomic characteristics.

	RESULTS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Environmental Conditions and Data Analysis
The 2004 and 2005 environmental conditions varied considerably, particularly with regard to disease pressures. Environments in 2004 were cooler than average, with Casselton, Carrington, and Lisbon having lower monthly mean temperatures from May to August compared to the long-term Prosper average. In 2004 environments were also drier than average with monthly precipitation in April, June, July, and August compared to the long-term Prosper average. Similarly, environments in 2005 were warmer and wetter than average in the month of June enhancing disease pressures. The mean temperatures in June 2005 at Prosper and Carrington for instance, were 20°C and 19°C, respectively, compared to a long-term average of 18°C. Similarly, the precipitation at Prosper and Carrington measured in 2005 was 161 mm at each location in the month of June compared to a long-term average of 91 mm. Total precipitation for the growing season was lower than average at four of the five environments compared to the long-term average. Rainfall at the Prosper environment in 2005 was 41 mm above the long-term Prosper average of 345 mm.

Environment had a significant effect on all agronomic traits reported in this study (Table 2 ). In 2004 the genotypes performed better for almost every trait compared with 2005. The diseases in 2005, particularly FHB and leaf rust, caused significant losses in grain yield and adversely affected most agronomic traits. Among all the environments, Casselton in 2004 had the best growing conditions. At this site, the highest grain yield, GVW, and TKW means were achieved. In 2005 Prosper produced the lowest GVW and TKW means, while Carrington produced the lowest grain yield. The annual rates of improvement for the different agronomic traits calculated for each individual environment were quite variable (Table 3 ).

Grain yield increased from 2.79 to 4.21 Mg ha^–1 since 1968 (Table 1), an annual increase of 30.4 kg ha^–1 yr^–1 since 1968 (Table 3). This represents an annual gain in grain yield of 1.3% yr^–1. The annual increases in grain yield ranged from 15.3 to 60.0 kg ha^–1 yr^–1 depending on the environment (Table 3). Grain yield differed significantly among genotypes and also interacted significantly with environment (Table 2). The NDSU line ND 803 was consistently the highest yielding, outyielding other genotypes at three of the five environments. ND 751 and Briggs were top yielders at one environment each. Grain yield showed a significant and positive correlation with year of release, GVW, and kernel number spike^–1 (Table 4). Grain yield also was correlated with reduced lodging (r = 0.39, P < 0.05), leaf rust resistance (r = 0.70, P < 0.01), and FHB resistance (r = 0.46, P = < 0.01).

Grain-Volume Weight and Yield Components
The mean GVW of all genotypes over all environments was 767 kg m^–3 (Table 1). The GVW of cultivars released before 1980 was 746 kg m^–3, between 1980 and 1989 was 758 kg m^–3, between 1990 and 1999 was 767 kg m^–3, and from 2000 and beyond was 784 kg m^–3. Among the 10 highest GVW genotypes, 8 were from the modern group. This includes four NDSU cultivars (Alsen, Steele-ND, Glenn, and Howard), the Agripro cultivar Hanna, and the three NDSU advanced lines (ND 751, ND 802, and ND 803). A total of 26 cultivars showed a significant increase in GVW compared with Waldron.

Over the five environments, GVW increased significantly by 1.25 kg m^–3 yr^–1 since 1968 (Table 3). This amounted to a mean annual gain in GVW of 0.2% yr^–1 since 1968. The GVW differed significantly among genotypes and environments (Table 2). Increases were significant for each environment but varied from 0.73 to 1.95 kg m^–3 yr^–1. The genotype-by-environment interaction was significant for GVW, most likely because FHB affected grain filling of the more susceptible genotypes in 2005. Glenn, a high GVW cultivar, however, maintained the highest GVW in all environments. The GVW was positively correlated with year of release and grain yield (Table 4).

The mean TKW over the five environments was 30.6 g (Table 1). Among the 10 genotypes with the highest TKW, 7 were from the modern group. This includes two NDSU cultivars (Dapps and Howard), the SDSU cultivar Briggs, the Agripro cultivar Hanna, and the three advanced lines (ND 751, ND 802, and ND 803).

Over the five environments, TKW increased significantly by 0.08 g yr^–1 since 1968 (Table 3), a mean annual gain of 0.3%. The mean annual gain values for TKW varied significantly with genotype and environment (Table 2). The increase with environment ranged from 0.04 to 0.15 g yr^–1 (Table 3). Carrington 2004 and Prosper 2005 were the only environments that showed a significant regression coefficient between TKW means and year of release. Overall TKW was positively correlated with year of release (Table 4).

The overall mean kernel number spike^–1 averaged over all environments was 23.9 (Table 1). No significant correlation occurred between kernel number spike^–1 and year of release for either individual or overall environments (Table 3). Kernel number spike^–1, however, was positively correlated with grain yield (Table 4).

Total plant, stem, and spike densities averaged 260 seedlings, 582 stems, and 512 spikes m^–2, respectively (Table 1). No significant correlation occurred between plant, stem, and spike counts with year of release for overall environments (Table 3). However, Prosper in 2005 did show a significant increase in plant and stem counts with year of release. Plant and stem density were significantly correlated to each other, as were stem and spike density (Table 4).

Plant Height and Days to Heading
The plant height mean of genotypes was 98.3 cm over all environments (Table 1). No significant correlation occurred between plant height and year of release for individual or across environments (Table 3). Similarly, there were no significant correlations between plant height and grain yield or between plant height and any other agronomic traits (Table 4). Genotypes differed significantly in height, ranging from 89 to 108 cm. Plant height decreased a modest 0.14 cm yr^–1 since 1968 (Table 3).

The overall mean days to heading was 63.8 d (Table 1). Cultivars released before 1980 averaged 64.8 d to heading, while it took 64.3 d for cultivars released between 1980 and 1989, 63.5 d for cultivars released between 1990 and 1999, and 62.9 d to heading for the modern cultivars. Among the 10 earliest maturing genotypes, six were from the modern group. This includes four NDSU cultivars (Alsen, Dapps, Steele-ND, and Glenn), the SDSU cultivar Briggs, and the advanced line ND 803.

Means over four environments did not show a significant decrease in days to heading overall since 1968 (Table 3). The decrease in days to heading varied significantly with environments, ranging from 0.01 to 0.10 d yr^–1. Among environments, Casselton 2004 and Prosper 2005 showed a significant negative regression coefficient for days to heading on year of release. Days to heading was not correlated with any agronomic traits (Table 4).

Lodging and Response to Diseases
In 2004 minor lodging occurred at Casselton and Carrington. In 2005, however, more severe lodging occurred at Prosper due to the high amount of rainfall during August before harvest. These three environments were used in overall lodging means to obtain regressions and correlations with other traits. Due to selection by breeders against lodging, susceptibility has decreased in the recently released cultivars when lodging potential is significant in an environment (Table 1). Cultivars have significantly decreased lodging scores by a level of 0.4% yr^–1 since 1968 (r = .26, P < 0.01). Lodging resistance showed a positive correlation with grain yield (r = 0.39, P < 0.05), and TKW (r = 0.38, P < 0.05). However, lodging resistance was negatively correlated with plant height (r = 0.41, P < 0.05) and the number of days to heading (r = 0.72, P < 0.01).

Epidemics of leaf rust occurred in both 2004 and 2005. Leaf rust notes were taken in all but the Carrington 2004 environment. High levels of leaf rust infection were noted at Casselton 2004, Prosper 2005, and Carrington 2005; but lower levels of infection were noted at Lisbon 2004. Leaf rust infection levels in 2004 and 2005 were significantly lower in the modern cultivars compared with cultivars released before 2000 (Table 1). Based on the scale combining the percent disease severity (0–100) and reaction (R, MR, MS, and S), in 2004 the modern cultivars had a leaf rust average of 10 MR, whereas the cultivars released before 2000 had a 40 MS response. Similarly in 2005 the modern cultivars had a 10 MR response, whereas the earlier cultivars had a 30 MS response. Leaf rust response based on disease severity was significantly correlated with grain yield (r = 0.70, P < 0.01), GVW (r = 0.71, P < 0.01), and TKW (r = 0.52, P < 0.01).

In 2004 and especially 2005, significant FHB infection levels were observed. Although notes on FHB severity were taken only in two environments in 2005 (Carrington and Prosper), the data allowed a clear discrimination between genotypes for FHB response. The levels of FHB infection at these two environments were not significantly different, with FHB severity averaging 26.1% at Prosper and 26.7% at Carrington. The data revealed that even the most FHB-resistant cultivars currently available, such as Alsen and Glenn, showed some disease development in a year with severe disease pressure, such as in 2005 (Table 1). However, resistance to FHB was significantly improved in the modern HRSW genotypes compared with genotypes released in the previous three decades. Cultivars and lines with the ‘Sumai 3’ background, such as Alsen, Glenn, ND 751, and ND 803, averaged an FHB infection level of 12.5% per experimental plot, which was much improved compared with the overall mean for FHB infection of 27% (Table 1). The FHB resistance was positively correlated with grain yield (r = 0.46, P = < 0.01) and GVW (r = 0.37, P < 0.05).

	DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Many changes have occurred during the last three to four decades with yield gains and some agronomic traits improved as a result of both genetic improvements of cultivars and new production technology factors (Feyerherm et al., 1984). The new production technology factors, which play a major role in yield gains, include larger equipment, herbicides, and fertilizers (Deckard et al., 1987). In addition to the better production technology that has occurred since 1968, changes in climate, new disease pressures, and different pathogen populations can affect the productivity of HRSW cultivars in North Dakota. This study is a comparison between the different HRSW cultivars released by NDSU since 1968, but it takes place with today's agricultural technology, disease populations, and climate. The cultivars in this study were not protected with fungicides, to see how the cultivars responded to biotic stresses without protection. This ultimately impacted the results obtained, as the modern cultivars had better resistance to the current prevalent races of the leaf rust pathogen. Similarly, since 2000, some cultivars have improved FHB resistance relative to earlier released cultivars in the study. This is in agreement with Cox et al. (1988), who reported that genetic progress is greatly enhanced under disease epidemics for traits such as grain yield for which breeders, geneticists, and pathologists have been targeting. This increased rate of change, however, shows that progress has been made not only in improvement of agronomic traits and by modernized production technology but also in increased resistances for multiple pests.

The 2004 and 2005 agronomic data show that recently developed NDSU HRSW genotypes, as well as two lines from other breeding programs (Briggs and Hanna) have greater grain yields, GVW, and TKW compared with older released cultivars, particularly Marquis. The significant increase and large mean annual gain in grain yield, GVW, and TKW in this study could be attributed partially to better resistance to leaf rust and FHB of modern cultivars compared with the previous cultivars. The disease resistances provided an advantage for the modern cultivars to achieve a greater grain yield, GVW, and TKW than the susceptible genotypes.

Nevertheless, grain yield was significantly correlated with year of release and showed a mean annual gain of 30.4 kg ha^–1 yr^–1 since 1968, an annual gain in grain yield of 1.3%. These results are in agreement with previous results from North Dakota (Deckard et al., 1985) and other wheat-growing regions such as Mexico and Argentina (Waddington et al., 1986; Calderini et al., 1995) that grain yield has been increasing in the recently developed cultivars. The grain yield gains obtained in this study are equal to or more than that reported in previous studies (Deckard et al., 1985; Cox et al., 1988; Donmez et al., 2001). Hence, this study shows that grain yield of HRSW in the northern Great Plains has not reached a plateau, which agrees with previous reports from other regions of the United States (Feyerherm et al., 1984; Cox et al., 1988; Donmez et al., 2001).

Our study also found that GVW significantly increased, and the trend, although larger, is similar to the significant increase in GVW found by Cox et al. (1988). Similarly, TKW significantly increased and was equivalent to twice as much of a mean annual gain as reported by Cox et al. (1988) for winter wheat cultivars. Donmez et al. (2001) found no change in TKW in winter wheat. Waddington et al. (1986), however, reported that TKW in spring wheat actually declined in the newer cultivars released between the years 1950 and 1982.

Across all environments, there was no significant correlation between kernel number per spike, and plant, stem, and spike densities with year of release. Donmez et al. (2001), however, found that winter wheat cultivars grown in Kansas have increased kernel number per spike. Total plants, stems, and spikes per square meter tend to be higher in the modern cultivars even though Prosper 2005 was the only environment to show a significant regression coefficient for plant and stem count with year of release. Previous work with winter wheat also showed that spikes per square meter has not increased in the recently developed cultivars (Donmez et al., 2001).

The results from this study did not show a significant decrease in plant height with the genotypes included in our study. Other studies have reported that plant height has decreased significantly over the years with the incorporation of semidwarf genes in modern cultivars (Cox et al., 1988; Donmez et al., 2001). Generally, cultivars released for high rainfall conditions are shorter cultivars. However, our study included both tall cultivars released for the dryland western region of North Dakota, as well as short cultivars, as all NDSU released cultivars since 1968 were included in this research. We did not observe a negative correlation between plant height and grain yield reported in previous literature (Donmez et al., 2001). This could reflect the lack of a significant decrease in plant height for the genotypes included in the study. We did not find a significant decrease in days to heading either, which was also reported in previous studies by Cox et al. (1988) and Donmez et al. (2001).

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
From the data generated in this study, we can draw a number of conclusions. First, no evidence of a decline over time has occurred in the improvement of most agronomic traits, including grain yield, in HRSW genotypes. Therefore, a genetic barrier for agronomic traits in HRSW grown in the northern Great Plains has not yet been reached. Second, overall, a significant increase in grain yield, GVW, TKW, and lodging resistance was observed in HRSW cultivars released since 1968. Third, there was improved resistance in the newly released cultivars for both leaf rust and FHB resistance, as the modern cultivars are currently resistant to the prevalent leaf rust races and some cultivars contain Sumai 3-derived FHB resistance. Finally, spring wheat breeders should be able to continue improving agronomic traits by introgressing favorable alleles, particularly for disease resistance.

This work is dedicated to Mr. James Faller, who passed away recently, for his dedication and continuous support in accomplishing this work and all other research projects at the HRSW breeding program at NDSU. The authors would like to extend their deep gratitude to the NDSU HRSW breeding and laboratory quality staff (J. Halley, T. Olson, K. McMonagle, Rachel Olson, and Amal Mergoum) for all their help and support in carrying out this work.. Thanks to Drs. Deckard, Meyer, Elias, and many other faculty members who reviewed or provided many constructive and pertinent suggestions to improve the quality of this research.

All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

Received for publication January 12, 2007.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 

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