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Published online 8 January 2009
Published in Agron J 101:124-130 (2009)
DOI: 10.2134/agronj2008.0187
© 2009 American Society of Agronomy
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Growth, Yield, and Yield Component Changes among Old and New Soybean Cultivars

Jason L. De Bruin* and Palle Pedersen

Dep. of Agronomy, Iowa State Univ., 2104 Agronomy Hall, Ames, IA 50011-1010

* Corresponding author (jsndbrn{at}iastate.edu).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soybean [Glycine max (L.) Merr.] yield has increased at a rate of 25 to 30 kg ha–1 yr–1 due in part to improved genetic gain, and has been further advanced by the addition of resistance to soybean cyst nematode (Heterodera glycines Ichinohe; SCN) in new cultivars. The objective was to determine specific growth changes that explain the yield improvement from old to new cultivars and the further yield improvement gained from the addition of SCN resistance. Studies were conducted at three Iowa locations during 2005 and 2006. Two old and two new SCN-susceptible, and two new SCN-resistant cultivars were evaluated for total dry matter (TDM) accumulation and leaf area index (LAI) through the season along with yield and yield components at harvest. New cultivars produced yields superior to older cultivars due to increased crop growth rate (CGR) culminating in greater TDM 105 days after emergence (DAE). Yield was strongly associated with the number of seeds produced m–2 and this yield component accounted for almost all of the yield differences among cultivars. Seeds m–2 was positively related to CGR between 42 and 105 (growth stage R1–R5.5) DAE and to LAI 105 DAE. New SCN-resistant cultivars produced yields 17 to 19% greater than new susceptible cultivars across three locations. Increased TDM and CGR explained the yield response at the low-yield location, but not at the high-yield locations. Apparent harvest index (HI) was similar among all cultivars at each location. Selection for increased yield has indirectly selected for increased TDM and CGR with a similar amount partitioned to seed dry weight. Future yield gains will be made by (i) increasing the amount and the rate of dry matter (DM) and (ii) through the increased production and duration of leaf area.

Abbreviations: CGR, crop growth rate • DAE, days after emergence • DM, dry matter • HG, Heterodera glycines • HI, harvest index • LAI, leaf area index • MG, maturity group • Pi, initial SCN population density • SCN, soybean cyst nematode • TDM, total dry matter

Received for publication May 31, 2008.
   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
CONTRIBUTION from genetic and agronomic improvements (Specht et al., 1999) have increased soybean yield in the United States by an average of 22.8 and 26.4 kg ha–1 yr–1 in Iowa between 1924 and 2007. In side-by-side comparisons, genetic gain has been estimated at 17 to 29 (Specht and Williams, 1984) and 30 kg ha–1 yr–1 for privately released cultivars in maturity groups (MG) II and III (Specht et al., 1999). This is similar to the 30 kg ha–1 yr–1 for cultivars released after 1976 in the MG 000 to 0 reported by Voldeng et al. (1997). Recently, De Bruin and Pedersen (2008a) reported genetic gain of 25.4 kg ha–1 yr–1 and similar yield stability for new cultivars in MG II and III compared with cultivars released between 1938 and 1951.

Yield can be studied as the product of TDM and HI (Egli, 1998). The relationship among these components has shown to be inconsistent in the literature and particularly not well-defined for genotypes grown in the upper Midwest of the United States. Both Shibles and Weber (1966) and Weber et al. (1966) reported that TDM at stage R5.5 to R6 (Fehr and Caviness, 1977) was not correlated to yield and tended to be negatively associated with HI, while yield increases were positively associated with HI. From these data they found that DM at the same developmental stage was not a good predictor of soybean yield. However, these two studies were conducted with a single cultivar at multiple plant populations at one location over multiple years and did not allow for large changes in TDM and yield. Others have reported that TDM at R5 (Board and Modali, 2005) and CGR (Egli and Zhen-wen, 1991) are closely linked to yield. Pedersen and Lauer (2004) found HI to be positively correlated with yield and this agrees with Shibles and Weber (1966), but does not agree with the negative association (Board and Modali, 2005) or no association (Cregan and Yaklich, 1986) between HI and yield.

Yield improvements between old and new cultivars in the MG 00 and 0 have been related to the fact that newer cultivars maintained leaf area longer, produced a greater amount of TDM during the seed-filling period, and had a greater HI (Kumudini et al., 2001). Cultivars released in the 1940s and 1950s yielded 11% greater than cultivars released between 1910 and 1938 due to an 11% increase in TDM with no change in HI (Cregan and Yaklich, 1986). Frederich et al. (1991) also identified TDM, not HI, as the contributing factor toward yield improvement. Specht and Williams (1984) attributed some yield improvement for MG II and III to larger seed mass and reduced lodging for newer cultivars, but did not evaluate HI or TDM.

There is strong evidence to suggest that enhanced plant growth during the seed set period (Egli and Zhen-wen, 1991; Vega et al., 2001) is a primary factor contributing to yield formation; however, growth and remobilization during the seed-filling period also have the potential to make significant yield improvements (Borrás et al., 2004). Determination of the factors most critical to yield formation is debatable as source strength during the early reproductive and late reproductive stages have been shown to place a potential limit on yield depending on the time of stress during the growing season and the production environment (Borrás et al., 2004; Egli, 2006).

Soybean cyst nematode reduces soybean yield more than any other pathogen (Wrather and Koenning, 2006) and has been an increasing problem for producers in Iowa following its arrival in 1978 (Edwards, 1988). Cultivars resistant to SCN provide significant yield advantages in environments where SCN has been identified (Chen et al., 2001; De Bruin and Pedersen, 2008b). Careful observation has identified that TDM of SCN-susceptible cultivars can be reduced 4 to more than 50% compared with SCN-resistant cultivars (Wang et al., 2003). Recently, De Bruin and Pedersen (2008b) reported a 14% yield improvement for SCN-resistant cultivars compared with SCN-susceptible cultivars regardless of the SCN population density or H. glycines (HG)-Type (defined as the ability of a SCN population to reproduce on a specific set of seven cultivars; Niblack et al., 2002). Resistant cultivars also show greater yield stability across environments (De Bruin and Pedersen, 2008a). Growth parameters that control this consistent yield improvement and stability among environments have not been defined.

De Bruin and Pedersen (2008b) report notable differences for seed yield and the number of seeds m–2 among old and new cultivars, but not for seed mass or reaction to SCN for old and new susceptible cultivars. We hypothesize that yield differences among new vs. old cultivars and SCN-resistant vs. SCN-susceptible cultivars will be explained by a combination of improved TDM, CGR, increased LAI, and more efficient partitioning of TDM to reproductive material for new and SCN-resistant cultivars. Information regarding yield improvement in soybean would potentially be of interest to plant breeders as selection criteria to improve yield and to agronomists for the selection of production practices that increase yield. The objective was to determine yield, yield components, canopy formation, and DM accumulation for six cultivars that vary in year of release and resistance to soybean cyst nematode.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The experiment was established at three locations in 2005 and 2006; eastern Iowa near De Witt (Tama, fine-silty, mixed, superactive, mesic Typic Argiudolls); central Iowa near Nevada (Webster clay loam, fine-loamy, mixed, superactive, mesic Typic Endoaquolls); and western Iowa near Whiting (Salix, fine-silty, mixed, superactive, mesic Typic Hapludolls). Experimental design was a randomized complete block in a split-plot arrangement with four replications. The main plot was six cultivars and the split-plot was six sampling periods separated by 3 wk following emergence. New cultivars used were the SCN-susceptible cultivars S25J5 and NK S32-G5 (Syngenta Seeds, Minneapolis, MN), the SCN-resistant cultivars PB291N (Prairie Brand Seeds, Story City, IA) and SOI2642NRR (Sands of Iowa, Marcus, IA). The two older SCN-susceptible cultivars were Richland and Harosoy. Richland was released in 1938 from a plant introduction and Harosoy was released in 1951 from a cross of AK Harrow x Mandarin (Ottawa) (Luedders, 1977).

Before planting pre-emergent herbicides s-metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide] and metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one] were applied at 1.8 kg a.i. ha–1 and 0.4 kg a.i. ha–1, respectively at Whiting and De Witt in 2005 and 2006 and Nevada in 2006. No pre-emergent herbicide was used at Nevada in 2005. Fields were cultivated to a depth of 10 cm to incorporate herbicides and provide a level seed bed following herbicide application. During the growing season, glyphosate [N-(phosphonomethyl)glycine] was applied twice at a rate of 865 g a.e. ha–1 to the glyphosate-tolerant cultivars. The combination of acifluorfen [5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid] at a rate of 300 g a.i. ha–1 and sethoxydim [2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one] at a rate of 400 g a.i. ha–1 was applied once to nonglyphosate-tolerant cultivars. Plots were kept weed free by hand weeding during the rest of the growing season. The insecticide Lorsban [Chlorpyrifos 0,0-diethyl-0-(3,5,6-trichloro-2-pyridinyl) phosphorothioate] was applied at a rate of 840 g a.i. ha–1 to the experiment at De Witt twice in 2005 for the control of spider mites (Tetranychus urticae) and soybean aphids (Aphis glycines) and to Nevada in 2006 for the control of bean leaf beetles (Cerotoma trifurcate Forster). To control bean leaf beetles at Whiting in 2005, the insecticide Baythroid, [cyfluthrin,cyano (4-fluoro-3-phenoxyphenyl) methyl-3-(2,2-dichloroethenyl)-2,2-dimethyl-syclopropanecarboxylate] was applied at 490 g a.i. ha–1.

Before planting and following harvest, each plot was sampled for initial SCN density (Pi) by taking 20 soil cores to a 15 to 20 cm depth. Egg number 100-cm–3 soil was determined by extracting the cysts (egg-filled dead females) and crushing the cysts to remove the eggs, followed by counting the eggs under a microscope at a 1:100 dilution as outlined by Tabor et al. (2003). Tests to determine HG-Type were conducted following the procedures outlined by Niblack et al. (2002). HG-Type is determined based on the percent female reproduction on seven indicator lines and compared with female reproduction on the susceptible cultivar Lee 74. Indicator lines with reproduction values >10% of the standard susceptible cultivar indicate that the nematode population present in the field is virulent to the indicator line. Soybean cyst nematode population densities (eggs 100 cm–3) with HG-Type listed in parentheses were 4000 (0) and 12,600 (7) at De Witt, 2005 and 2006; 4000 (1.2.5.7) and 650 (2.5.7) at Nevada, 2005 and 2006; and 100 (2.7) and 1000 (2.7) at Whiting, 2005 and 2006.

Plots were seeded at a rate of 432,000 seeds ha–1 at a row spacing of 38 cm. Plot size was 10.6 by 6.7 m with a total of 14 rows established with a seven-row grain drill (Almaco, Nevada, IA). Planting occurred the last week of April, except in 2005 at Nevada when planting occurred on 9 May. Seeds were inoculated with Bradyrhizobium japonicum (EMD Crop BioScience, Brookfield, WI).

Plots were 14 rows wide, rows 9, 10, and 12 and 13 were used for growth analysis sample collection. Sampling areas were two consecutive meters of row length, equaling a total area of 0.76 m2. At 3-wk intervals following emergence, plants were cut at the soil surface from the sampling area and counted. Three representative plants were selected from the sample population and separated into stems, leaves, and pod components. Samples and three plant subsamples were dried in a forced-air dryer at 60°C. Stem, leaf, and pod dry weights were recorded for the subsample along with the remainder of the DM collected from the sampling area. The weights of the subsample were added to the population sample to determine TDM for each sample. Pods from the subsample were divided into immature and mature (pods with seeds >0.5 cm) pods. Pods were counted, and weighed, and the seeds were removed from the mature pods, counted, and weighed to determine seed number per pod. Leaf area index was measured at each sampling period using a LAI-2000 leaf area meter (Li-Cor, Lincoln, NE). A series of four measurements were taken with one above the canopy and three measurements below the canopy at 0, 25, and 50% the distance of the row spacing. This series of measurements was taken four times within each plot.

At maturity, yield was determined by harvesting rows 2, 3, 4, and 5 of the total 14 rows with an Almaco small plot combine and adjusting plot weight to 130 g kg–1 moisture. Seed mass was determined by weighing 300 seeds. Moisture was determined for the seed mass sample at the same time and seed number m–2 was determined by dividing yield m–2 by seed mass once yield and seed mass had been adjusted to equal moisture content. Apparent HI was calculated by dividing the seed mass by the TDM (seed, stem, and pod) from the three plant subsample collected at R8 (Cregan and Yaklich, 1986). Source available per seed was estimated by dividing the final seed number m–2 by the LAI 105 DAE. Plant growth stage 105 DAE was approximately R5.5, this growth stage occurs approximately 7 d after R5 and represents a stage where node development has ceased, seed number has been determined, and seeds are beginning to fill (Fehr and Caviness, 1977).

Crop growth rate was calculated from the equation of the line describing the change in TDM for TDM samples collected 42, 63, 84, and 105 DAE (approximately R1–R5.5). Values were plotted against time in days to determine CGR for each plot.

Statistical analysis was conducted using Proc Mixed in the SAS program (SAS Institute, 2003). The restricted maximum likelihood method for variance component estimation indicated that the error variances for locations were heterogeneous so they were analyzed separately. Years were random in determining the appropriate F tests. Samples collected during the season were analyzed as subplots (Gomez and Gomez, 1984). When cultivar main effects or cultivar x sampling time interactions were significant (P ≤ 0.05) for TDM or LAI contrast statements were used to further separate the effect. Comparisons were made between old and new (both SCN-resistant and susceptible) cultivars and new SCN-resistant and new SCN-susceptible cultivars at specific sampling periods. Regression analysis to determine the relationship among seeds m–2 with CGR and LAI 105 DAE were determined in Proc Mixed. Cultivar was a fixed effect and either CGR or LAI were included as a continuous variable. The interaction between cultivar and the continuous variable (CGR or LAI) was included to test for a common slope based on the P value of the interaction (Littell et al., 2006). When the interaction term was P ≥ 0.05 the interaction was considered nonsignificant and a model with a common slope was used to describe the relationship.


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Seed Yield and Yield Components
New cultivars produced yields superior to old cultivars at each of the three locations with an average yield increase of 55, 40, and 35% at De Witt, Nevada, and Whiting, respectively (Table 1 ). At Whiting, the location with the smallest Pi (<1000 eggs 100 cm–3), new SCN-resistant cultivars produced 17% greater yields than new SCN-susceptible cultivars (Table 1). At De Witt and Nevada, Pi were greater but the evidence of improved yield performance from SCN-resistant cultivars was not as strong even through resistant cultivars yielded 20% more (P = 0.09) at De Witt and 19% more at Nevada (P = 0.07). Averaged across the three environments yield of new cultivars increased 44% compared with old cultivars and resistance to SCN increased yield 18% over new susceptible cultivars. Yield increases for both old vs. new and SCN resistant vs. SCN susceptible were the result of more seed m–2 produced (Table 1). Seed mass was similar across all cultivars at each location (Table 1). There were differences in the number of seeds pod–1 at De Witt and Whiting, but not at Nevada (P = 0.08). At De Witt, new cultivars produced 0.38 more seeds pod–1 and at Whiting the difference was 0.20 seeds pod–1.


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  		Table 1. Seed yield and yield components of two old soybean cyst nematode (SCN)-susceptible (Old), two new SCN-susceptible (NS), and two new SCN-resistant (NR) soybean cultivars measured at three locations in Iowa, 2005–2006.

 
Plant Growth
A cultivar x sampling time interaction was significant at each location for both TDM and LAI (Table 2 ). Separation of the interaction indicated that between 21 and 85 DAE there were no differences (P < 0.05) among old and new cultivars for TDM (Fig. 1 ). New cultivars had 131, 179, and 225 g m–2 more TDM 105 DAE (Fig. 1) than the old cultivars at De Witt, Nevada, and Whiting, respectively (Fig. 1). New SCN-resistant and new SCN-susceptible cultivars at the higher yield, SCN-infested, locations of De Witt and Whiting produced similar TDM but new SCN-resistant cultivars produced 83 g m–2 more TDM 105 DAE compared with the susceptible cultivars at Nevada, a lower yield, SCN-infested location.


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  		Table 2. Source of variation for leaf area index (LAI) and total plant dry mater (TDM) sampled throughout the season at three locations in Iowa, 2005 to 2006.

 

Figure 1
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  		Fig. 1. Total plant dry matter taken during the season at (a) De Witt, (b) Nevada, and (c) Whiting, 2005 to 2006. Each point is the average of two new soybean cyst nematode (SCN)-resistant (NR), two new SCN-susceptible (NS), and two old SCN-susceptible (OS) cultivars. Symbols *, **, *** indicate significant differences (P = 0.05, 0.01, and 0.001, respectively) and ns indicates no significant difference among new (N) and old (O) cultivars and NR and NS cultivars at the same sampling date.

 
Leaf area index was similar among cultivars at all locations for the 42 and 64 DAE measurement (Fig. 2 ). New cultivars had greater LAI than old cultivars at 84 and 105 DAE at Nevada and at 105 DAE at De Witt and Whiting. Differences ranged from 0.5 at Whiting to as much as 1.2 at Nevada 105 DAE indicating that the loss in leaf area was greater for old cultivars compared with new cultivars. Soybean cyst nematode resistant cultivars had 0.7 greater LAI compared to new SCN-susceptible cultivars at Nevada 105 DAE (Fig. 2b).


Figure 2
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  		Fig. 2. Leaf area index (LAI) taken between 42 and 105 days after emergence (DAE) at (a) De Witt, (b) Nevada, and (c) Whiting, 2005 to 2006. Each point is the average of two new soybean cyst nematode (SCN)-resistant (NR), two new SCN-susceptible (NS), and two old SCN-susceptible (OS) cultivars. Symbols *, **, *** indicate significant differences (P = 0.05, 0.01, and 0.001, respectively) and ns indicates no significant difference among new (N) and old (O) cultivars and NR and NS cultivars at the same sampling date.

 
Crop growth rate between 42 and 105 DAE varied among cultivars at Nevada and Whiting (Table 3 ). At Whiting, new cultivars produced 3.5 g m–2 d–1 more DM compared with old cultivars and at Nevada this differences was 2.4 g m–2 d–1 (Table 3). There were no differences among new SCN-resistant and SCN-susceptible cultivars at DeWitt or Whiting, but at Nevada SCN-resistant cultivars accumulated 1.5 g m–2 d–1 more DM than susceptible cultivars.


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  		Table 3. Harvest index (HI), crop growth rate (CGR) between 63 and 105 days after emergence (DAE), and leaf area index 105 DAE per seed of two old soybean cyst nematode (SCN)-susceptible (Old), two new SCN-susceptible (NS), and two new SCN-resistant (NR) soybean cultivars measured at three locations in Iowa, 2005 to 2006.

 
Harvest index of new cultivars averaged 53.6, 51.4, and 51.1% while old cultivars averaged nonsignificantly less (P > 0.08; Table 3) at 51.8, 50.4, and 49.1% at De Witt, Nevada, and Whiting, respectively. Lack of significant differences among cultivars was not the result of high coefficient of variation (CV) as CVs were 6, 5, and 13% at De Witt, Nevada, and Whiting, respectively. Source capacity seed–1 was similar across all cultivars within the three locations before the start of the seed-filling period (Table 3).

Production of seeds m–2 was positively related to CGR between R1 and R5.5 (Fig. 3 ). Response among seeds m–2 and CGR between R1 to R5.5 for each cultivar shared a common slope but a different intercept (Fig. 3). Among all cultivars, for each 1 g m–2 d–1 increase in TDM between R1 and R5.5 the number of seeds produced m–2 increased by 98 (Fig. 3). Due to the similarity in source seed–1, greater LAI values corresponded to greater established seeds m–2. Cultivars shared a common slope but different intercepts indicating that as LAI 105 DAE increased by one unit, seed number m–2 increased by 219, regardless of cultivar (Fig. 4 ).


Figure 3
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  		Fig. 3. Relationship between crop growth rate and seeds m–2 for six cultivars grown at three locations in Iowa during 2005 and 2006. The model Seeds m–2 = cultivar + CGR + cultivar x CGR was evaluated in Proc Mixed to determine the relationship among CGR (R1–R5.5) and the number of seeds m–2. The cultivar x CGR interaction was not significant (P = 0.96) but the cultivar term was significant (P ≤ 0.001) indicating cultivar response models shared a common slope (b1 = 96.0) but different intercepts. Each point represents the mean of n = four observations.

 

Figure 4
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  		Fig. 4. Leaf area index (LAI) 105 DAE and seeds m–2 for six soybean cultivars grown in Iowa at three locations during 2005 to 2006. The model seeds m–2 = cultivar + LAI + cultivar*LAI was evaluated in Proc Mixed to determine the relationship among LAI 105 DAE and the number of seeds m–2. The cultivar*LAI interaction was not significant (P = 0.44) but the cultivar term was significant (P ≤ 0.001) indicating that cultivar response models shared a common slope (b1 = 219) but different intercepts. Each point represents the mean of n = four observations.

 

   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Estimated genetic gain for this set of cultivars was 0.7% yr–1, in general agreement with previous findings from Kumudini et al. (2001), Luedders (1977), Specht and Williams (1984), Specht et al. (1999), and Voldeng et al. (1997). Yield improvement was related to improved TDM production. New cultivars produced DM at a more rapid rate between 42 and 105 DAE compared with old cultivars. A positive relationship between seeds m–2 and CGR agrees well with both Vega et al. (2001) at the plant level and Egli and Zhen-wen (1991) at the canopy level. Regression of seeds m–2 on CGR documented that the response pattern to increasing CGR was similar for all cultivars and has not changed over time (Fig. 3). In the current study, the nonsignificant change in HI among cultivars would indicate that yield improvement was almost entirely a function of improved TDM production for new cultivars.

Greater TDM production increased seed number per plant during the seed set period (Vega et al., 2001) and can increase seed mass during the seed-filling period (Borrás et al., 2004); however, there was no effect in the current study on seed mass. These findings make soybean a source limited crop through most of the growth period as increased growth during different stages has the capacity to increase yield. In the present article we have shown that yield increases due to breeding or SCN resistance have been achieved through a higher CGR during the seed set (R1–R5.5). Higher CGRs around the R1 to R5.5 periods increased seed number, and this higher sink number was fulfilled because genotypes established a similar source-sink balance (Table 2). Our results show newer cultivars established more seeds and produced more DM before the seed-filling period, maintaining the seed-filling source to sink ratio at a similar level, allowing for greater seed DM production during the seed-filling period. This is in agreement with Specht et al. (1999), who showed that yield increases were a result of higher seed number and higher DM productions during seed filling. Because cultivars established a similar source to sink ratio during seed filling it comes as no surprise that there were no differences in the apparent HI measured at crop maturity (Table 1).

Early in the season all cultivars were able to produce LAI at a similar rate and differences only existed late in the season as old cultivars lost more leaf area than new cultivars from 84 to 105 DAE (Fig. 2). At the higher yielding locations (Whiting and De Witt) and at the lower yielding site (Nevada) old cultivars maintained a leaf area of 3.2 and 2.8 through the R5.5 growth stage, respectively. This LAI was sufficient to capture more than 95% of the incoming solar radiation at Whiting and De Witt but not at Nevada (Shibles and Weber, 1965). This finding helps explain the reduced CGR at Nevada for old cultivars as TDM accumulation is a linear function of light interception (Shibles and Weber, 1965).

Specht et al. (1999) and Kumudini et al. (2001) noted that accelerated senescence and reduced LAI at the end of the cycle have a concomitant effect over the DM accumulated during the seed-filling period in old cultivars. Our results agree with their findings, as newer cultivars maintained a greater LAI longer in the season and greater TDM production during seed filling; however, this is the result of new cultivars establishing greater seed number before R5 and the maintenance of a similar source to sink ratio during seed filling. To maintain this source to sink ratio during seed filling new SCN-susceptible and resistant cultivars produced more DM during seed filling.

It is interesting that the number of seeds produced within each pod was greater for new cultivars at two of the testing locations as this yield component is reported to be very stable (Egli, 1998). Improvement of seeds pod–1 would follow the outcome of greater seeds m–2 and is in agreement with the findings of Board et al. (1999) as they identified seeds pod–1 as positively influencing seeds m–2.

Yield was strongly related to the production of TDM by the R5.5 growth stage and increased at a linear rate for each cultivar, even to TDM as great as 1200 g m–2 at Whiting for SCN-resistant cultivars (data not shown). Old cultivars included in the current study accumulated as much as 800 g m–2 DM (Fig. 1) and yield increased at a linear rate with increasing DM at R5.5 as well (data not shown). Shibles and Weber (1966) and Weber et al. (1966) both published that increased DM production had little effect on seed yield leading them to conclude that for an individual cultivar, seeded at various populations, DM production was not a good prediction of seed yield. However, HI was reduced by increasing DM production (Shibles and Weber, 1966). In the current study, HI was not reduced by increased DM production for new cultivars compared with old cultivars at any location leading to the conclusion that greater TDM production was the factor responsible for the yield improvement among old and new cultivars as well as SCN-resistant and SCN-susceptible cultivars.

Greater DM production 63, 84, and 105 DAE at Nevada, an SCN-infested, lower-yielding location by SCN-resistant cultivars is in agreement with the finding of Wang et al. (2003) that DM accumulation was not reduced for susceptible cultivars until 70 d after planting. This supports that yield losses from SCN is related to a restriction in source capacity early (R1–R2) in the reproductive stages. The data also indicate that SCN-resistant cultivars produce greater yields at SCN-infested, higher yielding locations (regardless of Pi density and HG-Type) through a combination of small, nonsignificant CGR, TDM, and LAI changes to produce a greater number of seeds m–2 with similar seed mass. This is in agreement with the findings of De Bruin and Pedersen (2008a) that tested a broader range of SCN-resistant and SCN-susceptible cultivars.


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
New cultivars produced higher yields as a result of improved TDM accumulation at both low and high yielding locations with no improvement in apparent HI. This higher yield was only a consequence of higher seeds m–2 since individual seed mass remained unchanged. Higher seeds m–2 were related to increased CGR from R1 to R5.5 and the production and duration of leaf area. SCN-resistant cultivars produced more DM and leaf area at the lower-yielding location, providing the basis for the higher stability these genotypes are known to show. The ability to produce a larger amount of DM by R5.5 suggested this is a critical component for increasing seed number m–2, because cultivars established a similar seed filling source to sink ratio and seed mass remained unchanged.


   ACKNOWLEDGMENTS
 
The authors thank Jodee Stuart and Adriana Murillo-Williams for their technical assistance and Lucas Borrás for his review of the manuscript before submission. This research was funded by the Iowa Soybean Association.


   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.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
The SCI Journals Crop Science Vadose Zone Journal
Journal of Natural Resources
and Life Sciences Education		 Soil Science Society of America Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome