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Crop Science 42:1081-1087 (2002)
© 2002 Crop Science Society of America

CROP BREEDING, GENETICS & CYTOLOGY

Heterosis of Agronomic Traits in Alfalfa

Heathcliffe Riday* and E. Charles Brummer

Dep. of Agronomy, Iowa State Univ., Ames, IA 50011

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


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Increasing forage yields remains a top priority of most alfalfa (Medicago sativa L.) breeding programs. Studies of crosses between dormant to moderately dormant M. sativa subsp. sativa and M. sativa subsp. falcata suggest a heterotic pattern for yield exists between the two subspecies. However, other agronomic traits need to be considered in addition to yield, especially when trying to develop breeding material from nonadapted sources. The objective of this study was to quantify the agronomic performance of sativa x falcata crosses (SFC) in relation to sativa x sativa crosses (SSC) and falcata x falcata crosses (FFC). Nine elite sativa clones and five falcata clones were crossed in a diallel mating design. Progeny were space planted in 1998 at Nashua and Ames, IA. During the 1999 growing season, winter injury, spring regrowth, vigor, growth habit, maturity, height, midseason regrowth, and autumn regrowth were measured. The 14 parental genotypes differed for general combining ability (GCA) for all traits; specific combining ability (SCA) was noted for height, maturity, winter injury, and vigor. Sativa x sativa crosses were superior to FFC for all traits except winter injury and vigor. Sativa x falcata crosses per se had slightly increased agronomic performance over the expected mid-subspecies for many traits. Most of the hybrids are intermediate to SSC and FFC, suggesting potential agronomic weaknesses of falcata germplasm in a breeding program. Improving regrowth, height, and growth habit of falcata breeding material would likely be needed to create commercially successful sativa–falcata semihybrid cultivars.

Abbreviations: SSC, sativa x sativa crosses • SFC, sativa x falcata crosses • FFC, falcata x falcata crosses • GCA, general combining ability • SCA, specific combining ability • HS-heterosis, halfsib heterosis


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
IN THE PAST FEW DECADES, alfalfa yields in the USA have been stagnant (Riday and Brummer, 2002). Brummer (1999) suggested that a semihybrid system, which could capture natural hybrid vigor found between certain alfalfa populations, could be a means to increase yield. By identifying specific heterotic patterns in alfalfa germplasm, populations could be produced by recurrent phenotypic selection, and semihybrid cultivars could be developed from interpopulation crosses. Medicago sativa subsp. falcata (hereafter referred to as "falcata") is yellow flowered alfalfa that is decumbent, winter hardy (Lesins and Lesins, 1979), and one of the nine initial alfalfa germplasm groups introduced into the USA (Barnes et al., 1977). Studies of falcata germplasm show it to have slower regrowth, earlier dormancy, and a more decumbent growth habit than the more commonly cultivated subsp. sativa (Julier et al., 1995). The Iowa State University forage breeding program has initiated a long-term breeding program to develop improved pure falcata germplasm that could be used in semihybrid breeding programs (Brummer et al., 1997; Brummer, 1999). Desirable agronomic field traits include rapid re-growth, erectness, height, stand persistence, appropriate maturity, winterhardiness, and early spring recovery, all these traits help maximize yield and stand life (Sheaffer et al., 1988).

Crosses between Medicago sativa subsp. sativa (hereafter referred to as "sativa") and falcata show heterosis for forage yield (Westgate, 1910; Waldron, 1920; Sriwatanapongse and Wilsie, 1968; Riday and Brummer, 2002). In addition to inter-subspecific crosses, hybrids between diverse sativa germplasm also expressed heterosis in some cases (Yazdi-Samadi and Stanford, 1969; Busbice and Rawlings, 1974; Hill, 1983). High parent heterosis on a halfsib basis was often observed for yield in SFC (Riday and Brummer, 2002). However, agronomic field traits of the sativa x falcata crosses (SFC) have not been thoroughly investigated. In an early study, Burton (1937) examined the progeny of a cross between falcata and hairy Peruvian sativa genotypes. He did not report the means of the progeny but instead discussed the correlation of the agronomic traits measured with plant yield and found that height and leaf shape of SFC were correlated with yield in a field setting. Other than this report, we are unaware of any studies specifically examining the performance of a sativa–falcata hybrid in relation to its parental subspecies.

Our objective in this study was to compare the performance of sativa–falcata hybrids to intra-subspecific sativa or falcata crosses for agronomic field traits including height, maturity, growth habit, winter injury, spring regrowth, midseason regrowth, autumn regrowth, and vigor. Sativa germplasm from three different commercial companies was included to determine if within-company crosses differed from between-company crosses.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
Plant Materials
Nine sativa and five falcata genotypes were crossed in a half-diallel mating design. The nine elite sativa genotypes included ABI408, ABI311, ABI419, and ABI314 from ABI Alfalfa, Inc. (Lenexa, KS); C96-514, C96-673, and C96-513 from Forage Genetics (West Salem, WI); and FW-92-118 and RP-93-377 from Pioneer Hi-bred International (Des Moines, IA). The five falcata genotypes included WISFAL-4 and WISFAL-6 from the semi-improved falcata population, WISFAL (PI560333; Bingham, 1993); C25-6, a semi-improved falcata population developed in Colorado (PI578248; Townsend et al., 1995); and two genotypes visually selected for vigor from plant introductions that had been planted in the field near Ames, IA: PI214218-1, derived from an accession collected in Denmark in 1954 and PI502453-1, derived from the Russian cultivar Pavlovskaya.

The 14 selected parents were crossed in a half-diallel mating design. Progeny were germinated in March 1998 in the greenhouse and stem cuttings of parental genotypes were made at this time. A total of 110 entries was included in this experiment (91 crosses; 14 parental clones; and 5 check varieties [Vernal, 5454, Innovator +Z, Ladak, and Legendairy]). More detail on greenhouse work is given in Riday and Brummer (2002).

Experimental Design and Agronomic Trait Measurements
Field experiments were planted at the Agronomy and Agricultural Engineering Research Farm west of Ames, IA, in a Nicollet loam soil (fine-loamy, mixed, superactive, mesic Aquic Hapludolls) on 20 May 1998 and at the Northeast Research Farm south of Nashua, IA, in a Readlyn loam (fine-loamy, mixed, mesic Aquic Hapludolls) on 22 May 1998. The plot design at Ames was a quadruple {alpha}-lattice, with 10 plots in each of 14 incomplete blocks for 560 total plots. At Nashua, the design was a quadruple {alpha}-lattice, with 9 plots in each of 14 incomplete blocks for a total of 504 total plots. Ten plants per plot were planted 30 cm apart within rows spaced 90 cm apart. Entries were separated by 60 cm within rows. Further details on the field design are described in Riday and Brummer (2002).

Biomass yield was measured on 18 Aug. 1998, 16 Oct. 1998, 27 May 1999, 7 July 1999, and 1 Sept. 1999 at Ames and on 20 Aug. 1998, 20 Oct. 1998, 6 June 1999, 15 July 1999, and 10 Sept. 1999 at Nashua (Riday and Brummer, 2002). Winter injury was scored after new stems had emerged on 12 April 1999 at Ames and on 19 April 1999 at Nashua. Winter injury measured crown health and evenness of regrowth and was scored on a 1 = least damaged to 5 = most damaged scale (McCaslin and Woodward, 1995). Regrowth, which scored the amount and rate of regrowth, was measured on a 1 = least to 5 = most scale four times during 1999. Spring regrowth, was measured on 12 April 1999 at Ames and on 19 April 1999 at Nashua; midseason regrowth, on 5 June 1999 and 21 July 1999 at Ames and on 26 June 1999 and 26 July 1999 at Nashua; and autumn regrowth on 7 Sept. 1999 at Ames and on 17 Sept. 1999 at Nashua. Vigor was scored by a 1 = least to 5 = most scale on 11 May 1999 at Ames and on 18 May 1999 at Nashua. Vigor scores were based on the density and amount of vegetative growth in each plot. Growth habit was visually scored on a 1 = most decumbent to 9 = most erect scale for each plot on 11 May 1999 and 30 Aug. 1999 at Ames and on 18 May 1999 and 9 Sept. 1999 at Nashua. Plot height was measured on 25 May 1999, 7 July 1999, and 30 Aug. 1999 at Ames and on 3 June 1999, 14 July 1999, and 8 Sept. 1999 at Nashua. The tallest point was measured on five random plants per plot as they stood naturally and an average plot height was calculated. Maturity was scored on a 1 = early vegetative to 9 = ripe seed pod scale (Kalu and Fick, 1981) on 26 May 1999, 9 July 1999, and 30 Aug. 1999 at Ames and on 3 June 1999, 14 July 1999, and 8 Sept. 1999 at Nashua.

Data Analysis
Calculations of Entry Means
For each field trait measured, the MIXED procedure of the SAS statistical software package (SAS Institute, 2000) was used to calculate least squares means for each entry at each measurement date and location. Each measurement date–location combination was treated as a single environment for the analysis of height, growth habit, midseason regrowth, and maturity. Locations or environments, replications and blocks were considered to be random effects and entries were fixed. Because of questionable field performance of parental genotypes, which were derived from cuttings rather than seedlings, the following analyses were based on progeny only (Riday and Brummer, 2002).

Combining Ability Analysis
General (GCA) and specific combining ability (SCA) were calculated using SAS (Zhang and Kang, 1997). The analysis used model I method 4 from Griffing (1956), which includes F1 progeny, but not reciprocal crosses or parents and in which genotypes are fixed.

Subspecies Mean Comparisons
To compare the different types of crosses for each agronomic trait, the 91 crosses from the 14 parent half-diallel were divided into three categories: (i) sativa x sativa crosses (SSC), (ii) sativa x falcata crosses (SFC), or (iii) falcata x falcata crosses (FFC). Comparisons among the three groups were calculated by means of linear contrasts (SAS Institute, 2000). A mid-subspecies mean was calculated as the average of the SSC and the FFC means. The mid-subspecies mean was linearly contrasted with the SFC mean (SAS Institute, 2000). If the comparison between them was significant, a deviation percentage was calculated, which represents average heterosis or mid-subspecies heterosis. The SSC were split into within-company and between-company crosses and the two groups were linearly contrasted (SAS Institute, 2000).

Halfsib Family Heterosis
The mean halfsib family performance of each parental genotype for each agronomic trait was calculated for both SFC and within subspecies crosses. The two halfsib means, sativa x falcata halfsib mean and within-subspecies halfsib mean for each genotype were linearly contrasted (SAS Institute, 2000). High parent heterosis on a halfsib basis was calculated by linearly contrasting the parental genotype's sativa x falcata halfsib mean with the larger of the following: (i) the parental genotype's within-subspecies halfsib mean or (ii) the within subspecies cross mean (SSC or FFC) of the subspecies in which the parental genotype was not found (SAS Institute, 2000). Low parent negative heterosis on a halfsib basis was determined in an analogous manner.

Mean heterosis on a halfsib basis (HS-heterosis) was calculated by comparing each parental genotype's sativa x falcata halfsib mean performance to the average performance of intra-subspecies crosses (Riday and Brummer, 2002). Using HS-heterosis allowed us to determine heterosis without relying on data obtained from parental cuttings, which may be unreliable, as well as to generalize heterosis values on the basis of half-sib family performance rather than on individual cross performance between the two heterotic groups.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
Subspecies Analysis
The FFC had equivalent vigor to SSC, but SFC were superior to both. Vigor was highly correlated with yield (r = 0.74) and these results are congruent with those seen for yield (Riday and Brummer, 2002). For winter injury, SFC and FFC were equivalent, and both had less injury than SSC (Table 1). The SSC had more rapid spring, midseason, and autumn regrowth; a more erect growth habit; and more rapid maturity than FFC (Ta-ble 1). The SFC mean fell in the range between SSC and FFC means for these traits (Table 1). The SSC were taller than FFC in all environments. In May 1999 at Nashua and in September 1999 at Ames, SFC were equivalent to SSC for height, but at other measurement dates, SFC were intermediate to FFC and SSC. Cross type x environment interactions were observed only for maturity and midseason regrowth; however, these interactions were due to magnitude effects.


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  		Table 1. Mean vigor, winter injury, spring regrowth, midseason regrowth, autumn regrowth, growth habit, height, and maturity, for crosses between and within M. sativa subsp. sativa and subsp. falcata, mid-subspecies heterosis (MS-heterosis), and combining ability analysis measured in 1999 across two Iowa locations.*

 
The 14 parental genotypes differed for GCA effects for all traits, but SCA effects were only significant for vigor, winter injury, and height. In autotetraploids, significant GCA indicates mostly additive gene action while significant SCA indicates non-additive gene action (Levings and Dudley, 1963). The majority of crosses displaying advantageous SCA were SFC (Figs. 1, 2, and 3) . Crosses displaying advantageous SCA for vigor and height are above the expectancy line (Fig. 1 and 2), while crosses with advantageous SCA for winter injury are below the expectancy line (Fig. 3). The observed pattern of superior SFC performance compared to expectation supports the idea that sativa and falcata form a heterotic pattern (Riday and Brummer, 2002; Brummer, 1999).



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  		Fig. 1. Observed vs. expected alfalfa vigor score (1 = least to 5 = most) for sativa x sativa, sativa x falcata, and falcata x falcata crosses, across two Iowa locations in May 1999.

 


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  		Fig. 2. Observed vs. expected alfalfa height for sativa x sativa, sativa x falcata, and falcata x falcata crosses, across two Iowa locations and three height measurements (May, July, and September) in 1999.

 


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  		Fig. 3. Observed vs. expected alfalfa winter injury score (1 = least to 5 = most) for sativa x sativa, sativa x falcata, and falcata x falcata crosses, across two Iowa locations in April 1999.

 
Mid-subspecies heterosis represents the average superiority of SFC compared to intra-subspecific crosses. All agronomic traits except growth habit and midseason regrowth exhibited mid-subspecies heterosis (Table 1). The spring traits of winter injury, spring regrowth, and vigor showed positive mid-subspecies heterosis ranging from 8 to 12%. Beginning in late summer, crosses containing a falcata parent went into dormancy earlier than SSC, exhibiting slower, more decumbent growth than they had earlier in the season. Consequently, autumn regrowth showed negative heterosis (i.e., slower regrowth) of about -5% (Table 1). Average maturity and height, which were measured three times throughout the growing season, had mid-subspecies heterosis values of 4 and 7%, respectively (Table 1). The increased maturity rates of SFC over the mid-subspecies mean was only 4%, or less than one half point on the scoring scale. Plant height also exhibited heterosis, and this is likely one of the causes of increased yield and, by extension, heterosis for yield, which we have observed previously (Riday and Brummer, 2002).

Elite Sativa Analysis
Sativa x sativa crosses between different companies had increased height over within-company crosses, which suggests that commercial companies may have germplasm divergent enough to constitute heterotic groups. However, dry matter yield, forage quality traits, and other agronomic traits measured did not show any differences between the groups (Riday and Brummer, 2002; Riday et al., 2002). The hypothesis that ABI Alfalfa, Forage Genetics, and Pioneer Hi-bred International germplasms form distinct heterotic groups would have to be tested by means of a larger germplasm sample. If heterosis exists between company germplasm pools, it appears on a smaller scale than sativa–falcata heterosis. Even this slight heterosis between commercial germplasm pools suggests that developing separate elite-sativa populations for use in semihybrid breeding may have merit (Brummer, 1999).

Halfsib Analysis
Winter injury and vigor showed high parent heterosis on a halfsib basis in some genotypes (Table 2). The traits of midseason regrowth, autumn regrowth, growth habit, and maturity had sativa x falcata halfsib mean means for each parental genotype that were intermediate to that genotype's within-subspecies halfsib mean and the overall mean of the other species' intra-subspecific crosses (Table 2). Despite the SFC mean falling in a range between the SSC and FFC means for autumn regrowth, growth habit, height, and maturity, some of the fourteen parental genotypes tested displayed significant HS-heterosis for these traits (Table 3). The general lack of high parent heterosis on a halfsib basis and lower or negative HS-heterosis values for these agronomic traits (Table 3) are in marked contrast to forage yield where HS-heterosis values averaged around 18% and high parent heterosis on a halfsib basis was observed in a majority of genotypes (Riday and Brummer, 2002).


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  		Table 2. Inter (SFHS) and intra-subspecific (WHS) halfsib family means for vigor, height, growth habit, maturity, winter survival, spring regrowth, regrowth and fall regrowth for fourteen genotypes across two Iowa locations during 1999.

 

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  		Table 3. Halfsib heterosis (HS-heterosis) of vigor, height, growth habit, maturity, winter survival, spring regrowth, regrowth and fall regrowth for fourteen alfalfa genotypes grown across two Iowa locations during 1999.

 
The majority of genotypes exhibited HS-heterosis for height and vigor, which is similar to the pattern seen for forage yield (Table 3) (Riday and Brummer, 2002). Several genotypes that had negative HS-heterosis values for growth habit had positive HS-heterosis values for height (Table 3). The differences seen between the height and growth habit HS-heterosis values probably arose because growth habit scored the overall growth form of the plants, encompassing both height and spread of the plants. Progeny of both faster and slower maturing parental genotypes exhibited HS-heterosis, indicating that maturation in SFC was more rapid than expected (Table 3). Halfsib-heterosis for winter injury, spring regrowth, autumn regrowth, and growth habit was very genotype dependent with both negative and positive values detected (Table 3).

Potential of Sativa–Falcata Hybrids
The poor agronomic performance of falcata germplasm per se is a major limitation to its use in traditional alfalfa breeding programs. Even though SFC produce higher yields (Riday and Brummer, 2002), their intermediate performance for most field traits discussed in this paper suggests falcata germplasm may need to be improved before it can be used in commercial semihybrids. However, when mid-subspecies heterosis for agronomic traits was found, it was usually positive. The HS-heterosis values were highly variable and in the case of winter injury, autumn regrowth, and growth habit, changed signs among genotypes. Therefore, careful selection of parents for desirable agronomic traits within heterotic groups is important. The lack of SCA for all field traits except height, vigor, and winter injury indicates that they behave in an additive manner. Additive genetic control should make selection for desirable genotypes easier.

Variation within falcata germplasm is large, but finding individuals with adequate performance levels for various traits is difficult. Currently most falcata germplasm is more decumbent, slower regrowing, and slower maturing (Barnes et al., 1977); identifying falcata germplasm that breaks this trend is desirable. In our study, WISFAL-6 is a good example of a desirable falcata geno-type; it grew erect and was morphologically more similar to sativa than most other falcata. The sativa x WISFAL-6 hybrids were robust, with the dense vegetation and erect growth habit similar to SSC; their only weakness was slower regrowth (Table 2). Currently, the Iowa State University forage breeding program, in a collaborative effort with other research sites, is attempting to improve falcata germplasm (Brummer et al., 1997).

Given the slower regrowth and decumbent nature of falcata germplasm, the sativa–falcata yield heterosis we observed in space plants (Riday and Brummer, 2002) may disappear in a sward planting or under a more intensive cutting regime. Since only five falcata genotypes were used in this study, it would be helpful to expand this study to a larger germplasm sample of falcata to determine if some existing falcata germplasm is better suited to a semihybrid breeding system. Examining a broad range of geographically and morphologically distinct falcata germplasm in sativa–falcata hybrids might aid prediction of heterosis. Developing improved falcata germplasm represents a genetically sound strategy for long term breeding objectives. It would not only increase the genetic base of commercial breeding germplasm, but would also allow the development of heterotic groups.

Advantageous sativa–falcata heterosis is not only seen for forage biomass but is observed for the traits of vigor, winter injury, spring vigor, height, and maturity as well. Fall regrowth is affected negatively in sativa–falcata hybrids. As observed for forage biomass sativa–falcata hybrids outperform their parental subspecies for vigor and winter injury. Although heterosis is observed for spring regrowth, height, and maturity, sativa–falcata hybrid performance still falls in a range between parental subspecies performance. Thus, improving falcata germplasm would create opportunities to utilize fully sativa–falcata heterosis.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Journal Paper No. J-19238 of the Iowa Agric. Home Econ. Exp. Stn., Ames, IA, project No. 2569, supported by Hatch Act and State of Iowa Funds.

Received for publication April 17, 2001.


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


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