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Crop Science 42:716-723 (2002)
© 2002 Crop Science Society of America

CROP BREEDING, GENETICS & CYTOLOGY

Forage Yield Heterosis 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, but yield trends suggest yield has stagnated over the past two decades. Little effort has been invested into capturing heterosis in alfalfa, but semihybrid breeding systems are a possible solutions to overcome forage yield stagnation. Development of alfalfa semihybrids will require identification of heterotic groups. Studies of crosses between dormant M. sativa ssp. sativa and M. sativa ssp. falcata suggest a heterotic pattern exists between the two subspecies. The objective of this study was to measure heterosis in elite sativa x falcata crosses (SFC) in relation to elite 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 Ames and Nashua, IA, and harvested for forage yield twice in 1998 and three times in 1999. A definite sativa–falcata heterotic pattern was observed. Sativa–falcata heterosis was observed at the subspecies, halfsib, and individual cross level calculated using subspecies comparisons, halfsib heterosis analysis, and combining ability analysis. On average, intersubspecific crosses yielded 18% more than the average of intrasubspecific crosses. The sativa–falcata heterotic pattern is a potentially useful resource in alfalfa breeding programs.

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, heterosis on a Halfsib basis


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
ALFALFA REPRESENTS about 2.5% of the total agricultural hectarage in the USA with approximately half of the alfalfa hectarage in the upper Midwest and northern Great Plains (USDA, 2001). Yields of alfalfa remained flat from 1919 until about 1955, increased steadily from about 1955 until about 1982, and then leveled off through 2000 (Fig. 1 ; USDA, 2001). The upper Midwest–northern Great Plains states showed a pattern similar to the U.S. average, except that around 1982 alfalfa yield not only leveled off, but began to decline slightly (Table 1).



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  		Fig. 1. USDA on-farm alfalfa yield (Mg ha-1) from 1919 to 2000 for the USA and upper Midwest and northern Great Plains (USDA, 2001).

 

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  		Table 1. Yearly changes in alfalfa yield during three periods in the 20th century for the USA and the upper Midwest and northern Great plains, determined on the basis of on-farm data (USDA, 2001).

 
Estimates of yield increases in alfalfa range from 0.15 to 0.3% per year (Hill et al., 1988; Holland and Bingham, 1994), but these were calculated on the basis of the superiority in yield of modern varieties relative to older check varieties grown in a common experiment. What appears to be happening is that yields of older varieties are declining, perhaps because of to increased pathogen or pest pressure, while newer varieties are only maintaining their former yield level (Central Alfalfa Improvement Committee, annual data, 1975–1998). One explanation offered for the yield stagnation is that breeding programs have focused on increased pest resistance and other non-yield traits at the expense of breeding for yield. Maintaining or improving many different desirable traits has made concurrent yield improvement difficult (Hill et al., 1988).

Clearly, increasing yield remains an important goal in alfalfa breeding. The current method of alfalfa breeding is almost exclusively based on recurrent phenotypic selection, which involves intercrossing selected parents to produce a synthetic variety (Hill, 1987). Brummer (1999) suggested the use of a semihybrid system to capture natural hybrid vigor found in crosses between certain alfalfa populations. After identifying specific heterotic patterns in alfalfa germplasm, populations could be developed by (reciprocal) recurrent phenotypic selection, and semihybrid varieties could be developed from interpopulation crosses.

The two primary criteria required to manifest heterosis are partial to complete dominance at loci controlling the trait of interest and differing allele frequencies between the two populations to be crossed (Falconer and Mackay, 1996; Hallauer and Miranda, 1988; Woodfield and Bingham, 1995). Breeding complementary, or "heterotic," populations with differing allele frequencies would relieve some of the breeding burden of trying to increase yield along with all other desirable traits. The semihybrid cross, made in a seed production field, would bring together the independently improved populations differing in allele frequencies, thereby increasing yield (Brummer, 1999).

Three hypothetical heterotic groups in alfalfa include: (i) M. sativa subsp. falcata (hereafter, "falcata"), (ii) dormant or moderately dormant M. sativa subsp. sativa (hereafter, "sativa"), and (iii) nondormant sativa (Brummer, 1999). Heterotic patterns may also exist within elite sativa germplasm. However, to identify heterotic groups, crosses between populations need to be made and evaluated in multiple environments.

Some early studies showed heterosis between sativa and falcata germplasm (Westgate, 1910; Waldron, 1920; Sriwatanapongse and Wilsie, 1968). Falcata, yellow flowered alfalfa, tends to be more winterhardy, to have more prostrate growth, and to yield less in the late summer and early autumn when compared with sativa. Westgate (1910) examined M. sativa subsp. varia, which is an intermediate subspecies formed by natural intercrossing of falcata and sativa, and determined that in many cases it outperformed pure sativa or falcata. Waldron (1920) reported 47.5% higher yields in sativa x falcata crosses than in crosses within the parental populations. Sriwatanapongse and Wilsie (1968) showed that crosses of two different sativa cultivars with ‘Kuban,’ a falcata cultivar, each showed heterosis, while the sativa x sativa crosses did not. In all of the above studies, falcata was suggested as a germplasm source to be introgressed into improved sativa populations to increase yields. In addition to intersubspecific crosses, hybrids between diverse sativa germplasm also expressed heterosis in some cases (Yazdi-Samadi and Stanford, 1969; Busbice and Rawlings, 1974; Hill, 1983).

In this study, we tested the hypothesis that heterosis for forage yield would be expressed in crosses between (i) falcata and elite sativa genotypes and (ii) elite sativa genotypes derived from different commercial alfalfa breeding programs.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
Plant Materials
Fourteen genotypes were used as parents in this experiment. The nine elite sativa genotypes included ABI408, ABI311, ABI419, and ABI314 from ABI Alfalfa, Inc. (12351 W. 96 Terrace, Suite 101, Lenexa, KS 66215); C96-514, C96-673, and C96-513 from Forage Genetics (N5292 S. Gills Coulee Road, West Salem, WI 54669); and FW-92-118 and RP-93-377 from Pioneer Hi-bred International (400 Locust Street, Suite 800, P.O. Box 14453, Des Moines, IA 50306). The five falcata genotypes included WISFAL-4 and WISFAL-6 from the semiimproved falcata population, WISFAL (PI560333; Bingham, 1993); C25-6 from a semiimproved falcata population developed in Colorado (PI578248; Townsend, 1995); and two genotypes selected visually for vigor from plant introductions that had been planted in a 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 parental genotypes were crossed in the greenhouse during autumn 1997 in a half diallel mating design, without reciprocals. Florets were hand emasculated to limit accidental self-pollination. In April 1998, seed from the 91 crosses and five check cultivars (Vernal, 5454, Innovator +Z, Ladak, and Legendairy) were planted in the greenhouse. Stem cuttings of the 14 parents were made at the same time. A total of 110 entries were included in this experiment (91 crosses; 14 parental clones; and 5 checks).

Experimental Design
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 a-lattice, with 10 plots in each of 14 incomplete blocks for 560 total plots. At Nashua the design was a quadruple a-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. Two months after transplanting, the seedlings were cut at approximately 7.5 cm above the ground and the forage was discarded. Subsequently, harvests for biomass yield were taken on 18 August and 16 October 1998 in Ames and on 20 August and 20 October in Nashua. Each 10-plant plot was hand-harvested and the total plot biomass was dried for 5 d at 60°C in a forced-air dryer and then weighed. In 1999, harvests were taken on 27 May, 7 July, and 1 September at Ames and on 6 June, 15 July, and 10 September at Nashua. Plots were subsampled by clipping several randomly selected stems from each plant; subsamples were weighed wet, dried for 5 d at 60°C, and then weighed dry. Whole plots were harvested and weighed wet during each harvest and a dry matter yield on a per plant basis was calculated on the basis of the dry matter percentage determined from the subsamples.

Data Analysis
Calculations of Entry Means
Analyses were conducted on dry matter yield at each individual harvest and on total yearly dry matter yield. Replication, blocks, locations, and years were considered random effects. The MIXED procedure of the SAS statistical software package (Littell et al., 1996) was used to calculate least squared means for each entry at Ames and at Nashua in 1998 and in 1999. Differences among entries, calculated using the entry least square means, were determined by year, by location, and across the four year x location combinations by the GLM procedure of the SAS (SAS Institute, 2000). Homogeneous variances were assumed among different entries.

Combining Ability Analysis
General (GCA) and specific combining ability (SCA) were calculated with 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, 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 using 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 per se 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 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. The HS-heterosis values were determined as follows (see example, Fig. 2) :

[1]
where, i = parental genotype, i = 1 to 14; SFHS = the half-sib family performance of parent i crossed with genotypes from the other subspecies; SS = if parent i is a sativa, then SS = within-subspecies halfsib mean; if parent i is a falcata, then SS = SSC mean; FF = if parent i is a falcata, then FF = within-subspecies halfsib mean; if parent i is a sativa, then FF = FFC mean.



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  		Fig. 2. Example of a halfsib heterosis (HS-heterosis) calculation for genotype ABI408. HS-heterosis is equal to the mean of ABI408 x falcata crosses (SFHSABI408) minus the mean of ABI408 crossed to other sativa genotypes (WHSABI408) and the falcata x falcata cross mean (FFCmean). This difference is divided by the mean of WHSABI408 and FFCmean.

 
HS-heterosis for yield of each individual harvest and for total yearly yield was calculated for each genotype–location–year combination. Differences among entries for HS-heterosis values were analyzed as fixed effects models by PROC GLM (SAS Institute, 2000).

Stability Analysis of Halfsib Heterosis
To assess the variability of HS-heterosis for parental genotype i over locations and harvests, we estimated HS-heterosis stability variance of genotype x harvest , genotype x location , genotype x location x harvest , and genotype x environment (where a environment is a harvest–location combination) components for each parent. HS-heterosis stability variance components were calculated by extending Shukla's (1972) stability variance . Since {sigma}2GL + {sigma}2GH + {sigma}2GLH {approx} {sigma}2GE, we assume {sigma}2iL + {sigma}2iH + {sigma}2iLH {approx} {sigma}2iE. Using this result, we decomposed {sigma}2iE into {sigma}2iL, {sigma}2iH, and {sigma}2iLH for each genotype.

Significance of all results was assessed at the 5% probability level unless noted otherwise.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
Parental Performance vs. Progeny Performance
The hybrid progeny yielded from 7 to 148% higher than the mid-parent (data not shown). All three cross types exhibited mid-parent heterosis, but the SFC had the highest values (SFC = 73%; SSC = 52%; and FFC = 43%). The performance of the parents in the field may have been depressed as a result of clonal propagation, thereby producing unreasonably high levels of heterosis. Because of this possible confounding factor, we used comparisons among hybrid progeny to assess heterosis.

Subspecies Analysis
Sativa x falcata crosses produced more total yield than either SSC or FFC across years and locations, in each location, and in each year (Table 2). In all environments, SSC had higher yields than FFC except at Nashua, where FFC and SSC were equivalent. Yield of the three cross types did not interact with locations or years (data not shown). All three cross types yielded more at Ames than at Nashua reflecting an environmental pattern also observed in statewide variety trials (Brummer and Smith, 2000). As expected, yields were higher in 1999 than during the establishment year of 1998. Analysis of yield at individual harvests reflected the overall trend of SFC outperforming both SSC and FFC. The only exception to this pattern was the July 1999 harvest, in which SSC produced more yield than SFC (Table 2). This was the harvest with the shortest regrowth period, perhaps reflecting a weakness of SFC for recovery ability (Riday, 2001). No differences were found in any environment for comparisons of within-company and between-company SSC (data not shown).


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  		Table 2. Mean alfalfa dry matter yield for crosses between and within M. sativa subsp. sativa and subsp. falcata, mid-subspecies heterosis (MS-heterosis), and general and specific combining ability (GCA and SCA), at two Iowa locations in 1998 and 1999.

 
Overall mid-subspecies heterosis was 18% above the mid-subspecies mean with a range from 11 to 20% across years, locations, and harvests (Table 2). The consistency of mid-subspecies heterosis despite fluctuation in intrasubspecific cross means indicates the presence of a sativa–falcata heterotic pattern.

On the basis of progeny performance, we were able to separate additive yield effects (GCA) from non-additive yield effects (SCA) (Griffing, 1956). General combining ability and SCA were present for locations, years, and across all harvests, except for the July 1999 harvest (Table 2). To visualize SCA deviations, we plotted the expected yield of each cross against its observed yield (Fig. 3) . Only SFC showed significant SCA effects. Further, most SFC tended to fall above the expected yield line, clearly indicating that crossing falcata and sativa produces a positive heterotic response for forage yield. By contrast, observed yields of SSC tended to fall near their expectation, and importantly none of them showed significant SCA effects. The FFC showed the most variation in yield relative to expectations, with one-half performing worse than expected.



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  		Fig. 3. Observed versus expected experimental mean alfalfa dry matter yield (g plant-1), across two Iowa locations and 2 yr for sativa x sativa crosses, sativa x falcata crosses, and falcata x falcata crosses.

 
Halfsib Analysis
To examine heterosis of individual genotypes, we compared the within-subspecies halfsib mean to the sativa x falcata halfsib mean of each genotype. The combined analysis over locations and years showed that sativa x falcata halfsib means were higher than the within-subspecies halfsib means for most genotypes, the only exceptions being three of the four ABI genotypes (Table 3). High parent heterosis on a halfsib basis was seen for all genotypes when the sativa x falcata halfsib mean outperformed the within-subspecies halfsib mean, except for genotype C25-6. The stronger performance of sativa x falcata halfsib means compared with their within-subspecies halfsib means was generally observed in 1999 and at Nashua, but less often in 1998 or at Ames, and high parent heterosis on a halfsib basis mirrored that result.


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  		Table 3. Inter (SFHS) and Intra-subspecific (WHS) halfsib family dry matter yield means for fourteen alfalfa genotypes measured in two Iowa locations and 2 yr, 1998 and 1999.

 
The August and October 1998 harvests closely reflected the results seen for total yield in 1998, and the May 1999 harvest closely reflects the overall pattern (Table 3). Given the high yield of the first harvest in the first full production year, this is not surprising. However, no high parent heterosis on a halfsib basis was observed in the July 1999 harvest. September had a somewhat similar pattern to October 1998, with sativa x falcata halfsib means superior to their within-subspecies halfsib means for six genotypes, of which few exhibited high parent heterosis on a halfsib basis (Table 3). No genotypes had sativa x falcata halfsib means that were inferior to their within-subspecies halfsib means in any environment, except for sativa genotypes in July 1999. Low parent negative heterosis was never observed in any genotype or environment.

Halfsib Heterosis
Overall, HS-heterosis values ranged from 2.3% (ABI419; not significantly different from 0%) to 29.4% (PI502453-1), with an average of 18.4% (Table 4). Significant location, genotype x location, and genotype x year HS-heterosis effects were observed. The genotype x location interactions were primarily due to magnitude differences between the two locations, and the few changes in rank among genotypes did not affect the interpretation of the results. All genotypes except ABI419 and ABI314 had positive HS-heterosis effects in both Ames and Nashua. Some genotypes expressed HS-heterosis only in one year (C25-6, ABI314) or only during certain harvests (ABI311, ABI419, C96-514, WISFAL-4, WISFAL-6, C25-6, PI214218-1). Among harvests, average HS-heterosis was lowest for the July 1999 harvest (12.4%) and this number is high only because of the very poor performance of FFC. However, HS-heterosis was greater than 19% for October 1998, May 1999, and September 1999 (Table 4).


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  		Table 4. Halfsib heterosis (HS-heterosis) of dry matter yield for fourteen alfalfa genotypes grown across two Iowa locations and 2 yr.

 
Stability Analysis
Genotypes ABI314, ABI311, C25-6 had the largest variances indicating that they were less stable than the other genotypes (Table 5). Location affected ABI314 most, while harvest was the major influence on ABI311. Of the 14 genotypes, WISFAL-6 was most affected by the harvest x location interactions (Table 5). Most of the {sigma}2iE for ABI408, ABI311, ABI419, and C96-513 was due to harvest variation (Table 5). Nearly equal proportions of {sigma}2GH, {sigma}2GL, and {sigma}2GLH contributed to the mean {sigma}2iE (i.e., {sigma}2GE).


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  		Table 5. Stability variance estimates and proportions of total environmental stability variance for each genotype that are due to harvest, location, and location x harvest effects for halfsib heterosis (HS-heterosis) of dry matter yield of 14 alfalfa genotypes across ten harvests taken over 2 yr and two Iowa locations.

 
On the basis of stability analysis of HS-heterosis, no single genetic or environmental effects emerged that would help determine what factors contribute to consistently stable heterosis for the sativa–falcata heterotic pattern. Developing semihybrid (sativa x falcata) cultivars that show consistent heterosis across environments would require extensive environmental testing. This analysis supports the notion that predicting heterotic performance is difficult since heterosis is the combination of many different factors that affect different genotypes in different ways.

Sativa–Falcata Heterotic Pattern
Using several analysis methods, we have clearly demonstrated that M. sativa subsp. sativa and subsp. falcata represent distinct heterotic groups. The average heterosis value was 18%, though there was variation among genotypes and environments. High parent heterosis on a halfsib basis was frequently observed and HS-heterosis values remained consistently positive across environments, the July 1999 harvest being an exception. In that harvest, SFC were intermediate to SSC and FFC. Therefore, we concluded that the expression of heterosis was due to harvest timing and not due to stronger parents carrying weaker parents. The May 1999 harvest showed higher heterosis than summer or fall harvests.

Several genotypes failed to follow the overall heterotic pattern. In particular, ABI311, ABI419, and C25-6 lacked heterosis in most environments and were never parents of SFC with significant SCA. We suggest that the ABI germplasm used in this study could have more falcata introgression, at least at loci responsible for dry matter yield, than the Forage Genetics or Pioneer Hi-bred International genotypes, which would lower or eliminate the sativa–falcata heterotic pattern. C25-6 is from a very broad based, semiimproved population derived from falcata plant introductions whose germplasm background is unknown (Townsend, 1995). Interestingly, PI214218-1 had yellow flowers with a green tinge, indicating a variegated background, and it had among the lowest HS-heterosis values of the remaining genotypes.

Basis of Heterosis
Genetically, heterosis is thought primarily to be the result of accumulating dominant (or partially dominant) alleles that are in repulsion at (linked) loci (Hallauer and Miranda, 1988). Thus, one parent contributes beneficial alleles at some of the loci, which the other parent "complements" by providing desirable alleles at the remaining loci. The presence of at least some of these loci in tightly linked blocks or linkats (Demarly, 1979) accounts for the difficulty of accumulating all the favorable alleles in a single plant, particularly if many loci are contributing to the trait of interest. In tetraploid alfalfa, the linkat concept was expanded by Bingham et al. (1994) and Woodfield and Bingham (1995).

Traditionally, inbreeding depression has been considered the opposite of heterosis (Falconer and Mackay, 1996). The causes of inbreeding depression are not completely known; however, some is due to major deleterious alleles that are exposed as self-fertilization forces loci towards homozygosity (Lynch and Walsh, 1998; Willis, 1999). Reversing this process allows the masking of these alleles and restores vigor.

Although the phenotypic effects of heterosis are striking, the mechanisms of restoring vigor and thereby increasing yield are unclear. What the alleles–loci actually do is as yet unresolved. Presumably, the hybrid's complementary sets of alleles inherited from its parents provide for better carbon acquisition, translocation, and/or storage by affecting physiological processes and/or morphological development. Heterosis could occur in the progeny of morphologically similar plants. In this case, the hybrid combination of alleles increases the size and/or number of the organs contributing to yield. A second type of heterosis can also be envisioned, that of crossing two plants with different morphologies. If the loci controlling the morphologies are partially to completely dominant and each affects yield positively, then the morphologically different contribution of each parent in the hybrid may create higher yielding progeny by combining morphologies.

In this study we observed heterosis in sativa–falcata crosses between genotypes with a variety of morphologies. Visual observation suggested that the thicker stems of sativa combined with increased stem numbers of falcata to produce a sativa–falcata hybrid with many thick stems, and hence, high yield. Examining relationships between yield heterosis data, morphological trait data, and molecular marker data may provide a partial understanding of the basis of the sativa–falcata heterotic pattern (Riday, 2001). We observed a moderate correlation (r = 0.50) between SCA and the average morphological differences between parental genotypes, suggesting that the expression of sativa–falcata heterosis is caused by both differential and non-differential morphologies between parents (Riday, 2001).

Further introgression of SFC into elite populations or crossing among SFC to develop populations for subsequent selection should be avoided to maintain the sativa–falcata heterotic pattern. Since the subspecies themselves already offer a means to capture high yield, they should not be mixed in breeding populations until methods are available to select the desirable alleles on a genome-wide basis. Combining all germplasm into one population would theoretically allow selection of individual genotypes with superior yield alleles at all loci. However, if many loci are involved in yield, increasing favorable allele frequencies at all or most loci simultaneously would be very difficult. Improving separate populations that produce favorable allele combinations in their offspring, when crossed together, would appear to be the most successful approach at the current time (Brummer, 1999).

Some future questions remain. First, this experiment was conducted on spaced plants in a three cut system. Future research needs to examine whether similar results would be obtained in a broadcast seeded situtation and/or under different harvest managements. Foster (1971a)(b) demonstrated in ryegrass (Lolium perenne L.) that population hybrids grown in swards still showed similar yield increases observed in space plants. Second, moving to a more frequent harvest regime may alter the benefits of sativa–falcata crosses from currently available falcata germplasm, since half of the sativa parental genotypes had within-subspecies halfsib means that outperformed sativa by falcata halfsib means in the July 1999 harvest, which had a short regrowth interval. Third, the sativa–falcata heterotic pattern was evaluated only in the establishment and first production year. Whether or not this pattern will remain after several years needs to be determined. Fourth, only 14 parents were used in our study, so the universality of our results to other sativa–falcata combinations is unclear. Fifth, if forage quality is adversely affected this may negate any yield gains.

Finally, a major obstacle to successful semihybrid production is developing elite falcata germplasm that can be crossed with sativa germplasm and reliably produce heterosis. Currently there are few semiimproved pure falcata populations available. Long-term work is needed to breed falcata germplasm to acceptable levels. With heterosis values around 20%, this effort may be justified.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Journal Paper No. J-19239 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 March 23, 2001.


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




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H. Riday and E. C. Brummer
Heterosis in a Broad Range of Alfalfa Germplasm
Crop Sci., January 1, 2005; 45(1): 8 - 17.
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M. D. Casler, M. Diaby, and C. Stendal
Heterosis and Inbreeding Depression for Forage Yield and Fiber Concentration in Smooth Bromegrass
Crop Sci., January 1, 2005; 45(1): 44 - 50.
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E. C. Brummer
Applying Genomics to Alfalfa Breeding Programs
Crop Sci., November 1, 2004; 44(6): 1904 - 1907.
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H. Riday and E. C. Brummer
Heterosis of Agronomic Traits in Alfalfa
Crop Sci., July 1, 2002; 42(4): 1081 - 1087.
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H. Riday, E. C. Brummer, and K. J. Moore
Heterosis of Forage Quality in Alfalfa
Crop Sci., July 1, 2002; 42(4): 1088 - 1093.
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The SCI Journals Agronomy Journal Vadose Zone Journal
Journal of Plant Registrations Soil Science Society of America Journal
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