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Published online 1 March 2007
Published in Crop Sci 47:665-671 (2007)
© 2007 Crop Science Society of America
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CROP BREEDING & GENETICS

Combining Abilities and Heterosis for Forage Yield among High-Yielding Accessions of the Alfalfa Core Collection

H. S. Bhandari, C. A. Pierce, L. W. Murray and I. M. Ray*

H.S. Bhandari, C.A. Pierce, and I.M. Ray, Dep. of Plant and Environmental Sciences, New Mexico State Univ., Las Cruces, NM 88003; L.W. Murray, University Statistics Center, New Mexico State Univ., Las Cruces, NM 88003

* Corresponding author (iaray{at}nmsu.edu).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Characterization of genetic parameters in plant germplasm repository populations can facilitate their utilization in commercial breeding programs. This study investigated genetic combining abilities and heterosis for forage yield among diallel hybrids derived from nine previously selected high-yielding accessions of the USDA-ARS National Plant Germplasm System core collection of alfalfa (Medicago sativa subsp. sativa L.). These accessions originated from Turkey, South Africa, Greece, Afghanistan, Israel, Uzbekistan, Mexico, Morocco, and the USA. Forage yield response of the parents and their hybrids was determined near Las Cruces, New Mexico. General combining ability (GCA) and specific combining ability (SCA) effects were significant. The magnitudes of the GCA effects were about 5% (<0.71 Mg ha–1) relative to the mean yield of the parents (15.4 Mg ha–1). The Turkey, Afghanistan, and Uzbekistan accessions demonstrated significant positive GCA effects. Accessions from Greece and Morocco demonstrated negative GCA effects. Six hybrids outperformed the average of six commercial cultivars. Five of these six hybrids demonstrated positive SCA effects. Midparent heterosis ranged from –17 to 17%. The results indicate that crossing between alfalfa populations possessing high per se performance and different fall regrowth responses is likely to produce a higher proportion of high-yielding hybrids in environments experiencing intermittent subfreezing winter conditions.

Abbreviations: GCA, general combining ability • GRIN, Germplasm Resources Information Network • HPH, high-parent heterosis • LPSRC, Leyendecker Plant Science Research Center • MPH, midparent heterosis • NPGS, USDA-ARS National Plant Germplasm System • PI, plant introductions • SCA, specific combining ability.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
ALFALFA BREEDING programs have made excellent progress in improving multiple pest resistance as a mechanism to realize yield potential (Kehr et al., 1972; Sorensen et al., 1972; Lamb et al., 2006). However, less progress has been made in genetically improving forage yield per se. Evaluation of forage yields of populations representing different eras (1898–1985) of breeding in the USA showed an annual increase of 0.18% (Holland and Bingham, 1994). This is very low compared to the yield gains made in other crops during the same period (Hill et al., 1988; Duvick and Cassman, 1999). It has been proposed that increased emphasis on improving pest resistance, and less emphasis on improving yield per se, in many alfalfa breeding programs might have been responsible for the slow rate of gain in forage yield improvement (Barnes et al., 1977; Hill and Elgin 1981; Hill, 1983).

Genetic improvement of alfalfa forage yield in the USA before the 1950s appears to have resulted from accumulation of favorable alleles (Holland and Bingham, 1994). Improvement after 1950 resulted primarily from exploitation of nonadditive types of gene action including heterosis, which likely reflects an increased emphasis on intermating between multiple alfalfa germplasms as a means to introgress resistance to multiple pests (Barnes et al., 1977). Early studies in alfalfa documented significant amounts of hybrid vigor for forage yield (Tysdal and Kiesselbach, 1944; Sriwatanapongse and Wilsie, 1968). Subsequent research has demonstrated the potential of exploiting both additive and nonadditive gene effects, including higher-order allelic and complementary gene interactions associated with autotetraploid inheritance, as a means to improve alfalfa production (Busbice and Rawlings, 1974; Hill and Elgin, 1981; Bingham et al., 1994; Woodfield and Bingham, 1995).

The importance of GCA and SCA in determining forage yield in alfalfa has been documented. Several studies reported a significant amount of GCA contributing to genetic variation of forage yield, but SCA was not significant (Dudley et al., 1969; Hill 1983; Groose et al., 1988). It was suggested that the inability to detect SCA effects might have been influenced by the use of half-sibs as parents instead of clones in some experiments (Hill, 1983). Moreover, detection of SCA effects is more difficult in autotetraploids than in diploids (Dudley et al., 1969). Busbice and Rawlings (1974), Riday and Brummer (2002), and Segovia-Lerma et al. (2004), however, reported significant SCA effects in crosses between diverse genotypes and populations of alfalfa. The parent germplasms used in those studies included nine primary germplasm contributors to contemporary North American cultivars, as well as other M. sativa subsp. sativa and subsp. falcata populations. Segovia-Lerma et al. (2004) observed that SCA effects were usually not detected in studies that used related or highly recombined parent populations to generate hybrids. Rather, SCA effects were primarily detected when divergent and genetically distinct populations were used to produce hybrids. To optimize heterotic response in alfalfa, they suggested that breeders need to generate hybrids between genetically distinct populations, many of which likely reside outside of conventional breeding populations.

Parent populations/genotypes used in studies that have documented SCA effects for alfalfa forage yield possessed a wide range of yield potentials and fall dormancy response. Physiological dormancy in alfalfa is an adaptation response resulting in decreased vegetative biomass production that occurs in response to declining photoperiod and temperature (McKenzie et al., 1988). It is used by some plants to survive winter conditions. With such diverse sets of parents, it should be relatively easy to detect significant amounts of GCA and SCA. Most breeders working in plant genetic improvement, however, prefer to utilize superior parents such as high-yielding germplasms or advanced populations.

A large number of alfalfa germplasms have been collected worldwide and are stored in germplasm repositories, such as the USDA-ARS National Plant Germplasm System (NPGS). Some of these populations likely have unique genetic potentials that could contribute toward improving alfalfa forage yield (Maureira et al., 2004). In a 4-yr replicated field study that evaluated forage yield potential of the 198 accessions of the NPGS alfalfa core collection in southern New Mexico, several germplasms were identified that yielded similarly to some commercial cultivars (I. M. Ray, unpublished data, 1998–2001). These germplasms also demonstrated differing levels of fall dormancy. We selected the nine highest-yielding accessions as candidates in the current study for evaluating the contribution of additive and nonadditive gene effects for improving forage yield in alfalfa.

We hypothesized that these high-yielding plant introduction accessions would produce hybrids with significant yield heterosis. Therefore, the objective of this study was to test this hypothesis by evaluating population-based diallel hybrids derived by intercrossing selected germplasms. The specific objectives were (i) to estimate genetic parameters associated with forage yield, (ii) to evaluate diallel hybrids for forage yield heterosis response, and (iii) to demonstrate the potential of using core collection accessions to improve alfalfa forage yield.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Diallel crosses involving selected populations of nine alfalfa core collection plant introductions (PIs) (PI 170532, PI 199279, PI 206574, PI 212104, PI 262544, PI 403954, PI 422246, PI 467910, and PI 516895) were evaluated in this study. These germplasms were introduced to the USA between 1948 and 1983 and represent a broad range of geographic origins and differing levels of improvement status. For simplicity they have been referred to in this article by the abbreviated names of their country of origin (Table 1). These nine germplasms were selected for the current study based on previous research that demonstrated that they were the top-yielding germplasms from among 198 accessions of the alfalfa core collection that were evaluated during 1998 through 2001 at the Leyendecker Plant Science Research Center (LPSRC), near Las Cruces, New Mexico (I. M. Ray, unpublished data). This study environment frequently experiences nighttime temperatures below 0°C during November through March. Therefore, information for three crop descriptors (fall regrowth, frost damage, and winter injury) from the Germplasm Resources Information Network (GRIN) database was included in Table 1 to provide additional insight into the winter survival potential of each population. These three descriptors reflect data collected on field plots of >1000 alfalfa accessions that were planted at Rosemont, MN, in spring 1989 (www.ars-grin.gov/cgi-bin/npgs/html). According to the fall regrowth descriptor from the GRIN database, the nine germplasms used in our study would conventionally be classified as semidormant to nondormant.


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  		Table 1. Origin, year of introduction to the USA, improvement status, fall dormancy, winter injury, and frost damage descriptors of nine alfalfa plant introductions (PI).

 
Each of the nine germplasms used for our diallel analysis was represented by 12 to 15 genotypes. These Cycle 1 genotypes were selected visually for overall plant vigor in 2001 from the 4-yr-old Cycle 0 field plots of the previously mentioned yield trial. Selected genotypes were dug from the field nursery in August 2001 and transferred to the greenhouse. Each genotype from each germplasm was randomly intermated by hand with each of the other eight germplasms to generate 36 F1 populations. For each cross combination, a total of 60 racemes, 30 racemes from each parent germplasm, were crossed. An equal number of F1 seeds, from each of the two parent populations involved in a cross, were mixed to form each balanced F1 composite. Seeds representing each Cycle 1 parent population were generated by randomly intercrossing all the selected genotypes from within a given population.

The 36 F1 crosses, their nine selected parent populations (Cycle 1), and the corresponding nine original populations (i.e., unselected, Cycle 0), were planted at the LPSRC in fall 2002 and evaluated for forage yield during 2003 and 2004. The original seed lots of the nine progenitor populations (Cycle 0) were obtained from the NPGS, Western Regional Plant Introduction Station, Pullman, WA. They were included in the study to measure the selection progress on mean forage yield for the selected parent germplasms (Cycle 1, hereafter referred to as parents and/or varieties). The results of selection progress in the parent populations, however, will be reported elsewhere. Six check cultivars and advanced experimental lines were also included for comparison of parent and hybrid performance relative to elite varieties. The experiment was planted during September 2002 using a randomized complete block design with three blocks. Each population was planted in a three-row plot, 1.5 m long, with rows spaced 30 cm apart and plots spaced 60 cm apart. Three hundred seeds (100 seeds per row) were planted to each plot. The experimental field was irrigated every 14 d from April through October each year. Before planting, fertilizer was applied at the rate of 25:115:0 kg ha–1 (N:P2O5:K2O, respectively), and 45 Mg ha–1of cow manure was incorporated into the soil during field preparation.

Forage was harvested four times during 2003 (May 14, June 6, July 9, and August 14), and three times during 2004 (May 4, June 15, and July 7). Forage yield data were collected using a flail harvester that cut at a 5-cm height. Fresh forage samples (minimum of 300 g fresh weight) were randomly collected from 10% of the plots at each harvest, dried at 60°C for 48 h, and used to adjust the yield data to a dry matter basis. During the June harvest in 2003, it was observed that many plots demonstrated delayed regrowth in one of the rows. The affected rows consistently occurred on the right side of the forage harvester indicating that some kind of harvester implement effect (most likely a wheel track effect) occurred during the previous May harvest. This was reflected by an inflated coefficient of variation in the June 2003 harvest data. Therefore, the June 2003 data were excluded from the analysis. There was no notable difference among the rows of the affected plots during the subsequent harvests. In 2004, an extended period of rain occurred immediately after the first irrigation in April, 2 wk before the first harvest. This resulted in soil water logging, which was manifested in a significant delay in forage regrowth in the plots following the first harvest. The severity of the impact on forage regrowth was scored 3 wk after the first harvest in 2004 and was used as a covariate to adjust all yield data collected during 2004. Consequently, data from the May, July, and August 2003 harvests and the May, June, and July 2004 harvests were used for the combined analysis.

Data Analysis
Data were adjusted for minor field trend effects using nearest neighbor analysis following an adjacent residual approach in AGROBASE software (AGRONOMIX, Inc., Portage La Prairie NB, Canada). Corrected seasonal total forage yield data (i.e., the sum of the three harvests from each year) were analyzed with the MIXED procedure in SAS release 8.2 (SAS Institute, Cary, NC) using entries and years as fixed effects and blocks as random effects. Entry was the main plot and year was the split plot factor. Before conducting the diallel analysis, data from the check cultivars and original Cycle 0 populations were excluded. The data of the Cycle 1 parents and their hybrids were then subjected to a variety diallel analysis following analysis II and III of Gardner and Eberhart (1966) to detect the effects of parents/varieties and heterosis. The analysis II heterosis effect was further partitioned into average heterosis, variety heterosis, and specific heterosis. Analysis III (Gardner and Eberhart, 1966; Murray et al., 2003) was used to detect the effects of parents/varieties, crosses, and varieties versus crosses. Variation among crosses/hybrids was partitioned into GCA and SCA. Mean squares for these components were obtained using GLM procedures in SAS. Significance of the effects of different genetic parameters as well as the differences between the entries were assessed at the 5% level.

As previously reported by Murray et al. (2003), analysis II variety effects are confounded with GCA effects. Also, the estimates for average heterosis and specific heterosis for analysis II are identical to the varieties versus cross effect and SCA effect, respectively, of analysis III. Therefore, the discussion of analysis II results will be limited to heterosis and variety heterosis only, which cannot be obtained from analysis III. Effects of different components of variation were tested using the "general linear hypothesis" approach as described by Murray et al. (2003). The mathematical formulas to estimate diallel component effects for parents, heterosis, varieties versus crosses (i.e., average heterosis), variety heterosis, GCA, and SCA have been described in detail (Murray et al., 2003; Segovia-Lerma et al., 2004). These effects were obtained by estimate statements with appropriate contrasts constructed in the MIXED procedure in SAS. The contrasts were constructed as a linear function of the sample means,

Formula
where c is the contrast coefficient subject to the constraint {Sigma}cj = 0, and yj is the mean of the jth population (i.e., parent or cross) or of the jth population x year combination, and t is the number of populations in the contrast.

Midparent heterosis (MPH), which describes the performance of crosses relative to the average performance of their parents, and high-parent heterosis (HPH), which describes the performance of crosses relative to the highest-yielding parent, were computed as follows:

Formula
where F1 is the performance of the hybrid population obtained from crossing parents P1 and P2, and HP is the performance of the highest-yielding parent for a given cross. The absolute MPH and HPH effects were tested using the following t statistic,

Formula
where L represents the contrast estimates for the effect of absolute heterosis combined over years, MSE is the mean square error for the main plot (mean square block x entries), r denotes the number of blocks, k denotes the number of years, and {sum}j=1tCj2 is the sum of squares of the contrast coefficients for the corresponding heterosis effects.

Forage yields of the hybrids and their parent populations were also compared with the average performance of the commercial checks using contrast statements in the MIXED procedure of SAS that ignored the diallel arrangement.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Performance of the Parent Germplasms and Their Hybrids
Parent populations differed significantly in mean forage dry matter yield (Table 2), which ranged from 12.7 Mg ha–1 (MEX) to 17.2 Mg ha–1 (AFG). The yield of AFG was significantly higher than the average yield of the check cultivars. The yields of SAF, UZB, TRK, GRC, and USA were comparable to the average yield of the checks. The yields of ISR, MEX, and MRC were significantly below the average yield of the checks.


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  		Table 2. Forage dry matter yield (Mg ha–1) of parent germplasms (diagonal, underlined), and their diallel hybrids (above diagonal) across 2 yr (2003, 2004) and six harvests.

 
Among the 36 F1 populations, UZB x USA, GRC x UZB, SAF x AFG, TRK x AFG, TRK x USA, and AFG x MEX had forage yields greater than the average of the checks (Table 2). These six crosses and one additional hybrid, ISR x UZB, performed similarly to the best check cultivar. Among these seven hybrids, three crosses involved UZB and three involved AFG as one of the parents, suggesting that these two germplasms may have unique genetic potential for our study environment.

Genetic Components of Variation
Analysis of variance averaging over years according to analysis III (Gardner and Eberhart, 1966; Murray et al., 2003) demonstrated that the effects due to varieties and crosses significantly contributed to variation in forage dry matter yield (Table 3). Varieties versus crosses (equivalent to average heterosis) was not significant. The variation among crosses was attributed to both GCA (2004, and averaging over years) and SCA (both years). A heterosis effect (analysis II) was present averaging over years, attributable to both variety heterosis and specific heterosis. When data were analyzed within each year according to analysis II, the only component contributing to the variability in forage yield heterosis was specific heterosis (p ≤ 0.05).


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  		Table 3. Mean squares for dry matter yield (Mg ha–1) from a diallel involving nine alfalfa germplasms within and across 2 yr according to analysis III of Eberhart and Gardner (1966).

 
Significant interaction between entries and years was detected (Table 3). However, the magnitude of this mean square was about one-third of that due to the entry main effect. The entry x year interaction was not attributed to the variation in parent performance or average hybrid performance in 2003 and 2004, because both variety x year and GCA x year were not significant. Rather, the entry x year interaction appeared to be associated with differential performance of individual hybrids in 2003 and 2004, as indicated by the significant SCA x year effect. This interaction was largely attributed to the MEX x MRC hybrid, which demonstrated the highest reduction in yield over time (Table 4). When this hybrid was excluded from the analysis, the entry x year component of variation was not significant (data not shown). Both parents of this hybrid were relatively nondormant and highly susceptible to frost damage and winter injury, according to the GRIN database (Table 1). This hybrid may have suffered injury during winter 2003/2004 and subsequent yield reduction in 2004. Two other hybrids, GRC x AFG and GRC x MRC, also showed a relatively large negative rank change from 2003 to 2004. This may reflect that the GRC population was one of the poorest combiners in the study, with five of its eight hybrids being among the lowest-yielding hybrids in 2004. The AFG x MRC hybrid demonstrated a large positive change in rank from 2003 to 2004. A similar, although less dramatic, response was observed in the AFG x MEX hybrid. Both hybrids involved a nondormant parent and may have improved in their relative performance over time as a consequence of the AFG parent contributing adequate levels of winter hardiness to each hybrid. Each of the six highest-ranking crosses (UZB x USA, GRC x UZB, SAF x AFG, TRK x AFG, TRK x USA, and AFG x MEX) also possessed a parent (either USA, GRC, or AFG) that demonstrated relatively low fall regrowth and greater tolerance to winter injury and frost damage (Table 1). These results indicate that including a relatively winter-hardy parent could be of advantage in providing hybrid yield stability over time in evaluation environments similar to ours, which do not experience extremely harsh winter conditions per se, but do routinely experience subfreezing nighttime temperatures.


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  		Table 4. Harvestwise forage dry matter yields (Mg ha–1) of nine alfalfa germplasms and their diallel hybrids.

 
Highly significant GCA and SCA effects suggested that both additive and nonadditive effects were important in determining forage yield in alfalfa. This implies that forage yield improvement can be achieved by accumulating favorable alleles and exploiting heterosis effects. The mean square due to GCA was nearly twofold higher compared to that for SCA, suggesting a relative greater importance of additive effects over nonadditive effects (Table 3). The predominance of GCA in determining forage yield of alfalfa has been reported elsewhere (Tysdal and Kiesselbach, 1944; Song and Walton, 1975; Hill, 1983; Segovia-Lerma et al., 2004). In studies where significant SCA was detected, its mean square ranged from 33-fold less to threefold greater than GCA (Busbice and Rawlings, 1974; Song and Walton, 1975; Riday and Brummer, 2002; Segovia-Lerma et al., 2004).

Estimation of Genetic Effects
Results from analysis III revealed that AFG, SAF, UZB, and TRK demonstrated significant positive variety effects (Table 5). The overall performance of MEX and MRC was poor. The GCA effects of the parents averaged over 2 yr ranged from –0.8 Mg ha–1 for MRC to 0.7 Mg ha–1 for TRK. Three parents, TRK, UZB, and AFG, demonstrated positive GCA effects, while those for MRC and GRC were negative. Busbice and Rawlings (1974) also observed that accessions from Afghanistan demonstrated significant GCA for forage yield. As previously discussed, differences among alfalfa genotypes and populations for GCA effects have been reported. Of those studies, the one which was most similar to ours in terms of the type of population evaluated was the study of Segovia-Lerma et al. (2004). The estimates of GCA effects in our study were much smaller in magnitude compared to their results. This likely reflects that the parent populations evaluated by Segovia-Lerma et al. (2004) possessed a much wider yield range and fall dormancy response. For example, one of their parents was M. sativa subsp. falcata, which had very poor per se performance, as did its diallel hybrids. The smaller range of GCA was not unexpected in our study because the selected high-yielding parents possessed a much narrower forage yield range.


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  		Table 5. Estimates for effects of varieties (v), average heterosis (h), general combining ability (GCA), specific combining ability (SCA), and their respective standard errors (in parentheses), for forage dry matter yield (Mg ha–1) in a diallel among nine alfalfa germplasms.

 
Five of the 36 hybrids, GRC x UZB, UZB x USA, AFG x MEX, SAF x AFG, and TRK x USA, demonstrated positive SCA effects (Table 5). These five hybrids were among the six highest-yielding crosses in the whole experiment. The single high-ranking hybrid without a significant SCA effect was derived from the high-yielding TRK and AFG parents, which had positive GCA effects. Forage yields of these six hybrids were superior to the average yield of the six check cultivars. Four other hybrids, AFG x UZB, GRC x USA, GRC x MEX, and SAF x ISR had negative SCA effects and were among the seven lowest-ranking hybrids in the experiment.

Average heterosis was not significant (Table 3), indicating that the average yield of all the crosses (mean = 15.8 Mg ha–1; range = 13.6 to 17.9 Mg ha–1) did not differ from the average yield of the parents (mean = 15.4 Mg ha–1; range = 12.7 to 17.2 Mg ha–1). This was also reflected by the mean forage yields of 27 hybrids that did not show any deviation from their respective midparent values (Table 6). This supports the preponderance of additive types of gene action described above. These results could indicate either a state of genetic linkage equilibrium among the germplasms (less likely), or the distribution of equal frequencies of favorable and unfavorable dominant alleles among the nine parents. The later scenario is supported by the presence of both positive MPH in seven hybrids and negative MPH in two hybrids. Moreover, MPH (%) across harvests changed differently for different hybrids (Table 7), attributable to changes in parental performance over time or differences in hybrid stability. Four of the hybrids demonstrating significant positive MPH, AFG x MEX, UZB x USA, GRC x UZB, and TRK x USA, were among the seven hybrids that were comparable to the best check. Each of these hybrids was derived from crosses between parents that possessed different fall regrowth capabilities. These hybrids consistently involved one relatively dormant parent, and at least one parent demonstrated high per se performance. Two other high-yielding hybrids that did not demonstrate significant MPH (TRK x AFG and SAF x AFG) were derived from parents that possessed the same characteristics as described above. Their mean forage yields were similar to that of their highest-yielding parent. The three remaining hybrids that demonstrated positive MPH involved MEX as a parent. One of these, MEX x MRC, was also the only hybrid with positive HPH. Heterotic behavior of MEX was detected in analysis II, which indicated that MEX was the only population demonstrating a significant variety heterosis effect (Table 5). None of these later three MEX hybrids were exceptional, and their high MPH and HPH values reflected the fact that the MEX and MRC parents were the lowest-yielding populations in this study. This illustrates that parent selection based strictly on MPH or HPH may sometimes be misleading. In plant breeding programs, the choice of hybrids should be based on their performance relative to commercial check cultivars rather than their absolute or relative heterosis response.


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  		Table 6. Percent relative midparent heterosis (MPH, above diagonal) and high-parent heterosis (HPH, below diagonal) for forage dry matter yield in diallel crosses among nine alfalfa germplasms.

 

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  		Table 7. Harvestwise midparent heterosis (%) for forage dry matter yield of diallel hybrids derived from nine selected alfalfa germplasms.

 
Two hybrids, GRC x USA and AFG x UZB, demonstrated negative MPH (Table 6). Both parents of GRC x USA were relatively fall dormant and had average per se performance and GCA effects. The use of two parents that both possess greater levels of fall dormancy seems counterproductive in hybrid production for our environment. The negative MPH of AFG x UZB, with both parents having high per se performance and GCA, is somewhat difficult to explain, although generation of inferior hybrids from superior parents is not uncommon. Riday and Brummer (2002) reported significant negative heterosis for fall regrowth for some hybrids derived from M. sativa subsp. sativa half-sib lines, as well as from subsp. falcata lines. Such a response in our study would likely have been manifested as reduced yield. Segovia-Lerma et al. (2004) also reported a significant negative SCA for a hybrid derived from two parent germplasms, Chilean and African, that individually possessed significant positive GCA. It is also perhaps worth noting that the AFG parent was collected from a market in Khanabad, Afghanistan (www.ars-grin.gov/cgi-bin/npgs/acc/display.pl?1175394). This site is approximately 50 miles from the Uzbekistan border, suggesting that these two populations might share a common lineage.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
This study demonstrated that the development of high-yielding hybrids, which equal or exceed the performance of elite commercial cultivars, is possible by crossing between high-yielding M. sativa subsp. sativa germplasms. The contributions of both GCA and SCA were important in explaining variation in forage yield among these hybrids. Ten of the 11 highest-yielding crosses involved at least one parent that possessed significant positive GCA effects. All 11 hybrids possessed at least one parent with significantly high per se performance. Even though the magnitude of GCA effects was two times larger than that for SCA, 5 of the 6 highest-yielding crosses demonstrated significant SCA effects indicating the relative importance of SCA effects in determining hybrid yield. These results clearly indicate that yield can be improved by exploiting both additive and nonadditive gene effects.

Among the top ranking 11 hybrids in this study, 10 were also derived from parents that possessed differing fall regrowth capabilities. Eight of these 10 hybrids involved one relatively dormant parent. Five of these 8 hybrids were the only crosses that demonstrated significant positive SCA. These results suggest that populations possessing varied fall dormancy responses may represent different gene pools, which have evolved under dissimilar climatic conditions and affiliated spatial/temporal isolation. It is also important to consider these results in the context of our evaluation environment, which does not experience severe winter conditions, but does routinely experience frosts over an extended period of time. From a physiological perspective, therefore, the data suggest that the incorporation of traits related to winter hardiness, through carefully chosen parents, may improve hybrid performance over time.

In summary, intrapopulation recurrent selection within populations that possess different fall dormancy response and high GCA and SCA effects, followed by intercrossing between such germplasms, would help to capitalize on favorable additive, dominance, and epistatic effects to improve alfalfa forage yield. Strategies for developing such hybrids in alfalfa have been previously described (Brummer, 1999; Segovia-Lerma et al., 2004). Direct selection within the hybrid populations developed in this study would also appear to be a useful short-term approach to improve alfalfa performance.


   ACKNOWLEDGMENTS
 
This research was supported by U.S. Department of Agriculture Hatch funds provided to the New Mexico Agriculture Experiment Station.

Received for publication June 15, 2006.


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





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