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Published online 25 July 2006
Published in Crop Sci 46:1948-1954 (2006)
© 2006 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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CROP BREEDING & GENETICS

Genetic Variation in Dry Matter Production and Nutritional Characteristics of Meadow Bromegrass under Repeated Defoliation

Kevin B. Jensen*, J. G. Robins, Blair L. Waldron and Michael D. Peel

USDA-ARS, Forage and Range Research Lab., 695 North 1100 East., Logan, UT 84322-6300

* Corresponding author (kevin{at}cc.usu.edu)


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Meadow bromegrass (Bromus riparius Rehm.) has gained interest as a highly productive pasture grass that can be used under management intensive grazing with limited irrigation. In 1999, 28 cloned parents and half-sib families of meadow bromegrass were each planted in a randomized complete block design to evaluate genetic variation of meadow bromegrass for dry matter yield (DMY) (Harvests 1 to 6), forage quality under repeated defoliation (Harvests 1, 3, and 5), and investigate intercharacter correlations. Narrow-sense heritability estimates and their standard errors for DMY at Harvests 2, 3, and 4 were 0.89 ± 0.32, 0.59 ± 0.28, and 0.53 ± 0.29, respectively. However, at Harvest 1 and 5, standard errors of the heritability estimates for DMY were equal to or greater than the estimates. All heritability estimates for crude protein (CP) were at least twice their standard errors at Harvests 1, 3, and 5. High narrow-sense heritabilities at Harvest 5 for acid detergent fiber (ADF) suggest that selection on forage harvested later in the growing season might be more efficient than forage harvested earlier in the season. Combined across years and within harvests, narrow-sense heritability estimates and standard errors for neutral detergent fiber (NDF) were 0.66 ± 0.29, 0.47 ± 0.30, and 0.71 ± 0.28, respectively. Based on correlations, it seems reasonable to assume that when selecting for increased DMY that ADF and NDF will increase and that CP and IVTD will decrease in this population.


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
INTEREST in utilizing less productive agricultural land, often associated with periods of reduced irrigation, soil salinity, and low fertility, is gaining greater interest as an alternative source of forage to public grazing. Productivity of these lands can be increased through genetically improved plant materials selected for more intensive management practices under abiotic and biotic stresses. One such species is meadow bromegrass due to its early seasonal forage production and rapid regrowth after defoliation.

Meadow bromegrass is a moderately creeping perennial grass, native to southeastern Europe, the Caucasus, Turkey, and Central Asia (Knowles et al., 1993), that was first introduced to North America in 1957. Meadow bromegrass is predominantly cross-pollinating, however, it does have the ability to produce seed under self-pollination (Knowles et al., 1993). Meadow bromegrass cultivars are decaploids (2n = 10x = 70) compared to smooth bromegrass, which is an autoallooctaploid (2n = 8x = 56) (Tzvelev, 1976, Armstrong, 1987, Armstrong, 1991).

Unlike its counterpart smooth bromegrass [B. inermis Leyss.], which regrows slower after defoliation, meadow bromegrass is known for its rapid regrowth of tillers after defoliation (Jensen et al., 2001). Due to meadow bromegrass's short rhizomes, it is less aggressive than smooth bromegrass in perennial grass–legume mixtures (Ferdinandez and Coulman, 2000). In the central Great Plains, meadow bromegrass is not as productive as smooth bromegrass (Vogel et al., 1996). Under a four-cut system in Canada, meadow bromegrass had lower initial yields than smooth bromegrass but had higher regrowth yields (Knowles et al., 1993). Under a six-harvest management and multiple irrigation levels in Northern Utah, meadow bromegrass significantly out yielded smooth bromegrass on total DMY by 28% (Jensen et al., 2001).

Estimates of heritability have varied among traits in grasses, with lower estimates reported for traits under complex genetic control, such as forage yield (Tan et al., 1978; Casler 1998) and fiber concentration (Tan et al., 1978), compared to high estimates for more simply inherited characters, such as plant height (Ross et al., 1970). Under a single harvest management system and open-pollination, narrow-sense heritability estimates and standard errors in 48 half-sib families of meadow bromegrass were 0.33 ± 0.21, 0.08 ± 0.29, and 0.21 ± 0.26, for DMY, CP, and NDF, respectively (De Araujo and Coulman, 2002).

The relationships between DMY and CP have generally been negative (Asay et al., 1968; Vogel et al., 1981; Berg and Hill, 1983) in perennial grasses. De Araujo and Coulman (2002) reported a nonsignificant relationship between DMY and CP, ADF, or NDF in meadow bromegrass, but did find a negative correlation between CP and ADF and NDF concentrations. In Timothy (Phleum pratense L.), Berg and Hill (1983) concluded that simulataneous selection for increased DMY, in vitro dry matter disappearance, and CP would probably lead to undesirable correlated responses. Van Soest (1965), proposed that NDF best estimated cell wall constituents (CWC), which are comprised of partially digestible cellulose, hemicellulose, and indigestible residue which includes lignin. Mertens and Van Soest (1973) reported a correlation (r = 0.73) between CWC and DMY intake in grass samples.

The majority of the selection efforts in meadow bromegrass have been restricted to combining genotypes with increased seed yield (Alderson and Sharp, 1994). The initial breeding work on meadow bromegrass in North America began with the development of a 15 clone synthetic cultivar Regar (Foster et al., 1966) originating from PI 173290. Two cultivar releases, Fleet and Paddock, were developed by Agriculture Canada Saskatoon, SK, with emphasis on increased seed yields over Regar (Knowles, 1990a, 1990b). The cultivar Montana is a 96 clone synthetic with one cycle of selection for recovery after clipping and seed yield (Cash et al., 2002). Multiple cycles of selection for increased regrowth under reduced irrigation resulted in the release of an 18-clone synthetic cultivar Cache (Jensen et al., 2004).

The principle objectives of this research were to evaluate (i) genetic variability for DMY and forage quality, (ii) effect of date of harvest in a multiple-harvest sequence on estimates of genetic parameters for forage yield and forage quality traits, and (iii) associations between forage yield and forage quality traits on each harvest date.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
In 1999, 28 cloned parents and half-sib families of a meadow bromegrass population were each planted in a randomized complete block design with four replications of the parents and eight replications of half-sib families to evaluate the effects of repeated clipping on heritability estimates. The original space-plant source nursery was comprised of medow bromegrass cultivars Fleet (Knowles, 1990a), Paddock (Knowles, 1990b), and Regar (Alderson and Sharp, 1994), which is similar to the population generated by De Araujo and Coulman (2002). On the basis of a selection index that included total seed yield and 100-seed weight, seed from 12 open-pollinated plants were selected from 1200 plants (Jensen et al., 2004). A completely randomized nursery comprised of 1200 plants (100 from each selection) was evaluated for visual plant vigor. Twenty-eight plants were selected and seed harvested for inclusion in this study. The experiment was located at the Utah State University Evans Experimental Farm, approximately 2 km south of Logan, UT (41°45' N, 111°8' W, 1350 m above sea level). Soil at the site was a Nibley silty clay loam (fine, mixed, mesic Aquic Argiustolls). Total precipitation (excluding irrigation) received from October through September was 385, 410, and 311 mm for 1999, 2000, 2001, respectively. Irrigation water plus rainfall was monitored from April through October in 2000 and 2001. Amounts of irrigation water plus rain applied to the plots were 25.1 mm wk–1 in 2000 and 23.2 mm wk–1 in 2001. Fertilizer applications, each 56 kg N ha–1, were made on 27 April, 16 June, 10 August, and 2 October in 2000 and on 9 May, 10 July, 13 August, and 22 October in 2001. Mean minimum and maximum monthly temperatures for 2000 and 2001 are in Table 1.


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  		Table 1. Minimum and maximum temperatures.

 
Forage Characteristics
Following establishment in 1999, determinations of DMY (Mg ha–1) were based on a plot mean basis. Plots were harvested to an 8-cm stubble height by a Swift Current sickle bar harvester (Swift Machining & Welding LTD, Swift Current, SK) from five parent plants (clones) and 10 half-sib family plants, arranged on 0.5-m centers within rows and 1-m spacing between rows. At Harvest 1, plants were at the boot stage (Moore et al., 1991) with all subsequent harvests occurring when plant regrowth reached 25 to 30 cm in height from 24 May through 13 September in 2000 and 30 May through 18 September in 2001. Harvest dates were 24 April, 24 May, 14 June, 12 July, 9 August, and 13 September in 2000 and 4 May, 30 May, 10 July, 8 August, and 18 September in 2001. After harvest-1, plants remained in the vegetative stage with only the occasional reproductive tiller being expressed. Plant height was measured from the ground to the top of the extended leaves. Forage samples used to estimate DMY were dried at 60°C in a forced-air oven to constant weight.

Samples used for DMY estimations were initially ground in a Wiley mill, and then finely ground in a Cyclone mill to pass through a 1-mm screen. Ground plant samples were scanned with a near infrared reflectance spectroscopy (NIRS) instrument (Model 6500; Pacific Scientific Instruments, Silver Spring, MD). Representative samples were selected from each year and harvest and used as a validation data set for actual chemical analysis. This data set consisted of 152 samples each for CP, ADF, NDF, and in vitro true digestibility (IVTD). These samples were not part of the original NIRS grass equation. The r2 values for validation of the CP were 0.92 combined across years and harvests. Corresponding r2 values were 0.90 for ADF, 0.92 for NDF, and 0.79 for IVTD.

Samples used for calibration were analyzed for N using a LECO CHN-2000 Series Elemental Analyzer (LECO Corp., St. Joseph, MI). Multiplying N by 6.25 determined levels of CP. Acid detergent fiber, NDF, and IVTD were determined using procedures described by Goering and Van Soest (1970). Analysis for ADF and NDF were made with an ANKOM-200 Fiber Analyzer (ANKOM Technology Corp., Fairport, NY). The first stage of the IVTD procedure consisted of a 48-hr in vitro fermentation in the ANKOM Daisy II incubator (ANKOM Technology Corp, Fairport, NY). The residual dry matter was then exposed to the NDF procedure.

Variance components for forage DMY and nutritional characteristics were estimated considering replications, parent clones, and half-sib families, and years as random variables using the REML option of PROC MIXED (SAS Institute, 1999). Broad-sense heritabilities were computed based on the variation among clonal lines as {sigma}c2/{sigma}2ph, where ßdßd{sigma}c2 is the total genetic variance arising from differences among clones of the parental lines and {sigma}2ph is the phenotypic variance among entries. The phenotypic variance is defined as {sigma}2ph = {sigma}c2 + {sigma}2rc/r + {sigma}2cy/y + {sigma}2rcy/ry, where {sigma}2rc, {sigma}2cy, and {sigma}2rcy are the variance components for replications x entries, entries x years, and replications x entries x years, respectively and r and y are number of replications and years, respectively. Narrow-sense heritabilities were computed based on the variation among half-sib families as {sigma}g2/{sigma}2ph, where {sigma}g2 is the total genetic variance arising from differences among half-sib families and {sigma}2ph is the phenotypic variance among entries (Hallouer and Miranda, 1981; Fehr, 1987). Broad sense (H2) and narrow sense (h2) heritability were estimated on a phenotypic mean basis. Approximate standard errors of H2 and h2 estimates were computed as a ratio of the standard error of the genetic variance over the corresponding phenotypic variance (Dickerson, 1969).

The relative contribution (%) of the genotype x year ({sigma}gy2/{sigma}2ph) to the phenotypic variance was estimated and compared to narrow-sense heritability. Genetic correlations, and corresponding standard errors, between total DMY and DMY at the individual harvests, and forage quality characteristics and individual harvest was done using the complete data set, considering entries as random using SAS MIXED and IML procedures, based on code from Holland (2006).


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
Forage Yield
Combined across years and harvests, h2 estimate for total DMY was 0.58 ± 0.28 (Table 2). This is in contrast to De Araujo and Coulman (2002) who reported zero variation and heritability for DMY in polycross progeny of meadow bromegrass. Total DMY in the half-sib families ranged from 0.52 to 9.88 Mg ha–1 with a mean of 2.44 Mg ha–1 (Table 2). Broad-sense heritability estimates in the parental clones was 0.33 ± 0.30 (Table 2), which is lower than those reported for meadow bromegrass (De Araujo and Coulman, 2002), Festuca arundinacea (Burton and De Vane, 1953; Hovin et al., 1976), F. pratensis (Aastveit and Aastveit, 1990), and Phalaris arundinacea (Asay et al., 1968). The increase in h2 over H2 estimates may be a result of increased precision (80 versus 20 plants) and increased genetic variation possibly due to transgressive segregation (Table 2) in the progeny.


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  		Table 2. Variance components and means for parental clones and half-sib families of meadow bromegrass for dry matter yield.

 
Narrow-sense heritability estimates were higher than {sigma}2gy/{sigma}2ph ratio for total DMY (Table 2), indicating that the environment had limited effect on total DMY in this study. However, the effect of environmental variation between growing seasons was much more apparent in the parental plots where the {sigma}2gy/{sigma}2ph ratio was 0.53 (Table 2). The presence of heterosis in the half-sib families, might result in a buffering effect that compensates for environmental changes not present in the parents. Given the potential influence of the environment, it is advisable to evaluate and select among half-sib families in multiple environments.

Parent-progeny coefficient of determination (r2) is a useful statistic that provides a measure of the probable effectiveness of selection based on the variation present in the progeny accounted for by corresponding covariation in the parents (Kneebone, 1958). The coefficient of determination of 0.51 and moderate h2 estimates in half-sib families for DMY indicate that considerable success could be expected from simple selection for total DMY in this meadow bromegrass population.

Selection efficiency for DMY could be increased if DMY from a single harvest, rather than multiple harvests, could identify superior genotypes with increased total DMY. Estimates of h2 were 0.89 ± 0.32, 0.59 ± 0.28, and 0.53 ± 0.29 at Harvests 2, 3, and 4, respectively (Table 2). High genetic variation ({sigma}2g), increased h2 estimates at Harvests 2, 3, an 4, and a high genetic correlation between total DMY and Harvests 3 and 4 (0.66 ± 0.15 and 0.86 ± 0.08, respectively), suggests that in meadow bromegrass, selection for increased DMY may be more effective as a measure of regrowth after the initial spring growth rather than total DMY.

Crude Protein
Mean CP concentration ranged from 107 g kg–1 at Harvest 3 to 281 g kg–1 in Harvest 1 (Table 3). Combined across years and harvests, h2 was 0.62 ± 0.28 in meadow bromegrass. In contrast, De Araujo and Coulman (2002) reported h2 estimates for CP ranging from –0.10 to 0.08. A combination of a high coefficient of determination and h2 estimates (Table 3) for CP across harvests indicates that considerable success could be expected with simple recurrent selection for increased CP in this population.


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  		Table 3. Variance components and means for parental clones and half-sib families of meadow bromegrass for crude protein (CP), acid detergent fiber (ADF), neutral detergent fiber (NDF), and in vitro true digestibility (IVTD).

 
Narrow-sense heritability estimates were higher than the {sigma}2gy/{sigma}2ph ratio for CP (Table 3), indicating that the environment had even less of an effect on forage CP concentrations than forage DMY. Progeny x harvest variance component was significant (data not shown) for CP; however, the progeny x year variance component was not significant. This would suggest that selection for CP probably does not need to be done over years, but that it might require data from multiple harvests. De Araujo and Coulman (2002) attributed low heritability estimates and large standard errors of those estimates in meadow bromegrass to variation in temperature, N, and soil fertility, which can vary throughout the growing season.

Narrow-sense heritability estimates ranged from 0.45 ± 0.15 to 0.65 ± 0.28 for CP across Harvests 1, 3, and 5 (Table 3). Genetic variation among parental clones and half-sib families was highest at Harvest 1; however, h2 estimates in the half-sib families were significant only at Harvests 3 and 5 (Table 3). Based on moderate to high h2 estimates and the limited effect of the environment across all harvests, selection for CP in forage harvested throughout the growing season would probably be effective. Genetic correlations for CP between Harvests 3 and 5 were 0.90 ± 0.15 and Harvests 1 and 5 were 0.72° ± 0.17. However, there was a trend toward increased h2 standard errors for CP concentrations from forage harvested during the middle of the growing season.

Acid Detergent Fiber
Acid detergent fiber is the percentage of highly indigestible plant material present in forage. Acid detergent fiber had moderate to high h2 estimates and low standard errors at individual harvests and when computed over harvests (0.60 ± 0.29) indicated that expected progress for selection for lower ADF would be achievable (Table 3). Low ADF values generally mean forages with increased energy and high digestibility. Combined across years, H2 estimates ranged from 0.70 ± 0.28 in Harvest 1 to 0.84 ± 0.28 in Harvest 5 (Table 3). The estimates of H2 for ADF were similar to values reported by De Araujo and Coulman (2002) for meadow bromegrass.

Based on the {sigma}2gy/{sigma}2ph ratio, 43, 18, and 14% of the variation for ADF at Harvests 1, 3, and 5, respectively (Table 3), were attributed to possible environmental affects. High h2 estimates and a high coefficient of determination (0.81) at Harvest 5 suggests that selection for ADF concentration based on forage harvested later in the growing season would be more efficient than forage harvested earlier in the season. Genetic correlations were highest for ADF between Harvests 3 and 5 (0.90 ± 0.15). Hovin et al. (1976), alluded to this concept, that there may be advantages in using only regrowth forage, which is less affected by maturity differences in genetic studies of reed canarygrass for forage quality.

Neutral Detergent Fiber
Neutral detergent fiber represents all of the structural or cell wall material in the forage. The NDF of forage is inversely related to the amount that a cow or calf is able to consume; thus, forages with low NDF will have higher intake rates than those with high NDF. Mean NDF concentrations in the half-sib families ranged from 363 g kg–1 in Harvest 1 to 569 g kg–1 in Harvest 3 (Table 3). Combined across years and within harvests h2 estimates and standard errors for NDF concentrations were 0.66 ± 0.29, 0.47 ± 0.30, and 0.71 ± 0.28 for Harvest 1, 3, and 5, respectively, compared to 0.62 ± 0.28 over years and harvests (Table 3). In meadow bromegrass De Araujo and Coulman (2002) reported h2 estimates of 0.21 ± 0.26 and –0.08 ± 0.42 for NDF. High h2 estimates and a coefficient of determination of 0.86 at Harvest 5 suggest that it may be more efficient to select for NDF in regrowth forage harvested later in the growing season. Genetic correlations for NDF were highest between Harvests 3 and 5 (0.89 ± 0.16) when forage regrowth was used.

Orchardgrass and tall fescue forage harvested later in the growing season had significantly (P < 0.05) lower NDF concentrations than the early and midseason harvests (Asay et al., 2002; Jensen et al., 2003). This decrease in NDF concentration can be attributed to the reduction in stem development after the initial spring growth and subsequent defoliation (Jensen et al., 2001). Based on the data, one could speculate that it is more efficient to select for reduced NDF concentrations when forage is in the vegetative stage versus stem elongation. The difference between h2 estimates and the {sigma}2gy/{sigma}2ph ratio for NDF concentrations (Table 3) was greater at Harvest 5.

In Vitro True Digestibility
Mean IVTD concentrations ranged from 789 g kg–1 in Harvest 2 to 965 g kg–1 at Harvest 1 (Table 3). Combined over years and harvests, h2 was 0.81 ± 0.29 (Table 3). Genetic variation ({sigma}2g) and h2 estimate of 0.78 ± 0.28 were highest at Harvest 5 (Table 3). However, the coefficient of determination was 0.17 at Harvest 5, suggesting that a large portion of the variation for IVTD present in the progeny could not be accounted for by the covariance of the parents. At Harvests 1 and 3, the coefficient of determination was 0.80 and 0.86, respectively.

Genetic correlations for IVTD were greater between Harvests 1 and 3 (1.02 ± 0.24). A combination of large h2 estimates (Table 3) and reduced environmental impact at Harvest 1, suggests that selection for increased IVTD based on forage harvested earlier in the growing season would be more efficient than forage harvested in the middle to latter part of the growing season.

With the exception of Harvest 3, h2 estimates were several magnitudes higher than {sigma}2gy/{sigma}2ph ratio for IVTD (Table 3). Despite the low coefficient of determination at Harvest 5, selection for IVTD probably does not need to be done at multiple locations, and should be concentrated on forage harvested early and later in the growing season.

Intercharacter Correlations
At Harvest 1, there was no correlation between DMY and ADF and NDF (Table 4), whereas at Harvests 3 and 5, there was a strong association; as ADF (r = 0.86 and r = 0.77, respectively) and NDF (r = 0.79 and r = 0.75, respectively) increased so would the correlated response to DMY increase (Table 4). With the exception of Harvest 1, the correlation between ADF and NDF exceeded r = 0.94 (Table 4). Based on the observed correlated responses, selection for increased DMY would increase ADF and NDF concentrations and selection would probably be more effective on regrowth forage than early season growth. De Araujo and Coulman (2002) reported no correlations between DMY and ADF and NDF under a one harvest treatment in meadow bromegrass. Positive and significant correlations between DMY and ADF and NDF concentration were reported by (Falkner and Casler, 1998). This is not surprising, as higher DMY would likely result from larger plants with a greater percentage of more highly lignified stems. The lack of association between DMY and fiber at Harvest 1, may imply that selection for increased DMY on forage produced early in the season might not adversely affect fiber concentration.


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  		Table 4. Intercharacter correlations on 25 half-sib families of meadow bromegrass. Progeny (upper diagonal) and Parents (lower diagonal).

 
Within each harvest, there was a significant (P < 0.01) negative correlation between DMY and CP and DMY and IVTD (Table 4). The strongest negative correlation was observed at Harvest 3 for CP (r = –0.72, P < 0.01) and IVTD (r = –0.88, P < 0.01), respectively. These correlations suggest that selecting for traits associated with increased DMY will result in decreased CP and IVTD regardless of when the forage is harvested throughout the growing season. De Araujo and Coulman (2002) reported no correlation between DMY and CP in meadow bromegrass. As expected, correlations between CP and IVTD were positive and significant at Harvests 3 and 5 (Table 4). At Harvest 1, there was no observable associated response between CP and IVTD (Table 4). There was a negative association (P < 0.0001) between CP and ADF and CP and NDF at Harvests 3 and 5, which confirms correlations previously reported by De Araujo and Coulman (2002). Correlations between these traits and others at Harvest 1 were less conclusive and additional study is needed to define correlated response between CP and IVTD with DMY and fiber concentrations on forage harvested early in the growing season. Marum et al. (1979) reported that concentrations of CWC were correlated with ADF. Hovin et al. (1976) reported correlation coefficients between CWC and in vitro digestible dry matter as negative and ranged from –0.81 to –0.91. Often associated with selection for increased CP concentrations, is a reduction in forage DMY (Casler, 1998; Casler and Vogel, 1999).

Based on these correlations, it seems reasonable to assume that when selecting in this meadow bromegrass population for increased DMY that ADF and NDF will increase and that CP and IVTD will decrease, resulting in a poorer quality forage. Selection for CP and IVTD may be more efficient when evaluated on forages produced earlier in the growing season. If decreased ADF and NDF are the selection objectives, it may be more efficient to select on forage that was produced later in the growing season.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Joint contribution of the USDA-ARS and the Utah Agric. Exp. Stn. Utah Agric. Exp. Stn. Journal Paper No. 7798. Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by the USDA or Utah State University.

Received for publication March 15, 2006.


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





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