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Dep. of Horticulture & Crop Science, The Ohio State Univ., Columbus, OH 43210
* Corresponding author (sulc.2{at}osu.edu)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Abbreviations: WHS, winter-hardy species • WSS, winter-sensitive species • VNS, variety not stated • NDF, neutral detergent fiber • CP, crude protein
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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There is increasing interest in utilizing small grains and other annual crops for forage in northern regions, including their use as double-crops within traditional grain crop rotations (Thelen and Leep, 2002). The integration of different annual forage species into grain crop rotations in the Midwest USA would provide supplemental forage harvesting and grazing opportunities that might reduce livestock winter feeding costs and increase net return on the same land base. For example, broadcast seeding of oat, cereal rye, and Brassica species over a corn (Zea mays L.) field in late summer with an airplane would allow time for forage establishment before corn grain harvest, provided sufficient moisture is available for seed germination. Once the grain harvest was completed, livestock could graze the high quality annual forage along with the corn stover. Integrating annual forages into grain crop rotations is also possible by planting oat or cereal rye after harvesting corn silage, early maturing soybean [Glycine max (L.) Merr.] cultivars, or wheat for grain. Since wheat grain harvest occurs in July in the Midwest, a forage species could be planted by late July or early August, providing ample time for many annual species to produce forage for late autumn use.
Important factors to consider when choosing annual crop species and cultivars for extending forage supplies from autumn to spring include forage yield potential, dependability and seasonal distribution of forage, pest resistance, and forage quality characteristics (Bruckner and Raymer, 1990). Cereal grains vary in growth characteristics that affect seasonal yield distribution. For example, winter-hardy cereal cultivars have limited forage production in the autumn, but offer abundant forage production in spring. In an Arkansas study, forage yield in autumn was negatively correlated with winter hardiness of small grain species (West et al., 1988). In Wisconsin, August-planted spring cereals had significantly greater forage yield in late autumn than August-planted winter-hardy species (Maloney et al., 1999). Thus, mixtures of winter-sensitive (autumn active) and winter-hardy (spring active) species may provide optimal forage yield distribution for late autumn and early spring (Maloney et al., 1999).
Nutrient requirements for different classes of livestock should be considered when selecting species and cultivars for forage production. Differences in growth characteristics can affect the feeding value of the forage. For example, Maloney et al. (1999) reported that winter-sensitive cereal grains had greater neutral detergent fiber (NDF) and lower crude protein (CP) concentrations in the autumn compared with winter-hardy cereal species. The winter-sensitive cereals grew more actively and had elongated stems in the autumn whereas the winter-hardy cereals were comprised primarily of leaves.
In addition to small grain species, other annual forage species have demonstrated potential use for extending forage supplies during cool months in the U.S. Midwest. In Missouri, Kallenbach et al. (2003) reported winter yields of 825 to 2356 kg DM ha–1 from a cold-tolerant annual ryegrass cultivar planted in early September. Cold-tolerant annual ryegrass cultivars are able to grow when average daily temperatures are less than 6°C (Keating et al., 1980; Cherney and Robinson, 1985). In regions with mild winters or where consistent snow cover provides protection from extreme cold temperatures, annual ryegrass can survive the winter and provide forage in the spring. Annual ryegrass forage is high in CP and relatively low in NDF. Information is lacking on productivity of autumn-sown annual ryegrass in the Ohio Valley region.
One disadvantage of using annual ryegrass within a cropping system is its tendency to become a weed in cereal grain crops and the potential for development of herbicide-resistant biotypes (Stanger and Appleby, 1989; Perez and Kogan, 2003). Harvesting or grazing the annual ryegrass before it produces viable seed would be an important management practice, along with an aggressive herbicide program before planting and during early establishment of the subsequent grain crop.
Rape and other Brassica species are also worthy of consideration for extending forage supplies during cool months. Rape planted in late summer has potential to produce high forage yields with high CP and low NDF concentrations (Reid et al., 1994; Kunelius et al., 1989; Smith and Collins, 2003). When rape is grazed, livestock should receive supplementary fiber to ensure good rumen function (Penrose et al., 1996). Planting rape in combination with a cereal grain may provide sufficient fiber to meet livestock requirements.
Information is limited on forage yield and nutritive value of annual species and mixtures of species during the autumn and subsequent spring in the Ohio valley region. Although studies have shown the potential of annual ryegrass and rape planted as monocultures in late summer (August), it is not clear how well they would perform when planted in mid-September, as would be the case when following harvests of soybean for grain or corn for silage. Information is also lacking on the adaptability of annual ryegrass and rape planted in mixtures with winter-hardy cereal grains. We were particularly interested in finding species combinations that produced well in both late autumn and the subsequent spring after being planted in mid-September.
The objective of this study was to evaluate yield, CP, and NDF of forage harvested from monocultures and binary mixtures of several winter-hardy cereals and winter-sensitive annual species planted in mid-September and harvested in late autumn and the subsequent spring in Ohio. The information would be useful for selecting species combinations that could be planted in late summer or early autumn to provide supplemental forage from late autumn to early spring. The information would also provide insight into which species are most suitable for double-cropping within grain crop or corn silage production systems in the Ohio valley region.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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In 2001 and 2002 the WHS included ‘Winter King’ rye, ‘Trical 815’ winter triticale and ‘Hopewell’ winter wheat, which were seeded alone and in binary mixtures with the WSS ‘Armor’ oat, ‘Trical 2700’ spring triticale, ‘Abundant’ annual ryegrass, and ‘Dwarf Essex’ rape. In 2002, a common rye cultivar, i.e., variety not stated (VNS), was added and sown alone and in binary mixtures with the four WSS (oat, spring triticale, ryegrass, rape). In addition, each of the WSS was sown in monoculture stands in 2002. All cultivars were commercially available and seeding rates and seeding costs are listed in Table 1. Seeding rates for each species were the same in monocultures and mixtures.
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Before establishment, lime and fertilizer were applied in accordance with guidelines based on soil test results (Vitosh et al., 1995). The sites were chisel plowed, disked and cultimulched to provide a uniform seedbed and incorporation of applied fertilizer. Nitrogen was broadcast applied at 56 kg ha–1 (as ammonium nitrate) at the initial tillering stage in the autumn and in early March. Plots were seeded with a Hege plot drill equipped with press wheels (Hege Equipment, Inc, Colwich, KS). The cereal grain seed was metered out by a rotating cone to hoses that dropped the seed between the double-disk openers of the drill. Ryegrass and rape seed was metered out through a bulk seed box (with on/off mechanism) to hoses that dropped the seed behind the double disk openers and immediately in front of the press wheels of the drill. In this way, all seed intended for a specific plot was planted simultaneously to the appropriate depth. The cereal grain species seed was placed 3.5 cm deep, while the ryegrass and rape seed was placed 0.5 cm deep. Because two of the WSS (annual ryegrass, rape) had to be seeded through the bulk seed box to achieve proper planting depth, the WSS treatments were assigned to whole plots and the WHS to subplots. The split-plot restriction on randomization provided a practical and efficient method for planting the treatments while making the best use of the available experimental area. Individual subplots consisted of seven rows, each spaced 15 cm apart and 6 m in length.
Forage yield was measured in each subplot and forage samples were collected from both sites in the autumn and spring. The autumn harvests occurred on 16 Nov. 2001 and 4 Nov. 2002 at South Charleston and 8 Nov. 2002 at Ripley. Autumn harvests were timed to coincide with significant slowing of the growth rate based on predicted onset of cold temperatures. The spring harvest occurred on 18 Apr. 2002 and 28 Apr. 2003 at South Charleston and 16 Apr. 2003 at Ripley. The timing of spring harvest was based on obtaining forage of good nutritive value (before inflorescence development) and to allow sufficient time for preparing the field for timely planting of a subsequent grain crop such as corn or soybean. Subplots were cut with a Swift sicklebar harvester (Swift Machine & Welding LTD., Saskatchewan Canada) to 7.5-cm stubble height. Forage samples were hand-clipped from three random locations in each subplot in three replicates immediately before harvesting for yield. Sample weights were recorded and added to the total subplot fresh weight. Forage samples were dried at 55°C to a constant weight to determine dry matter concentration for converting subplot fresh weight to dry weight. Samples were ground through a 1-mm screen (Thomas-Wiley Laboratory Mill Model 4, Arthur H. Thomas Co., Philadelphia, PA) before conducting forage CP and NDF analyses. Forage NDF was determined according to ANKOM Technology's filter bag method (Anonymous, 2003). Concentration of CP in the forage was calculated as 6.25 x total N, which was determined by the Dumas method (AOAC, 1990).
All data were analyzed using mixed-model methodology, as implemented in PROC MIXED of SAS with the NOBOUND option for removing boundary constraints on covariance parameters. The Satterthwaite method was used for computing the denominator degrees of freedom for the tests of fixed effects. The WHS and WSS treatments and their interaction were considered fixed effects, whereas replicates, main-plot error, and experimental error were considered random effects. Data were analyzed within each environment (year-location combination) because additional treatments were added to the 2002–2003 experiment. In addition, when common treatments were included in a combined analysis across years and locations, treatment x location and treatment x year interactions (P < 0.05) were found for most variables. The PDIFF option in the LSMEANS statement of PROC MIXED was used to compare treatment means when the ANOVA indicated significant differences (P < 0.05) were present. To provide the reader with information for making treatment means comparisons, Fisher's protected LSD (P = 0.05) values were included in the tables of results. The LSD values were calculated using the appropriate standard error of the difference, as provided in the output from the PDIFF option of the LSMEANS statement.
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RESULTS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Forage Yield
South Charleston, 2001 to 2002
In the experiment established at South Charleston in 2001, there was a WSS x WHS interaction for forage yield in the autumn (Table 2). The interaction was primarily a result of changes in magnitude of differences among treatments rather than from changes in ranking of treatments (Table 3). Within each WSS treatment, the WHS ranked consistently for autumn yield: Winter King rye > winter triticale > wheat, with few exceptions. The differences among WHS were not significant when the WSS companion was spring triticale and between winter triticale and wheat when planted with oat (Table 3). Within WHS treatments, ranking of WSS for autumn yield was also fairly consistent: spring triticale > oat > all other treatments (Table 3). The exception to that general pattern occurred within Winter King rye, as the only significant difference for autumn yield occurred between the Winter King monoculture (2.07 Mg ha–1) and the spring triticale + Winter King mixture (2.63 Mg ha–1). Annual ryegrass and rape mixtures with WHS were similar in yield to the corresponding WHS monocultures, except rape + wheat yielded more than the wheat monoculture by 0.43 Mg ha–1.
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South Charleston, 2002 to 2003
In the experiment established at South Charleston in 2002, WSS and WHS main effects were observed for autumn yield (Table 2). Ranking of WSS treatments was: oat mixtures > spring triticale mixtures = rape mixtures > annual ryegrass mixtures = WHS monocultures. Within WHS treatments, mixtures of WSS with Winter King yielded 0.42 Mg ha–1 more forage on average than all other mixtures and the mean for WSS monocultures in the autumn (Table 4). There was a WHS main effect for forage yield the following spring and for total forage yield (Table 2, 4). The WSS monocultures yielded 37 to 51% less spring forage and 40 to 56% less total forage than the mixtures of WSS with WHS (Table 4). Winter King rye mixtures yielded 40% more than the wheat and VNS rye mixtures in the spring and 22 to 40% more total forage than the other WHS mixtures.
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There was a WSS x WHS interaction for forage yield in the spring at Ripley (Table 2), primarily because of inconsistencies among WSS treatments within the WHS treatments (Table 5). For example, Winter King and VNS rye monocultures had similar yield as all of their mixtures with WSS, while the winter triticale monoculture yielded 1.1 Mg ha–1 more than the winter triticale + oat treatment and the wheat monoculture yielded 0.71 Mg ha–1 more than the wheat + oat treatment but 1.29 Mg ha–1 less than the wheat + rape treatment. In contrast, the ranking for yield among WHS treatments within each WSS was fairly consistent: Winter King rye > winter triticale = VNS rye > wheat = WSS monoculture ("None" WHS treatment, Table 5). The exceptions to that pattern were: the spring triticale monoculture yielded the same as all of its corresponding mixtures except for the Winter King mixture, and the rape monoculture had intermediate yield which was the same as its mixtures with winter triticale and VNS rye.
There was a WSS x WHS interaction for total combined forage yield at Ripley (Table 2). Differences among WSS treatments were not significant within Winter King rye and only one was significant within winter triticale (spring triticale vs. annual ryegrass). Magnitude of differences among WSS treatments was intermediate within wheat and VNS rye, and greatest within the "None" WHS treatment (i.e., WSS monocultures) due to the low yield of annual ryegrass and oat monocultures. Despite the interaction, consistent trends were observed. Including a WSS with a WHS failed to increase total forage yield compared with the corresponding WHS monoculture in 12 of 16 comparisons (Table 5). Exceptions to that pattern occurred for spring triticale and rape mixtures with wheat and VNS rye. Consistent differences were found among the WHS within each WSS treatment for total yield at Ripley: Winter King rye > Winter triticale = VNS rye > wheat = WSS monocultures ("None" WHS treatment). Winter King rye yielded 3.21, 3.36, and 5.41 Mg ha–1 more than winter triticale, VNS rye, and wheat, respectively, averaged over all WSS treatments.
Forage Crude Protein and Neutral Detergent Fiber
There was a WSS x WHS interaction for CP in autumn at South Charleston in the 2001–2002 experiment (Table 2); however all treatments had very high forage CP levels (271 to 301 g kg–1) and treatment differences were small and likely not meaningful when considered in the context of animal CP requirements. In the 2002–2003 experiments, there were WSS effects at both locations and a WHS effect for forage CP at Ripley in the autumn (Table 2); however, all treatments had CP concentrations between 300 and 340 g kg–1 (data not shown) and the small treatment differences were likely not meaningful in terms of animal response. In the spring, mean forage CP ranged from 196 to 226 g kg–1 across the three environments (data not shown). There was a WHS effect for forage CP in the spring in all environments (Table 2). Winter King rye treatments had the lowest mean forage CP in all environments (170 to 210 g kg–1), which ranged from 21 to 41 g kg–1 lower than CP concentration of forage in the other WHS treatments. There was also a WSS effect for forage CP at Ripley in spring 2003 (Table 2). Rape and the monoculture WHS treatments were lower in CP (189 g kg–1) than the oat, spring triticale, and annual ryegrass treatments (200 g kg–1).
There was a WSS x WHS interaction for forage NDF in the autumn at South Charleston in 2001 (Table 2, 6). Differences in forage NDF among WSS treatments were larger within winter triticale (75 g kg–1 range) and wheat (99 g kg–1 range) than within Winter King rye (47 g kg–1 range). Oat and spring triticale mixtures with winter triticale and wheat had greater forage NDF (mean 331 g kg–1) than monocultures of winter triticale and wheat (mean 278 g kg–1) and rape mixtures (mean 251 g kg–1). There were no differences in forage NDF among WHS treatments within oat (Table 6). When comparing WHS, Winter King rye forage had greater forage NDF in 7 of 10 comparisons, averaging 324 g kg–1.
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A significant WHS main effect was observed for forage NDF in spring in both South Charleston experiments (Table 2). Winter King rye had the greatest forage NDF (512 and 499 g kg–1), which averaged 87 g kg–1 greater than the NDF concentration of the other WHS (data not shown). There was a WSS main effect for forage NDF in spring in the South Charleston 2002–2003 experiment (Table 2). The mean NDF for treatments containing rape (384 g kg–1) was lower than for all other WSS treatments (422 to 441 g kg–1), except those containing annual ryegrass (401 g kg–1). The rape monoculture had very low spring forage NDF (282 g kg–1) at South Charleston. There was a WSS x WHS interaction for forage NDF in the spring at Ripley in the 2002–2003 experiment (Table 2), primarily because of the effect of rape and Winter King rye on forage NDF concentrations. The rape monoculture had very low NDF (235 g kg–1) and all rape mixtures had low NDF concentrations (373 to 394 g kg–1), except for the Winter King + rape mixture (522 g kg–1). All Winter King rye treatments had high forage NDF (477 to 529 g kg–1) relative to the other treatments.
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DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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–5). The lower incidence of autumn yield improvement from adding WSS to Winter King rye was the result of its vigorous autumn growth and high yield. Oat and spring triticale mixtures with WHS usually had the greatest autumn yield. Oat and spring triticale mixtures with WHS increased autumn yield over the corresponding WHS monocultures in 91% of all comparisons, the primary exceptions being their mixtures with Winter King rye which usually had similar autumn yield to the Winter King monoculture. A similar trend was observed in Wisconsin where mixtures of winter and spring cereals produced greater autumn yield than the corresponding winter cereal planted alone (Maloney et al., 1999). The WHS + WSS mixtures used in our study usually had autumn forage yields that were equal to or greater than the yield of the corresponding WSS monocultures (Tables 4, 5). In contrast, winter and spring cereal mixtures in the Wisconsin study yielded less forage in the autumn than the corresponding spring cereal monocultures (Maloney et al., 1999). The Wisconsin experiments were planted about a month earlier (mid August) than ours. The later planting dates, different environmental conditions, and specific cultivars in our study may have affected the relative competitiveness of winter and spring cereal cultivars as compared with the Wisconsin study results. Forage yield of mixtures was usually similar to that of WHS monocultures in the spring and for total yield summed across autumn and spring harvests (Tables 4 and 5). Thus, adding a WSS to WHS to improve autumn production did not appear to be detrimental to spring productivity. The only exception to that trend was for the oat + winter triticale and oat + wheat treatments at Ripley in spring 2003 (Table 5). Oat was very competitive with those two species the previous autumn, as evidenced by the oat mixtures vs. corresponding winter triticale and wheat monoculture treatments (Autumn 2002, Table 5). The oat competition in the autumn appeared to result in reduced forage yield the following spring in winter triticale and wheat, which were essentially left as weaker monocultures since oat had winter killed. The WSS + WHS mixtures rarely produced more total forage (autumn + spring) when compared with WHS monocultures; the average advantage of the mixtures was primarily to increase autumn forage production. Spring yield of WSS monocultures tended to be lower than their corresponding mixtures with WHS (Tables 4, 5), except when compared with the wheat mixtures at Ripley. It was not clear why the wheat had such low yield at Ripley in spring 2003 (Table 5), but an unconfirmed viral infection was suspected.
Forage Yield of Monocultures
Winter King rye had high forage yield in both autumn and spring, which was greater than the yield of most other WHS treatments (Tables 3–5). Winter King rye is a forage-type cultivar that demonstrated vigorous autumn growth compared with VNS rye, winter triticale, and wheat. Winter King rye had the greatest combined autumn plus spring yield (Tables 4 and 5), a result of its greater autumn productivity and earlier spring growth, compared with the other WHS cultivars. Winter King rye was more mature than the other WHS at the spring harvest each year, which also contributed to its greater spring yield. Winter King was at Feekes (Beuerlein et al., 2005) stage 10 (boot stage), whereas the remaining WHS and spring triticale were at Feekes stage 7 (second node visible). The high yield performance of Winter King rye was consistent with previous trials conducted in Ohio (Samples et al., 1996).
Oat and spring triticale were the greatest yielding WSS in the autumn followed by rape and annual ryegrass (Tables 3–5). Oat and spring triticale monocultures had greater autumn yield than winter triticale, wheat, and VNS rye monocultures at both locations in the 2002–2003 experiment. The oat monoculture yielded 0.4 Mg ha–1 more (P < 0.05) autumn forage than the Winter King rye monoculture at South Charleston in 2002 (WSS x WHS interactions means not shown) but was similar in autumn yield to the Winter King monoculture at Ripley (Table 5). Autumn growth and yield of rape was lower than expected, probably due to the late planting date (9 and 18 Sept. 2002). Bartholomew (1992) reported autumn yields of 3.5 to 4.5 Mg ha–1 for rape planted in July and harvested 
90 d later in southern Ohio. In the current study, annual ryegrass yields were lower than those reported by Kallenbach et al. (2003) in Missouri. The later planting date and cultivar selected may have contributed to the lower autumn yield observed in our study.
Forage Nutritive Value
Crude protein concentrations in all forage treatments were exceptionally high, especially in the autumn. Concentrations of NDF in the forage were generally much lower in the autumn than in the spring. Many statistically significant differences in forage CP and NDF were observed among species treatments; however, most differences were not considered to be biologically meaningful in the context of animal diets, given the high CP and low NDF concentrations observed in the forage from all treatments. The most notable differences were the greater NDF and lower CP concentrations of Winter King rye in the spring and low NDF for rape in both seasons compared with other species. The greater NDF and lower CP for Winter King rye treatments in the spring was attributable to its advanced maturity (Feekes, stage 10) at harvest compared with the other WHS (Feekes, stage 7). Although forage NDF levels for Winter King rye in the spring were relatively high (mean of 520 g kg–1), levels below 550 g kg–1 in grasses are considered to be sufficiently low to meet the digestible forage needs of ruminant animals having high nutrient requirements (Weiss et al., 1999). Cherney and Martin (1982) reported that in vitro dry matter digestibility of grasses was 650 g kg–1 when cell wall constituents were near 500 to 520 g kg–1. Winter King rye also had the lowest mean CP concentrations in the spring; however, those CP levels (>170 g kg–1) were within an acceptable range for most classes of livestock. Rape NDF levels in the autumn and spring were very low, which is common for Brassica species. Adding a WHS to rape helped increase NDF levels in the harvested forage, which is desirable when feeding rape to large ruminants.
Seed Cost Implications
The seed cost ha–1 varied greatly among the different species and cultivars used in our study (Table 1). While a detailed economic analysis is beyond the scope of this report, some general observations can be made regarding the impact of seed cost on marginal returns for the different species combinations. Oat and spring triticale mixtures nearly always had greater autumn forage yield than the corresponding WHS monocultures; however, the low seed cost ha–1 of oat (Table 1) resulted in a considerably lower cost per unit of forage produced relative to the spring triticale treatments (data not shown). Rape had the lowest seed cost ha–1, but rape mixtures produced lower autumn yield, resulting in slightly greater cost per unit of forage produced in the autumn relative to the oat treatments (data not shown). The autumn yield improvement from mixing oat with a forage-type rye cultivar like Winter King was not large on average; however, the cost per unit of forage produced was near breakeven compared to Winter King planted alone, with the added benefit of more consistent autumn production. For example, the Winter King + oat mixture yielded significantly more than the Winter King monoculture at South Charleston in both 2001 (Table 3) and 2002 (WSS x WHS means not shown), resulting in a lower cost per unit of forage produced for the oat mixture in those two environments.
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SUMMARY AND CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Winter King rye produced high forage yield in the autumn, demonstrating that rye cultivars with an active autumn growth habit are excellent options to fill both autumn and early spring forage needs. Mixtures of the forage type Winter King rye with oat provided the greatest combined autumn and spring yield at an economical seed cost. If sufficient land resources are available, then oat and rye could be planted in separate blocks or fields. That practice offers the option for a larger proportion of the total area to be planted to oat for increasing autumn forage supplies and a smaller acreage planted to rye alone or oat + rye for both autumn and spring forage. Limiting the rye acreage would facilitate grazing and harvest management in the spring, which is often a challenge with rye because of its early and rapid reproductive development resulting in a rapid decline of forage nutritive value.
Although this study was not conducted under grazing, the results demonstrated the potential for extending the grazing season in the Ohio Valley region with mixtures of WSS and WHS. Future studies should include evaluations of the seasonal yield distribution and regrowth potential of various WSS and WHS combinations under multiple grazing cycles. Additional studies would also be useful to determine optimal seeding rates and nitrogen fertilization rates for different species combinations in terms of forage yield, forage nutritive value, and marginal economic returns.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Received for publication June 21, 2005.
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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