Published in Agron. J. 96:1540-1544 (2004).
© American Society of Agronomy
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Production Papers
Comparative Morphology of Caucasian Old World Bluestem and Native Grasses
Keith R. Harmoneya,* and
Karen R. Hickmanb
a Kansas State Univ. Agricultural Research Center, Hays, KS 67601
b Dep. of Plant and Soil Sci., Oklahoma State Univ., Stillwater, OK 74078
* Corresponding author (kharmone{at}ksu.edu)
Received for publication March 12, 2004.
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ABSTRACT
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Caucasian old world bluestem (OWB) [Bothrichloa bladhii (Retz) S.T. Blake] has been widely introduced in the southern and central Great Plains as a perennial warm-season grass for forage and soil conservation. As a result of its widespread introduction, it has escaped into some native rangelands. Once established in native pastures, observations suggest that Caucasian OWB may mature earlier than native vegetation and may be avoided by grazing animals in mixed rangelands, thus altering grazing distribution and over utilizing native species. More advanced morphological development in grasses is also associated with lower forage quality and lower palatability. This study was conducted to examine if morphological development of Caucasian OWB and native perennial warm-season grass species differs. Monoculture stands of Caucasian OWB, big bluestem (Angropogon gerardii Vitman), little bluestem [Schizachyrium scoparium (Michx.) Nash], and side-oats grama [Bouteloua curtipendula (Michx.) Torr.] were established in the spring of 2000. After the initiation of first growth in 2002 and 2003, vegetation was hand clipped each week for 8 wk and quantified for morphological stage of development according to the Nebraska staging method. In 2002, a drought season, Caucasian OWB had a greater mean stage weight (MSW) than the native species during Weeks 3 through 7 (P < 0.001). In 2003, Caucasian OWB began the season at a similar morphological stage as the native species, but had a rate of development that was 5 to 11 times greater than the native species. The last 3 wk of the sampling period, MSW of Caucasian OWB was greater than all three native species (P < 0.001). With more advanced morphological development in Caucasian OWB than the native grasses, declining forage quality could affect grazing preference and distribution patterns in mixed swards of Caucasian OWB and native species. The introduction of Caucasian OWB into native rangelands may have impacts that have yet to be understood.
Abbreviations: DOY, day of year • MSC, mean stage count • MSW, mean stage weight • OWB, old world bluestem
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INTRODUCTION
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CAUCASIAN OLD WORLD BLUESTEM (OWB) has been widely introduced in the southern and central Great Plains of the USA as a warm-season perennial grass for soil stabilization in Conservation Reserve Program fields, road rights-of-way, and as pasture or hay for animal consumption. With proper management, monocultures of Caucasian OWB are capable of producing abundant dry matter for hay or grazing (Sanderson et al., 1999). However, Caucasian OWB has been observed as an exotic species in some native rangelands in Kansas, Oklahoma, and Texas and may be an undesirable plant community component because of unknown effects on utilization, growth, and reproduction of native vegetation (Wilson and Shay, 1990; Lodge, 1993; Simberloff, 1996; Sakai et al., 2001).
In native rangelands where Caucasian OWB has already established, we have observed cattle (Bos sp.) avoiding the Caucasian OWB plants, which may alter grazing distribution and increase utilization of the native vegetation. Furthermore, our observations suggest that a potential reason for avoidance of Caucasian OWB is that these plants are more mature than the native vegetation. Plant maturity is the primary factor affecting forage quality (Nelson and Moser, 1994), and Caucasian OWB forage quality has been found to rapidly decline with advancing maturity (Dabo et al., 1988). Additionally, plant maturity can impact herbage preference of grazing animals (Briske, 1991). Differences in plant maturity or rates of morphological development could affect grazing preference and distribution patterns in rangelands that have mixed stands of native species and introduced Caucasian OWB. Berg and Sims (1984) alluded to similar palatability concerns with yellow OWB [Bothriochloa ischaemum (L.) Keng], also used for conservation and forage production. Animals may selectively graze and overutilize native vegetation if less mature than Caucasian OWB.
A comparison of development between Caucasian OWB and native species has been performed on seedlings. Compared with seedlings of Indiangrass [Sorghastrum nutans (L.) Nash], sand bluestem (Andropogon hallii Hack.), blue grama [Bouteloua gracilis (Willd. ex Kunth) Lag. ex Griffiths], and switchgrass (Panicum virgatum L.), Caucasian and yellow OWB seedlings produced more biomass, leaf area per plant, tillers, and leaves per tiller than the native species when grown in a greenhouse environment (Coyne and Bradford, 1985). A comparison of morphological development in established stands of native species and Caucasian OWB has not been documented.
Once already established in mixed stands of native species in central Kansas, Caucasian OWB appeared to be more mature and underutilized. Differences in plant maturity were suspected as being a major contributor to observed Caucasian OWB grazing avoidance, but information regarding a direct comparison of morphological development of Caucasian OWB and native grass species was lacking. Morphology of switchgrass and big bluestem (Angropogon gerardii Vitman) has been successfully quantified and predicted by day of year and accumulated growing degree days using the Nebraska staging system (Mitchell et al., 1997). This study was conducted to quantify differences in developmental morphology of Caucasian OWB and three native grass species during the early summer growing period in central Kansas.
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MATERIALS AND METHODS
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Research was conducted on an Armo loam soil (fine-loamy, mixed, mesic Entic Haplustolls) with 1 to 3% slope belonging to Fort Hays State University in central Kansas (38°51'29''N lat, 99°22'14''W long, elev. 611 m above sea level). Precipitation and temperature data were collected daily at a permanent weather station located 3 km from the study on the Kansas State University Agricultural Research Center at Hays. Pure stands of Caucasian OWB, ‘Kaw’ big bluestem, ‘Aldous’ little bluestem, and ‘El Reno’ side-oats grama were seeded in the spring of 2000 with a no-till seeder in a completely randomized design with two replications. Seasonal growth was allowed to accumulate on the plots each of the first two growing seasons, 2000 and 2001. Residual dry matter was mowed and removed in the spring of 2001 and 2002 before the onset of new growth. In 2003 residual dry matter was burned before the onset of new growth because of favorable climatic and soil moisture conditions. Individual plots were 5 by 10 m in size and were not fertilized in any year. Plots were randomly divided into eight subplots at the beginning of the 2002 and 2003 seasons to correspond with harvest locations for eight sampling dates. Sampling occurred from 4 June to 24 July in 2002 and from 13 May to 1 July in 2003. Harvest dates differed between years because of vast precipitation and temperature differences that altered initiation of seasonal growth. Vegetation within two 0.1 m2–frames were harvested at ground level from one subplot per main plot each week for 8 wk. Samples from each frame were placed in individual bags and refrigerated at 4.5°C until morphological development could be determined. Individual live tillers from each sample were grouped by stage according to the Nebraska staging method, which includes four main phases of grass tiller development: (i) vegetative, (ii) elongative, (iii) reproductive, and (iv) seed ripening (Moore et al., 1991; Moore and Moser, 1995). Grass tillers for each respective stage were counted and placed in a forced air oven at 50°C for 48 h. Dry weights for each respective stage of grass tillers were recorded. Overall mean stage count (MSC) of a grass sample by tiller count was calculated by the following formula (Moore et al., 1991; Moore and Moser, 1995).
where Si is the growth stage, from 1 to 4.9, Ni is the number of tillers in stage Si, and C is the total number of tillers. To calculate MSW, N and C were replaced with the total dry weight for the tillers in each stage and the total dry weight for all tillers. All samples from a harvest were staged before the next harvest, and calendar day of year (DOY) was determined for each harvest date.
Dry matter yield, tiller density, MSC, and MSW were analyzed by the general linear model of SAS (SAS Inst., 1995). Traits were considered to be statistically different if P < 0.05, unless otherwise noted. Linear contrasts were used to determine if grass development over time was different than a slope of 0, and if slopes of development trends for each species were different from each other. If slopes of development were determined to be different than 0, regression analysis on individual sample observations of each species and DOY of each harvest date within a season was performed using the REG or NLIN procedure of SAS (SAS Inst., 1995). Fisher's protected least significant differences were also used to compare harvest date or species x harvest date means within years at P < 0.05.
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RESULTS AND DISCUSSION
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Climatic Conditions
Precipitation pattern for the two seasons of sampling differed (Fig. 1). Much less precipitation fell throughout the 2002 season compared with 2003. In 2002, April was the only month before or during sampling that had normal precipitation levels. From March to June, 14.2 cm of precipitation was received, only 56% of the long-term average of 25.2 cm. In addition, the mean high and low temperatures (Fig. 2) for June and July were 4% above the normal long-term average and compounded the low rainfall total. Lack of precipitation resulted in delay of initial spring grass growth and development. In 2003, above normal precipitation fell in March, April, and June. Average high and low temperatures were also slightly below normal (3%) for May and June. Adequate precipitation and soil moisture were present to expect normal late spring and early summer growth and development in 2003.
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Fig. 1. Precipitation from March through August for 2002 and 2003, and the long-term normal mean for Hays, KS.
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Fig. 2. Average high and low temperatures for March through August in 2002 and 2003, and the monthly long-term normal mean for Hays, KS.
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Developmental Morphology
Comparing MSC and MSW values for all species, a significant positive linear relationship was found between MSC and MSW. Grass MSW tended to be slightly greater than MSC for the same sample, with values just below the 1:1 line as evidence (Fig. 3), but the correlation coefficient shows that the two measurable traits were highly correlated (r = 0.98) through the range of values found in this study. Therefore, MSW data were presented to indicate similar trends found between MSC and MSW.
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Fig. 3. Relationship of mean stage weight (MSW) to mean stage count (MSC) for Caucasian old world bluestem (OWB), big bluestem, little bluestem, and side-oats grama from eight harvests per season in 2002 and 2003 at Hays, KS.
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Biomass yield and MSW had a significant species x harvest x year interaction; therefore, data were sorted according to year and analyzed. At the start of 2002 harvests, MSW of grasses was near 1.5, except for side-oats grama, which had a MSW value near 1.2 (Fig. 4). Caucasian OWB development increased rapidly the first half of the sampling period, reaching a MSW of 2.2 by the fourth week of harvest. From calendar Day 169 to 196, for a total of five harvest dates, Caucasian OWB did have significantly greater MSW than the three native species (P < 0.001). However, a decline in maturity at the end of the season resulted in a MSW similar to the native species at the last sampling period, and total season development of Caucasian OWB followed a quadratic trend (Table 1). Little bluestem had a decreasing trend in MSW during 2002, while big bluestem and side-oats grama MSW showed no trend and remained at a constant level through the season (Fig. 4). The Nebraska staging system quantified live tillers, so the decline in morphological stage for Caucasian OWB may be explained by tiller senescence from elevated temperatures and inadequate late season moisture. Tillers with more biomass have been found to use more water to remain viable (Baker and Hunt, 1961). Caucasian OWB developed more rapidly and had greater biomass, so greater transpiration and greater soil moisture depletion may have resulted in tiller senescence and lower MSW at the last harvest. Tiller density slowly declined for Caucasian OWB after the first harvest, and the last harvest had only 25% of the tillers counted in the first week (data not shown). Tillers accumulate biomass as they mature (Hendrickson et al., 1998; Mitchell et al., 1997), so maintaining yield for Caucasian OWB during the first four harvests while tiller density declined was likely from an increase in individual tiller weight (Table 2). The three native species also had continually lower tiller numbers after Harvests 2 through 4 (data not shown), and subsequently yields declined from Week 3 or 4 through the rest of the season. No species x harvest date interaction resulted for yield in 2002.
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Fig. 4. Mean stage weight (MSW) trends of Caucasian old world bluestem (OWB), big bluestem, little bluestem, and side-oats grama at eight harvest dates in 2002 at Hays, KS. * indicates dates at which MSW of Caucasian OWB was greater than all three native species, LSD(0.05) = 0.24.
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Table 1. Mean stage weight equation statistics for Caucasian old world bluestem (OWB), big bluestem, little bluestem, and side-oats grama regressed over day of year (DOY) for 2002 and 2003. Equations significant at the P < 0.05 level (little bluestem in 2003, P = 0.08).
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Table 2. Dry matter production of Caucasian old world bluestem (OWB), big bluestem, little bluestem, and side-oats grama from eight harvests during 2002 and 2003.
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In 2003, all grasses started with similar MSW values. The three native species had increasing linear trends (little bluestem, P = 0.08) for MSW values through the season (Table 1) and did not significantly differ from each other in rate or stage of development. Low coefficients of determination resulted from very little slope increase in MSW for the native species. Caucasian OWB development was best described by a nonlinear function. Midway through sampling, slope of Caucasian OWB maturity appeared to increase to 5 to 11 times greater than that of the native species (Fig. 5). By Harvest 5, Caucasian OWB had advanced to a greater maturity than little bluestem and side-oats grama. At Week 6 on calendar Day 168, Caucasian OWB morphology was greater than all three native species (P < 0.001) and remained greater throughout the rest of the sampling period (Fig. 5). Caucasian OWB finished the sampling period with a MSW value of 2.5, while the three native species finished the sampling period with MSW values below 1.7. During the last 3 wk of harvests, Caucasian OWB was at least 0.7 stage units greater than the three native species for MSW. All grasses had linear trends for dry matter production throughout the season and reached peak production by Harvest 7 (Day 176) or Harvest 8 (Day 182) (Table 2). Caucasian OWB had greater gains in yield than the native species as the season progressed. All grasses had tiller densities at the end of the sampling period that were similar to densities from the first sampling period (data not shown).
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Fig. 5. Mean stage weight (MSW) trends of Caucasian old world bluestem (OWB), big bluestem, little bluestem, and side-oats grama at eight harvest dates in 2003 at Hays, KS. * indicates dates at which MSW of Caucasian OWB was greater than all three native species LSD(0.05) = 0.25.
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Big bluestem did not develop as predicted by the MSW equation of Mitchell et al. (1997) in either year—even in 2003, where above normal precipitation was available. None of the native grasses showed any advancement in development in 2002, likely because of the large deficit in precipitation from March to June. In 2003, big bluestem was nearly 0.3 MSW units more mature at the first harvest and 0.3 MSW units less mature at the last harvest than what was predicted by using the MSW equation of Mitchell et al. (1997). Differences in big bluestem development between our study and that of Mitchell et al. (1997) could have been a result of differences in elevation between the studies, stress caused by the lack of precipitation in 2002, timing of precipitation in 2003, or nutrient status. Although moisture status was drastically different between years, Caucasian OWB had greater maturity than other grasses beginning near calendar Day 168 both years. Caucasian OWB had a greater proportion of the stand with culmed and reproductive tillers, while the native grass stands remained mostly vegetative. The high proportion of vegetative tillers late in the season is consistent with two other native warm-season grasses observed in the Nebraska sandhills (Hendrickson et al., 1998). Forage morphological development affects foraging behavior (Stuth, 1991), and with more advanced morphological development in Caucasian OWB than the native grasses, declining forage quality (Dabo et al., 1988) indeed could affect grazing preference and distribution patterns in mixed swards of Caucasian OWB and native species. The introduction of exotic Caucasian OWB into native rangelands may have impacts on native vegetation that are yet to be understood.
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NOTES
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Journal Paper no. 04-293-J of the Kansas Agric. Exp. Stn., Manhattan, KS.
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REFERENCES
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ABSTRACT
INTRODUCTION
