Agronomy Journal Journal of Natural Resources and Life Sciences Education
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

Published online 7 February 2006
Published in Agron J 98:270-279 (2006)
DOI: 10.2134/agronj2005-0100
© 2006 American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Belesky, D. P.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Belesky, D. P.
Agricola
Right arrow Articles by Belesky, D. P.
Related Collections
Right arrow Forage Management
Right arrow Other Forage Crops
Right arrow Crop Ecology

Production Papers

Regrowth Interval Influences Productivity, Botanical Composition, and Nutritive Value of Old World Bluestem and Perennial Ryegrass Swards

David P. Belesky*

USDA-ARS, Appalachian Farming Systems Research Center, 1224 Airport Road, Beaver, West Virginia 25813

* Corresponding author (david.belesky{at}ars.usda.gov)

Received for publication April 5, 2005.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS and METHODS
 RESULTS and DISCUSSION
 SUMMARY
 REFERENCES
 
Botanical composition of pasture in much of the Appalachian Region varies as weather patterns interacting with abiotic and biotic factors create conditions allowing a range of plant species to occupy temporal and spatial gaps in the stand. Field-grown old world bluestem [Bothriochloa caucasia (Trin.) C.E. Hubb.] or perennial ryegrass (Lolium perenne L.) were defoliated at fixed time intervals (1-, 2-, or 4-wk for old world bluestem and 2-, 4-, or 6-wk intervals for perennial ryegrass). Herbage mass, sward botanical composition, and estimates of nutritive value were determined. White clover (Trifolium repens L.), black medic (Medicago lupulina L), and various common forbs and grasses volunteered in all stands. Old world bluestem yields ranged from 3.38 Mg ha–1 (1-wk regrowth interval in 2002) to 9.42 Mg ha–1 (4-wk regrowth interval in 2000). Perennial ryegrass yield increased with increasing regrowth interval in 2002, ranging from 3.16 to 7.29 Mg ha–1. The longest regrowth interval did not necessarily lead to the greatest productivity. Old world bluestem swards clipped at 4-wk intervals decreased 60% by the third growing season, whereas perennial ryegrass swards clipped at 6-wk intervals increased by 37%. The proportion of encroaching species increased substantially in perennial ryegrass but not old world bluestem swards by 2002, suggesting that combined effects of repeated defoliation and weather influenced sward composition and productivity. Choice of plant resource, timing and intensity of defoliation, and very likely nutrient inputs coinciding with requirements of sown species could influence sward composition and help manage diversity to sustain productivity.


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS and METHODS
 RESULTS and DISCUSSION
 SUMMARY
 REFERENCES
 
ARELIABLE SUPPLY OF FORAGE is a critical requirement for successful livestock production operations. In much of Appalachia, temperature determines the limits of growth intervals, and variation in short-term weather patterns complicates the predictability of herbage production and botanical (floristic) composition of the sward. Landscape position (Belesky et al., 2002b) and disturbances that favor certain species, such as defoliation strategy or nutrient inputs (White et al., 1997), also affect productivity and sward composition. For example, productivity and seasonal distribution of cool- (Belesky and Fedders, 1994) and warm-season (Belesky and Fedders, 1995a) grass yield in central Appalachia were influenced by defoliation frequency and intensity under modest nutrient input conditions. Total annual productivity was comparable for cool- and warm-season grasses, but the interval and occurrence of productivity was complementary, and sward botanical composition changed with time (Belesky and Fedders, 1995b).

Maintaining monospecific stands of sown forages in pasture is difficult in the Appalachian region. Weather patterns, management, and competition among plants in the stand interact to create conditions allowing a range of native and naturalized forbs, grasses, and legumes to occupy gaps in the sward (Campbell et al., 1996). Fluctuations in botanical composition over years often were correlated with climatic conditions (Silvertown et al., 1994) and management. Management practices, such as modest nutrient inputs or ill-timed defoliation, can interact with weather patterns to create conditions allowing a range of forbs, grasses, and legumes to occupy temporal and spatial resource gaps in the stand. Simply managing the extent of sward removal could shift the dynamics of competition for space in grass canopies (Barthram et al., 2005). Producers could use complementary growth and seasonal yield distribution patterns of cool- and warm-season grasses to improve resource acquisition and sward productivity as conditions vary within and among growing seasons.

Swards sown to warm- or cool-season perennial forages often change with time into mixtures that vary in botanical composition, herbage productivity, and nutritive value. Cool-season swards clipped or grazed intensively can be invaded by warm-season species, such as crabgrass (Digitaria spp.), and warm-season swards can be invaded by a number of cool-season species adapted to a site, especially in spring and late summer. Some warm-season grasses, such as ‘Caucasian’ old world bluestem [Bothriochloa caucasica (Trin.) C.E. Hubb., referred to herein as old world bluestem], benefit from close grazing (Jung et al., 1985; Christiansen and Svejcar, 1988) and form dense swards that offer few sites for volunteer species encroachment. Old world bluestem was more productive than comparably managed warm-season grasses in southern West Virginia (Belesky and Fedders, 1995a) and central Pennsylvania (Jung et al., 1990). Swards in West Virginia were virtually devoid of weeds, regardless of clipping frequency or intensity, and stand density increased as clipping frequency increased (Belesky and Fedders, 1995a).

Perennial ryegrass (Lolium perenne L.) is grown widely in temperate regions and is the mainstay of pastures in much of Europe and New Zealand. Documentation of the agronomic and ecophysiologic attributes of Lolium species for growing conditions in eastern North America is limited (Jung and Shaffer, 1993; Jung et al., 1996; Hall et al., 1998; Kunelius et al., 2004). Jung and Shaffer (1993) found that perennial ryegrass grown in mixture with alfalfa (Medicago sativa L.) produced greater yield and contributed to stands with fewer weeds compared with stands containing timothy (Phleum pratense L.).

Grazing imposes additional disturbance and acts as an agent of change in sward structure through selectivity among sward components and differential removal based on plant morphology and grazing animal behavior. Herbage intake is generally regulated by concentration of energy related to chemical composition of the sward. Ontogeny, morphology, and management influence herbage composition, and the desired nutritional composition of a diet often is achieved with mixtures of plant resources. Consequently, estimates of herbage energy, such as metabolizable energy (ME) or total digestible nutrients (TDN), expressed relative to herbage nitrogen, are needed to improve nutrient-use efficiency of the grazer on pasture where strategic supplementation may be difficult or economically unfeasible (NRC, 1996; Moore et al., 1999).

A field plot experiment was conducted to determine yield distribution and botanical composition of monospecific swards sown to a bunch-type cool- or warm-season grass clipped to create short or long regrowth intervals. Although the sown grasses chosen for this experiment have adapted, typically they are not grown in the region and are unlikely to be represented in the soil seed bank. Thus, the sown swards provided a backdrop against which the changes in sward composition could be evaluated, and production data for two species adapted to but not widely grown in the region at this time could be obtained. Grazing was not imposed because grazer selectivity and differential removal are difficult to control and confound sward production and botanical composition estimates. Encroachment by naturalized white clover (Trifolium repens L.), forbs, and other grasses was considered a feature of treatment effects.


   MATERIALS and METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS and METHODS
RESULTS and DISCUSSION
 SUMMARY
 REFERENCES
 
Site Characteristics
Plots were established on an upland site with convex topography and 0 to 5% slope, located on the Allegheny Plateau of southern West Virginia (37°46' N lat; 81°00' W long; 870 m elevation above sea level). Soil was a Clymer series, channery loam (coarse-loamy, siliceous, active, mesic Typic Hapludult). Existing vegetation was a well established stand of tall fescue (Festuca arundinacea Schreb.) that was clipped occasionally for hay. Presence of other plant species in the original sward was minimal (<5% frequency).

Establishment and Management
Glyphosate [N-(phosphono-methyl) glycine] was applied at 2.52 kg a.i. ha–1 in early June, 1997 to eliminate the existing tall fescue. The area was sown to black oats (Avena strigosa Schreb.) at 65 kg seed ha–1 using a Tye no-till seeder in June 1997. The entire area was fertilized with 1 Mg ha–1 of 10–20–20 commercial N–P–K fertilizer. Initial pH was 7.03 in the surface 15 cm of soil, with moderate amounts of P (about 15 kg ha–1) and ample K (about 250 kg ha–1). Oats grew to maturity and in mid-August were rolled and cultipacked onto the soil surface. Black oats reseeded and grew along with a late-season flush of broadleaf weeds. The area was sprayed with 1 L ha–1 2,4-D amine (2,4-dichlorophenoxy acetic acid) to control broadleaves. Black oats were clipped and chopped, with the residue retained in situ as a thatch for winter cover. Glyphosate (2.52 kg a.i. ha–1) was applied in April 1998 to control weeds and any reseeding black oats. The area was surface-tilled and lightly compacted before seeding.

Three replicate plots (5 by 21 m) of ‘Caucasian’ old world bluestem or black oats were established in mid May 1998 by broadcasting 10 kg live seed of old world bluestem or 65 kg seed ha–1 black oats on respective plots, followed by cultipacking. Seeded plots received 235 kg ha–1 of 19–19–19 commercial N–P–K fertilizer in July 1998. Plots containing black oats were surface tilled, and commercially available ‘Linn’ perennial ryegrass (hence referred to as perennial ryegrass) was sown in late August 1998 at 34 kg seed ha–1. Fertilizer (240 kg ha–1 of 19–19–19 commercial N–P–K fertilizer) was applied in a split application each year with half applied in mid April (perennial ryegrass) or mid May (old world bluestem) and half in mid July to all plots. Establishment year herbage samples were not included in productivity, seasonal distribution, or nutritive value data analysis. Old world bluestem plots were clipped at 2-, 4-, and 6-wk regrowth intervals in 1999, but data were not included in the analysis and are not reported here. The 6-wk regrowth interval was omitted for old world bluestem swards and a 1-wk interval added in 2000.

Treatments and Analyses
Sown species main plots were split to accommodate three regrowth intervals of 2-, 4-, or 6-wk for perennial ryegrass and 1-, 2-, and 4-wk intervals for old world bluestem in a randomized, complete-block design. Species had the 2- and 4-wk regrowth intervals in common. The initial clip was made in spring when mean sward height reached 20 cm. Plants were clipped to a 5-cm residue height, and regrowth intervals were imposed thereafter. In 2000, removal frequency for old world bluestem included 1 wk to simulate intensive canopy use, and the 6-wk removal interval was omitted. A yield strip was collected from the center of each plot with a bag-equipped rotary mower. Samples were dried at 65°C in a forced-air drying oven.

Botanical composition was determined by visual estimation (Warren-Wilson, 1959). Herbage residue (soil surface to 5 cm) was estimated from micro-swards (10 cm diameter) collected in late July or early August each year. Three micro-swards were collected from each treatment within each replicate. Herbage from each micro-sward was collected to soil surface, separated into species and senesced components, dried, and weighed. Components were designated as seeded grass (old world bluestem, perennial ryegrass), white clover, other grasses, and forbs. The white clover, grasses, and forbs volunteered from the naturalized population seed bank in the soil. No attempt was made to characterize the soil seed bank in this experiment.

Nitrogen was determined by combustion of dry plant tissue (Carlo Erba EA 1108 CHNSO analyzer; Fisons Instruments, Beverly, MA) and expressed as crude protein (CP) (N g kg–1 x 6.25). Total nonstructural carbohydrates were determined by an autoanalyzer (Alpkem RFA 300; Astoria-Pacific, Intl. Clackamas, OR) procedure (Denison et al., 1990). Nutritive value determination included ash (AOAC, 1990) and in vitro organic matter disappearance (IVOMD) (Moore 1970). The IVOMD procedure used rumen fluid obtained from rumen-cannulated steers offered orchardgrass (Dactylis glomerata L.)–alfalfa hay. Computations for estimates of nutritive value included CP and metabolizable energy of feed (ME) as (MJ kg–1 dry matter [DM]) = 0.0157 (IVOMD) (AFRC, 1993). Values for TDN were calculated from ME data (NRC, 1996) as TDN (%) = {[(ME/4.184)/0.82]/4.409} x 100.

Statistical Analysis
Data for yield (each harvest), cumulative dry matter yield, dry matter distribution, botanical composition, protein concentration, and IVOMD were analyzed as a split-plot design using SAS-MIXED procedure (Littell et al., 1996). Regrowth interval, sown species, and harvest dates were considered fixed effects and replication random in the model. Harvest dates were normalized for 30-d intervals. Years were analyzed separately within the mixed model because {chi}2 test indicated heterogeneity of variance. Denominator degrees of freedom were calculated using the Satterthwaite option of MIXED analysis to determine appropriate degrees of freedom. Treatment effects were considered significant at P < 0.05.


   RESULTS and DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS and METHODS
 RESULTS and DISCUSSION
SUMMARY
 REFERENCES
 
Weather Conditions
Minimum temperatures were slightly above the 30-yr mean for the location. Maximum temperatures were slightly above the mean in April through June and below from July through October (Fig. 1 ). Precipitation was near the 30-yr mean, but distribution varied within and among years (Fig. 1). Distribution of precipitation within a growing season is probably more important than total precipitation as an influence on herbage production and botanical composition of mixed-species canopies (Dunnett and Grime, 1999).


Figure 1
View larger version (42K):
[in this window]
[in a new window]
  		Fig. 1. Mean monthly maximum and minimum air temperatures, monthly precipitation, and 30-yr mean values for each parameter at Beckley, WV.

 
Occasional extreme weather events can influence patterns of vegetation change and disrupt the typical seasonal progression of development. In this experiment, three consecutive nights with subfreezing temperatures in May 2002 damaged vigorously growing perennial ryegrass and new regrowth of old world bluestem. Subfreezing conditions influenced the total number of harvests and duration of the 2002 growing season.

Total Herbage Dry Matter Yield
Total DM yield (sum of all herbage for a given treatment) differed significantly among years for the interaction of sown species and regrowth interval (Table 1). Regrowth interval did not influence total DM yield in a predictable manner and as such was an agent of diversity. Yields from plots sown to old world bluestem ranged from 3.38 Mg ha–1 (1-wk regrowth interval in 2002) to 9.42 Mg ha–1 (4-wk regrowth interval in 2000) and were similar to old world bluestem yields obtained from swards clipped on canopy development criteria (Belesky and Fedders, 1995a).


View this table:
[in this window]
[in a new window]
  		Table 1. Total yield mean ± standard error of mean (Mg ha– 1) of swards sown to old world bluestem (OWB) and perennial ryegrass (PRG) as a function of regrowth interval (RI) at the end of each of 3 yr (2000–2002).

 
Perennial ryegrass yields differed among regrowth intervals, but trends were not consistent in 2000 and 2001 (Table 1). Yield increased with increasing regrowth interval in 2002 and ranged from 3.16 Mg ha–1 to 7.29 Mg ha–1 (Table 1). Yield was least when swards were clipped repeatedly at 2-wk intervals and greatest when clipped at 6-wk intervals. Changing sward botanical composition and variation in productivity within and among years contributed to differences in total yield. Total yields were comparable to those reported by Spitaleri et al. (2001) for Linn perennial ryegrass clipped at approximately 4-wk intervals and produced under similar conditions in eastern Kentucky. Yields also were comparable to those of ryegrass as a component of alfalfa–ryegrass mixtures growing in central Pennsylvania (Jung and Shaffer, 1993).

Comparatively dry conditions late in 2002 and severe frost in mid-spring led to a short growing season and correspondingly fewer harvests (Fig. 2 ). Dry matter yield was influenced by seeded species, with regrowth interval, harvest, and interactions among factors having significant, although less, impact. There was substantial variation in total yield for perennial ryegrass each year as a function of regrowth interval, whereas variation was less for old world bluestem (2000 being the exception) (Fig. 2). For example, perennial ryegrass swards clipped at 2-wk intervals had similar productivity (~5 Mg ha–1) in 2000 and 2001 but less (3 Mg ha–1) in 2002. Conversely, productivity of swards clipped at 4-wk intervals fluctuated, and swards clipped at 6-wk intervals increased from 5 Mg ha–1 in 2000 and 2001 to 7 Mg ha–1 in 2002. Yields represent the mass of all herbage in the sward and reflect botanical composition. The longest regrowth interval was detrimental to total productivity of old world bluestem swards but was beneficial to perennial ryegrass yield.


Figure 2
View larger version (35K):
[in this window]
[in a new window]
  		Fig. 2. Cumulative yield of all herbage in plots sown to old world bluestem (OWB) and perennial ryegrass (PRG) as a function of regrowth interval (RI) during the 2000, 2001, and 2002 growing seasons. Analysis of variance includes RI, species (S), harvest date (H), and the interactions. Each point is the mean of three replicates.

 
The duration of old world bluestem sward growth was less than that of perennial ryegrass in any given year (Fig. 3 ), although there was some variation attributable to regrowth interval. The influence of regrowth interval was not consistent for sown species. Productivity of old world bluestem with the greatest regrowth interval (4-wk) decreased 60% by the third growing season, but that of perennial ryegrass clipped at 6-wk intervals increased 37% (Fig. 3). A 6-wk regrowth interval was detrimental to the excellent site filling characteristics of old-world bluestem (data not shown) and was discontinued in 1999; it was replaced with a 1-wk regrowth interval treatment in 2000. Total DM yield was similar among regrowth intervals for old world bluestem but varied with interval for perennial ryegrass (Fig. 2).


Figure 3
View larger version (35K):
[in this window]
[in a new window]
  		Fig. 3. Cumulative yield of seeded species (old world bluestem [CB] and perennial ryegrass [PRG]) as a function of 2- and 4-wk regrowth intervals (RI) during the 2000, 2001, and 2002 growing seasons. Analysis of variance includes RI, species (S), harvest date (H), and the interactions. Each point is the mean of three replicates.

 
Seasonal Distribution of Dry Matter
Some of the variation in DM yield was caused by changing proportions of sward components. Seeded species predominated in swards in the first growing season and decreased as a proportion of total sward mass with each successive year in the case of perennial ryegrass or varied irrespective of year in the case old world bluestem (Fig. 4 ). The general decline in seeded species is typical of forage swards in the region, regardless of whether the sward is defoliated on fixed time intervals or on morphologic criteria (Jung and Shaffer 1993; Belesky and Stout, 1994; Belesky and Fedders, 1994, 1995a; Belesky et al., 1999, 2000, 2002a, 2002b). Perennation of seeded grasses in well managed pasture depends predominantly on the occurrence and expression of vegetative propagules, such as tillers, rhizomes, and stolons, associated with a given species (Benson et al., 2004). The expression of vegetative propagules depends as much on the genetic potential of the plant as on management and environment. Consequently, seasonal production patterns and sward composition are likely to vary with growth characteristics displayed by individual species and cultivars.


Figure 4
View larger version (64K):
[in this window]
[in a new window]
  		Fig. 4. Distribution of estimated dry matter (DM) based on sward botanical components as a function of regrowth interval (RI) and time (day-of-year) during the 2000, 2001, and 2002 growing seasons. Analysis of variance includes RI, species (S), harvest date (H), and the interactions.

 
Distribution of DM production, across and among growing seasons, differed with sown species as expected. Production patterns of the seeded species were complementary (Fig. 4). The production interval for warm-season grasses is relatively short in portions of northeastern North America (Madakadze et al., 1998) and ranges from 90 to 110 d in the central Appalachian highlands, depending on the occurrence of frost (Belesky and Fedders, 1995b). The relative contribution of each seeded species to total sward yield varied as a function of time during the growing season, reflecting different requirements for growth. The influence of sown species on distribution of growth within a growing season differed among years and probably reflects weather conditions within a year as much as the cumulative effects of regrowth interval and age of the stand. For example, the relatively mild, wet conditions that occurred in July of each year of the experiment seemed to favor the persistence of white clover.

Changes in sward composition caused by defoliation can, in some cases, be relatively minor compared with floristic changes caused by rainfall distribution patterns (Archer and Smeins, 1991). In most years, periodic drought and relatively warm temperatures slow production of cool-season forages common in pastures of the Northeastern USA. Conversely, warm-season species are likely to maintain productivity under these conditions. Carlassare and Karsten (2003) noted that grazing regimen influenced botanical composition and seasonal distribution of herbage mass but not total productivity. They noted that a productive, invasive species, quackgrass (Elytrigia repens L., Nevski), was relatively insensitive to dry periods and made substantive contributions to sward production.

Regrowth interval, harvest date, and sown species interacted to influence DM distribution among sward components, but the significance of interactions was not consistent among years (Fig. 4). Sown species tended to have the greatest influence (P > F2000 = 47.91; P > F2001 = 29.04; P > F2002 = 39.50) on DM distribution among sward components. Legumes generally made a greater contribution to estimated sward DM in perennial ryegrass than old world bluestem swards (Fig. 4). Legume presence was greatest in 2001 regardless of regrowth interval or sown species and reflects relatively ample precipitation and moderate temperatures occurring that year.

The relative proportion of legumes (volunteer plants of white clover and black medic) and forbs (various species, including Plantago spp.; Stellaria media L.; Cerastium vulgatum L.; Polygonum pensylvanicum L.; Lamium amplexicaule L. predominated) surpassed the sown grass component of perennial ryegrass swards by 2002 (Fig. 4). The proportion of forbs increased in the sward by the third growing season regardless of regrowth interval, reflecting combined effects of repeated defoliation, weather, and time. The decline in perennial ryegrass over time was compensated by an increase in other species (i.e., forbs and grasses [Festuca arundinacea Schreb., Poa annua L., Digitaria spp.]).

Residual Herbage
Dry matter distribution of sward residue was influenced by regrowth interval in 2000 but not in subsequent years (Fig. 5 ). The influence of seeded species on sward residue composition decreased with time. Old world bluestem mass was greater when swards were clipped at 1- compared with 4-wk intervals. There was more white clover in perennial ryegrass than in old world bluestem sward residue in 2000 and 2001 and no apparent difference between the seeded species in 2002. Relative to perennial ryegrass, old world bluestem swards tended to form dense sods and as such would compete with white clover for space, limiting white clover encroachment. Differential competition would occur for water across the growing season, especially in summer when warm-season species would be at an advantage among sward components.


Figure 5
View larger version (62K):
[in this window]
[in a new window]
  		Fig. 5. Dry matter distribution of sward residual mass (soil surface to 5-cm defoliation height) of old world bluestem (OWB) and perennial ryegrass (PRG) as a function of regrowth interval (RI) during the 2000, 2001, and 2002 growing seasons. Analysis of variance includes RI, species (S), and the interaction.

 
Residual herbage mass (soil surface to 5-cm residue height) could influence the size and number of gaps, leading to encroachment by other plants and eventually floristic change in the sward. Other grasses and forbs contributed about 50% of sward residue mass irrespective of year, seeded species, or defoliation treatment. Mass reflects tiller numbers that are a function of how a particular species is expressed and responds to defolition and changing light conditions in the sward (Matthew et al., 2000; Christiansen and Svejcar, 1988), influencing competition for space and resources. Tillering is related to temperature-driven leaf appearance and senescence processes (Nelson, 2000), clearly linking climate and management.

Nutritive Value
The TDN/CP quotients varied among years and within a growing season for perennial ryegrass and old world bluestem (Fig. 6 ). The quotients for perennial ryegrass clipped at 2-wk intervals represent herbage that may have insufficient energy relative to CP (Fig. 6). In general, TDN/CP quotients ranged from about 2.0 to 4.0 for perennial ryegrass plots clipped at 2-wk intervals and from 2.5 to 5.0 for plots clipped at 4-wk intervals. The quotients for plots sown to perennial ryegrass are probably a product of substantial white clover encroachment contributing to herbage mass and nutritive value with correspondingly high N concentrations. Quotients for perennial ryegrass swards represent herbage with insufficient energy for efficient N use (NRC, 1984).


Figure 6
View larger version (39K):
[in this window]
[in a new window]
  		Fig. 6. Seasonal distribution of available herbage total digestible nutrient (TDN): crude protein (CP) of old world bluestem (OWB) and perennial ryegrass (PRG) as a function of regrowth interval (RI) during the 2000, 2001, and 2002 growing seasons. Analysis of variance includes RI, species (S), harvest date (H), and the interactions.

 
The quotients for old world bluestem, irrespective of clipping frequency, approximated those recommended for medium-frame, growing beef cattle (NRC, 1984). The TDN/CP quotients of old world bluestem very often were greater than those of perennial ryegrass for a given point in the growing season. The TDN/CP of old world bluestem ranged from 3.0 to 5.0 for swards clipped at 2-wk intervals and from about 4.0 to 7.5 for swards clipped at 4-wk intervals. Forwood et al. (1988) reported acceptable nutritive value throughout the summer for old world bluestem growing in Missouri.

Nutritive value associated with changing sward composition could affest grazer productivity by influencing selectivity among sward components and nutrient use. Feedstuff energy expressed as TDN (NRC, 1996) provides an estimate of digestible energy content on a per-unit and nutrient-requirement basis. A TDN/CP range of 5.0 to 7.0 is likely to meet animal and rumen microorganism needs while allowing for variation in forage system management and seasonal growing conditions (NRC, 1996; Moore et al., 1999). Values less than 5.0 suggest excess herbage N, leading to questionable N-use efficiency. Forages with TDN/CP values less than 5.0 might not be selected by grazers when offered a choice (Mayland et al., 2000).

Fluctuations in floristic composition (variable environment theory; see Chesson, 1994) and sward productivity are a function of environment (Silvertown et al., 1994) and should be factored into the design of pasture systems for the humid eastern USA (Carlassare and Karsten, 2003). Assemblages of plant functional types for pasture can operate at several spatial and temporal scales simultaneously. For example, combinations of functional types serve ecologists evaluating system responses to global change (Epstein et al., 1997), while producers might use similar mixtures to meet dietary needs of grazing livestock as requirements and plant growth patterns vary within or among seasons. Mixtures can buffer variation in productivity associated with growing conditions (Belesky et al., 2002a), landscape positions (Belesky et al., 2002b), and microsite (Feldhake et al., 2004), although absolute benefits of mixtures in grazed system considered in terms of resilience and productivity are the focus of continuing debate.


   SUMMARY
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS and METHODS
 RESULTS and DISCUSSION
 SUMMARY
REFERENCES
 
Pastures often are sown to achieve a desired floristic composition; however, these tend to change naturally over time into mixtures differing from the intended sward structure. The dynamic sward structure is a function of sown species development and persistence and volunteer plants interacting with environment, management, and time. Changing sward composition complicates management strategy and utility of the sward in forage-based livestock systems. Regrowth interval and season effects on cool- and warm-season functional groups, represented by perennial ryegrass and old world bluestem, differed in terms of impacts on productivity and sward composition. Old world bluestem was competitive and persistent and was a dominant component of harvested forage, regardless of regrowth interval, in an environment favoring cool-temperate origin species. Swards sown to perennial ryegrass maintained productivity, but sward composition changed, and yield was a function of increasing amounts of volunteer species and decreasing perennial ryegrass. Changes in sward composition were reflected in nutritive value where perennial ryegrass swards rich in white clover had lesser energy-to-protein quotients. Distribution of productivity and nutritive value of old world bluestem and perennial ryegrass, grown in adjacent plots, were complementary within years. The pattern observed would likely differ if cultivars with different growth strategies and competitive abilities were used.

In part, yield was a function of weather patterns that dictate duration of the growing season and within-season production patterns for different plant functional groups. Changes occurred regardless of sown species or regrowth interval imposed by clipping. Although clipping does not completely represent the effects of grazing, it does minimize the confounding effects of selectivity and differential removal intensities and frequencies on sward composition that are associated with grazing. Well managed pasture offers established forage plants little opportunity to reproduce by seed and thus minimizes contributions to the soil seed bank and at the same time suppresses weed encroachment (Tracy et al., 2004). Weed encroachment occurred in perennial ryegrass swards regardless of management-confirming observations reported by Jung and Shaffer (1993). Old world bluestem seems to be a vigorous competitor, whereas the perennial ryegrass cultivar used was less so. Choice of plant resource and strategically timed nutrient inputs coinciding with requirements of sown species could provide a means to influence sward change.


   ACKNOWLEDGMENTS
 
This work was accomplished with the excellent support of Joyce M. Ruckle in all phases of experiment design and data analysis and the diligent technical assistance of Matthew L. Huffman. Kenneth E. Turner, research animal scientist, and Kenneth N. Harless provided in vitro nutritive value estimates. The authors thank the reviewers of this manuscript for their excellent assistance.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS and METHODS
 RESULTS and DISCUSSION
 SUMMARY
 REFERENCES
 
Trade names are used for the convenience of the reader and do not imply USDA endorsement or preference over comparable products and services.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS and METHODS
 RESULTS and DISCUSSION
 SUMMARY
 REFERENCES
 





This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Belesky, D. P.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Belesky, D. P.
Agricola
Right arrow Articles by Belesky, D. P.
Related Collections
Right arrow Forage Management
Right arrow Other Forage Crops
Right arrow Crop Ecology

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
The SCI Journals Crop Science Vadose Zone Journal
Journal of Natural Resources
and Life Sciences Education		 Soil Science Society of America Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome