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FORAGE & GRAZINGLANDS

Canopy Structure and Neutral Detergent Fiber Differences among Temperate Perennial Grasses

U.S. Dairy Forage Research Center, 1925 Linden Drive West, Madison, WI 53706

^* Corresponding author (gebrink{at}wisc.edu).

	ABSTRACT

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

 
Intake of animals grazing grass pasture is in part influenced by canopy density. Our objective was to determine the vertical distribution of dry matter and neutral detergent fiber (NDF) within temperate perennial grass swards. The study was conducted in 2004 and 2005 on a Plano silt loam (fine-silty, mixed, superactive, mesic Typic Argiudolls). When leaf height of each grass reached 25 cm in spring, summer, and fall, canopy layers were harvested from 20 to 25, 15 to 20, and 10 to 15 cm. Differences in canopy density were usually not significant when precipitation was below normal. With abundant spring precipitation, quackgrass [Elymus repens (L.) Gould] had greater upper canopy density (1.9 mg dry matter [DM] cm^–3) than any of the other grasses studied. Grasses typically grazed at shorter heights such as bluegrass (Poa pratensis L.) and perennial ryegrass (Lolium perenne L.) had greater density in the lower canopy layer (1.0–1.3 g DM kg^–1). While soft-leaf tall fescue (Festuca arundinacea Schreb.), reed canarygrass (Phalaris arundinacea L.), and meadow fescue (Festuca pratensis Huds.) had lower density in the upper canopy than timothy (Phleum pratense L.) and quackgrass in the spring, the reverse was true in the summer. Herbage NDF of grasses generally increased from the upper to lower canopy layer (mean difference of 50 g kg^–1); perennial ryegrass and meadow fescue often had the lowest NDF throughout the canopy (range of 392–452 and 418–469 g kg^–1, respectively). Pasture intake may benefit from a diverse species composition because animals can graze a mixture having optimum density and NDF throughout the canopy.

Abbreviations: DM, dry matter • NDF, neutral detergent fiber

Canopy Structure and Neutral Detergent Fiber Differences among Temperate Perennial Grasses

U.S. Dairy Forage Research Center, 1925 Linden Drive West, Madison, WI 53706

^* Corresponding author (gebrink{at}wisc.edu).

Intake of animals grazing grass pasture is in part influenced by canopy density. Our objective was to determine the vertical distribution of dry matter and neutral detergent fiber (NDF) within temperate perennial grass swards. The study was conducted in 2004 and 2005 on a Plano silt loam (fine-silty, mixed, superactive, mesic Typic Argiudolls). When leaf height of each grass reached 25 cm in spring, summer, and fall, canopy layers were harvested from 20 to 25, 15 to 20, and 10 to 15 cm. Differences in canopy density were usually not significant when precipitation was below normal. With abundant spring precipitation, quackgrass [Elymus repens (L.) Gould] had greater upper canopy density (1.9 mg dry matter [DM] cm^–3) than any of the other grasses studied. Grasses typically grazed at shorter heights such as bluegrass (Poa pratensis L.) and perennial ryegrass (Lolium perenne L.) had greater density in the lower canopy layer (1.0–1.3 g DM kg^–1). While soft-leaf tall fescue (Festuca arundinacea Schreb.), reed canarygrass (Phalaris arundinacea L.), and meadow fescue (Festuca pratensis Huds.) had lower density in the upper canopy than timothy (Phleum pratense L.) and quackgrass in the spring, the reverse was true in the summer. Herbage NDF of grasses generally increased from the upper to lower canopy layer (mean difference of 50 g kg^–1); perennial ryegrass and meadow fescue often had the lowest NDF throughout the canopy (range of 392–452 and 418–469 g kg^–1, respectively). Pasture intake may benefit from a diverse species composition because animals can graze a mixture having optimum density and NDF throughout the canopy.

Abbreviations: DM, dry matter • NDF, neutral detergent fiber

	INTRODUCTION

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

 
CANOPY STRUCTURE can be defined as the distribution and arrangement of aboveground plant parts within a community, including the weight and percentage contribution of these parts to the sward, and their spatial and angular distribution (Rhodes and Collins, 1993). While canopy structure strongly influences the photosynthetic capacity of a plant community (Monsi et al., 1973; Sheehy and Cooper, 1973), the structure of a pasture canopy can also have a large impact on herbage intake by grazing animals (Barrett et al., 2003). Individual bite weight of a grazing animal is a product of bite volume and canopy density (Casey and Brereton, 1999), which is influenced by canopy height, tiller density, proportion of leaf and stem, and physical characteristics of the leaf such as the angle of attachment to the tiller, dimensions, and rigidity (Rhodes and Collins, 1993; Duru et al., 2004). The canopy structure of grasses also influences the energy required to graze them (Griffiths and Gordon, 2003; Illius et al., 1995).

Due to its productivity, forage quality, and prevalence in major grazed temperate grasslands, the canopy structure of perennial ryegrass (Lolium perenne L.) has probably been studied more than any other forage grass species. Rhodes (1971a) found significant differences in vertical distribution of leaf area index within the canopy strata of two contrasting perennial ryegrass cultivars, which he suggested was the reason for a differential response of cultivars to cutting frequency. Selection for contrasting combinations of leaf rigidity and tiller angle demonstrated that ryegrass productivity under frequent cutting was greatest in genotypes with greater leaf area index in the basal layers of the canopy (Rhodes, 1971b). Within a ryegrass canopy, dry matter (DM) and chemical composition vary as well. From the upper (>15 cm) to the lower layer (0–5 cm), Delagarde et al. (2000) reported vertical gradients in DM (increase of 80 g kg^–1 fresh grass), crude protein (decrease of 100 g kg^–1 organic matter), and neutral detergent fiber (NDF, increase of 250 g kg^–1 organic matter). Depending on the ploidy (diploid vs. tetraploid) and maturity (early, intermediate, or late), Gilliland et al. (2002) noted significant differences in canopy structure (proportion of lamina, green leaf mass, sward and tiller height, and bulk density) and herbage nutritive value (water-soluble carbohydrate concentration and proportion of linoleic and -linolenic fatty acids).

The relationship between canopy structure and intake by grazing animals has been established in both cattle and sheep. McGilloway et al. (1999) reported that sward height was the principal determinant of bite weight in lactating dairy cows grazing perennial ryegrass, but that the influence of sward bulk density on intake became greater as sward height declined. Casey and Brereton (1999) reported similar findings when the bulk density of perennial ryegrass was altered by removing tillers; bite weight increased as sward height and density increased and the effect of height was greater than that of density. The effect of sward density, however, increased as sward height decreased. In sheep, pasture intake was related to sward height when tiller density was constant, and to bulk density at similar sward height (Black and Kenney, 1984).

Although pasture intake is influenced primarily by DM allowance, NDF of available pasture has relevance in grazing-based dairy systems because it is negatively associated with potential intake (Vazquez and Smith, 2000). The general role of NDF in the diet of dairy cows was described by Mertens (1997): if the forage consumed has excessive NDF, energy density may be low and intake and productivity may be reduced. Thus, the amount of dietary fiber can have an impact on pasture utilization.

Depending on management goals and pasture composition, practitioners of managed intensive rotational grazing typically have a target sward height at which they begin and end grazing. For tall-growing temperate grasses, current recommendations suggest that grazing begin at a 25-cm height and end when a 10-cm stubble is reached (Undersander et al., 2002). Given these criteria, the objective of this study was to determine seasonal differences in canopy density and NDF among temperate grasses common to the Midwest and Northeast USA.

	MATERIALS AND METHODS

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

 
The study was conducted in 2004 and 2005 at the University of Wisconsin's Arlington Agricultural Research Station (43.30°N, 89.35°W) on a Plano silt loam (fine-silty, mixed, superactive, mesic Typic Argiudolls). Mean and monthly precipitation during the experiment are presented in Fig. 1 . Mean daily soil temperature at 5-cm depth is presented in Fig. 2 . The site had a pH of 6.5, 52 mg kg^–1 P, and 117 mg kg^–1 K (Bray P1). In April 2003, ‘Park’ Kentucky bluegrass (Poa pratensis L., KBG), ‘Mara’ perennial ryegrass (PRG), ‘Bronc’ orchardgrass (Dactylis glomerata L., OGR), ‘Itasca’ timothy (Phleum pratense L., TIM), ‘Johnstone’ tall fescue (Festuca arundinacea Schreb., TFK), ‘Barolex’ soft-leaf tall fescue (TFS), ‘Bartura’ meadow fescue (Festuca pratensis Huds., MDF), ‘Lincoln’ smooth bromegrass (Bromus inermis Leyss., SBG), ‘Rival’ reed canarygrass (Phalaris arundinacea L., RCG), and common quackgrass [Elymus repens (L.) Gould, QGR] were broadcast seeded in 4.56- by 6.08-m plots on a prepared seedbed. The seeding rate for each species was the following: bluegrass, 16.8 kg ha^–1; perennial ryegrass, 28 kg ha^–1; orchardgrass, 11.2 kg ha^–1; timothy, 9.0 kg ha^–1; tall and meadow fescue, 11.2 kg ha^–1; smooth bromegrass, 17.9 kg ha^–1; reed canarygrass, 6.7 kg ha^–1; and quackgrass, 22.4 kg ha^–1. Plots were separated by 1-m alleys of ‘Phoenix’ turf-type tall fescue and were arranged in a randomized complete block design with four replicates. Plots were clipped to a 10-cm stubble and fertilized with 56 kg N ha^–1 as NH₄NO₃ in June and August 2003.

View larger version (15K): [in this window] [in a new window]   Figure 1. Monthly precipitation during 2004 and 2005, and 30-yr mean at Arlington, WI.

View larger version (13K): [in this window] [in a new window]   Figure 2. Mean daily soil temperature at 5-cm depth during the experiment at Arlington, WI, and mean sampling date of grasses in 2004 and 2005.

Herbage from each layer was ground to pass a 1-mm screen in a Wiley mill. Samples were analyzed for NDF concentration by calibrated near-infrared reflectance spectroscopy. The NDF of calibration samples was measured by the method of Mertens (2002) using 600-mL beakers. Calibration statistics for NDF were the following: standard error of prediction corrected for bias [SEP(C)] = 1.1, R² = 0.98.

Canopy structure was expressed as canopy layer density (mg DM cm^–3). Density and NDF were analyzed by analysis of variance using a split-plot-in-time model (Steel et al., 1997). Block and year were assumed to be random effects, while species and season were assumed to be fixed effects. Means for each grass were compared using Fisher's LSD (P 0.05).

	RESULTS AND DISCUSSION

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

 
Canopy layer density and NDF were analyzed by year and by season (spring, summer, fall) due to significant (P 0.05) year x grass and season x grass interactions. Not all grasses were present during each season of each year; precipitation, temperature, and apparent winter injury in two cases had marked effects on grass growth and persistence. Due to below-normal precipitation in August and September of 2004 (Fig. 1), Kentucky bluegrass, perennial ryegrass, timothy, and smooth bromegrass produced insufficient growth in the fall. Perennial ryegrass and smooth bromegrass also suffered apparent injury during the 2004–2005 winter. Although smooth bromegrass stands were inadequate for sampling in 2005, perennial ryegrass plots were uniformly recovered by the fall of 2005. Timothy produced inadequate growth for sampling in the summer of 2005, and as others (Riesterer et al., 2000) observed for the Midwest region, quackgrass produced little growth during the late summer and fall, and was not sampled either year.

Canopy Layer Density
Quackgrass is regarded as a noxious weed in cropping systems, but it is considered a valuable early season forage in temperate grazing systems (Asay and Jensen, 1996). Its potential value in pasture settings was observed here as well; quackgrass had greater density (1.89 mg DM cm^–3) in the upper canopy layer (20–25 cm) than any other grass in the spring of 2004 (Fig. 3 ). When sampled at 25-cm height, the leaf blades of quackgrass appeared to droop, whereas the leaf blades of the other grasses were more rigid in either the horizontal or vertical position. Quackgrass canopy density was also greater than all grasses except Kentucky bluegrass, smooth bromegrass, and timothy in the middle canopy layer (15–20 cm). Because of the influence of canopy density on bite weight of a grazing animal (Casey and Brereton, 1999), this density advantage over other grasses would have a significant, positive effect on intake. In the lower canopy layer (10–15 cm), however, grasses commonly exhibiting a more prostrate growth habit (Kentucky bluegrass and perennial ryegrass) had greater density in the spring of 2004 than all grasses except quackgrass (Fig. 3). Kentucky bluegrass and perennial ryegrass are often grazed at a shorter initial height and more closely than the other grasses. Because the influence of canopy density on intake becomes greater as sward height declines (McGilloway et al., 1999; Casey and Brereton, 1999), intake of these more prostrate grasses may differ little from that of other grasses if livestock are forced to graze to a 10-cm stubble.

View larger version (27K): [in this window] [in a new window]   Figure 3. Dry matter density of vertical canopy layers within temperate perennial grasses (KBG, Kentucky bluegrass; PRG, perennial ryegrass; OGR, orchardgrass; TIM, timothy; TFK, Kentucky 31-type tall fescue; TFS, soft-leaf tall fescue; MDF, meadow fescue; SBG, smooth bromegrass; RCG, reed canarygrass; QGR, quackgrass) in the spring of 2 yr. Error bars indicate one standard error.

Precipitation during the early spring of 2005 (Fig. 1) was below normal and mean daily soil temperature (Fig. 2) was generally lower than that in 2004. These conditions probably explain why few differences in canopy layer density were measured among grasses in the spring of 2005, although the general ranking of grasses within a canopy layer tended to be similar to that within the same layer in 2004. Kentucky bluegrass had the greatest density in the 20- to 25-cm layer, but no significant differences were found among the other grasses in this layer, or among a majority of grasses in the 15- to 20- or 10- to 15-cm layers (Fig. 3). As might be expected, canopy density appears to have the same sensitivity to the effects of environment as total herbage yield.

Ranking of grasses for the upper canopy layer density in the summer of 2004 differed considerably from the spring. While soft-leaf tall fescue, reed canarygrass, and meadow fescue had lower density in the upper canopy layer (20–25 cm) than timothy and quackgrass in the spring (Fig. 3), the reverse was true in the summer (Fig. 4 ). With the exception of reed canarygrass, which consisted primarily of pseudostem tissue in the middle and lower canopy layers, soft-leaf tall fescue and meadow fescue also had greater density from 15 to 20 and 10 to 15 cm than all grasses except Kentucky bluegrass and perennial ryegrass. Despite favorable precipitation (Fig. 1) and temperature (Fig. 2) conditions during the summer of 2004, the canopy density of orchardgrass in all layers was significantly lower than that of any other grass (Fig. 4).

View larger version (29K): [in this window] [in a new window]   Figure 4. Dry matter density of vertical canopy layers within temperate perennial grasses (KBG, Kentucky bluegrass; PRG, perennial ryegrass; OGR, orchardgrass; TIM, timothy; TFK, Kentucky 31-type tall fescue; TFS, soft-leaf tall fescue; MDF, meadow fescue; SBG, smooth bromegrass; RCG, reed canarygrass; QGR, quackgrass) in the summer of 2 yr. Error bars indicate one standard error.

Unlike the spring and summer, the pattern of vertical DM distribution in the fall generally increased from the top to the bottom of the canopy in both years, probably due to decreased day length that increased tillering (Leopold, 1949). Tall fescue is noted for its fall growth and suitability as a stockpiled grass (Riesterer et al., 2000). Superior fall growth relative to the other grasses was reflected in the density of soft-leaf tall fescue in 2004 and Kentucky 31-type tall fescue in 2005, particularly in the middle and lower canopy layers (Fig. 5 ). When precipitation was more abundant before sampling in 2005 than in 2004 (Fig. 1), however, several grasses produced sufficient growth for sampling that were not sampled the previous year. For example, the upper layer density of orchardgrass was greater than that of any grass except soft-leaf tall fescue, and the middle and lower layer density of perennial ryegrass was equivalent to or greater than any grass (Fig. 5).

View larger version (24K): [in this window] [in a new window]   Figure 5. Dry matter density of vertical canopy layers within temperate perennial grasses (KBG, Kentucky bluegrass; PRG, perennial ryegrass; OGR, orchardgrass; TIM, timothy; TFK, Kentucky 31-type tall fescue; TFS, soft-leaf tall fescue; MDF, meadow fescue; SBG, smooth bromegrass; RCG, reed canarygrass; QGR, quackgrass) in the fall of 2 yr. Error bars indicate one standard error.

In the spring of both years, Kentucky bluegrass had greater NDF throughout the canopy than any other grass (Table 1 ). In a pasture environment, Kentucky bluegrass is typically grazed at a lower initial height than the height at which it was harvested in this experiment (25 cm), and the herbage would consist primarily of leaves. When harvested at 25-cm height, stems were present in each canopy layer of bluegrass in the spring, which increased total NDF (Karn et al., 2006). In contrast, perennial ryegrass, the other grass typically grazed at a lower initial height, had the lowest NDF throughout the canopy in 2004 due to its inherently high forage quality relative to other temperate grasses (Jung et al., 1996). Among the remaining, more erect grasses, smooth bromegrass had greater NDF in the upper and middle layers than all grasses except quackgrass, while meadow fescue had lower NDF throughout the canopy than any grass except reed canarygrass in the spring of 2004 (Table 1). The lower NDF of meadow fescue may explain why Casler et al. (1998) found that apparent intake of meadow fescue grass and tall fescue was similar despite a significant advantage in available forage by endophyte-free tall fescue. With the exception of Kentucky bluegrass, however, these results were not repeated in the spring of 2005, when few differences in NDF existed among the grasses.

	CONCLUSIONS

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

Studies of perennial ryegrass have demonstrated that canopy structure significantly influences herbage utilization by grazing animals. Differences in canopy density translate practically into differences in available forage, which, depending on the extent to which a pasture is utilized, is positively associated with pasture intake (Holden et al., 1994) due to greater DM intake per bite (Casey and Brereton, 1999). Neutral detergent fiber concentration of herbage, however, is also a determinant of dietary intake because cell walls contribute to rumen fill (Jung and Allen, 1995). Depending on the season, differences in canopy layer NDF among grasses often exceeded the 50 g kg^–1 threshold necessary to produce a tangible effect on an animal's dietary intake (D.R. Mertens, personal communication, 2006). For all grasses, the general pattern of increasing NDF from the upper to the lower canopy layer implies lower intake potential as animals are forced to graze lower in the canopy. Thus, pastures with a diversity of particular species may benefit not only in terms of total herbage production (Sanderson et al., 2005), but also in terms of potential intake because animals may graze a forage mixture having optimum density and NDF throughout the canopy.

	NOTES

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

 
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Received for publication January 24, 2007.

	REFERENCES

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

 

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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Brink, G. E. Articles by Hall, M. B. Search for Related Content PubMed Articles by Brink, G. E. Articles by Hall, M. B. Agricola Articles by Brink, G. E. Articles by Hall, M. B. Related Collections Grazing Management Other Forage Crops Crop Ecology