Describing the botanical composition of a mixed species Northeastern U.S. pasture rotationally grazed by cattle. (Forage & Grazing Lands).
Total herbage, herbage harvested or consumed, ground cover or basal cover, tiller density, and number of species are often measured in pasture studies (Evans and Love, 1957; Mitchell and Glenday, 1958; Hodgson, 1979; Defosse' et al., 1990; Forage and Grazing Land Committee, 1991; Elgersma et al., 1998). Pasture researchers have defined herbage mass as the total amount of herbage dry matter per ground area cut at ground level. Herbage consumed or herbage harvested is defined as the forage defoliated by grazing animals or by mechanical harvesting, respectively, per ground area (Hodgson, 1979; Thomas, 1980). Ground cover and percentage cover of herbaceous species as estimated by the point intercept or step-point method or visual estimates of cover are also used to describe the dominant species in pastures and rangeland (Evans and Love, 1957; Thomas, 1980; Hopkins, 1986; Martz et al., 1999). Ground cover or basal cover describes species contribution to the canopy and ground, and tiller/leaf density describes the population density of growing units (Mitchell and Glenday, 1958; Jones et al., 1982; Hopkins, 1986; Brock and Hay, 1993; Karsten and Fick, 1999). Although total herbage, ground cover, and tiller density describe and rank the dominant pasture species from a plant production perspective, ranking the dominant forage species from the herbivore and pasture manager's perspective on the basis of herbage consumed or harvested may produce a different ranking and appreciation of which species dominate in terms of providing forage (Casler et al., 1998).
Much of the pasture dynamics research has been conducted in places such as New Zealand and England, where pastures are relatively homogenous and dominated by perennial ryegrass (Lolium perenne L.) and white clover. Total herbage, tiller density, and ground cover have been used to describe botanical composition, and the relative ranking of dominant species did not differ much in these relatively homogenous pastures (Mitchell and Glenday, 1958; Jones et al., 1982; Hopkins, 1986; Brock and Hay, 1993).
Although some pastures in the northeastern USA may also be homogenous, rotationally stocked pastures in the northeastern USA often include a number of pasture species with different growth forms and seasonal production patterns. A typical mixed-species pasture in Pennsylvania and the northeastern USA includes forage species that differ morphologically (Hoveland, 1992; Tracy and Sanderson, 2000; Balasko et al., 1995). Upright bunchgrasses, tail legumes, prostrate rhizomatous grasses, and stoloniferous legumes grow to different heights with different dry matter distribution in the sward, and therefore provide different amounts of forage accessible to grazing animals.
The forage consumed by a grazing animal from a given plant may vary depending on plant species morphology, the species and class of grazing animal, grazing management, climatic conditions, time of the grazing season, and the ease of forage prehension (Arnold and Dudzinski, 1967; Allden and Whittaker, 1970; Hodgson, 1981; Casler et al., 1998). Some researchers have found that total herbage does not predict animal intake of pasture as well as tiller or sward height, green dry matter, leaf length, or digestible organic matter (Arnold and Dudzinski, 1967; Allden and Whittaker, 1970; Hodgson, 1981). In Wisconsin, Casler et al. (1998) measured animal apparent intake of pure grass or grass-alfalfa stands on the basis of pre- and post-grazing estimates of available,, total herbage for 91 varieties from 15 forage grass species. Apparent intake differed among grass species that provided the same total herbage; and there were significant differences between total herbage and apparent animal intake among grass varieties (Casler et al., 1998).
In this study, we compared how different one-time sample collection methods of estimating botanical composition describe the relative ranking of species in a mixed-species pasture. We hypothesized that some descriptions of botanical composition are misleading for describing the forage contribution to grazing animals of plants with different morphologies typical of Pennsylvania and the northeastern USA. Further, the botanical composition in one season does not accurately represent the seasonal contribution of each plant species. We acknowledge that although animal selectivity is another important factor, and that although others have illustrated its importance, we did not measure it. Instead, we compared one-time sample collection methods with similar time and labor requirements. Sampling animal intake with pre- and post-grazing samples would require twice as much time to sample and separate herbage samples on a botanical basis.
We estimated botanical composition in spring, summer, and fall on the basis of (i) herbage harvested at 7-cm stubble height (the prescribed post-grazing height for managing for orchardgrass vigor), (ii) total herbage (all plants cut at ground level), (iii) tiller/leaf density of all plants cut to ground level, and (iv) ground cover. In 1999 and 2000, we also harvested bluegrass herbage at 5-cm stubble height to account for bluegrass grazing height differences that we measured in the sward in 1998.
MATERIALS AND METHODS
Between May 1998 and April 2000, we sampled a pasture at the Hailer Beef Research Farm near State College, PA, (40[degrees]51' N lat., 77[degrees]51' E long.). We estimated botanical composition in a mixed-species rotationally grazed 1.4-ha pasture that included orchardgrass, bluegrass species, quackgrass, alfalfa (Medicago sativa L.), red clover (Trifolium pratense L.), white clover, dandelion, and other mixed perennial and annual grass and forb species. The soil series was Hagerstown silt loam (fine, mixed, mesic Typic Hapludalf) with 3 to 8% slope. Pasture management during the 10 yr before our experiment was lax: pastures were grazed four to five times per year with 590 kg beef cow-calf (Bos taurus L.) pairs at 1.2 AU (animal units) [ha.sup.-1] during a 6-mo grazing season, and pastures were fertilized only with 56 kg [ha.sup.-1] of nitrogen annually in April.
Four paddocks (19 by 93 m) were sampled for botanical composition before grazing in spring, summer, and autumn. Cattle were moved into paddocks when the average orchardgrass height was 27 cm and removed when average stubble height was 7 cm. Paddocks were grazed seven times in 1998 and six times in 1999. Pastures were stocked at 2.7 AU [ha.sup.-1]] during a 6-mo grazing season with no more than 10 Black Angus-Simmental cross cow-calf pairs (590 kg) and for no more than 36 h. At the end of June 1998 and July 1999, 56 kg [ha.sup.-1] of nitrogen were applied to all pastures. Soil pH ranged from 6.5 to 6.8 across the four experimental blocks in the pasture. In late October 1998, we applied 2240 kg [ha.sup.-1] of granular lime, which increased soil pH to 6.8.
Pastures were sampled for herbage harvested, total herbage, and tiller/leaf density on 5 May 1998, 12 July 1998, 28 Aug. 1998, 1 May 1999, 27 June 1999, 24 Sept. 1999, and 28 Apr. 2000. Ground cover was also measured on 27 June 1999, 24 Sept. 1999, and 28 Apr. 2000. Bluegrass harvested herbage cut at 5-cm stubble height, was also sampled on the same dates that ground cover was measured: 27 June 1999, 24 Sept. 1999, and Apr. 28, 2000.
Each of the four paddocks was evenly subdivided into six subplots (15 by 13 m) along a grass species dominance gradient domain. Areas of the paddocks within 5 m from the water trough and 2 m from the fence line were excluded from the sampling. To avoid sampling an area more than once, each of the six subplots was divided into 60 sampling areas of (2.5 by 1.3 m). One sampling area was randomly chosen and sampled before each grazing event in each of the six subplots.
All of the botanical composition samples were collected within the same six sampling area (2.5 by 1.3 m) in each block. Within each sampling area, herbage harvested was estimated by cutting forage at 7 cm (the prescribed stubble height for orchardgrass) from two randomly selected quadrats (40 by 9.5 cm) in 1998 and from three sampling quadrats in 1999 and 2000. Total herbage and tiller density were sampled by cutting at ground level two randomly selected quadrats (40 by 9.5 cm) in 1998, 1999, and 2000. Fouling areas and plants not grazed by animals were excluded from sampling.
In the laboratory, the herbage harvested and the total herbage samples were separated into orchardgrass, bluegrass, quackgrass, dandelion, tall legumes (alfalfa and red clover), and white clover. A few plants of other species were present and were grouped together into the category of "other species." These included timothy (Phleum pratense L.), perennial ryegrass, downy brome (Bromus tectorum L.), green foxtail [Setaria viridis (L.) P. Beauv., SETVI], barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.], broadleaf plantain (Plantago major L.), bull thistle [Cirsium vulgare (Savi) Tenore], and occasionally other species. Senescent plant material was separated as dead material. Plant material was dried at 70[degrees]C and weighed.
To determine the tiller and leaf density, tillers from the total herbage samples (cut at ground level) were counted. Numbers of grass tillers were counted by grass species. Individual leaves of white clover and dandelion were also counted. Although a single leaf does not constitute the plant unit of dandelion or white clover, at the axil of each leaf there is an axillary bud that contains meristematic tissues from which a new shoot branch and a new plant unit can develop (Forbes and Watson, 1992). Individual stems of the tall legumes (alfalfa and red clover) were counted. Each stem contained multiple growing points; however, most apical and axillary buds on the stems are removed at each grazing event. Therefore, the number of individual alfalfa and red clover shoots were counted as an indicator of potential new shoots that would emerge from axillary buds at the base of a stem or from crown buds.
In summer and autumn of 1999 and spring 2000, we visually estimated the percentage of ground covered by orchardgrass, bluegrass, quackgrass, dandelion, white clover, and other species in each of the (2.5 by 1.3 m) sampling areas. In this measure of ground cover, we included alfalfa and red clover in the group "other species." Therefore, when we compared the four methods of describing botanical composition, for purposes of comparing the same categories of species with the other methods (herbage harvested, total herbage, tiller/leaf density), we added the tall legumes to the other species and compared the more inclusive group of "other species and tall legumes."
Post-grazing heights of orchardgrass and bluegrass were measured on 18 plants of each species per block after each grazing event. In 1998, we learned that the cattle grazed the bluegrass down to 5-cm stubble height and orchardgrass to 7-cm stubble height. Therefore, in 1999 and 2000 the additional 2 cm of a bluegrass was collected from a randomly selected monoculture bluegrass quadrat (40 by 9.5 cm) within each sampling area (2.5 by 1.3 m) to estimate the additional bluegrass forage harvested by the cattle. This additional 2 cm of bluegrass was used to correct the amount of harvested bluegrass herbage to bluegrass harvested at the 5-cm stubble height in 1999 and 2000. Bluegrass harvested at 5 cm was included in the comparison of the four methods of describing botanical composition.
Data were analyzed following the procedure for a split-split plot on a randomized complete block experimental design with four replications. Year was the main plot, seasons were the sub-plots, and methods of describing the botanical composition the sub-subplot. A mixed effects model was used in GLM of SAS (SAS Institute, 1999, Cary, NC). Methods of estimating botanical composition were fixed effects, and year and seasons were treated as random effects. Preplanned comparisons were made between spring and summer, and spring and autumn for each species. Tukey's multiple range test was used to test for significant differences between species and seasons. When three methods of describing botanical composition were compared, sample sizes for Tukey's tests ranged from 26 to 28 for comparing methods and from 22 to 35 when seasons were compared. When four methods and bluegrass 5-cm stubble heights were compared, the samples size for Tukey's test was 12, when methods were compared, and 16, when seasons were compared. Effects and differences were considered significant when P < 0.05.
The average precipitation and average temperatures are illustrated in Fig. 1. The percent contribution of each species did not differ among years. However, the percent contribution of orchardgrass, bluegrass, white clover, tall legumes (alfalfa and red clover), and the other mixed species differed among methods of describing botanical composition (Fig. 2a,b). The interaction between the methods and seasons was significant for white clover and other species when four methods were compared (Fig. 3a,b). The percent contribution to the sward from quackgrass, white clover, and bluegrass differed among seasons when three methods were compared (Fig. 4).
[FIGURE 1 OMITTED]
Harvested Herbage, Total Herbage, and Tiller/Leaf Density Comparison
Orchardgrass was the dominant species based on harvested herbage cut at 7-cm stubble height (48%). Ouackgrass, bluegrass, and the other mixed species contributed similar amounts of harvested herbage (15, 11, 10%, respectively). The tall legumes (alfalfa and red clover) and dandelions each provided 7%, and white clover's contribution to harvested herbage was minor (2%, Fig. 2a).
[FIGURE 2 OMITTED]
The percentage that orchardgrass, bluegrass, and tall legumes contributed to total herbage, and the relative rankings of the species differed from harvested herbage. In terms of total herbage, orchardgrass did not dominate the: sward, but rather contributed a similar percentage as bluegrass, averaging about one third of the total herbage (34 and 27%, respectively, Fig. 2a). The percentage of total herbage that bluegrass contributed to the sward was more than two-fold its contribution to harvested herbage. By contrast, the percent contribution of quackgrass to total herbage did not change relative to harvested herbage, and therefore quackgrass ranked behind bluegrass in terms of total herbage, contributing half as much as bluegrass. Dandelion and the other mixed species each contributed similar percentages to total herbage as they did to harvested herbage (8%), as did white clover (4%). However, tall legumes contributed half as much to total herbage as they did to harvested herbage (3%; Fig. 2a).
The percent contribution of orchardgrass, bluegrass, white clover, and other mixed species to the tiller/leaf density also differed from the harvested herbage and total herbage percent contributions. On the basis of tiller/leaf density, bluegrass dominated the sward (54%), and orchardgrass and other mixed species both contributed smaller percentages than they did to either harvested herbage or total herbage (15 and 4%, respectively). Tall legumes contributed a smaller amount than they did to harvested herbage (3 vs. 7%), but a similar percentage as they did to total herbage. White clover contributed a greater proportion than it did to either harvested herbage or total herbage (7%). Only quackgrass and dandelions contributed similar proportions to the total tiller-leaf density (12 and 6%) as they had to harvested herbage and total herbage.
Harvested Herbage, Total Herbage, Ground Cover, and Tiller/Leaf Density Comparison
When we included the measures of ground cover and bluegrass harvested at 5-cm stubble height in 1999 and 2000, the relative ranking of species and the percentage they contributed to ground cover was similar to the total herbage percentages for all species except quackgrass (Fig. 2b). Orchardgrass and bluegrass each contributed about a third of the ground cover (37 and 29%, respectively), quackgrass and the other mixed species (which in this comparison included tall legumes) each contributed about 11%, and white clover and dandelions both contributed 5.5%. Ground cover percentages were different from harvested herbage for orchardgrass, bluegrass, and white clover, and were similar to harvested herbage for quackgrass, dandelion and other species (which included the tall legumes). Ground cover also differed from the tiller and leaf density percentages for all species except quackgrass and dandelion (Fig. 2b).
Accounting for the bluegrass herbage harvested at 5-cm stubble height, increased the percentage of harvested herbage that bluegrass contributed on average across seasons from 5.7% (when bluegrass was harvested at 7-cm stubble, data not shown) to 10.4% (when bluegrass was harvested at 5-cm stubble, Fig. 2b). Accordingly, the percentage that orchardgrass, quackgrass and other species each contributed to the sward decreased only slightly (by 1-3%; Fig. 2b).
Method x Season Interaction
There was a significant interaction between the four methods and seasons for white clover and the other species (which included the tall legumes and other mixed species) (Fig. 3a,b). The percent contribution of white clover to harvested herbage did not differ among the seasons. However from spring to summer, the percent contribution of white clover as estimated by total herbage and leaf density increased by 4.6- and 2.9-fold, respectively. By contrast, the contribution of white clover to the total ground cover decreased in summer to one third of spring ground cover (Fig. 3a).
[FIGURE 3 OMITTED]
The total herbage, ground cover, and leaf density of other species did not differ among seasons. However, harvested herbage of other species increased significantly in autumn. Harvested herbage in autumn was 150% more than in spring, and the contribution of other species in autumn was almost double their contribution to harvested herbage in summer (28 vs. 15%; Fig. 3b).
Preplanned contrasts of spring compared with summer and autumn revealed that the mean percentage of quackgrass, white clover, and bluegrass differed from spring in some seasons (Fig. 4). The mean percentages of orchardgrass, tall legumes, dandelion, and other species did not differ from spring in summer and autumn. Compared with spring, quackgrass contributed on average across methods 4% less in summer than in spring and autumn. The average contribution of white clover across methods doubled in summer compared with spring; and bluegrass on average across methods contributed 8.5% less in autumn than in spring and summer (Fig. 4). With ground cover and the bluegrass harvested at 5-cm stubble height included in the comparison of four methods, the main effect of season alone was not significant for any of the species.
[FIGURE 4 OMITTED]
The relative ranking of dominant species and the percent contribution of orchardgrass, bluegrass, white clover, tall legumes (alfalfa and red clover), and the other mixed species differed among the methods we used to describe the pasture botanical composition, as hypothesized. Although each method may be an indicator of plant productivity, the botanical composition of the harvested herbage, the estimate that most closely represented grazed herbage, was not accurately represented by the percent contribution of the species to total herbage, ground cover, or tiller/leaf density. Granted, we did not measure actual animal intake and therefore the botanical composition estimates do not account for which species animals might consume more due to selective grazing. Instead, we compared common one-time sample collection methods of estimating botanical composition. Collecting pre- and post-grazing samples would have required twice as much time for sampling and botanical separation. When we adjusted our sampling method in 1999 and 2000 to account for cattle grazing bluegrass to a 5-cm stubble height rather than 7-cm, bluegrass percent contribution to the sward increased by almost 5% (from 5.7-10.4%). However, even when the additional 2 cm of grazed bluegrass was accounted for, bluegrass still accounted for only 22.5% of the harvested herbage contributed by orchardgrass (46.5%).
The percent contribution and relative ranking of the tiller/leaf density, an indication of population and growing point density, also was not indicative of the relative contribution of species to the total herbage. Differences between harvested herbage and the other descriptions of botanical composition were mainly due to differences in species' growth forms and how accessible the herbage was to the grazing animal.
Tall, upright growing species, including orchardgrass and tall legumes (alfalfa and red clover), accounted for a larger percentage of the total harvested herbage than they contributed to total herbage or total tiller and leaf density proportion. The minor species that were grouped together into "other species" also accounted for a larger proportion of the harvested herbage than they contributed to the tiller/leaf density (Fig. 2a,b). The other species group was dominated by tall, upright growing species such as timothy, perennial ryegrass, downy brome, barnyardgrass, and thistles. And the tall legumes were included in the other species group when four methods were compared.
[FIGURES 2A & 2B OMITTED]
By contrast, bluegrass and white clover, the shorter, more prostrate species with rhizomes and stolons that spread and store reserves below or at the soil surface, accounted for much more of the total herbage, ground cover, and tiller/leaf density than they contributed to harvested herbage (Fig. 2a,b). Casler et al. (1998) also found that the apparent intake of orchardgrass relative to orchardgrass total herbage was higher than the apparent intake of the more prostrate bluegrass relative to bluegrass total herbage.
Quackgrass and dandelion were the only two species that contributed similar proportions to the botanical composition among methods. Their growth forms appear intermediate in comparison to the taller more upright or shorter more prostrate plant growth forms (Fig. 2a,b). Although quackgrass is rhizomatous, it produces larger tillers than bluegrass, and therefore a higher proportion of the quackgrass total herbage was accessible to the grazing animal than that of bluegrass.
Although we did not measure animal intake, our finding that tall species contributed more to harvested herbage than the more prostrate species contributed to total herbage is supported by others, who have also found that total herbage and ground cover were poor predictors of how much herbage grazing animals consumed (Arnold and Dudzinski, 1967; Allden and Whittaker, 1970; Hodgson, 1981; Casler et al., 1998). Allden and Whittaker (1970) attributed differences between total herbage and animal consumption as largely due to differences in sward height and forage accessibility or ease of prehension for the grazing animal. Hodgson et al. (1977) found that total herbage alone was not a good indicator of animal intake; sward height and proportion of green material also had significant effects on pasture intake. Similarly, during the short grazing period characteristic of rotational stocking in this study, cattle could harvest more dry matter from the taller, upright orchardgrass and tall legumes, and other species, than they could from the more prostrate bluegrass and white clover. Granted, if the grazing management had not been rotational stocking, and cattle had been kept in the paddock for a longer time and forced to graze the sward closer, the botanical composition of the harvested herbage probably would have been more similar to the botanical composition of the total herbage.
A significant interaction between methods and seasons was observed for white clover and other mixed species when four methods were compared. The percentage that white clover contributed to harvested herbage did not change significantly with season, although it tended to increase in summer (Fig. 4). However, according to total herbage and leaf density estimates the contribution of white clover in the sward increased in summer as would be expected when the daily average temperature reached white clover's optimum growth temperature in summer (24 [degrees]C; Mitchell, 1956; Fig. 1, 3, and 4b). Ground cover percentage of white clover was lower in summer compared with spring. Although white clover contribution to leaf density increased, its contribution to the total cover apparently did not increase in comparison to species.
Further, according to total herbage, ground cover, and tiller/leaf density other species did not change over the season. However, the other species contributed significantly more harvested herbage in autumn than they did in spring and summer. Since the proportion of white clover and other species in the harvested herbage and the percentages measured with the other methods changed with the seasons and were not consistently correlated with harvested herbage over the seasons, they would not be useful predictors of harvested herbage (Fig. 3a,b).
[FIGURES 3A & 3B OMITTED]
In addition, on average white clover contribution doubled from spring to summer, quackgrass percent contribution decreased by 4% in summer, and bluegrass percentage decreased by 8% in autumn (Fig. 4). The contribution of bluegrass may have decreased from spring and summer to fall because relative to other species, bluegrass growth decreased more than other species after the hot, dry summer conditions, and in comparison to the spring when tiller production and growth is usually high due to reproductive tiller development (Mitchell, 1956; Wedin and Huff, 1996). Bluegrass may also have allocated more reserves to belowground rhizomes in autumn. The contribution of quackgrass, a cool-season grass, decreased by 4% on average across methods in summer, probably due to the warm and drier summer conditions. Therefore, describing the botanical composition of a pasture on the basis of sampling once during one or even two seasons did not accurately represent the contribution of some species during spring, summer, and autumn.
We compared four one-time sample collection methods for estimating pasture botanical composition in a mixed-species pasture that was rotationally grazed by cattle. We did not measure actual animal intake and account for animal grazing selectivity by collecting twice as many samples before and after grazing. However, when we sampled at the prescribed stubble heights (7 and 5 cm for bluegrass), the relative ranking of species differed among the methods. Depending on the growth form of a species, the total herbage, ground cover, and tiller density either underestimated or overestimated the contribution of a plant species to harvested herbage. Total herbage and ground cover percentages of prostrate species with a high proportion of dry matter close to the soil surface (bluegrass and white clover), overestimated the proportion that these species contributed to herbage harvested for rotationally grazing cattle. Total herbage and ground cover percentages of orchardgrass, the tall legumes, and other tall, upright species underestimated their contribution to harvested herbage. Visual estimates of ground cover were similar to total herbage (with the exception of quackgrass) suggesting that in some cases a rapid visual estimate of ground cover botanical composition might provide an estimate of the botanical composition of the total herbage in a mixed species pasture. However, neither total herbage nor ground cover were accurate descriptions of the relative contribution of all of the plant species to harvested herbage. Further, the relationship between harvested herbage and the other measures of botanical composition changed with season for white clover, a species sensitive to climatic changes, and a mixed group of other species. And the average percent contribution of quackgrass, bluegrass, and white clover also differed with season. Sampling in spring alone or any one season did not represent the contribution of all species to harvested herbage in all seasons.
Ideally pasture botanical composition will be described on the basis of animal intake measurements from pre- and post-grazing samples. However, if time and labor constraints do not permit animal intake measurements, when describing the botanical composition of rotationally grazed, mixed-species pastures that include different growth forms, we predict that total herbage, ground cover, or tiller/leaf density will not accurately represent what percentage each plant species is contributing to forage for the grazing animal. Describing pasture botanical composition according to harvested herbage (i.e., harvested above the prescribed grazing residual height), can improve pasture managers' understanding of the relative contribution of plant species to grazing cattle. Additionally, if the plant species present are sensitive to seasonal and climatic changes, harvested herbage should be sampled in different seasons to accurately represent species forage contribution during each season. An improved understanding of which species provide the most forage for grazing animals will help pasture managers identify which plant species to sow and maintain for optimal animal production.
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Heather D. Karsten, Dep. of Crop and Soil Sciences, 116 ASI Building, The Pennsylvania State Univ., University Park, PA 16802; and M. Carlassare (firstname.lastname@example.org). Received 26 April 2001. * Corresponding author (email@example.com).
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|Author:||Karsten, H.D.; Carlassare, M.|
|Date:||May 1, 2002|
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