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Dep. of Crop and Soil Sci., 116 ASI Bldg., The Pennsylvania State Univ., University Park, PA 16802
* Corresponding author (hdk3{at}psu.edu)
Received for publication June 6, 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: HI, harvest index
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Current grazing height recommendations for the northeastern USA are based on plant morphology and animal production (Blaser et al., 1986; Emmick and Fox, 1993; Hall, 1998). Grazing height recommendations for tall grasses (e.g., orchardgrass) are higher than those for sod-forming grasses (e.g., Kentucky bluegrass) (Blaser et al., 1986; Emmick and Fox, 1993; Hall, 1998). Several studies have demonstrated that herbage height before grazing influenced daily herbage intake and animal performance (Allden and Whittaker, 1970; Hodgson et al., 1977; Hodgson et al., 1981). Animal intake increases curvilinearly with pasture height because herbage accessibility and ease of removal increases for the grazing animal (Allden and Whittaker, 1970; Hodgson et al., 1977). Under rotational stocking, Le Du et al. (1979) and Mayne et al. (1987) reported that animal intake and milk production decreased when a perennial ryegrass (Lolium perenne L.) pasture was grazed down to a mean height <8 cm.
In the northeastern region of the USA, orchardgrass and Kentucky bluegrass are two common species in mixed pastures (Hoveland, 1992; Tracy and Sanderson, 2000). Orchardgrass is a tall-growing, cool-season bunchgrass desirable for its consistently high productivity and high quality (Belesky and Fedders, 1994; Casler et al., 1998). Frequent and severe grazing, however, can cause orchardgrass stands to gradually deteriorate (Griffith and Teel, 1965; Clark et al., 1974; Mislevy et al., 1977). Kentucky bluegrass is a rhizomatous, short-growing, cool-season grass that is more tolerant of close grazing and often invades pastures of the Northeast (Smith et al., 1986). A major drawback of bluegrass is lower productivity compared with orchardgrass, particularly during hot and dry periods (Smith et al., 1986; Kanneganti and Kaffka, 1995; Casler et al., 1998).
Previous orchardgrass studies demonstrated that relatively small differences in defoliation height caused significant variations in plant persistence and productivity (Griffith and Teel, 1965; Clark et al., 1974; Mislevy et al., 1977). Current suggested grazing heights for pastures dominated by tall species (e.g., orchardgrass) range from 18 to 30 cm before grazing and from 5 to 7.5 cm after grazing (Blaser et al., 1986; Emmick and Fox, 1993; Hall, 1998). We hypothesized that within the range of grazing recommendations for pastures dominated by tall species, taller orchardgrass heights before and after grazing would improve orchardgrass production and persistence and increase overall pasture productivity and forage accessibility to grazing animals. To test this hypothesis, we compared two grazing regimes that are within the current management-intensive grazing recommendations for pastures dominated by tall plant species. We also wanted to understand: (i) the seasonal forage distribution of common pastures species, (ii) how the two grazing regimes would affect seasonal forage production in a mixed-species pasture, and (iii) the value of the harvest index (HI) for comparing forage availability to grazing cattle among plant species and between grazing height regimes.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Two grazing regimes were imposed that represented tall and short grazing height regimes within the current range of recommended grazing heights for management-intensive rotational stocking of pastures dominated by tall plant species. Average extended height (from soil surface to the tip of the leaf when extended vertically) of orchardgrass plants before and after grazing was used to define the two grazing height regimes. Tall pastures were stocked with cattle when orchardgrass height averaged 27 cm, and cattle were removed within 36 h when the average orchardgrass residual height was 7 cm; short pastures were stocked when orchardgrass height averaged 20 cm and grazed down to an average orchardgrass residual of 5 cm within 36 h.
The experiment was started in April 1998 and completed in May 2000. The total area of the pasture was subdivided into four blocks (38 by 93 m) along a gentle slope gradient. Each block was divided into two paddocks (19 by 93 m) to which the short and tall grazing regimes were randomly assigned. The eight paddocks were rotationally stocked by an average of ten 590-kg Angus-Simmental cows and their calves. In 1998, 52 and 94 kg ha-1 P and K, respectively, were applied according to soil test recommendations. Soil pH ranged from 6.4 to 6.8 in the four blocks. In the fall of 1998, 2242 kg ha-1 granular lime was applied to the blocks with a soil pH of 6.4. At the end of June 1998 and the end of July 1999, 56 kg N ha-1 was applied to all pastures as NH4NO3. Immediately after the first two grazing periods of both years, all pastures were clipped at 20 cm to remove tall grass inflorescences and rejected mature plants.
Sampling
Areas of the eight paddocks within 5 m from the water trough and 2 m from the fence line were excluded from the sampling areas. Each paddock was evenly subdivided into six subplots (15 by 13 m). Each subplot was divided into 60 sampling areas (2.5 by 1.3 m). Before and after each grazing event, one sampling area per subplot was randomly selected for sample collection and height measurements. Sampling areas were used only once during the duration of the study to avoid repeated measures of the same surface. Fouling areas and plants not grazed by animals were excluded from sampling.
At the beginning of the experiment, all paddocks were grazed (uniformly) when average herbage height reached 11 cm in a preliminary grazing period (referred to as period zero). Extended tiller height was measured on 30 randomly chosen orchardgrass plants per paddock to determine when a paddock was ready to graze and when animals should be removed. Actual grazing regime heights were calculated from the average of 18 orchardgrass plants per paddock (three chosen randomly from within each of the six sampling areas) that were measured promptly before and after grazing. We also measured the height of bluegrass, the other dominant and morphologically most different grass species, with the same sampling procedure as orchardgrass (18 plants per paddock). We calculated the actual average height of 18 bluegrass plants randomly chosen before and after grazing. Elongated flowering tillers were not measured.
Samples to determine herbage harvested were cut at the defined grazing regime residual heights promptly before each grazing event. Within six sampling areas (2.5 by 1.3 m), two (1998) or three (1999 and 2000) randomly selected quadrats (40 by 9.5 cm) were cut to 7 cm in the tall grazing regime and to 5 cm in the short grazing regime, for a total of 12 or 18 samples per paddock per grazing event to estimate herbage harvested. Only herbage rooted inside the frame was collected. In 1998, we found that the cattle grazed bluegrass on average 2 cm below the target grazing residual height. In 1998, the actual bluegrass residual height averaged 5 cm in the tall pastures and 3 cm in the short pastures. Therefore, in 1999 and 2000, the additional 2 cm of bluegrass grazed 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 mass of the additional 2 cm of bluegrass herbage grazed by the cattle and the total bluegrass herbage harvested by cattle in 1999 and 2000. We also calculated the average dry matter of the residual 2 cm of bluegrass grazed in spring, summer, and autumn and used the seasonal averages to adjust the estimates of bluegrass harvested at each grazing event in 1998.
Within the same sampling areas (2.5 by 1.3 m), two more quadrats (40 by 9.5 cm) were cut to ground level, for a total of 12 quadrats per paddock, to estimate herbage mass at each grazing event. Due to time limitations, herbage mass samples were not collected at ground level before the last grazing periods of 1998 and 1999 and the first period of 2000.
In the laboratory, all samples were separated into orchardgrass, bluegrass, quackgrass, dandelion, tall legumes [alfalfa (Medicago sativa L.) and red clover (Trifolium pratense L.)], white clover, and other species. Senescent tissues were separated from the green material and classified as dead dry matter. Development stage of orchardgrass, bluegrass, and quackgrass was assessed in May by counting the number of vegetative (from stage V0 to E0), elongating (from stage E1 to E3), and flowering (from stage R0 onward) tillers by species (Moore et al., 1991). Plant material was dried at 70°C and weighed.
Pasture and species harvest indices were calculated as the ratio of herbage harvested at the defined grazing height regime (7 or 5 cm) divided by the live herbage mass (cut at ground level) and averaged over all of the grazing events in the experiment. The bluegrass herbage harvested and HI were determined at 5- and 3-cm grazed residual heights in tall and short pastures, respectively. Harvest indices could not be accurately calculated for the tall legumes, alfalfa and red clover, because tall legumes often were not present in both samples of herbage harvested and herbage mass.
To estimate the percentage of rejected areas, after grazing, beginning with the second grazing period of 1999, two persons walked diagonally across transects of each of the six subplots and recorded the number of rejected plants intersected and the total number of steps taken while walking along the transect. Only plants that were entirely intact were counted as rejected; all others were identified as grazed.
In 1998, weather data were collected at the site of the experiment with a Campbell Scientific weather station. In 1999, precipitation and temperature values were collected at a weather station 11.3 km away, at the Pennsylvania State University, University Park campus.
Statistical Analysis
The experimental design was a split-block with four blocks; date of grazing was the whole plot, grazing regime the subplot, and species the sub-subplots. The whole-plot date represented the average date of grazing all four paddocks of both grazing regimes. Tall and short pastures were not grazed on the same dates. Therefore, the dates of grazing events within each grazing height regime that occurred within a 10-d period of each other were averaged together to represent one paired grazing date. Because short pastures were grazed a total of three more times during the experiment than tall pastures, the three grazing periods in the short system that were more than 10 d apart from any of the tall-pasture grazing events were left out of the paired analysis.
Analysis of variance was conducted using the general linear model procedure of SAS (SAS Inst., 1998). The model used was a split-block design for herbage harvested, herbage mass, actual orchardgrass and bluegrass heights, and the proportion of vegetative and flowering tillers and rejected areas. All grazing periods of each of the two regimes were summed to calculate total live herbage harvested and total live herbage mass. Harvest index was calculated by averaging across all of the grazing periods of each regime. The statistical model used to analyze the variance of the total sum of herbage harvested, total plant herbage mass, and HI over the entire experimental period was a randomized complete block design. Effects and differences were considered significant when P < 0.05, unless otherwise indicated.
Grazing regime paddocks were compared before the experiment began at grazing period zero (at the end of April 1998), and the paddocks did not differ in herbage harvested, herbage mass, and species tiller density (data not shown). Then, we began imposing the two grazing regimes. Grazing period zero thereafter was not included in the statistical analysis even though it is indicated in the figures and tables.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Grazing Management
Actual orchardgrass heights differed in the tall and short regimes, averaging 26.7 (SE = 0.8) and 20.1 (SE = 0.5) cm, respectively, before grazing and 7.5 (SE = 0.2) and 5 (SE = 0.1) cm, respectively, after grazing. Actual bluegrass heights averaged over the entire experiment also differed in the tall and short regimes at 9.8 (SE = 0.3) and 8.1 (SE = 0.2) cm, respectively, before grazing and 4.8 (SE = 0.1) and 3.6 (SE = 0.1) cm, respectively, after grazing. Although we cut the harvested herbage of both species at a slightly different stubble height than the actual postgrazing, the difference between the harvested (sampling) and actual grazed height of the herbage was no greater than 0.6 cm. Total live herbage mass at each grazing event averaged 1210 kg ha-1 in the tall regime and 965 kg ha-1 in the short regime. Grazing seasons lasted 153 d in 1998 and 181 d in 1999. Short grazing regime pastures (short pastures) were grazed eight times in both years while tall grazing regime pastures (tall pastures) were grazed seven times in 1998 and six times in 1999 (Table 2). Percentage of area rejected in both grazing regimes was not different. Animals rejected 20% of the pasture surface on average at each grazing event, with lowest amounts in the spring (14%) and the highest in the fall (26.5%), probably due to an increase in the number of plants that were mature and contaminated with manure from spring to fall.
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Dead Material Harvested
The interaction of grazing regime and date and the main effects grazing regime and date of grazing all influenced dead material accumulation. At the first spring cycle of each year, pastures had the least dead dry matter of the season (Fig. 1b). This was probably because dead material from the previous year had decayed and leaf appearance rates were generally higher than leaf death rates. During 1999, dead dry matter increased over the season in both regimes, a trend similarly reported by others (Korte et al., 1982; Kanneganti and Kaffka, 1995). There was twice as much dead tissue harvested above the grazing level in tall pastures as in short pastures (Fig. 1b). Others have also reported that longer regrowth periods and taller grazing heights resulted in the accumulation of senescent material in the sward (Bircham and Hodgson, 1983; Belesky and Fedders, 1994).
Orchardgrass Harvested
Orchardgrass herbage harvested was significantly influenced by the interaction of grazing regime and date as well as by the main effects grazing regime and date of grazing. In 1998, abundant precipitation in April and relatively uniform distribution of rainfall in June and July sustained orchardgrass herbage harvested, which peaked in mid-July in both grazing regimes. In short pastures, early beginning dates and more frequent grazing reduced and distributed orchardgrass herbage harvested more evenly over time compared with tall pastures. In tall pastures, the orchardgrass herbage harvested reached a maximum of 35% of the annual harvest at the first grazing period of 1999. Similar proportions were measured by Belesky and Fedders (1994) in orchardgrass swards that were cut throughout the growing season when 20 cm tall. Before 16 May and 7 June grazing periods in 1998, and the first grazing events of 1999 and 2000, orchardgrass herbage harvested in tall pastures was 2 to 3.5 times greater than orchardgrass herbage harvested in short pastures. At each grazing event, orchardgrass herbage harvested was on average 79% greater in tall pastures than in short pastures (Fig. 1c). Orchardgrass was the species with the largest increase in herbage harvested in the tall vs. short regime.
Previous studies reported that higher orchardgrass stubble led to increased plant carbohydrate reserves and leaf area index after defoliation, enhancing plant regrowth and final production (Ward and Blaser, 1961; Davidson and Milthorpe, 1966). Longer regrowth periods were found to increase orchardgrass yields compared with shorter intervals and more frequent defoliation (Clark et al., 1974; Mislevy et al., 1977; Belesky and Fedders, 1994).
Percentage of elongating and flowering tillers in the spring was similar in the two regimes (Fig. 2a) . Maximum percentages of flowering tillers (up to 12%) occurred before the second period of the season (mid-May) of every year.
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Bluegrass herbage harvested was significantly influenced by the interaction of grazing regime and date and the main effect date of grazing (Fig. 3a) . Weather conditions, plant stage of development, and the interaction of these two factors with grazing regime significantly influenced bluegrass herbage harvested at each grazing event. In general, bluegrass herbage harvested peaked in early spring (first 15 d of May) when weather conditions were favorable and reproductive tillers grew above the grazing level (Table 1 and Fig. 3a).
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The prostrate growth habit of bluegrass after the spring reproductive period might also explain the reduction in bluegrass herbage harvested. On 27 June 1999, bluegrass herbage mass cut at ground level was similar to previous grazing events (data not shown). However, herbage harvested at the grazing height was 62% lower than previous grazing periods because tillers were mainly concentrated below the bluegrass residual grazed height (Fig. 3a).
Grazing regime as a main effect did not explain the variation in bluegrass herbage harvested. In spring, there tended to be more bluegrass harvested from tall pastures than short pastures when the percentage of flowering tillers was higher in tall than in short pastures (Fig. 2b and 3a). Percentage of elongating and flowering tillers was also significantly higher in bluegrass than orchardgrass and quackgrass (17% compared with 12 and 4%, respectively; Fig. 2b).
Quackgrass Harvested
Tall pastures produced more quackgrass herbage harvested than short pastures (Fig. 3b). Date of grazing also significantly influenced quackgrass herbage harvested; the interaction of grazing regime and date was not significant. Although quackgrass constituted only 17% of the total pasture herbage mass, particularly during the dry summer periods, quackgrass often yielded as much or more herbage harvested as bluegrass, which produced 29% of the total pasture herbage mass (Fig. 3a and 3b). Quackgrass produced a higher percentage of vegetative tillers in spring compared with orchardgrass and bluegrass (96% compared with 88 and 83%, respectively; Fig. 4a, 4b, and 4c)
. The deeper root system of quackgrass relative to bluegrass (Troughton, 1957) and steady carbohydrate reserve levels during summer periods (Arny, 1932) could explain the tolerance of quackgrass to dry periods. Likewise, Kanneganti and Kaffka (1995) reported a 50% quackgrass increase in the pasture dry matter during the summer while bluegrass decreased by 50%. Under hay cutting, quackgrass was found to produce more mass than bluegrass and as much as orchardgrass (Malzer and Schoper, 1984; Casler and Goodwin, 1998).
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Significantly more dry matter of other species was harvested from tall pastures compared with short pastures (Fig. 3d); the effect of date was also significant. Grass species that contributed the most to the difference between regimes were downy brome, perennial ryegrass, and timothy in the spring grazing periods and green foxtail and barnyardgrass in the summer. Although other species contributed less than 10% of the total herbage mass, in several grazing periods, other species tended to contribute similar or more than bluegrass to the herbage harvested, particularly in 1998.
Legumes Harvested
There was significantly more legume herbage harvested in tall pastures than short pastures (Fig. 3e). Date of grazing also significantly influenced legume herbage harvested. Alfalfa and red clover, which constituted 77% of total legume herbage harvested, contributed most to the difference between grazing regimes. Differences were particularly visible in the second half of 1999 grazing season when legume herbage harvested from tall pastures doubled the amount from short pastures. Alfalfa and red clover are less tolerant to close and frequent grazing than white clover because they have more growing points and leaves in the upper canopy, resulting in the loss of a higher proportion of their growing points and leaf area at grazing (Barnes et al., 1995). Therefore, higher grazing heights and longer regrowth periods improved alfalfa and red clover performance compared with shorter and more frequent grazing.
More herbage tended to be harvested in short pastures of white clover than tall pastures, particularly in the spring 2000 (Fig. 3f). However, no differences were detected, probably due to the small percentage of white clover in the pastures and the relatively high standard errors of this data set.
Total Live Herbage Harvested
Total live herbage harvested summed over all grazing periods of all years was 23% greater from tall pastures than short ones (10130 vs. 8232 kg-1 ha, Fig. 4a) even though short pastures were grazed three more times. Total orchardgrass herbage harvested from tall pastures was 51% greater than from short pastures. Tall legumes were the only other group of species that significantly increased total herbage harvested under the tall regime compared with the short regime (Fig. 4a).
Total live herbage harvested from bluegrass, quackgrass, dandelion, white clover, and other species was similar between grazing regimes (Fig. 4a). More frequent grazing events in the short regime compared with the tall regime compensated for the lower quackgrass productivity in short pastures, such that total quackgrass herbage harvested in short and tall pastures was similar (Fig. 3b).
Orchardgrass contributed most to the total herbage harvested, constituting on average 42% of the total. Although orchardgrass produced only 8% more herbage mass (cut at ground level) than bluegrass, orchardgrass herbage harvested at grazing height was 2.45 times greater than bluegrass.
Total Herbage Mass
In contrast to total herbage harvested at grazing height, total herbage mass of pasture, orchardgrass, and tall legumes collected at ground level was similar in the two grazing regimes (Fig. 4b). Total herbage mass of bluegrass, dandelion, and other species also did not differ between regimes. Quackgrass and white clover total herbage mass was statistically greater in short pastures compared with tall ones (Fig. 4b).
Dead tissue production on average was 36% of the total material (green and dead) collected at ground level. Tall pastures produced 23% more total dead material than short pastures (Fig. 4b) without inhibiting total herbage production.
Harvest Indices
Harvest index in tall pastures tended to be higher than in short pastures, for all species individually and combined, but was statistically greater only for dandelion (P < 0.1) (Fig. 5) . The lack of significant difference may be due to variability inherent in the sampling methodology, particularly for the less common species. To preserve sample integrity, we collected herbage harvested and herbage mass samples from different locations within each sampling area, and in some cases, species were not present in both samples. Similar HIs between grazing regimes also indicates that the sward defoliation efficiency was similar between grazing regimes. In the tall grazing regime, the taller sward and stubble after grazing did not reduce harvest efficiency nor did it remove a larger proportion of the plant reserves needed for regrowth and higher productivity.
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The HIs of quackgrass (0.50) and dandelion (0.50) tended to be higher than that of bluegrass (0.37) but were not statistically higher (Fig. 5). Again, the cattle may have grazed quackgrass and dandelion closer to a lower residual height than we sampled (7 and 5 cm), in which case the actual HI for quackgrass and dandelion could be higher than what we measured and statistically higher than bluegrass. Although quackgrass has a rhizomatous growth habit like bluegrass, it had taller and larger tillers that appeared to be more accessible to grazing cattle. The HI of the other species (0.56) ranked second behind orchardgrass and higher than all of the other species (Fig. 5), probably because it was a heterogeneous group with some tall, upright species such as timothy, perennial ryegrass, downy brome, green foxtail, and barnyardgrass.
In general, plants that concentrate photosynthetic tissues close to the ground level increase their tolerance to grazing by avoiding grazing animals (Briske, 1991) and reducing overall leaf accessibility to grazing animals. Casler et al. (1998) reported the same limitation with rotationally stocked, bluegrass-dominated pastures in Wisconsin. Kentucky bluegrass ranked highest in ground cover among six very winter-hardy, moderate regrowth, cool-season grass species. But cattle grazed the least dry matter of bluegrass compared with the other grasses. When the researchers compared orchardgrass to three different moderately winter-hardy, vigorous regrowth grasses, orchardgrass produced a high herbage mass, and animals grazed a high proportion of the orchardgrass herbage (linear regression coefficient was 0.77; Casler et al., 1998).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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With the exception of orchardgrass and bluegrass, in this experiment, direct comparisons between species were not possible due to the different proportions of species present in the pasture. Nevertheless, even though quackgrass accounted for a smaller proportion of the total herbage mass than bluegrass, quackgrass, with its larger tillers and higher HI, produced similar herbage harvested as bluegrass. And although quackgrass and dandelion are often considered weeds, they were less sensitive to dry and warm periods than bluegrass. Therefore, these species could contribute to a more uniform distribution of pasture forage throughout the growing season.
Although actual residual grazing heights for each plant species and animal performance measurements are needed to assess how differences among plant species affect animal productivity, HI was a useful parameter to compare the potential contribution of species to the total herbage available above a target grazing height. Plant morphology was the most significant factor that influenced herbage harvested and species HI. Although prostrate species such as bluegrass and white clover produced significant amounts of herbage mass, even when the animals selectively grazed bluegrass more closely, the tall-growing plants such as orchardgrass and the tall legumes provided more upright herbage that could be easily harvested.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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