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Dep. of Crop and Soil Sciences, 116 ASI Building, The Pennsylvania State Univ., University Park, PA 16802
* Corresponding author (hdk3{at}psu.edu)
Received for publication November 29, 2001.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: AU, animal unit = 500 kg cow liveweight
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Several studies have found that under short grazing height regimes, grass tiller density increased and tiller weight/size decreased (Lambert et al., 1986; Davies, 1988; Bircham and Hodgson, 1983). Individual tiller size may change in a compensatory manner, usually described by the "self-thinning" law, until plants reach their physiological potential under the applied management and environmental conditions (Yoda et al., 1963; Sackville-Hamilton et al., 1995; Matthew et al., 1995). Extensive research on tiller population dynamics has been conducted mainly with perennial ryegrass (Lolium perenne L.) dominated pastures in the U.K. and New Zealand (Chapman et al., 1983; Korte et al., 1985; Korte, 1986; Davies, 1988). Few detailed tiller population studies have examined common pasture species in the northeastern USA, such as orchardgrass and Kentucky bluegrass.
A number of studies have found that when orchardgrass, a tall-growing bunchgrass, was grazed or cut frequently, close to ground level, its tiller density, stand persistence, and productivity were reduced. These results suggest that the physiological regrowth potential of orchardgrass was limited by short grazing height regimes (Griffith and Teel, 1965; Mitchell, 1967; Clark et al., 1974; Fales et al., 1995). By contrast, Kentucky bluegrass, a short, sod-forming grass, tolerated higher intensities of defoliation under rotational stocking better than orchardgrass; and Kentucky bluegrass tiller density and sward productivity actually increased under these conditions (Fales et al., 1995; Murphy et al., 1997). Not surprisingly, Kentucky bluegrass is one of the dominant species of continuously stocked pastures (Hoveland, 1992; Paine et al., 1999).
In this study, we hypothesized that under more frequent and close grazing, orchardgrass tiller density and herbage mass production in a mixed pasture would decrease over time. In contrast, Kentucky bluegrass or other species more tolerant to intensive defoliation would increase tiller density and dry weight, and replace orchardgrass in the sward. To test this, we compared tiller and leaf population density and dry weight of the major species in a typical Pennsylvanian mixed pasture dominated by orchardgrass and Kentucky bluegrass under two sward grazing height regimes. We report sward botanical composition and seasonal productivity in terms of species tiller and leaf density and weight, herbage mass, and the relationships among these parameters.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The experiment was started in April 1998 and completed in May 2000. The pasture was divided 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 with an average of 10 Angus and Simmental cow–calf pairs (cows averaged 590 kg) that remained in each paddock for no more than 36 h. Both in 1998 and 1999, immediately after the first two grazing cycles, all pastures were top clipped at 20 cm to remove tall grass inflorescences and mature rejected forage. In 1998, 52 kg ha-1 of P as triple superphosphate, and 94 kg ha-1 of K as muriate of potash, were applied according to soil test recommendations. Soil pH ranged from 6.4 to 6.8 in the four blocks. In fall 1998, 2242 kg ha-1 of granular lime was applied to all the blocks with a soil pH of 6.4. At the end of June 1998 and the end of July 1999, 56 kg ha-1 of N was applied to all pastures to imitate typical management practices of moderately intensive pasture-based livestock producers.
Grazing Procedures
Two rotational-stocking treatments represented short and tall grazing height regimes within the current range of recommended grazing heights for pastures dominated by tall species such as orchardgrass (Blaser et al., 1986; Emmick and Fox, 1993; Hall, 1998). Tall pastures were stocked with cattle when orchardgrass extended height averaged 27 cm and cattle were removed when orchardgrass residual height averaged 7 cm; short pastures were stocked when orchardgrass height averaged 20 cm and grazed down to a residual height of 5 cm. Average extended height (from soil surface to the tip of the leaf blade) of orchardgrass tillers before and after grazing was used to define two grazing intensities. Extended orchardgrass tiller height of the tallest tiller was measured on 30 plants per paddock by the same person, to determine when a paddock was ready to graze, or when animals should be removed. Actual grazing heights were calculated from the average of 18 orchardgrass plants (three chosen randomly from within six sampling subplots) promptly before and after grazing. Within the six sampling areas, we also measured the height of 18 randomly chosen Kentucky bluegrass plants, the other dominant, and morphologically most different, grass species. Elongated flowering tillers were not measured.
Sampling Procedure
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 period, one sampling area per subplot was randomly selected for sample collection and height measurements. Sampling areas were utilized only once during the study. Fouled areas and forage not grazed by animals were excluded from sampling. Areas within 5 m of the water trough and 2 m from fence lines were excluded from the sampling areas. At the beginning of the experiment, all paddocks were grazed when average herbage height reached 11 cm down to an average stubble height of 7 cm, in a preliminary grazing period (referred to as cycle zero).
In each sampling area (2.5 by 1.3 m), two sampling frames were tossed into the area to randomly select two quadrats (40 by 9.5 cm). The quadrats were cut to ground level to estimate species and total herbage mass at each grazing period. Only herbage rooted inside the frame was collected. In the laboratory, all samples were separated by hand into orchardgrass, Kentucky bluegrass, quackgrass, dandelion, white clover (Trifolium repens L.), and other species. Green entire grass tillers (any individual grass blades were not counted as tillers) or dandelion leaves were counted by species, and tiller or leaf density was calculated by dividing the number of entire tillers or dandelion leaves by the sampled area. Stage of development of orchardgrass, Kentucky 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). Senescent plant tissues were separated from the green material and classified as dead dry matter. Plant material was dried at 70°C, and weighed.
Although a single petiole and leaf of dandelion is not equivalent to a tiller, at the axil of each leaf there is an axillary bud that contains meristematic tissues from which a new shoot, and thus a new plant unit, can develop. Therefore, we considered leaf density an indirect measurement of potential growing points and vegetative reproduction. We calculated tiller or leaf weight by dividing each species green dry mass by the number of tillers or leaves.
Due to time limitations, samples were not collected at ground level before the last grazing cycles of 1998 and 1999, and the first cycle of 2000. Total herbage mass over the entire experiment was calculated by summing the species dry matter collected at each grazing period (except for the last grazing cycles of 1998, 1999, and the first cycle of 2000) under the two treatments.
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 located 11.3 km away at the University Park, PA.
Experimental Design
The experimental design was a split-block with four blocks, with grazing date as the whole plot, and grazing regime the subplot. To assess the cumulative effect of grazing regimes, we treated each grazing date as unique and not a replication of the previous season. The grazing date was the average date that the four paddocks for each regime were grazed. Tall and short pastures were not necessarily grazed on the same date. Therefore, when the average grazing date of both grazing regime cycles occurred within the same 10-d period, they were paired. Short pastures were grazed three times more than tall pastures. The three short grazing periods that were more than 10 d apart from any tall grazing periods were left out of the paired analysis of the tiller and leaf density and weight data. Summed herbage mass included the herbage mass of all of the short cycles (i.e., three more grazing periods than in the tall pastures).
Statistical Analysis
Analysis of variance was conducted on the data using the general linear model procedure of SAS (SAS Inst., 1998). The model was a split-block for grass heights, number of vegetative and flowering tillers, tiller and leaf density and weight, and species and pasture total green herbage mass. The error term for the F test for grazing date was the Grazing date x Block interaction; and the error term for the F test for grazing regime was the Grazing regime x Block interaction. The residual error term of the model was used to test the Grazing date x Grazing regime interaction. The model for the species and total herbage mass over the entire experimental was two factorial with grazing regime and blocks as the main effects. Effects and differences were considered significant at P < 0.10.
No differences in tiller/leaf density or weight were found between tall and short regime paddocks before the experiment began (Cycle 0, the end of April 1998). Cycle 0 was not included in the statistical analysis, but the data are included in the figures and tables. During the spring, grass tiller density and weight is uniquely influenced by the reproductive stage of development. Therefore, we conducted an additional analysis of variance on the stage of development, tiller and leaf density, and weight data with spring grazing cycles data excluded.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Grazing Management
Actual orchardgrass heights differed and averaged 26.7 cm (SE 0.8) and 20.1 cm (SE 0.5) before grazing, and 7.5 cm (SE 0.2) and 5.0 cm (SE 0.1) after grazing, in the tall and short grazing regimes, respectively. Kentucky bluegrass heights differed and averaged 9.8 cm (SE 0.3) and 8.1 cm (SE 0.2) before grazing and 4.8 cm (SE 0.1) and 3.6 cm (SE 0.1) after grazing, in the tall and short pastures, respectively. Grazing seasons lasted 153 d in 1998 and 181 d in 1999. Short pastures were grazed eight times each year, while tall pastures were grazed seven times in 1998 and six times in 1999 (Table 2). Percentage of rejected areas was similar in the two grazing regimes, averaging 20% of the pasture surface at each grazing period (Carlassare and Karsten, 2002).
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Orchardgrass
Orchardgrass tiller density was significantly influenced by grazing date and by the Grazing regime x Grazing date interaction, but not grazing treatment alone (Table 3). Weather and plant stage of development often differed among grazing periods for both grazing regimes, and contributed to tiller density fluctuations among grazing periods and between regimes over time.
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Average orchardgrass tiller dry weight was influenced by the grazing regime and the Grazing regime x Grazing date interaction (Fig. 1b). Differences between grazing treatments mainly occurred in early spring when the largest proportion of orchardgrass tillers were elongating and reproductive (Fig. 2a). Although tall pastures had similar percentages of elongating and flowering tillers as short pastures, tillers in tall pastures were at a more advanced stage of elongation and flowering compared with tillers in short pastures (E2 vs. E1, for instance). Although orchardgrass tillers in tall pastures were taller by grazing regime definition, tiller weight after the spring cycles was similar in the two grazing regimes.
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Although tiller population dynamics in response to climatic conditions were not consistent, Kentucky bluegrass fluctuations were consistently larger than orchardgrass and quackgrass. In summer 1998, Kentucky bluegrass tiller density declined by 70% in short pastures compared with 50 and 30% decreases in orchardgrass and quackgrass, respectively. In spring 1999, Kentucky bluegrass tiller density decreased twice as much as orchardgrass tiller density decreased, while quackgrass tiller density was stable (Fig. 1a, 3a, and 4a).
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Kentucky bluegrass tiller weight was significantly influenced by grazing regime, grazing date, and their interaction. Similar to orchardgrass, tiller weight was higher in tall pastures than short pastures in spring. However, after spring, tiller weight was similar in the two regimes (Fig. 3b). Kentucky bluegrass vegetative tillers in tall pastures were taller than in short pastures. But short tillers probably weighed similarly because they had more numerous or larger leaves than in tall pastures. In 1998, vegetative Kentucky bluegrass tillers weighed less than half as much as vegetative tillers did in 1999, and Kentucky bluegrass tiller density was 2.5-fold greater than in 1999. It appears that the dry early spring weather in 1999 limited tiller density and influenced Kentucky bluegrass tiller size–density compensation more than grazing regime.
Kentucky bluegrass herbage mass at each grazing was influenced by the main effects of grazing regime and grazing date. Kentucky bluegrass herbage mass averaged 17% higher in tall pasture than short pastures (Fig. 3c). In both regimes, Kentucky bluegrass productivity peaked when tiller density was highest, with the exception of 13 Aug. 1999, when high tiller dry weight in short pastures compensated for low tiller density (Fig. 3a, b, and c). However, more frequent grazing periods under the short regime compensated for lower Kentucky bluegrass productivity at each grazing, so that Kentucky bluegrass herbage mass summed over the entire experiment was similar under the two grazing regimes (Table 4).
Climatic conditions and defoliation timing explained the variable impact of height defoliation regimes in previous studies (Bryan et al., 2000; Graber, 1933). Bryan et al. (2000) found that during wet years Kentucky bluegrass produced more under a short defoliation regime than under a tall regime. But during the dry second and third years, Kentucky bluegrass productivity was similar in both grazing treatments. Graber (1933) observed that Kentucky bluegrass plots produced more under a short cutting regime in the first year, but not the second and third years.
Quackgrass
Quackgrass tiller density was higher in short pastures than tall pastures. Grazing regime differences were most visible in 1999 and spring 2000 (Fig. 4a). Fluctuations in quackgrass tiller populations during the growing season were not significant and were minimal compared with other species. In particular, in spring 1999, quackgrass tiller density remained constant in both regimes when orchardgrass, Kentucky bluegrass, and dandelion tiller or leaf number decreased (Fig. 1a, 3a, 4a, and 5a).
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Further, quackgrass is recognized as a troublesome invasive weed that spreads rapidly via aggressive rhizomes (Westra and Wyse, 1981). Evans and Ely (1935) found that quackgrass had longer rhizomes and spread faster than Kentucky bluegrass and Canada bluegrass. Troughton (1957) reported that quackgrass has a deeper root system than bluegrass species. In Missouri, Brown (1943) found that carbohydrate reserves declined in Kentucky bluegrass in early summer and during a drought period. By contrast, Arny (1932) did not observe a clear reduction in quackgrass carbohydrate content during the summer in Minnesota.
Quackgrass tiller dry weight was similar under the two grazing regimes; but varied significantly over the grazing season (Fig. 4b). A very small percentage of reproductive tillers developed in the spring (Fig. 2c), particularly in comparison to orchardgrass and Kentucky bluegrass (Fig. 2a and b). Nevertheless, in spring, quackgrass tiller dry weight was 18 and 55% higher in tall and short pastures, respectively, than in summer and fall grazing periods (Fig. 4b).
Quackgrass herbage mass at each grazing period varied significantly over time; the largest decline (30%) occurred in tall pastures during the first 15 d of July 1998 when orchardgrass and Kentucky bluegrass herbage mass reached a maxima. Quackgrass herbage mass before each grazing period did not differ between grazing regimes (Fig. 4c). Although not statistically significant, quackgrass tillers cut from tall pastures tended to be heavier than tillers from short pastures. This may have compensated for the lower tiller density in tall pastures compared with short pastures, resulting in similar quackgrass herbage mass at each grazing cycle under the two regimes. High variability of quackgrass herbage mass within the collected samples may also have limited the statistical power to detect differences (Fig. 4c). When herbage mass was summed over the entire experiment, quackgrass was the only grass species that produced more (25%) in short pastures (that were grazed more times), than in tall pastures (Table 4).
Dandelion
Dandelion leaf density was higher in short pastures than tall pastures, and varied significantly during the growing season (Table 3 and Fig. 5a). In both grazing regimes, the lowest leaf density occurred at the beginning of the experiment, and the highest leaf density occurred at the beginning of the 1999 grazing season. Similar to Kentucky bluegrass and orchardgrass, during the early spring dry spell of 1999, dandelion leaf density decreased from the previous grazing period by 75 and 45% in tall and short pastures, respectively. Leaf density also decreased noticeably (24%) in both regimes during the dry and hot late July 1998 (Fig. 5a).
Leaf density does not describe whether dandelion in short pastures increased in number of plants, or the number of leaves per plant, or both. Nevertheless, the data suggest that dandelion plant population in the sward was sensitive to this relatively small variation of grazing intensity, and that shorter canopies increased dandelion's potential success by increasing the overall number of meristematic points that are associated with leaf axillary buds. Dandelion's rosette growth habit concentrates meristematic tissues just above or slightly below the soil surface, which protects growing points below the grazing level and allows dandelion to tolerate close and frequent defoliation. Timmons (1950) found that dandelion was more competitive than sod-forming grasses such as bermudagrass [Cynodon dactylon (L.) Pers.], creeping bentgrass (Agrostis palustris Huds.), and buffalograss [Buchloe dactyloides (Nutt.) Engelm.] in field plots cut at 2.5 cm every 2 wk.
Taller pasture canopies also might have shaded and reduced the dandelion population. This theory is supported by Molgaard (1977), who found that dandelion plant density was higher in grass plots that were cut when 2 cm tall in comparison with grass plots cut when 5 or 10 cm tall. Shading reduced dandelion invasion in taller swards, including those with a sparse canopy and bare ground, favorable environments for new plant establishment (Molgaard, 1977). Phenotypic plasticity, as expressed in leaf size, morphology, and density, has been documented as playing an important role in the adaptability of dandelion to disturbance (Cox and Ford, 1987; Vavrek et al., 1997; Molgaard, 1977).
Dandelion leaf dry weight varied over time, but not between regimes (Fig. 5b). In 1998, leaves were twice as heavy as in 1999. This could reflect a difference in plant resources and age between the two seasons, and/or a response to the early drier spring conditions of 1999 compared with 1998. Smaller leaf dry matter in 1999 might be due to a higher proportion of small plants in the sward that established during 1998, the first year of intensive grazing. Dandelion herbage mass at each grazing period was similar in the two grazing regimes (Fig. 5c) and peaked in early spring (May) in correspondence with high leaf density (1999, Fig. 5a) or dry weight (1998, Fig. 5b). Dandelion herbage mass summed over years did not differ significantly (Table 4).
Total Live Herbage Mass and Dead Material
Total live herbage mass was influenced by grazing regime, grazing date, and their interaction. At each grazing period, tall pastures averaged 20% more total herbage mass than short pastures with a peak of higher productivity (61%) at the first grazing cycle in 1999 when a higher proportion of grass tillers were elongating and reproductive (Fig. 6a). After the first spring grazing in 1999, herbage mass was occasionally higher in short than tall pastures. Herbage mass before each grazing period averaged 1210 kg ha-1 in the tall system and 965 kg ha-1 in the short system. Orchardgrass, Kentucky bluegrass, and tall legumes (alfalfa and red clover, data not shown) were major contributors to the higher total herbage mass at each grazing period (Fig. 1a and 3a). However, pasture herbage mass summed across all grazing cycles over the entire experiment was similar in the two regimes (Table 4). More frequent grazing in short pastures (three additional periods) compensated for the lower productivity at each grazing period, and higher quackgrass herbage mass in short pastures appeared to compensate for lower orchardgrass herbage mass (Table 4).
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Since Kentucky bluegrass tiller density and herbage mass decreased most in response to dry and warm conditions, it is not recommended for consistent seasonal forage production for rotationally stocked pastures in the northeastern USA. In contrast, quackgrass and dandelion are more tolerant of warm and dry periods and intensive grazing, and spread rapidly in short pastures. The cumulative effect of more frequent and intensive grazing in short pastures compared with tall ones did not result in a significant deterioration of the orchardgrass stand. However, the tall grazing regime produced more total pasture and orchardgrass on average at each grazing period, and is more likely to limit the spread of quackgrass and dandelion, and benefit orchardgrass. Total pasture mass was similar under both grazing regimes, apparently due to changes in tiller and leaf size, and shifts in botanical composition, particularly of quackgrass. Limited precipitation and warm temperatures most consistently and significantly reduced orchardgrass, Kentucky bluegrass, and pasture productivity, regardless of season. Climate is a significant source of variability and risk in cool-season pastures that managers should take into account when designing grazing systems.
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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M. Carlassare (current address), Dep. of Environmental Agronomy and Crop Science, Univ. of Padova, Agripolis, Viale dell'Università,16, 35020 Legnaro (Padova), Italy. 
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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