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Productivity of Kentucky Bluegrass Pasture Grazed at Three Heights and Two Intensities

^a Div. Animal Vet. Sci., Morgantown, WV USA
^b Div. Plant Soil Sci., Morgantown, WV USA
^c Div. Res. Manage., West Virginia Univ., Morgantown, WV 26506-6108 USA

wbryan{at}wvu.edu

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Sward height before grazing and amount of forage harvested affect production, composition, and utilization of rotationally stocked pasture. In this 3-yr field study, yearling steers (Bos taurus L.) grazed 400-m² plots of Kentucky bluegrass (Poa pratensis L.)–white clover (Trifolium repens L.) pastures every time sward height reached either 11.8 cm (short), 13.7 cm (medium), or 15.3 cm (tall). Plots were grazed with enough steers to remove either 50 or 60% (grazing intensity) of the herbage mass in 4 h. The experimental design was a 3 x 2 (height treatment x grazing intensity) factorial with four replications. Sward heights before and after grazing periods were measured. Herbage mass, apparent growth rate, herbage removed, rejected area, and percent utilization were calculated. Average herbage mass before grazing ranged from 1855 to 2350 kg dry matter (DM) ha^-1. Average herbage mass after grazing varied from 855 to 1060 kg DM ha^-1. Apparent herbage growth rate (54 kg DM ha^-1 d^-1) and herbage removed (7520 kg ha^-1) were highest on the short height treatment with low grazing intensity (50% removal). Assuming a sigmoidal growth curve, maximum apparent growth rate occurred at 10.5-cm sward height. More than twice as much herbage was removed in the wettest year compared with the driest. In the dry year, height and intensity of grazing had little effect on herbage production. Thus, the benefits farmers can expect from more intensive grazing management will be more evident in a year of good growth.

Abbreviations: ADF, acid-detergent fiber • CP, crude protein • DM, dry matter • IVDMD, in vitro dry matter digestibility • NDF, neutral-detergent fiber

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
AGRICULTURE in the northern USA is dominated by animal production. Grazing systems are more likely to be economically viable than confinement feeding for animal production on small operations (Horn, 1998). Defoliation management of pasture was identified by Rayburn et al. (1998) as one of the research needs for pasture production in the Northeast. Rayburn (1994) identified Kentucky bluegrass as the second most frequently occurring grass species in more than 700 pastures sampled in New York, Vermont, New Hampshire, and Maine. Kentucky bluegrass and white clover are the two most important desirable species in West Virginia permanent pastures wherever soil fertility has been well maintained (Pierre et al., 1937).

Two aspects of defoliation management that can be readily varied are frequency and intensity of grazing. Grazing frequency can be described in rotational stocking as sward height immediately before a grazing period. In most circumstances, sward grazed when it reaches a lower height will be grazed more frequently than if allowed to reach a higher height at grazing. Intensity refers to the amount of DM removed at each grazing period in rotational stocking. It can be measured by estimating the herbage removed during the grazing period and is influenced by the number of animals and duration of grazing. How frequency and intensity of grazing affect pasture production (herbage accumulation and utilization) is of interest to grassland farmers because of its influence on animal production. However, the complex interactions between soil, plants, and grazing livestock are not well understood.

Much research on defoliation of pasture (Illius and Hodgson, 1996) has been conducted on clipped perennial ryegrass (Lolium perenne L.). Kentucky bluegrass is a low-growing grass similar to perennial ryegrass. Collins and McCarrick (1969) reported that Kentucky bluegrass and perennial ryegrass responded similarly to cutting frequency. Frequency of defoliation of perennial ryegrass produced no effect on yield in some experiments (Briseño de la Hoz and Wilman, 1981; Fulkerson and Michell, 1987). In other studies, perennial ryegrass pasture produced more herbage when cutting frequency was lower (Wilson and McGuire, 1961; Holliday and Wilman, 1965; Leaver, 1985). Results of these experiments were similar to reports of the effect of cutting frequency on taller-growing grasses, such as orchardgrass (Dactylis glomerata L.), tall fescue (Festuca arundinacea Schreb.), and bromegrass (Bromus inermis Leyss.) (Bryant and Blaser, 1961; Frame, 1973). Some researchers report interactions of defoliation management with season and fertilization. Mowing height affected DM production most in autumn; harvest interval affected it most in spring (Fulkerson and Michell, 1987). Holliday and Wilman (1965) reported that the reponse of a mixed perennial ryegrass, timothy (Phleum pratense L.), meadow fescue (Festuca pratensis L.), and white clover pasture to N fertilization was less at low than at high cutting frequency.

Most of the reported defoliation experiments with Kentucky bluegrass were conducted before 1955. Little effect of cutting frequency on yield was found (Ahlgren, 1930; Graber, 1933; Mortimer and Ahlgren, 1936; Brown, 1938). Where differences were found, lower frequency of defoliation gave higher yields. Mortimer and Ahlgren (1936) allowed Kentucky bluegrass swards to grow to 20 to 25 cm or 10 to 13 cm and cut them to ground level. They found that 5% more herbage mass accumulated at the taller height than at the shorter height.

Graber (1933), Mortimer and Ahlgren (1936), and Robinson and Sprague (1947) all found that yield of Kentucky bluegrass was highest when defoliated more intensely. Graber found that clipping 13 times to 1.5 cm during the first growing season resulted in a thin stand with many more weeds the following year, compared with clipping 13 times to 4 cm. He also reported that N availability and precipitation interacted with intensity of clipping, resulting in complex effects on growth during the season. A further complication is the increase in the legume proportion that may follow a change in grazing frequency or intensity. Mott (1944) reported that there was four times more legume in Kentucky bluegrass pasture clipped from 10 to 15 cm to 1.5 cm than in pasture clipped weekly to 1.5 cm.

Brougham (1956) compared the effect of clipping short-rotation ryegrass (L. perenne L. x L. multiflorum Lan.) to different heights on the length of time it took regrowth to reach maximum light interception. Because it took only 4 d for this grass to reach maximum light interception when cut to 12.5 cm, he recommended defoliation at that height. Most other research has shown that herbage accumulation is greater when pasture is defoliated more intensely (Bryant and Blaser, 1961; Reid, 1962). Fulkerson and Michell (1987), in Tasmania, found that lower mowing increased yield of perennial ryegrass in autumn, winter, and early spring, but reduced it in mid-spring. In a greenhouse experiment, Harrison (1931) found that the shorter Kentucky bluegrass was cut, the more its leaf area was reduced and the smaller quantity of roots was produced.

Production of herbage is only one goal of grazing management. Both overgrazing and undergrazing can influence herbage quality and efficiency of utilization by the grazing animal. Little work has been published in which grazing animals were used to apply management treatments to permanent Kentucky bluegrass pastures (Murphy et al., 1997). A better understanding of how frequency and intensity of grazing affect sward productivity, quality, and persistence will improve recommendations to farmers on how to manage hill-land pastures. This experiment was conducted to determine herbage growth, amount harvested, and nutritive value of Kentucky bluegrass when grazed at three sward heights and two intensities.

	Materials and methods

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
The experiment was located in a permanent pasture on the West Virginia University Livestock Farm, near Morgantown. Herbage was uniform in botanical composition and consisted of 91% grass (predominantly Kentucky bluegrass), 2% legume (predominantly white clover), and 7% other broadleaf plants (predominantly dandelion, Taraxacum officinale Weber ex F.H. Wigg). The grazing season for the experiment was from mid-April to mid-October. Soil type was Dormont silt loam (fine-loamy, mixed, mesic Oxyagnic Hapludalfs); slope was 15 to 25% with a northwest aspect. Soil pH was 6.2, with 89 kg ha^-1 available P and 368 kg ha^-1 available K (7.5-cm sample depth).

The experiment was started in April 1989 and was repeated in 1990 and 1991. The experiment was designed to determine pasture productivity when grazed rotationally on reaching three different heights to remove two different percentages of the available herbage (3 x 2 factorial design) with four replications. Individual plots (20 by 20 m) were grazed when they approximated the assigned height treatments of 11.5, 13.5, or 15.5 cm (designated as short, medium, and tall, respectively), with variable numbers of yearling steers (330 kg head^-1 average live weight) calculated to remove an amount of available herbage in a 4-h grazing period. Actual sward height was used to decide when to start grazing. Average actual sward heights were 11.8, 13.7, and 15.3. Target removal was 50% of available herbage for the low-intensity treatment and 60% for the high intensity. The design resulted in varying intervals between grazing periods. Plots were grazed in the morning. Steers were held overnight on a pasture of similar composition and small enough to restrict their intake, to allow for rapid defoliation of the test plot.

Sward height was estimated as the mean of 20 measurements in each plot, made with the Hill Farming Research Organization sward stick (Bircham, 1981) immediately before and after grazing. Measurements were made by pacing across the slope along four lines in each plot. Herbage in each measured area was visually classified as grazed or rejected (i.e., ungrazed or grazed very little), before and after grazing. Grazed sward height was the average of the measurements classified as grazed. Rejected sward height was the average of the measurements classified as rejected. Percentage area of the plot rejected was calculated by dividing the number of rejected measurements by the total number (20) and multiplying by 100.

Before each grazing, two random samples were clipped at 2.5 cm within a circular area of 0.2 m² in each plot. Samples taken before the first grazing each year were separated into grass, legume, other broadleaf plants, and dead fractions; subsequent clippings were separated into live and dead fractions. Visible soil contamination was removed by picking out contaminants during hand separation, and fractions were dried to constant weight at 60°C. Sward height for each clipped sample was estimated from 10 heights taken within the circle before clipping. Herbage mass was estimated from prediction equations developed from the clipped samples (Bryan et al., 1990). Apparent daily growth rate was calculated as the difference between herbage mass at grazing minus the herbage mass after the previous grazing, divided by the number of days of regrowth. For the first cycle each year, the herbage mass after the previous grazing was arbitrarily predicted from sward height on 1 April (approximately 4.5 cm). Herbage removed was determined by the difference between herbage mass before and after each grazing. Utilization was herbage removed divided by herbage mass before grazing, multiplied by 100. Herbage allowance was calculated as the herbage mass before grazing divided by the number of animals placed on the plot.

After drying and weighing, clipped samples were composited to give one sample per plot per grazing, and ground through a Wiley mill (1-mm screen). All separated fractions were included. Neutral-detergent fiber (NDF) and acid-detergent fiber (ADF) concentrations were determined by the method of Goering and Van Soest (1970) as modified by Van Soest et al. (1991). Nitrogen was determined by the Kjeldahl procedure (AOAC, 1984) and in vitro DM digestibility (IVDMD) by the procedure of Goering and Van Soest (1970). Crude protein (CP) was calculated as N x 6.25.

Data were averaged or totaled over grazing cycles within years and analyses of variance were performed using the General Linear Model (SAS Institute, 1990). The experiment was analyzed as a split plot, with each height treatment and grazing intensity as a main plot. Year was treated as a split plot. Because this procedure showed few significant interactions between main effects and year, multivariate analysis was not done. When effect of treatment height was significant (P < 0.05), the sums of squares were partitioned into two orthogonal comparisons: short vs. medium and tall, and medium vs. tall. When the effect of year was significant, sums of squares were partitioned into two orthogonal comparisons: 1989 vs. 1990 and 1991, and 1990 vs. 1991.

	Results and discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Rainfall in 1989 was higher than normal and well distributed during the growing season (Table 1) . The second and third years of the study were progressively drier. Average number of grazing cycles per treatment ranged from 4 in 1991 (medium and tall height treatments) to 9.5 in 1989 (short height treatment with 50% removal). Herbage growth rate in spring and early summer 1989 was more than 70 kg ha^-1 d^-1 and remained higher than in succeeding years for approximately 10 wk. Animals did not remove as much herbage as we would have liked in the first three cycles in that year. Consequently, in remaining cycles we increased animal numbers on medium and tall height treatment plots. The result was that in 1989 average herbage allowance was progressively lower on short to tall plots, whereas in 1990 and 1991 herbage allowance was similar on each sward height treatment. Herbage allowance was always less (P < 0.01) on more intensely grazed plots (7.1 kg animal^-1) than on those less intensely grazed (11.0 kg animal^-1).

View larger version (17K): [in this window] [in a new window]   Fig. 1 Apparent growth rate of sward herbage as affected by height treatment and grazing intensity. Interaction significant at P < 0.05

View larger version (24K): [in this window] [in a new window]   Fig. 2 Apparent growth rate of sward herbage as affected by height treatment and year. Interaction significant at P < 0.001

Although most herbage was removed (P < 0.05) when height treatment was short, there was an interaction (P < 0.07) between height treatment and grazing intensity, similar to that found for apparent growth rate (Fig. 1). Most herbage was removed at short height treatment with low grazing intensity and at medium height treatment with high intensity. These data support the sigmoidal form of herbage growth, in that highest growth rate and herbage removal can be expected when sward height is maintained in the range of maximum growth. When herbage is grazed at a short height, a low grazing intensity ensures that sufficient leaf area remains to allow rapid regrowth. When herbage is grazed at a tall height, intense grazing ensures that enough DM is removed to maintain leaf area in that part of the growth curve to allow for maximum growth rate over the longest time period. We found that medium height at high intensity also offers a good combination of growth rate and herbage removal. Herbage removed is an important measure of how much forage is consumed by animals; however, in this experiment no conclusions can be drawn about animal performance.

Year had a dramatic effect on herbage removed (Table 4). Removal in 1991 was less than half that in 1989. These results show a dramatic year-to-year fluctuation in harvestable DM, which most likely depends on amount and distribution of rainfall (Table 1). Farmers with enough stock to utilize 50% of their pasture in 1989 would be grossly overstocked in 1991. Those who stocked to utilize 60% of production in 1991 would have been grossly understocked in 1989. The interaction between height treatment and years shows that only in 1989 were there differences in herbage removed due to height treatment (Fig. 3) . In practice, in a wet year such as 1989, many more animals would be required than in a dry year to carry out the grazing management we practiced. Alternatively, a flexible grazing system (Bryan et al., 1997) could be used to harvest excess herbage as hay.

View larger version (30K): [in this window] [in a new window]   Fig. 3 Sward herbage removed as affected by height treatment and year. Interaction significant at P < 0.01

	Conclusions

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Published as West Virginia Agric. and Forestry Exp. Stn. Sci. Paper no. 2694.

Received for publication November 2, 1998.

	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 Similar articles in Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Web of Science (10) Citing Articles via Google Scholar Google Scholar Articles by Bryan, W. B. Articles by Lozier, J. Search for Related Content PubMed Articles by Bryan, W. B. Articles by Lozier, J. Agricola Articles by Bryan, W. B. Articles by Lozier, J. Related Collections Forage Management Crop Rotation Systems Other Forage Crops