Published in Agron J 91:889-897 (1999)
© 1999 American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
Agronomy Journal 91:889-897 (1999)
© 1999 American Society of Agronomy
FORAGES
Grass Species and Cultivar Effects on Establishment of Grass–White Clover Mixtures
Matt A. Sandersona and
Gerald F. Elwingera
a USDA-ARS Pasture Systems and Watershed Management Res. Lab., Bldg. 3702, Curtin Rd., University Park, PA 16802-3702 USA
mas44{at}psu.edu
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ABSTRACT
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Grasses may differ in their compatibility with white clover (Trifolium repens L.) during establishment. We conducted greenhouse and field experiments to evaluate the compatibility of early- and late-maturing cultivars of perennial ryegrass (Lolium perenne L.) and orchardgrass (Dactylis glomerata L.) with `Will' white clover during establishment. Monocultures and binary mixtures of each cultivar with white clover were established from seed in pots in two greenhouse studies. After an initial harvest at 6 wk of growth, plants were harvested every 2 or 4 wk at a 4- or 8-cm height for 8 wk. The same monocultures and mixtures were planted in field plots and harvested twice at 4 or 8 cm after an 11-wk establishment period. White clover produced more (P < 0.05) stolon + leaf mass and clover proportion of herbage yield was greater when grown with early-maturing than with late-maturing grass cultivars in the field and greenhouse. This indicates that early-maturity grasses were more compatible with white clover. Individual perennial ryegrass plants had about twice as many tillers per plant (P < 0.01) and yielded 24% more (P < 0.01) dry matter than orchardgrass in mixture with clover. Perennial ryegrass–clover mixtures yielded 20% more herbage and had 40% more tillers per unit area than orchardgrass–clover in the field. Clover plants in monoculture were heavier and more complex in structure than plants in mixture in both field and greenhouse. We conclude that maturity of grass cultivars has an effect on white clover establishment and that early-maturing cultivars of perennial ryegrass or orchardgrass are more compatible with Will white clover during the establishment phase.
Abbreviations: PPFD, photosynthetic photon flux density
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INTRODUCTION
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WHITE CLOVER
is the predominant legume of pastures in temperate climates, including the northeastern USA (Pedersen, 1994). Orchardgrass is commonly recommended for pastures in the Northeast (Van Santen and Sleper, 1996), because of its better drought tolerance and winterhardiness compared with perennial ryegrass (Christie and McElroy, 1994). Some dairy producers in the Northeast, however, have begun to use perennial ryegrass–white clover mixtures in pastures, because of the high nutritive value of this mix and because of popular press reports of the nutritive value and productivity of this mixture in Europe and New Zealand. Grass–white clover mixtures support more milk production per cow (Bos taurus) than grass monocultures (Phillips and James, 1998).
Establishing a productive and persistent grass–white clover sward requires a critical density of white clover seedlings and rapid development of those seedlings to achieve an adequate density of stolons (Haggar et al., 1985; Frame et al., 1998). This process depends in part on the compatibility of the grass species and cultivar with white clover (Haynes, 1980; Collins et al., 1996). A compatible mixture has been defined as one that supports a clover proportion sufficient to contribute both its N2 fixation and forage quality attributes within a highly productive grass matrix (Rhodes et al., 1994). Compatibility has been measured by the yield of legume in mixture, with higher legume yields indicating greater compatibility. Tall fescue (Festuca arundinacea Schreb.) cultivars reportedly differ in their compatibility with white clover (Pedersen and Brink, 1988). Orchardgrass lines with later maturity, reduced spring canopy height, and fewer tillers per plant were more compatible (measured as increased production of legume in mixture with grass) with birdsfoot trefoil (Lotus corniculatus L.) than were other lines (Short and Carlson, 1989).
Canopy height and structure can affect the quality of light penetrating the plant canopy, which in turn may affect plant morphology. The plant canopy selectively absorbs red wavelengths (
660 nm) more than far-red wavelengths (730 nm), such that the red:far-red ratio of light decreases toward the base of the canopy (Holmes and Smith, 1977). Common responses to a lower red:far-red ratio in the plant canopy include elongation of petioles, leaves, and stems and reduced tillering in grasses and reduced branching in legumes (Ballere et al., 1995).
Studies on grass–white clover competition have been conducted with containers of various sizes in the greenhouse (e.g., Turkington and Klein, 1991; Turkington and Jolliffe, 1996) and in field plots (e.g., Annicchiarico and Piano, 1994; Elgersma and Schlepers, 1997). Greenhouse studies allow greater control of environmental variables. We conducted two experiments in the greenhouse to compare a large number of treatments with adequate replication under controlled conditions, and a field experiment to confirm greenhouse results.
The objective of our study was to examine the effects of orchardgrass and perennial ryegrass cultivars on the morphology and yield of the grasses and white clover in binary mixtures during the establishment phase. We used cultivars differing in relative maturity to determine if this trait affects grass–legume compatibility, as suggested by Short and Carlson (1989). In addition, we varied defoliation frequency and intensity in the greenhouse to determine if these factors interact with species and cultivars to affect grass–legume compatibility. We then repeated the study in the field, varying only the cutting height.
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Materials and methods
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`Bastion' and `Rosalin' were the perennial ryegrass cultivars used. Bastion is earlier in maturity than Rosalin and both are tetraploid varieties of perennial ryegrass from the Netherlands (Alderson and Sharp, 1994; Adrian Vanderhave, Advanta Seeds Pacific, personal communication, 1997). `Dawn' orchardgrass is a medium-maturity cultivar; `Pennlate' is late maturing (Alderson and Sharp, 1994; Van Santen and Sleper, 1996). Both orchardgrasses were developed in the USA. Will white clover is a large-leaved cultivar developed and released by North Carolina State University (Caradus and Woodfield, 1997).
Greenhouse Experiments
In August 1996 (Run 1) and February 1997 (Run 2), plants of grasses and clover were started from seed in 28-cm-diam. by 30-cm-deep plastic pots in a greenhouse. The pots were filled with 10 kg of potting soil (Scots-Sierra Horticultural Products Co., Marysville, OH). A planting jig was used to mark 14 equidistant holes in the soil into which three seeds were placed, covered with 1 cm of soil, lightly packed, and then watered. Plants were thinned to one per hole (14 plants per pot) after emergence. Monocultures of each cultivar and species consisted of 14 plants per pot. Binary mixtures of each grass with white clover were established by planting grass and legume in alternate rows of holes (seven grass and seven legume plants). Data collection was confined to the four target plants in the center of the pot (four grass or clover plants in the monoculture pots or two grass and two clover plants in the mixture pots). Thus, the ten plants closest to the edge of each pot served as border plants. A total of 180 pots was planted in each run. Pots were watered daily and 0.8 g of a complete nutrient solution (Grow More all purpose plant food, NPK ratio of 10–15–10; Grow More, Gardena, CA) was added to each pot twice during the experiment.
The experimental design was a randomized complete block with five replicates. Treatments were arranged in a 9 x 2 x 2 factorial of nine forage species and cultivar treatments (monocultures of each species and cultivar plus mixtures of each grass species and cultivar with white clover), two cutting heights (4 and 8 cm), and two cutting frequencies (2 and 4 wk). Pots were grouped into five blocks of 36 pots each. Each block of pots was arranged in four rows of nine pots, all adjacent. Pots were rotated weekly within blocks to minimize edge effects.
All pots were harvested at 44 d after planting in 1996 and 49 d after planting in 1997. Pots were then harvested at a 2- or 4-cm cutting height at 2-wk intervals for 8 wk (a total of five harvests including the initial harvest) or 4-wk intervals for 8 wk (a total of three harvests including the initial harvest). The clipped material from the four target plants was separated into grass and clover portions and dried at 55°C for 48 h. At each clipping interval, grass tiller height (soil surface to extended leaf tip) and petiole length were measured, and grass tillers and clover growing points were counted on target plants.
At the end of the experiment, the plant material remaining below the cutting height was clipped at or slightly below ground level. The number of stolons, number of nodes per stolon, length of the longest stolon, and number of branches were measured for the clover plants removed from the soil. All tap roots and nodal roots were trimmed and discarded and the plants were dried at 55°C for 48 h.
The experiment was conducted from August to November 1996 and repeated during February to May 1997. Plants were grown under natural daylight and daylength during the first run. During the second run, natural light was supplemented (but the natural daylength was not extended) with artificial light from 400-W lamps providing 260 µmol PPFD m-2 s-1 at plant height during the entire experiment. Daylength during August to November at State College ranges from 14 to 10 h, respectively, and 10 to 14.5 h during February to May, respectively. Red:far-red ratio of the supplemental light was 1.61, compared with 1.31 for ambient light levels. Temperatures varied from 20 to 38°C during the day and 11 to 27°C during the night in fall 1996, and 22 to 41°C during the day and 13 to 24°C during the night in spring 1997. Grasses remained vegetative during both runs of the experiment.
Separate analyses of variance were conducted on herbage yield and structural components. Data were checked for normality and transformed as necessary. Data are presented on the original scale, with table footnotes indicating when significance tests were calculated on a transformed scale. Single-degree-of-freedom contrasts were constructed to compare main effect means.
Field Experiment
On 1 July 1998, the same nine grass and white clover monocultures and mixtures used in the greenhouse experiments were established in field plots at the Russell E. Larson Agricultural Research Center near Rock Springs, PA. Soil type at the research site is a Hagerstown silt loam (fine, mixed, mesic Typic Hapludalf). The previous crop was alfalfa (Medicago sativa L.). Soil analyses (15-cm depth) in the spring of 1998 indicated a pH of 6.0, available P at 123 kg ha-1, and available K at 128 kg ha-1. No fertilizer was applied during the study.
Each monoculture or mixture was seeded into 1.8- by 2-m plots (five blocks of nine plots each) with a Hege1
plot drill. Plots were 10 rows wide, with 18 cm between rows. Monoculture plots of orchardgrass and perennial ryegrass were seeded at 11.2 and 22.4 kg ha-1, respectively, and monoculture plots of white clover were seeded at 5.6 kg ha-1. Binary mixtures of orchardgrass and white clover were seeded each at 5.6 kg ha-1, and binary mixtures of perennial ryegrass and white clover were seeded at 11.2 and 5.6 kg ha-1, respectively. These rates are common in the U.S. Northeast (Penn State University, 1997). Plots were established on a tilled seed bed (moldboard plowed, disked, harrowed, and packed) and weeds were controlled manually. Each 1.8- by 2-m plot was divided into two subplots (five rows, 0.9 m wide by 2 m long), and cutting height treatments of 4 and 8 cm were randomly assigned to subplots. The experimental design was a randomized complete block with a split-plot arrangement of treatments. The nine monocultures or mixtures of grass and white clover were the whole plots and cutting heights were subplots.
On 21 August, the unextended height of two grass tillers and two white clover petioles in each subplot was measured and tillers were counted on one grass plant from each whole plot. Plots were harvested initially on 16 September and regrowth was harvested on 28 October. Before each harvest, the unextended height of two grass tillers and two white clover petioles in each subplot was measured. Stolon density was estimated by counting the number of stolons intersecting a meter rule laid parallel to the center row of each subplot. Before the 16 September harvest, four grass and four clover plants were dug up from each main plot. The number of tillers and leaves were counted on each grass plant; the number of stolons, stolon branches, and nodes (rooted and nonrooted) were counted and the length of the longest stolon was measured on each white clover plant. Before the 28 October harvest, a 0.1-m2 area centered over two rows was excavated to a 10-cm depth from each subplot and the number of grass tillers and clover plants were counted. Ten grass tillers were randomly selected, the number of green leaves per tiller was recorded, and the tillers were dried at 55°C for 48 h. Clover seedlings were classified as with (at least one stolon >1 cm long) or without stolons. Four clover plants with stolons were selected; the number of stolons, stolon branches, and nodes (rooted and nonrooted, counted only on the longest stolon) were counted and the length of the longest stolon was determined. At each harvest, herbage in a 0.3- by 1-m area (centered over two rows) in each subplot was clipped with battery powered shears at either 4 or 8 cm, and the herbage was separated into grass or clover and dried at 55°C for 48 h.
Growth rates of white clover stolons were measured during 21 August to 4 September, 4 to 16 September, 16 September to 1 October, and 1 to 28 October. The first two periods were preharvest measurements and the last two periods occurred between Harvests 1 and 2. White clover seedlings had just begun elongating stolons on 21 August. Two stolons (one each on separate plants) in each subplot were marked and measured in each period. Stolons were marked by placing a thin wire staple directly behind the stolon tip; the distance from the staple to the stolon tip was measured at the beginning and end of each period.
The amount of red and far-red light at the base of the plant canopy was measured with a Model LI-1800 spectroradiometer (Li-Cor, Lincoln, NE) on 27 August, and before and after the harvests on 16 September and 28 October. Skies were clear on each day, and measurements were made during 1300 to 1530 h. Measurements were taken by placing a remote cosine receptor (attached to the instrument via a fiber optic cable) at the base of the plant canopy midway between the two center rows in each subplot. The sensor was about 1 cm above the ground because of the sensor housing. Light spectral irradiance was recorded at 5-nm increments from 400 to 750 nm. One measurement was taken per subplot. The ratio of red to far-red light was calculated using the average spectral irradiances at 645 nm for red and 735 nm for far-red.
Separate analyses of variance were conducted for data from each date. The MIXED procedure of SAS (SAS, 1992) was used to analyze the split-plot experiment. This procedure provides more valid and efficient statistical estimates of error terms for split plot experiments where blocks are considered random and treatments fixed effects than does the General Linear Models procedure. Data were checked for normality and transformed as necessary. Data are presented on the original scale, with table footnotes indicating when significance tests were calculated on a transformed scale. Single-degree-of-freedom contrasts were used to compare main effect means.
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Results
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Greenhouse Experiments
Generally, plants were larger (more tillers, growing points, and leaves) and yielded more dry matter in the second run of the experiment than in the first run. This probably resulted from the increased light input during the second run. We reasoned that frequent overcast skies during February to May in central Pennsylvania would limit light input compared with the relatively clearer skies of fall, hence the decision to supplement light during the second run. The increased light levels in the second run generally affected all treatments similarly; however, there were a few instances of an interaction of treatments with run. In these instances, the interactions resulted from an increase in the magnitude of response and not from crossover effects. Thus, we present and discuss the data as means of both runs.
Herbage Yields
Herbage yield per pot was lower (P < 0.01) in grass–clover mixtures than in the monocultures at the initial harvest; however, the reverse occurred at regrowth harvests (Table 1)
. The increased yield of mixtures in regrowth probably resulted from increased clover proportion (Fig. 1)
. Orchardgrass yielded less than perennial ryegrass at the initial harvest but yields were similar during regrowth. Cultivar maturity did not affect herbage yields at the initial harvest.
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Table 1 Herbage yield per pot of grass and white clover grown in mixture or monoculture in the greenhouse. Initial harvest data are averages of two runs and two cutting heights. Regrowth data are totals for all regrowth harvests and are the averages of two cutting frequencies and two runs of five replicates each
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Fig. 1 Percentage of white clover in clipped herbage from grass–clover mixtures grown in the greenhouse. Data are averages of two clipping heights, two clipping frequencies (regrowth harvest only), and two runs each of five replicates. PRG, perennial ryegrass; OG, orchardgrass. Bastion and Dawn are early maturity; Rosalin and Pennlate are later maturity. *,** Significant at P < 0.05 and 0.01, respectively
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Early-maturing grass cultivars yielded more herbage than late-maturing cultivars in monocultures during regrowth (Table 1). Herbage yields during regrowth were greater (P < 0.01) when cut at 4 cm than at 8 cm. There was a cutting height x grass species and cultivar interaction, caused by a shift in the rank of species and cultivars. Yield of Bastion perennial ryegrass–white clover mixtures increased by 13% when cut at an 8-cm height, compared with 4 cm, whereas most other cultivars and species showed the opposite trend or did not change. The proportion of white clover in herbage of the initial harvest was greater in orchardgrass than perennial ryegrass mixtures and was greater with early- than late-maturing cultivars (Fig. 1).
Grass Plant Structure
Orchardgrass plants were taller, had fewer tillers, and yielded less than perennial ryegrass plants at the initial harvest (Table 2) . Early-maturing cultivars were taller in monoculture and had fewer tillers per plant in mixture than late-maturing cultivars. Cultivar maturity did not affect harvested dry mass per plant at the initial harvest.
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Table 2 Structure of individual grass plants at the initial harvest when grown in monoculture or in mixture with white clover in the greenhouse. Data are the averages of two cutting heights and two runs of five replicates each*
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During regrowth, grasses were taller when grown with white clover, and early-maturity cultivars were taller than late-maturity cultivars (Table 3)
. Orchardgrass was taller than perennial ryegrass in monoculture during regrowth. The number of tillers per plant in regrowth harvests was affected by species and cultivar, and an interaction of cutting height with species and cultivars (Table 3). The interaction resulted from an increase in tiller number for Bastion perennial ryegrass in monoculture and mixture and a decrease in tiller number for Dawn orchardgrass in mixture when cut at 8 cm compared with 4 cm. The number of tillers per plant was not affected by cutting height in other cultivars. Grasses had more tillers when grown in mixture than in monoculture, and Rosalin produced more tillers than Bastion in mixture. Harvest frequency had no effect (P > 0.05) on tiller numbers during regrowth. Perennial ryegrass plants yielded more dry matter than orchardgrass in mixture, and early-maturing cultivars were heavier than late-maturing cultivars in monocultures (Table 3). Cutting every 4 wk produced more (P < 0.01) dry matter (1.61 g plant-1) than cutting at a 2-wk interval (1.42 g plant-1; data not shown).
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Table 3 Structure of individual grass plants at regrowth harvests when grown in monoculture or in mixture with white clover in the greenhouse. Data are the averages of the two common harvest dates, two cutting frequencies, and two runs each of five replicates*
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Clover Plant Structure
Orchardgrass appeared to be more compatible than perennial ryegrass with white clover at the initial harvest. White clover plants yielded more leaf mass at the initial harvest, petioles were longer, and plants had more growing points when grown with orchardgrass than when grown with perennial ryegrass (Table 4)
. Maturity of the grasses had no effect on clover yield or structure at the initial harvest. During regrowth, however, it appeared that early-maturing cultivars of both grasses were more compatible with white clover than late cultivars. White clover plants yielded more leaf and stolon mass when grown with early than with late-maturing cultivars (Table 5)
. Grass species and cultivar maturity, however, did not affect the complexity (stolon number, structure, and branches) of white clover plants. In the regrowth harvests, clover petioles were longer when grown in monoculture than in mixture, and when grown with orchardgrass than with perennial ryegrass. Increased petiole length is a common response of white clover when grown with taller companion plants (Frame et al., 1998, p. 27–28).
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Table 4 Structure of individual white clover plants at the initial harvest when grown in monoculture or in mixture with grass in the greenhouse. Data are averages of two runs and two cutting heights
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Table 5 Structure of individual white clover plants at regrowth harvests when grown in monoculture or in mixture with grass in the greenhouse. Data for leaf mass, petiole length, and leaf number are averages of the two common harvest dates, two cutting frequencies, and two runs each of five replicates. Data for stolon characteristics are from the final harvest
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White clover had significantly more and longer stolons with more nodes and branches in monoculture than in mixture (Table 5). Stolons were more numerous and longer (P < 0.01) when plants were clipped every 4 wk (5.3 stolons, 9.4 cm) than every 2 wk (5.0 stolons, 7.8 cm). Clover stolons did not extend over the edge of the pots.
Field Experiment
Air temperatures during July, August, and October were very near normal, whereas temperature during September was 1.8°C above normal (Table 6)
. Maximum temperatures during the first 30 d after planting ranged from 21 to 29°C and 48 mm of rain fell during the first week after planting. This resulted in excellent germination and emergence of seedlings. Rainfall during August to October was 100 mm below normal, which may have stressed the seedlings.
There were no interactions of cutting height with species or cultivar for any trait measured in the field. Therefore, only main effect means are presented and discussed.
Herbage Yields
Early-maturing grass cultivars yielded less than late-maturing cultivars in monoculture at both harvest dates (Fig. 2)
. At the regrowth harvest on 28 October, orchardgrass yielded less than perennial ryegrass. The proportion of herbage as white clover was greater in mixture with perennial ryegrass than orchardgrass at the initial harvest on 16 September; however, the reverse occurred at the regrowth harvest. The proportion of white clover in both perennial ryegrass and orchardgrass mixtures was greater with early than late-maturing grass cultivars at both harvests, indicating that early-maturing cultivars may be more compatible with Will white clover at establishment (Fig. 1 and 2).
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Fig. 2 Herbage dry matter yields and percentage clover in grass–white clover mixtures and monocultures at two harvest dates in the field. Data are averages of two cutting heights and five replicates. Btn, Bastion perennial ryegrass (early maturity); Ros, Rosalin perennial ryegrass (late maturity); Dwn, Dawn orchardgrass (early maturity); Pnn, Pennlate orchardgrass (late maturity); Will, white clover
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Grass Plant Structure
There was no difference in height among grass species or cultivars on 21 August (Table 7)
; however, orchardgrass was taller than perennial ryegrass on 16 September. Perennial ryegrass had nearly twice as many tillers per plant than did orchardgrass on 21 August and 16 September, similar to the greenhouse results (Tables 2 and 3). Orchardgrass had more leaves per tiller than did perennial ryegrass. Tiller mass followed the opposite pattern of tiller number.
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Table 7 Structure of individual grass plants in monoculture or in mixture with white clover in the field. Data are averages of two cutting heights and five replicates
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Early-maturing grass cultivars were shorter than late-maturing cultivars at the regrowth harvest in October (Table 7). This was opposite to the response in the greenhouse (Table 3). Tillers per unit area and tiller mass in October followed the same pattern as tillers per plant and tiller mass at the September harvest. Orchardgrass had more leaves per tiller than perennial ryegrass and the early-maturing grass cultivars had more leaves than later cultivars in mixture in October.
Clover Plant Structure
White clover grown in monoculture had more stolons, nodes per stolon, and growing points per plant than did white clover grown in mixture at the initial harvest in September (Table 8)
. White clover plants were also heavier (greater leaf + stolon mass) in monoculture than in mixture. No branches or rooted nodes were observed on any stolon examined at the initial harvest. Petiole length averaged 9.4 cm and was similar among species and cultivars at the initial and regrowth harvests (data not shown).
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Table 8 Structure of white clover plants grown in mixture with grasses or in monoculture in the field at the initial harvest on 16 Sept. Data are averages of two cutting heights and five replicates*
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At the regrowth harvest in October, white clover grown in monoculture had greater seedling (with stolon development), stolon, and growing point density and greater total plant mass (total of stolons and leaves) than white clover grown in mixture (Table 9) . There were no significant differences, however, in the structure of the individual stolons. There was an average of three rooted nodes on the longest stolon at the regrowth harvest with no significant difference among species and cultivars (data not shown). There were very few branches observed on stolons from any treatment at the regrowth harvest (data not shown).
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Table 9 Structure of white clover plants grown in mixture with grasses or in monoculture in the field at the regrowth harvest on 28 October. Data for species and cultivars are averages of two cutting heights. Data for cutting heights are averaged for species and cultivars
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White clover produced a greater stolon + leaf mass per unit area when grown with early-maturity than with late-maturity grass cultivars (Table 9), which confirms results in the greenhouse (Table 5). This may indicate that early-maturity cultivars are more compatible with white clover than the late cultivars. Despite significant differences between grass species and cultivars in plant structure (Table 7), these differences did not seem to affect clover structure in the field (Tables 8 and 9). The trend, however, was for most structural components of clover to be numerically greater with early than late-maturity grass cultivars.
Stolon growth rate was greater (P < 0.05) in white clover monoculture (47 mm d-1) than in mixture (33 mm d-1) during 21 August to 4 September. Growth rates did not differ (P > 0.05) among treatments at other dates (data not shown). Stolon growth rate decreased from an average of 36 mm d-1 in August to 15 mm d-1 in October.
The red:far-red ratio of light penetrating the plant canopy was significantly lower (more far-red light penetrating, indicating more shading) in plots cut at 8 cm than at 4 cm after the initial harvest and both before and after the regrowth harvest (Table 10) . The red:far-red ratio was lower at the base of the canopy of late-maturing cultivars than early cultivars in monoculture before harvest on 16 September. The ratio was lower in white clover monoculture compared with the average of the mixtures and lower in monocultures of perennial ryegrass compared with orchardgrass after harvest on 16 September.
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Table 10 Ratio of red to far-red light at the base of the canopy in white clover grown in monoculture or mixture with grasses in the field. Data for species and cultivars are averages of two cutting heights. Data for cutting heights are averaged for species and cultivars
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Discussion
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Herbage yield of mixtures and monocultures in the greenhouse generally reflected productivity in the field (Table 1; Fig. 1 and 2). There were fewer significant differences, however, among species and cultivars in white clover plant structure in the field than in the greenhouse (Tables 4, 5, 8, and 9), perhaps reflecting the dry field conditions. The field experiment was planted relatively late and seedlings probably were water stressed during the fall. Except for one instance, the structural differences in the grasses did not seem to affect the light environment at the base of the canopy (Table 10), which probably accounted in part for the lack of differences in clover plant structure in the field. Changes in canopy structure that increase shading (a lower red:far-red light ratio) of clover generally increase petiole and stolon length and reduce branching (Frame et al., 1998, p. 27–28).
Our results agree with those of Collins et al. (1996), who reported that substantial competition occurs between seedlings of grasses and white clover during establishment. Our results showed that white clover seedlings were larger and more complex when grown in monoculture than in mixture with grasses, indicating significant competitive effects of the grass. In contrast to Collins et al. (1996), however, we found few differences in white clover seedling complexity in mixture with different cultivars of perennial ryegrass and orchardgrass in the greenhouse and none in the field. We did find that white clover produced more stolon + leaf mass (Tables 5 and 9) and that clover proportion of herbage yield was greater (Fig. 1 and 2) when grown with early-maturing than with late-maturing grass cultivars, indicating that early-maturity grasses are more compatible with Will white clover. This might be partly explained by fewer tillers per plant with early cultivars in the greenhouse (Tables 2 and 3), but that was not the case in the field (Table 7).
Our data support the results of Gilliland (1996) and Gooding et al. (1996), who reported that early-maturing cultivars of perennial ryegrass (e.g., Bastion, used in both studies) were more compatible with white clover in 2-yr grazing evaluations. Increased compatibility was associated with reduced sward density in both studies. Early-maturing diploid cultivars of annual ryegrass (Lolium multiflorum Lam.) were shown to be more compatible with alfalfa than were late-maturing annual ryegrasses during establishment (Sulc and Albrecht, 1996). On the other hand, our results with seeding-year stands of grass and white clover are contrary to those of Short and Carlson (1989), who reported that late maturity in orchardgrass was associated with increased compatibility with birdsfoot trefoil in established stands. They also reported, however, that tiller number of orchardgrass was an important determinant of compatibility, with fewer tillers per plant associated with increased compatibility with birdsfoot trefoil. The late-maturing orchardgrass had longer and wider leaves than early lines, which they considered important in compatibility with legumes.
We conclude that cultivar maturity of the companion grass has an effect on white clover establishment and that early-maturing cultivars of perennial ryegrass or orchardgrass are more compatible with Will white clover than late-maturity cultivars during the establishment phase.SAS Institute 1992
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NOTES
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1 Mention of a trademark or proprietary product does not constitute a guarantee or warranty of the product by the USDA and does not imply its approval to the exclusion of other products that may also be suitable. 
Received for publication January 11, 1999.
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REFERENCES
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ABSTRACT
INTRODUCTION
