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Published in Agron J 91:916-921 (1999)
© 1999 American Society of Agronomy
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Agronomy Journal 91:916-921 (1999)
© 1999 American Society of Agronomy

FORAGES

Establishment of Temperate Pasture Species into Alfalfa by Frost-Seeding

Michael D. Caslera, David C. Westb and Daniel J. Undersandera

a Dep. of Agronomy, Univ. of Wisconsin, Madison, WI 53706-1597 USA
b Consumers' Coop., Richland Center, WI 53581 USA

mdcasler{at}facstaff.wisc.edu


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 Summary and conclusions
 REFERENCES
 
Mature alfalfa (Medicago sativa L.) stands are generally unsuitable as pastures because of their low plant density and the relative inability of alfalfa plants to tolerate grazing pressure. The objective of this research was to determine the potential for introducing new species into mature or declining alfalfa stands by frost-seeding. Experiments were conducted in 1995 and 1996 at the University of Wisconsin Agricultural Research Stations near Arlington, WI (four sites), and Lancaster, WI (three sites). Smooth bromegrass (Bromus inermis Leyss.), orchardgrass (Dactylis glomerata L.), perennial ryegrass (Lolium perenne L.), reed canarygrass (Phalaris arundinacea L.), timothy (Phleum pratense L.), and red clover (Trifolium pratense L.) were frost-seeded into mature alfalfa stands at seeding rates of 0, 55, 110, 220, 440, and 880 seeds m-2. Competition from existing vegetation was reduced by clipping the fields in the autumn prior to seeding and harvesting (by either grazing or clipping) during the seeding year. Seedling counts were taken approximately 60 and 90 d after seeding. Seedling densities were similar at 60 and 90 d after seeding in both years. There was variability in the magnitude of species establishment and density among sites, but species ranking order was fairly consistent across sites. Species ranking for seedling density was perennial ryegrass > orchardgrass, smooth bromegrass, and red clover > timothy and reed canarygrass (59, 28, 20, 22, 13, and 5 seedlings m-2, respectively). All species showed a linear or quadratic response to increasing seeding rate, but the response varied with species. Results of this research indicate that establishment of these species by frost-seeding is possible and that this technique can increase plant diversity within pastures.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 Summary and conclusions
 REFERENCES
 
PASTURE

as a primary source of feed has become increasingly important in the dairy industry. This can be explained by changes in milk price that are insufficient to offset increases in capital costs. To maximize pasture productivity, management may be intensified by increased fertilization, higher stocking rates, and a change from continuous grazing to rotational grazing. Growers often introduce new species if the existing vegetation is insufficient in yield and/or quality.

Several methods can be used to introduce new species. Tillage reduces existing plant competition and increases seed–soil contact (Ahlgren et al., 1944; Sprague et al., 1947). However, exposed soil is prone to erosion on sloping sites. No-tillage seeding alleviates erosion concerns, but requires investment in specific equipment. Frost-seeding in late winter is an alternative that minimizes equipment expenditures and erosion concerns while introducing new, more desirable species (George, 1984).

Improved pastures provide several agronomic benefits such as increased dry matter production (Knight, 1970), improved seasonal forage distribution, potential replacement of nitrogen requirements with legume introduction (Evers, 1985) and improved livestock performance (Bagley et al., 1988). Legumes frost-seeded into grass sod can improve dry matter yield and plant diversity despite marginal seeding conditions (Evers, 1995; George, 1984; Gettle et al., 1996). Grasses have been difficult to establish in sod due to excessive competition from resident plants (Sprague et al., 1947). Frost-seeding research to date has focused on introducing new species into grass sod. There is little known about the viability of frost-seeding legumes and grasses into a more open canopy such as alfalfa. Because alfalfa, typically machine harvested, is getting more frequent use in grazing systems, frost-seeding may be a low-cost and viable option to convert old alfalfa fields into productive pastures by increasing their species diversity, forage yield, and longevity. Mature or declining alfalfa stands could be rapidly converted into productive mixed-species pastures if grazing-tolerant cool-season grasses or legumes could be rapidly introduced without tillage. The objective of this research was to quantify the establishment, density, and seeding rate response of six cool-season pasture species frost-seeded into mature or declining alfalfa stands.


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 Summary and conclusions
 REFERENCES
 
Field experiments were conducted in 1995 and 1996 at the University of Wisconsin agricultural research stations near Arlington (43°18' N, 89°22' W) and near Lancaster (42°50' N, 90°47' W). In 1995, the experiment was conducted in three alfalfa fields (one at Lancaster and two at Arlington); all fields were harvested mechanically. In 1996, the experiment was conducted in four alfalfa fields (two at Arlington and two at Lancaster), with a grazed site and clipped site at each location. In both years and at both locations, the experiment was also conducted in pastures dominated by Kentucky bluegrass (Poa pratensis L.). The soil at all Arlington sites was Plano silt loam (fine-silty, mixed, superactive, mesic Typic Argiudolls). The soil at all Lancaster sites was Fayette silt loam (fine-silty, mixed, superactive, mesic Typic Hapludalfs).

The experimental design was a split plot in a randomized complete block replicated four times. Whole plots were six species and subplots were six seeding rates; subplot size was 1.22 by 6.71 m. The six species used were smooth bromegrass cv. Alpha, orchardgrass cv. Benchmark, perennial ryegrass cv. Madiera, reed canarygrass cv. Rival, red clover cv. Marathon, and timothy cv. Colt. The six seeding rates were 0, 55, 110, 220, 440, and 880 pure live seeds m-2. Each species was represented by a single seed lot, produced in 1994, which were tested for germination (AOSA, 1998) in February 1995 and 1996. Fields selected for the experiment had been in alfalfa for 2 to 5 yr or in permanent grass pasture for more than 20 yr. Alfalfa stand density was approximately 30 to 50 plants m-2. Field preparation consisted of clipping each site to a 5-cm stubble height in the autumn prior to the seeding year. Seeding took place in mid-March. A drill seeder was used, with the openers elevated above the soil surface to simulate broadcast seeding.

Nitrogen was applied to plots of all five grasses at a rate of 56 kg N ha-1 approximately 30 d after seeding. To reduce competition from preexisting vegetation, plots were clipped (hay treatment) or grazed (pasture treatment) to an 8-cm height throughout the seeding year whenever the maximum canopy height reached 35 cm. Following grazing, the fields were clipped to achieve a uniform stubble height of 8 cm. Grazing was accomplished by yearling heifers (Bos taurus) at a stocking rate sufficiently high to remove the desired amount of vegetation in less than 8 h. Approximately 60 and 90 d after seeding, when seedlings were deemed to be photosynthetically self-sufficient, seedling density was determined by counting the number of seedlings in four randomly placed 0.045-m2 circular sampling frames per plot. Soil bulk density samples were taken with a slide-hammer type sampler (Blake, 1965) before and after grazing to measure any changes in soil compaction. The bulk density samples were 8 cm in diameter and 8 cm long, representing soil depths of 0 to 8 cm.

Seedling numbers were analyzed with analysis of variance, for individual sites and combined over sites. Species was considered to be a fixed effect, while block, site, and seeding rate were considered random. Simple linear or quadratic regression was used to measure response of species to increased seeding rates. Contrasts were used to further test differences among species, rates, and species x rate interactions.

An economic analysis of frost-seeding was performed using two assumed constants. The 9-yr mean (1991 to 1999) retail price of certified seed was based on the cultivars we used or the most similar cultivars of that species marketed in southern Wisconsin. Seed number was determined by counting the number of seeds in three 1-g samples from the seedlot used in this study. These values were used to compute the seed cost for each seeding rate, as well as the seeding rate and dollar requirements to obtain 20 seedlings m-2.


   Results and discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
Summary and conclusions
 REFERENCES
 
Grass seedling establishment was unsuccessful in the bluegrass pastures, and establishment of red clover was sparse. This was due perhaps to the limited seed–soil contact in the dense ground cover and the high level of competition from the resident grasses. Because of this establishment failure, seedling counts were not made at the bluegrass sites.

Following frost-seeding into the alfalfa fields, all species became established but seedling density differed among species. Plant counts taken 60 and 90 d after seeding were similar in both overall means and individual treatment means. Mean seedling numbers for red clover, perennial ryegrass, orchardgrass, reed canarygrass, smooth bromegrass, and timothy at 60 d after seeding were 22, 28, 59, 5, 20, and 14 seedlings m-2, respectively; at 90 d after seeding, the means were 19, 32, 66, 5, 20, and 13 seedlings m-2, respectively. Because mean seedling numbers, treatment mean rankings, and all significance levels were similar for both seedling counts, only data from plant counts taken 60 d after seeding will be reported. The similarity of seedling count data between 60 and 90 d was an indication that establishment had occurred by 60 d after seeding and that seedlings observed on this date were autotrophic.

Climatic conditions were similar for the two locations. Spring of 1995 was characterized by mean temperatures 1°C above April and May normals (8 and 14°C, respectively) and mean precipitation 75 mm above the normal total for April and May (235 mm); the precipitation occurred in few but substantial rainfall events. In 1996, mean temperatures were approximately 2°C below April and May normals and rainfall was similar to the April and May normals, with a high frequency of low-rainfall events. There was variability in the magnitude of establishment and seedling density among locations, harvest managements and years, but this did not affect treatment mean rankings. Mean seedling densities were 29 and 22 seedlings m-2 in 1995 and 1996, respectively.

Grazed sites had fewer seedlings than clipped sites, with a mean of 15 vs. 28 seedlings m-2, respectively (P < 0.01). Grazing when soil moisture is high, a condition which may have occurred in 1996 due to the frequent rains, has been shown to cause seedling damage by a loss in soil structure, pan formation, a perched water table, or an increase in bulk density of the surface layer (Edmond, 1958; Federer et al., 1961; Mullen et al., 1974). However, bulk density samples taken prior to and after grazing had means of 1.39 and 1.43 g cm-3, respectively, indicating no increase in soil compaction (P > 0.05). This, coupled with the short grazing duration on these sites, suggests that the lower seedling numbers found in the grazed sites are best explained by site-to-site variation. We observed no visible treading damage to the existing vegetation, nor hoof imprints on the soil surface. Grazing was conducted under conditions sufficiently dry to minimize the apparent impact of treading. Furthermore, prior research suggests that neither light nor heavy treading of sheep (Ovis aries) on new seedings affected productivity of renovated pastures (Mullen et al., 1974).

Differential Response of Species
Mean seedling establishment percentages varied across sites, but there was a similar ranking of species within sites (Table 1) . All species became established, but the levels varied greatly among sites and were dependent on the individual species' establishment characteristics. For example, perennial ryegrass consistently had the most successful establishment, with a mean of 24.5% of the total seeds broadcast becoming autotrophic—more than double the next highest species. This high level of establishment can be attributed to rapid germination, early seedling development, and tolerance to a variety of environmental conditions (Jung et al., 1996). Orchardgrass had the next highest establishment, with a mean of 11.7%. Orchardgrass has good seedling vigor but low competitive ability during its juvenile phase (Kreuz, 1991). However, because of its shade tolerance (Blake et al., 1966), orchardgrass establishment in the presence of resident plants was superior to all other species, except perennial ryegrass.


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  		Table 1 Mean establishment percentages of six temperate pasture species following frost-seeding for seven sites in Wisconsin, identified by location, harvest management, and year of seeding. Means are over four replicates and five seeding rates

 
Red clover and smooth bromegrass had moderate establishment, with means of 9.5 and 8.1%, respectively. These values are considerably lower than typical values for conventional seeding methods, suggesting that fluctuations in seeding conditions associated with frost-seeding may substantially reduce establishment levels under suboptimal conditions (Gettle et al., 1996). Consistent establishment of smooth bromegrass was attributed to its excellent seedling vigor, a trait enhanced by early seeding and adequate moisture and temperature (Vogel et al., 1996).

The lowest establishment was found in timothy and reed canarygrass with means of 6.0 and 2.2%, respectively. Timothy seeds germinate quickly, but seedling development is slow (Berg et al., 1996), making it susceptible to competition from resident plants. Both germination and emergence of reed canarygrass are slow, reducing its competitive ability (Buxton and Wedin, 1970; Kroth et al., 1976). Poor germination in reed canarygrass has also been attributed to seed dormancy and a high proportion of immature seed (Marten, 1985). Because of the slow establishment of reed canarygrass, competition from resident plants, especially aggressively growing grasses and broadleaf weeds, reduced establishment to a greater extent than for the faster establishing species.

Establishment percentage decreased slightly as seeding rates increased for all species (Table 2) . This can be explained by a fixed level of competition from resident plants and relatively poor seed–soil contact, providing an apparent limit to the absolute number of seedlings that can be established at a given site. This is consistent with related work on surface seeding (Evers, 1995). Perennial ryegrass was the only species to show a significant decrease in establishment percentage as seeding rate increased (P < 0.05), suggesting that species with high levels of establishment may have a greater incidence of intraspecific competition due to greater seedling numbers compared with species with lower levels of establishment.


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  		Table 2 Mean establishment percentages of six temperate pasture species following frost-seeding at five seeding rates. Means are over four replicates and seven sites

 
Seedling density varied among sites with a range of 9 to 38 seedlings m-2, averaged over species (Table 3) . The range among species means, averaged over sites, was more than double this range (5–59 seedlings m-2) and was even greater within some sites. There were some slight changes in species ranking among sites, but not enough to change any conclusions. Thus, the analyses combining sites were sufficient to explain the species means and their response to increased seeding rate.


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  		Table 3 Mean seedling density of six temperate pasture species following frost-seeding at seven sites, identified by location, harvest management, and year of seeding. Means are over four replicates and five seeding rates

 
Perennial ryegrass had the highest seedling density mean of 59 seedlings m-2 (Table 3) and the greatest response to increased seeding rate (Fig. 1) . The linear slope for perennial ryegrass was 3.3 times greater than the mean slope of all other species and 2.4 times greater than the linear slope for orchardgrass, the next highest response (data not shown). Quadratic responses for both perennial ryegrass and orchardgrass were reported because these two species' responses were clearly not linear. These data suggest that, for low to moderate seeding rates up to 440 seeds m-2, an average of 2.51 ± 0.19 perennial ryegrass seedlings became established for every 10 seeds frost-seeded. For rates between 440 and 880 seedlings m-2, establishment rates were approximately halved, to 1.04 ± 0.09 seedlings per 10 seeds frost-seeded.



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  		Fig. 1 Effect of seeding rate on seedling density following frost-seeding of six temperate pasture species. Means are over four replicates and seven sites. Regressions were: y = 5.97 + 0.055x, R2 = 0.98 for red clover (RCL); y = 1.15 + 0.295x - 0.000145x2, R2 = 1.00 for perennial ryegrass (PR); y = 1.28 + 0.142x - 0.000080x2, R2 = 0.98 for orchardgrass (OG); y = 1.26 + 0.017x, R2 = 0.94 for reed canarygrass (RCG); y = 0.31 + 0.069x, R2 = 0.97 for smooth bromegrass (SB); and y = 1.66 + 0.038x, R2 = 0.97 for timothy (Tim). All regression coefficients were significant at P < 0.01

 
Orchardgrass, smooth bromegrass, and red clover (a bunch grass, a rhizomatous grass, and a legume, respectively) had moderate seedling density with respective means of 28, 20, and 22 seedlings m-2 (Table 3). Linear responses to increased seeding rates were similar for smooth bromegrass and red clover (Fig. 1) and for orchardgrass (linear response not shown). On average, frost-seeding smooth bromegrass and red clover produced approximately 0.69 ± 0.05 and 0.55 ± 0.04 seedlings for every 10 seeds frost-seeded, respectively. For low to moderate seeding rates of orchardgrass, up to 440 seedlings m-2, an average of 1.18 ± 0.11 orchardgrass seedlings became established for every 10 seeds frost-seeded. For rates between 440 and 880 seedlings m-2, our results suggest that establishment rates of orchardgrass were severely reduced, to 0.36 ± 0.07 seedlings per 10 seeds frost-seeded.

Seedling density means for these three species were similar, as were the combined seedling means of all grasses compared with red clover means. In addition, the combined seedling density means of rhizomatous grasses were similar to the means of bunchgrasses. This suggests that there is no establishment advantage to specific growth habits, or to red clover vs. grasses in general. This was surprising, because red clover is most often selected over grasses for frost-seeding, ostensibly because of superior establishment (Gettle et al., 1996; Sprague et al., 1947; Taylor and Smith, 1995). Frost-seeding into a fairly open alfalfa canopy revealed that red clover seedling density was less than perennial ryegrass, similar to orchardgrass, smooth bromegrass, and timothy, and greater than reed canarygrass.

Timothy and reed canarygrass had the lowest mean seedling density, with means of 13 and 5 seedlings m-2 (Table 3). Timothy and reed canarygrass also had the lowest average responses to increased seeding rate: 0.38 ± 0.04 and 0.17 ± 0.05 seedlings for every 10 seeds frost-seeded, respectively (Fig. 1).

Economics and Species Selection
Seed costs varied considerably among species and rates (Table 4) and were not correlated with the degree of establishment. Smooth bromegrass was the most expensive species, per seed, because of its large seed mass (4.167 mg seed-1). Reed canarygrass, despite its traditionally high cost per seed mass, was relatively inexpensive, per seed, because of its relatively small seed mass (0.758 mg seed-1). Timothy was the least expensive, per seed, because of its extremely small seed mass (0.424 mg seed-1) and low retail price.


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  		Table 4 Seed costs for frost-seeding six temperate pasture species at five seeding rates

 
The regressions shown in Fig. 1 were used to predict the seeding rate required to give an expected establishment density of 20 seedlings m-2. Because of differential establishment success rates, these predicted seeding rates ranged from 66 to 1102 seeds m-2 (Table 4). The lowest predicted seeding rates, on a mass basis, were for perennial ryegrass, orchardgrass, and timothy . These values represent 8, 15, and 23% of traditional recommended seeding rates for perennial ryegrass, orchardgrass, and timothy, respectively, with conventional seeding into tilled soil (Undersander et al., 1998). Because of the relatively low seeding rates, seed costs were extremely small, ranging from 3.02 to 5.42 $ ha-1. These extremely low seed costs, combined with the relatively low vegetative spreading capability of these three species, suggest that the threshold of 20 seedlings m-2 could be increased for these three species. A threshold of 40 to 50 seedlings m-2 would provide more consistent coverage of the introduced species and increase the opportunity for long-term survival of orchardgrass and timothy in frost-seeded pastures. The relatively high potential for winter injury to perennial ryegrass in southern Wisconsin and similar environments (Casler, 1988; Casler and Walgenbach, 1990) suggests a need for frequent reseeding to maintain perennial ryegrass in pastures, increasing long-term input costs.

Red clover and smooth bromegrass had predicted seeding rates of 5.20 and 11.88 kg ha-1, respectively, which is 46 and 66% of their traditional recommended seeding rates for conventional seeding into tilled soil (Table 4). This red clover seeding rate is very similar to that typically recommended and used in frost-seeding practice in southern Wisconsin. For smooth bromegrass, this rate could most likely be halved to a threshold of 10 seedlings m-2, giving a seeding rate of 5.92 kg ha-1. Its highly rhizomatous nature allows smooth bromegrass to colonize pastures that are managed with adequate rest periods between grazing events.

Finally, reed canarygrass had a predicted seeding rate of 8.35 kg ha-1, giving it the largest cost of 69.29 $ ha-1, due to the combined effects of relatively low establishment capacity and high retail seed price. Its predicted frost-seeding rate to provide 20 seedlings m-2 was 125% of that normally recommended for conventional seeding into tilled soil. As with smooth bromegrass, the rhizomatous nature and ability to colonize well-managed pastures should allow this threshold to be reduced to a range of 5 to 10 seedlings m-2, reducing seed costs to 17.32 to 34.65 $ ha-1. Nevertheless, these values are sufficiently high to constitute a serious risk with frost-seeding reed canarygrass. For two of the seven sites, the mean establishment percentage and mean number of seedlings per unit area were insufficient to register a positive value, averaged over seeding rates (Tables 1 and 3). Thus, recommending a low seeding rate of reed canarygrass simply to reduce seed costs may be counter-productive, increasing the probability of failure. Based on these results, it seems that frost-seeding reed canarygrass is sufficiently unreliable that it should not be recommended.


   Summary and conclusions
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Summary and conclusions
REFERENCES
 
The results of this research suggest that red clover and several grasses can be successfully established when frost-seeded into mature alfalfa stands. Though seed costs vary between species, and establishment levels are reduced by the marginal conditions associated with frost-seeding, these negative traits are mitigated by the reduction of fuel, equipment, and labor costs compared with reduced tillage or conventional pasture improvement techniques.

Individual treatment means and means over locations were similar at 60 and 90 d after seeding in both years, indicating autotrophy of seedlings. There was slight variation in the magnitude of establishment among sites and years, but the ranking order of species was relatively constant. All species responded to increasing seeding rate, but the slope and shape of response varied with individual species' establishment characteristics. Perennial ryegrass consistently had the highest establishment rate, seedling density, and response to increased seeding rate. Perennial ryegrass was effective from an establishment point of view, but marginal winter hardiness may require frequent reseeding to maintain its presence in forage mixtures.

Orchardgrass establishment, density, and response to increased seeding rate was about half as great as perennial ryegrass, but considerable seedling numbers were established. The cost of establishing orchardgrass was similar to that for perennial ryegrass, on a per-seedling basis. Smooth bromegrass and red clover both had moderate establishment and seedling density and a similar response to increasing seeding rate. Smooth bromegrass had the second-highest seed cost, but its persistence and adaptability to a variety of management practices make it cost-effective for use in frost-seeding. Seeding rates could be kept moderate (5 to 6 kg ha-1) if some time is allowed for established plants to spread by rhizomes.

Timothy had relatively poor establishment, seedling density, and response to increasing seeding rate. However, low seed costs can justify increasing seeding rates to improve seedling density. Reed canarygrass had the poorest establishment, seedling density, and response to increasing seeding rate. Establishment of reed canarygrass by frost-seeding requires moderate to high seeding rates (6 to 8 kg ha-1) and considerable patience in allowing time for established plants to spread by rhizomes, suggesting that it should not be recommended.

Received for publication February 25, 1999.
   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Summary and conclusions
 REFERENCES
 




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