frost seeding into aging alfalfa stands: sward dynamics and pasture productivity

Frost Seeding into Aging Alfalfa Stands: Sward Dynamics and Pasture ProductivityDaniel J. Undersander, David C. West, and Michael D. Casler*

ABSTRACT tential, maturity, and palatability among grass speciesand biotypes may justify introduction of grasses intoLittle is known about the potential to frost-seed cool-season pasturepasture swards to improve animal performance.species into mature alfalfa (Medicago sativa L.). Experiments were

conducted in 1995 and 1996 near Arlington, WI (four sites), and Lancas- No-tillage seeding allows introduction of new speciester, WI (three sites), to evaluate the establishment and response to while reducing erosion and minimizing risk of standseeding rates of five cool-season grasses and red clover (Trifolium failure and yield loss in the seeding year. However, no-pratense L.) frost-seeded into mature alfalfa (2- to 5-yr-old stands with tillage seeding has traditionally been limited by cost and30 to 50 plants m22 ). Smooth bromegrass (Bromus inermis Leyss.), availability of specialized seeding equipment. Surfaceorchardgrass (Dactylis glomerata L.), perennial ryegrass (Lolium per- broadcasting of seed in late winter (frost seeding) pro-enne L.), reed canarygrass (Phalaris arundinacea L.), timothy (Phleum

vides a mechanism to renovate pastures without tillagepratense L.), and red clover were frost-seeded into mature alfalfaand with minimal equipment expenditures. Aging alfalfastands at six seeding rates. Orchardgrass, perennial ryegrass, and redstands, which have become unproductive hay fields, areclover had higher densities and responses to seeding rate than smoothexcellent candidates for introduction of grasses by frost-bromegrass, timothy, and reed canarygrass in the seeding year, but

these differences were less pronounced in the postseeding year. Or- seeding techniques. Establishment of perennial grasseschardgrass contributed more grass dry matter in the seeding year by frost seeding into aging alfalfa stands may allow forbut was similar to smooth bromegrass and timothy and greater than development of productive, persistent, and species-richperennial ryegrass and reed canarygrass in the second year. Alfalfa and pastures without opening the sod to erosion. The objec-weed suppression were highest with orchardgrass due to its aggressive tives of this research were to determine the influencegrowth habit, high occurrence, and winterhardiness. Postseeding–year of species and seeding rates on sward component occur-mixture yields were high for smooth bromegrass, orchardgrass, timo-

rence, botanical composition, and forage yield of sixthy, and reed canarygrass, but low for perennial ryegrass and redtemperate pasture species frost-seeded into mature, orclover. Forage yield increased with seeding rate at sites with thedeclining alfalfa stands.greatest initial establishment. The results of this study suggest frost

seeding temperate pasture species into mature alfalfa can increaseMATERIALS AND METHODSplant diversity and forage yield while suppressing weeds.

Field experiments were conducted in 1995 and 1996 at theUniversity of Wisconsin Agricultural Research Stations near

Increased capital costs without commensurate in- Arlington, WI (438189N, 898229W), and near Lancaster, WIcreases in produce prices have caused many farmers (428509N, 908479W). In 1995 the experiment was conducted in

with livestock, especially dairy, to rely more on pasture three alfalfa fields; one at Lancaster and two at Arlington,as a low-cost source of forage. Rising land prices and and all were harvested mechanically. In 1996 the experimentincreased tax rates have caused many of the same farm- was conducted in four alfalfa fields—two at Arlington and

two at Lancaster—with a grazed site and clipped site at eachers to optimize both total production and seasonal distri-location. The soil at all Arlington sites was Plano silt loambution of production through increased management(fine-silty, mixed, mesic Typic Argiudolls). The soil at all Lan-and agronomic inputs. Unmanaged pastures tend tocaster sites was Fayette silt loam (fine-silty, mixed, mesic,have low yield, often contain undesirable species (Doll,Typic Hapludalfs).1981; Shaeffer et al., 1990), and may shift to less palat-

The experimental design was a split-plot in a randomizedable biotypes of grass species (Falkner and Casler, complete block with four replicates. Whole plots were six1998), while well-managed pastures maintain higher to- species and subplots were six seeding rates; subplot size wastal yield, seasonal yield distribution, and forage quality 1.22 by 6.71 m. The six species used were: smooth bromegrass(Paine et al., 1999). cv. Alpha, orchardgrass cv. Benchmark, perennial ryegrass cv.

Well-managed pastures are characterized by ade- Madiera, reed canarygrass cv. Rival, red clover cv. Marathon,quate fertilization, adequate pasture rest periods be- and timothy cv. Colt. The six seeding rates were 0, 55, 110,

220, 440, and 880 seeds m22 on a pure live seed basis. Thetween grazings, and introduction of new species. Pasture880 seeds m22 rate corresponded to an average rate of 14.5,improvement research has concentrated mainly on in-17.6, 6.1, 7.5, 29.4, and 3.3 kg ha21 for red clover, perennialtroduction of legume species that have been shown toryegrass, orchardgrass, reed canarygrass, smooth bromegrass,increase dry matter production (Knight, 1970) and im-and timothy, respectively (Smith et al., 1986). The fields se-prove seasonal forage distribution (Evers, 1985). Grasseslected for the experiment had been in alfalfa for 2 to 5 yr.have been difficult to establish in existing sod due to Alfalfa stand density was approximately 30 to 50 plants m22.

excessive competition from resident plans (Sprague et Field preparation consisted of clipping each site to a 5-cmal., 1947). However, the tremendous range in yield po- stubble height in the autumn before the seeding year. Seeding

took place in mid-March. A drill seeder was used with theopeners elevated above the soil surface to simulate broadcastD.J. Undersander and M.D. Casler, Dep. of Agronomy, Univ. of

Wisconsin, Madison, WI 53706-1597; and D.C. West, Consumers’ seeding while maintaining precise control of seeding rates.Coop., Richland Center, WI 53581. Received 23 Feb. 2000. *Corre-sponding author ([email protected]).

Abbreviations: NIRS, near-infrared reflectance spectrophotometer;SEC, standard error of calibration; SEV, standard error of validation.Published in Agron. J. 93:609–619 (2001).

609

610 AGRONOMY JOURNAL, VOL. 93, MAY–JUNE 2001

Nitrogen was applied to all plots (excluding the six red composition variables were analyzed by analysis of variancefor individual sites and combined over sites. Species was con-clover seeding rates) at a rate of 56 kg ha21 approximately

30 d after seeding and again the first week of August in the sidered a fixed effect, while block, site, and seeding rate wereconsidered random. Data from each year were analyzed sepa-seeding year, and at 78 kg ha21 in early April of the postseeding

year. To reduce competition from preexisting vegetation, plots rately. Simple linear or log-linear regression was used to mea-sure response of species to seeding rates. Regression modelswere clipped (hay treatment) or grazed (pasture treatment)

to 8 cm throughout the seeding year whenever the maximum were determined by visual inspection of plots with the objec-tive of using the best common model for all species. Linearcanopy height reached 35 cm. After grazing, the fields were

clipped to achieve a uniform stubble height of 8 cm. or log-linear regression coefficients were compared by t-test(Steel et al., 1997).Sward-component occurrence was determined in late Sep-

A short-term economic analysis of frost-seeding success wastember of the seeding year and in late May of the postseedingperformed using forage yield data from May of the postseedingyear by the line-intercept method. A 90-cm transect was placedyear. For each combination of seven sites–six species–fiveat three random positions per plot and the single species near-seeding rates, the value of additional hay due to frost seedingest to each of six evenly spaced points was recorded, giving(V 5 frost-seeding treatment mean-mean of the unseededa total of 18 observations per plot. Species were recorded incontrol), was computed using a hay price of $90 Mg21. Thefive groups: seeded species, alfalfa, annual grasses, unseededcost of seed (C) was computed using data from Casler et al.perennial grasses (any perennial grass not intentionally(1999). A successful frost seeding was defined as V 2 C . 0.seeded), and broadleaf weeds. Occurrence was computed asThe probability of frost-seeding success was computed forthe percentage of the 18 recorded observations.each of the 30 seeding rate–species combinations as the fre-Forage yield was determined in late September of the seed-quency of sites with V 2 C . 0. These probability estimates areing year and late May of the postseeding year. Forage yieldshighly conservative, because they only account for extremelyfrom the seeding year were obtained by randomly clippingshort-term profits (May of the postseeding year). Frost-seed-two 0.25-m2 areas per plot to a 5-cm stubble height. Forageing equipment, fuel, and labor costs were assumed to be negli-yields for the postseeding year were obtained by using a sickle-gible. The probability of frost-seeding success was modeledbar plot harvester to harvest the entire plot. A 1-kg grab-by linear, log-linear, or quadratic regression for each species.sample was taken at random from each plot.

Samples were oven-dried at 608C for approximately 3 d and RESULTS AND DISCUSSIONused to determine dry matter content. Two stratified randomgroups of forage samples (n 5 205 for the fall 1995 sites and Species 3 site and rate 3 site interactions were signifi-n 5 245 for the remaining harvests combined) were manually cant (P , 0.05) for all variables. Therefore, most resultsseparated into three components: grass, legume, and other are presented for individual sites. Species 3 rate interac-broadleaves. Separated samples were reconstituted after tions were significant (P , 0.05) only for grass andweighing the individual components. Samples from the fall of legume contributions to sward dry matter and for occur-1995 were ground twice to pass through a 2-mm and a 1-mm rence at both September and May harvests. The species 3screen, respectively. The remaining samples were ground once

rate 3 site interaction was not significant for these vari-to pass a 2-mm screen. All forage samples were scanned onables, so species 3 rate interaction data are presenteda near-infrared reflectance spectrophotometer (NIRS) andas means over sites. Finally, despite the presence ofseparated samples were used to calibrate NIRS equations tothese interactions, there were numerous significant (P ,predict the contribution of these components for all forage0.01) main effects for species and seeding rate. The mainsamples. Calibration and validation statistics (R2, SEC 5 stan-effect of species resulted from reasonably consistentdard error of calibration, and SEV 5 standard error of valida-

tion, respectively) for the fall 1995 harvest were: 0.52, 16.8 g species rankings across sites and rates. The significantkg21, and 19.4 g kg21 (grass); 0.77, 13.6 g kg21, and 16.9 g rate main effects resulted from the overwhelminglykg21 (legume); and 0.58, 15.7 g kg21, and 16.5 g kg21 (other large variation among rate means.broadleaves). Calibration and validation statistics (R2, SEC,

Variation among Speciesand SEV, respectively) for the remaining harvests were: 0.97,4.4 g kg21, and 5.5 g kg21 (grass); 0.97, 6.3 g kg21, and 7.6 g Seeding Yearkg21 (legume); and 0.96, 5.9 g kg21, and 6.9 g kg21 (other broad-

Occurrence of seeded species differed among yearsleaves).Forage yield, sward component occurrence, and botanical and sites within years (Table 1). Mean seeded species

Table 1. Mean seeded-species occurrence following frost seeding, determined by line-intercepts in September of the seeding year atseven sites, identified by location, harvest management, and year of seeding. Means are over four replicates and five seeding rates.

Location/management/year

Arlington(1) Arlington(2) Lancaster Arlington Arlington Lancaster LancasterClipped Clipped Clipped Grazed Clipped Grazed Clipped

Seeded species 1995 1995 1995 1996 1996 1996 1996 Overall mean

%Red clover 23.9 36.8 1.9 2.4 4.0 8.9 9.1 12.4Perennial ryegrass 24.7 30.3 22.0 9.8 11.8 18.8 31.5 21.3Orchardgrass 31.1 40.3 16.9 3.8 4.9 13.1 19.0 18.4Reed canarygrass 7.9 13.5 2.3 0.0 0.0 1.0 2.2 3.8Smooth bromegrass 10.7 19.8 5.7 10.5 1.4 2.8 4.5 7.9Timothy 12.4 17.6 3.9 0.5 0.7 1.4 4.7 5.9

Mean 18.5 26.4 8.8 4.5 3.8 7.7 11.8 11.6LSD (0.05) 6.3 7.7 3.6 3.7 2.9 4.9 5.3 5.1

UNDERSANDER ET AL.: FROST SEEDING PERENNIAL FORAGE GRASSES 611

occurrence was higher in 1995 compared with 1996 (17.9 5.5%, data not shown) than smooth bromegrass, reedcanarygrass, red clover, and timothy, which have lessvs. 7.0%, respectively). In 1995, temperatures and rain-

fall were normal through the period of seedling estab- aggressive seedlings (mean occurrence 5 7.5 to 7.9%;all P , 0.05 compared with perennial ryegrass). Suppres-lishment but the remainder of the growing season had

unusually high temperatures and below-average rainfall. sion of annual grasses by orchardgrass was greater thansmooth bromegrass (mean annual grass occurrence 5 6.0The Lancaster site may have endured greater climatic

stress because of its southern exposure and substantially vs. 7.9%: P , 0.05) for similar reasons. Suppression ofunseeded perennial grasses, primarily quackgrass [Ely-lower organic matter in the soil compared with the soil

at Arlington (24 vs. 39 g kg21 ), potentially reducing trigia repens (L.) Nevski] may be more difficult becauseof their extensive rhizomes. Orchardgrass had greateravailable water holding capacity. In 1996, rainfall was

above normal for spring, but below normal for July and quackgrass suppression than reed canarygrass (meanunseeded perennial grass occurrence 5 10.0 vs. 12.9%;August. Temperatures were well below normal, espe-

cially in April and May, possibly reducing seedling es- P , 0.05); there were no other species differences forunseeded perennial grass occurrence. Species with thetablishment and thereby affecting seeded-species occur-

rence at the end of the season. Seedling density 60 d highest component occurrence (Table 1)—such as or-chardgrass, perennial ryegrass, and red clover—reducedafter planting (Casler et al., 1999) was generally lower

for grazed sites, although this trend was not evident for broadleaf weed occurrence more than reed canarygrass(mean broadleaf weed occurrence 5 24.2 to 26.4 vs.seeded-species occurrence by September of the seed-

ing year. 33.6%; all P , 0.05).There was little difference in overall mean grass con-Species rankings for occurrence in September of the

seeding year were highly repeatable across sites (Table tribution between years (212 g kg21 in 1995 and 243 gkg21 in 1996), but considerable variation among sites1). Perennial ryegrass or orchardgrass ranked first or

second at all sites, with the exception of perennial rye- within years (Table 2). As with seeded-species occur-rence, variation among species was as great as variationgrass at Arlington-Clipped-1995 (no. 2) and orchardgrass

at Arlington-Grazed-1996. Similarly, reed canarygrass among sites. Orchardgrass contributed more grass in1995 than smooth bromegrass, reed canarygrass, andranked sixth (last) at six sites and fifth at the other site,

while timothy ranked fifth at four sites and fourth at timothy due to its aggressive growth and tolerance toheat and drought. Despite its high occurrence, perennialthree sites. The ranking of species means corresponded

closely to their ranking for seedling density 60 d after ryegrass provided dry matter contributions similar tothose for smooth bromegrass, reed canarygrass, and tim-planting (Casler et al., 1999) and to the seedling aggres-

siveness classification made by Blazer et al. (1956). othy at Arlington in 1995, where drought conditionswere less evident than at Lancaster. This is attributedPerennial ryegrass, due to its aggressive seedlings (Ta-

ble 1; Blazer et al., 1956; Casler et al., 1999), had greater to the short stature of perennial ryegrass compared withthe upright growth of smooth bromegrass, reed ca-suppression of annual grasses (mean occurrence 5

Table 2. Mean grass and legume contributions to sward dry matter following frost seeding, determined by NIRS analysis of samplesclipped in September of the seeding year at seven sites, identified by location, harvest management, and year of seeding. Means areover four replicates and five seeding rates.




g kg21

Grass component dry matter

Red clover 157 121 123 212 103 175 192 155Perennial ryegrass 242 276 202 318 156 378 394 281Orchardgrass 307 471 219 282 180 299 370 304Reed canarygrass 179 208 125 202 131 276 326 207Smooth bromegrass 211 298 139 221 158 231 386 235Timothy 219 204 117 246 87 179 333 198

Mean† 219 263 154 247 136 256 333 230Unseeded check 198 198 123 209 147 224 291 199LSD (0.05) 84 80 62 128 65 81 89 81

Legume component dry matter

Red clover‡ 457 632 720 221 851 762 769 630Perennial ryegrass 327 333 662 142 787 563 560 482Orchardgrass 260 215 666 157 742 635 564 463Reed canarygrass 299 341 712 128 848 669 614 516Smooth bromegrass 337 320 697 178 803 701 531 510Timothy 324 356 743 157 855 757 611 543


† Mean of frost-seeded treatments only.‡ Data in this row include both the frost-seeded red clover and preexisting alfalfa.


narygrass, and timothy. Species differences were less orchardgrass, similar to smooth bromegrass and red clo-evident in 1996 as a result of reduced establishment ver, and usually greater than timothy and reed ca-compared with 1995 (Casler et al., 1999) and cooler narygrass. Perennial ryegrass was not as prevalent intemperatures, resulting in more uniform growth of all the postseeding year, especially at the Arlington 1996seeded grasses. Across all seven sites, only three species sites where reduced duration of snow cover, comparedshowed evidence of significant (P , 0.05) increases in with Lancaster, increased the potential for winter injury.grass dry matter compared with the unseeded check Overall mean occurrence of perennial ryegrass in theplots: smooth bromegrass at two sites, orchardgrass at postseeding year was 1.4 times greater than in the seed-three sites and averaged across sites, and perennial rye- ing year, the lowest increase of all species. Substantialgrass at three sites and averaged across sites. over-winter increases of perennial ryegrass are not likely

Differences among species for legume dry matter con- in this temperate climate due to its lack of adaptationtributions (Table 2) were generally inverse of those for to Wisconsin winters with low potential for snow coverseeded-species occurrence (Table 1). All five grass spe- (Casler, 1988; Casler and Walgenbach, 1990). This sug-cies led to reduced legume dry matter compared with gests the potential benefits of frost seeding perennialthe red clover seedings (P , 0.05) at a minimum of ryegrass may be seen more in the seeding year andthree sites and averaged across sites. However, there persistence is likely to decrease in subsequent years,were few significant differences among grass species; prompting the need for occasional reseeding.the only reasonably consistent difference was: orchard- Red clover had a 2.2-fold increase in occurrence fromgrass , timothy, generally at sites with higher tempera- the seeding year to the postseeding year, similar to thattures, lower rainfall, and/or higher legume contribu- for most of the grasses (Table 3). Smooth bromegrass,tions, conditions under which orchardgrass was better timothy, and reed canarygrass were similar in that theyable to compete than timothy. Orchardgrass appeared to had the lowest establishment (Casler et al., 1999), butbe the most effective competitor against the established they also had the greatest increase in occurrence fromalfalfa plants. the seeding year to the postseeding year (2.9, 3.8, and

For all of the seven sites, there were no differences 3.4 times greater, respectively). These three species haveamong seeded species or for seeded species vs. unseeded the greatest long-term persistence of the six species usedcheck for total-sward forage yield in September of the in this study. These results indicate the general negativeseeding year (data not shown). association between short-term establishment rate and

long-term persistence. The mechanisms that promotePostseeding Year long-term persistence in these three species—rhizomes

or haplocorms—most likely require additional energySpecies ranks for seeded-species occurrence wereinputs and development time, reducing the establish-consistent among the seven sites, except for Arlington-ment rate for these species. These results support theGrazed-1996, where smooth bromegrass values weresuggestion that these species can be successfully frost-inflated due to the presence of smooth bromegrass be-seeded at lower rates than orchardgrass, red clover, orfore frost seeding (Table 3). However, there were someperennial ryegrass, due to their increased capacity fornotable changes in species abundance from that ob-vegetative reproduction (Casler et al., 1999).served in the seeding year (Table 1). Orchardgrass be-

Alfalfa occurrence was greatly reduced from the seed-came the most prevalent species, having the greatesting year (503 to 399 g kg21 from September to May,occurrence at all sites, except Arlington-Grazed-1996.data not shown) as a result of natural stand mortality,This was due to its aggressive growth in the seedingN applications favoring grass growth, and the advancingyear (Table 1; Casler et al., 1999), coupled with earliereffects of grass colonization. Generally, the grass speciesspring growth and increased response to supplementalwith the greatest occurrence had the most suppressiveN compared with the other species. The overall meaneffect on alfalfa occurrence (data not shown). Researchoccurrence of orchardgrass was 2.3 times greater in thehas shown orchardgrass to be more aggressive to alfalfapostseeding year compared with the seeding year.

The occurrence of perennial ryegrass was less than in binary mixtures than other cool-season grasses

Table 3. Mean seeded-species occurrence following frost seeding, determined by line-intercepts in May of the postseeding year at sevensites, identified by location, harvest management, and year of seeding. Means are over four replicates and five seeding rates.




%Red clover 56.0 64.3 15.5 8.4 12.5 14.0 24.1 27.8Perennial ryegrass 42.2 42.8 43.5 3.0 16.7 21.6 34.4 29.2Orchardgrass 66.3 66.9 60.5 5.8 18.3 28.5 51.3 42.5Reed canarygrass 22.2 36.8 9.2 1.7 1.2 2.8 16.5 12.9Smooth bromegrass 36.7 45.6 15.5 20.7 8.4 9.8 22.0 22.7Timothy 43.0 54.4 14.4 4.0 7.5 6.0 25.5 22.1

Mean 44.4 51.8 26.4 7.2 10.7 13.8 29.0 26.2LSD (0.05) 8.8 9.8 5.8 4.0 6.0 5.8 6.0 6.8


(Casler, 1988; Jung et al., 1982). This frost-seeding re- bromegrass, and timothy increased in grass dry matterfrom September to May, by an average of 53 to 92%.search can be interpreted similarly, but occurrence of

the colonizing species may be as important as their Reed canarygrass showed no change, while perennialryegrass dry matter declined by 22%, most likely duegrowth and establishment characteristics. Adjusting

seeding rates to obtain a desired density, regardless of to winter injury.The contribution of frost-seeded red clover to legumespecies, may help minimize the differences in sup-

pression. dry matter was greater than for all frost-seeded grassesat all 1995 sites, but only at Lancaster-Grazed-1996 (Ta-Orchardgrass and perennial ryegrass each suppressed

annual grasses more than the other species (annual grass ble 4). This was due to the generally greater stands ofseeded species at the 1995 sites, both immediately afteroccurrence 5 4.4 vs. 6.5 to 9.6%; all P , 0.05; data not

shown). The early spring growth of orchardgrass and establishment (Casler et al., 1999) and in Septemberof the seeding year (Table 2). Greater frost-seedingdense ground cover of both orchardgrass and perennial

ryegrass provided an unfavorable environment for an- establishment success of both grasses and red cloverresulted in a greater differential between grass and le-nual grasses. Reed canarygrass had the lowest suppres-

sion (annual grass occurrence 5 9.6%) because of its gume contributions for the grass vs. red clover seedingtreatments in 1995.low occurrence, providing adequate space for annual

grasses to grow. Quackgrass occurrence increased from Seeded-species rankings for broadleaf dry matter con-tribution were highly inconsistent among sites (data notthe levels found in the seeding year because of vigorous

rhizomes and supplemental N applications intended to shown). There were numerous significant differencesamong species, but they often involved large changespromote growth of the seeded species. Orchardgrass,

with its aggressive growth habit and high plant occurrence in rank among sites. Over all sites, there were no differ-ences among species or between seeded species andsuppressed quackgrass better than reed canarygrass or

smooth bromegrass (perennial grass occurrence 5 13.9 unseeded checks in broadleaf weed contributions tothe sward.vs. 24.5 and 23.0, respectively; both P , 0.05). Control

of these weeds has proven difficult, especially in forage Forage yield differences among species were variableamong sites, but perennial ryegrass and red clover seed-systems where legumes or other short-lived species de-

crease in occurrence and provide a niche for weed inva- ings usually had lower yield compared with smoothbromegrass, orchardgrass, timothy, and reed canarygrasssion. By manipulating the plant environment with the

addition of aggressive and/or high occurrence species, seedings (Table 5). This was observed for perennialryegrass at four sites where establishment and/orthe extent of weed invasion may be reduced.

Grass contribution to dry matter yield increased from seeded-species occurrence was relatively high, and prob-ably resulted from its short stature. In addition, thethe seeding year to the postseeding year at all sites,

except Arlington-Grazed-1996 and Lancaster-Clipped- occurrence of dandelions (Taraxacum officinale We-ber), which reduce yield potential due to their rosette1996 (Table 4 vs. Table 2). Orchardgrass, smooth

Table 4. Mean grass and legume contributions to sward dry matter following frost seeding, determined by NIRS analysis of samplesclipped in May of the postseeding year at seven sites, identified by location, harvest management, and year of seeding. Means areover four replicates and five seeding rates.



Species 1995 1995 1995 1996 1996 1996 1996 Overall mean

g kg21

Grass component dry matter

Red clover 56 124 110 147 132 150 20 105Perennial ryegrass 220 177 444 188 152 302 59 220Orchardgrass 403 483 526 258 230 375 251 361Reed canarygrass 185 266 290 195 200 320 98 222Smooth bromegrass 380 525 432 148 203 287 222 314Timothy 345 446 443 292 109 250 135 288


Legume component dry matter

Red clover‡ 404 514 647 74 677 749 889 565Perennial ryegrass 171 260 408 83 626 623 795 423Orchardgrass 133 127 354 76 568 588 544 341Reed canarygrass 162 224 524 109 614 635 741 430Smooth bromegrass 128 118 419 79 630 657 643 382Timothy 160 198 447 80 630 693 714 417


† Mean of frost-seeded treatments only.‡ Data in this row include both the frost-seeded red clover and preexisting alfalfa.


Table 5. Mean forage yield in May of the postseeding year for 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.




Mg ha21

Red clover 1.53 1.46 1.52 1.38 1.38 1.56 1.11 1.42Perennial ryegrass 1.57 1.25 1.71 1.53 1.49 1.57 1.04 1.45Orchardgrass 1.71 1.59 1.68 1.62 1.44 1.81 1.33 1.60Reed canarygrass 1.56 1.54 1.54 1.78 1.54 1.69 1.19 1.55Smooth bromegrass 1.93 1.94 1.61 1.75 1.74 1.71 1.22 1.70Timothy 1.67 1.89 1.65 1.73 1.47 1.74 1.23 1.63

Mean† 1.66 1.61 1.62 1.63 1.51 1.68 1.19 1.56Unseeded check 1.56 1.49 1.56 1.57 1.51 1.58 1.12 1.49LSD (0.05) 0.25 0.18 0.21 0.19 0.18 0.12 0.11 0.18

† Mean of frost-seeded treatments only.

growth pattern, was usually the highest in perennial Seeding Yearryegrass plots (data not shown). The dense canopy of A log-linear response of seeded-species occurrencehorizontal leaves associated with red clover has been to seeding rate (P , 0.01) was observed at five of sevenshown to shade more prostrate species, thus reducing sites (Table 6). Among sites, log-linear responsessward yield potential (Harris, 1974). Smooth brome- showed a sevenfold range of variation. Sites with thegrass ranked highest in forage yield at three of seven highest mean seeded-species occurrence had the great-sites and, overall, was significantly higher in sward for- est response to seeding rate. Increased seeding rate ledage yield than all seeded species, except timothy (P , to reduced occurrence of alfalfa, annual grasses, and0.05). Smooth bromegrass was the only species with broadleaf weeds at nearly all sites, but the magnitudemean forage yield higher than the unseeded check aver-

and significance of these responses were highly variable.aged over sites, due to its consistent high rankingSignificance (P , 0.05) of regressions for these compo-across sites.nents occurred only at one to four of the five sites withsignificant regressions for seeded-species occurrence,

Seeding Rate Responses and were not closely related to mean occurrence ofthese components.Simple linear regressions of mean botanical compo-

Averaged over sites, red clover, perennial ryegrass,nent densities on seeding rate did not fit well. On theand orchardgrass showed significantly contrasting seed-whole, the best overall model was the linear regressioning-rate responses for the contributions of grass vs. le-of mean component densities on the natural logarithmgume dry matter to swards (Fig. 1). For these threeof seeding rate—all such regressions were fitted withspecies, responses of grass and legume dry matter tothis model. Forage yield and botanical composition data

fitted best to simple linear regressions. seeding rate were all significant (P , 0.05). These three

Table 6. Summary statistics for linear regressions of sward component occurrences (%) measured in September of the seeding year onthe natural logarithm of seeding rate (seeds m22 ) following frost seeding at seven sites, identified by location, harvest management,and year of seeding. Regressions were computed from means over four replicates and six species.



Component/statistic 1995 1995 1995 1996 1996 1996 1996 Overall mean

Seeded speciesMean occurrence, % 18.5 26.4 8.8 4.5 3.8 7.7 11.8 11.6Slope 4.41* 5.48** 2.26* 0.76 1.13 2.23* 3.16** 2.88**R2 0.80 0.91 0.74 0.56 0.63 0.73 0.83 0.82

AlfalfaMean occurrence, % 18.6 20.1 58.1 18.9 72.5 54.4 49.3 41.7Slope 21.27* 20.97** 21.80* 20.24 20.83 21.25* 20.82 21.03*R2 0.70 0.85 0.68 0.06 0.16 0.68 0.35 0.77

Annual grassesMean occurrence, % 7.7 5.2 16.2 0.6 0.3 0.5 18.9 7.0Slope 21.02* 20.82** 20.49 0.11 0.06 20.07 22.00** 20.62**R2 0.79 0.89 0.50 0.42 0.33 0.24 0.90 0.96

Broadleaf weedsMean occurrence, % 50.3 39.5 15.4 60.7 11.0 7.8 14.4 28.4Slope 21.59 23.13* 20.02 20.68 20.06 20.12 20.44 20.84R2 0.51 0.75 0.00 0.27 0.01 0.03 0.46 0.41

* Slope is significantly different from zero at P , 0.05.** Slope is significantly different from zero at P , 0.01.


Fig. 1. Grass (closed circles ) and legume (open circles ) contributions to sward dry matter in September of the seeding year, 6 mo after frostseeding with six temperate pasture species. Means are over four replicates at each of seven sites. Presence of * or ** indicates significanceof linear regressions for grass (G) or legume (L) dry matter on seeding rate, or significance of the difference between linear regressioncoefficients for grass (bG ) vs. legume (bL ) at P 5 0.05 or 0.01, respectively.

September of the seeding year to May of the postseedingspecies also showed the greatest establishment 60 d fol-year (Table 7 vs. Table 6); each of these increases waslowing frost seeding (Casler et al., 1999). Although nonesignificant (P , 0.01). Increases in mean seeded-speciesof the grasses became the dominant component of theoccurrence ranged from 79 to 200% of the Septembersward, grass and legume concentration of the swardmean. At all but two sites (the two grazed sites), thewere equal at the highest orchardgrass seeding rate. Thelog-linear response to seeding rate increased (P , 0.05)short stature of perennial ryegrass, resulting in shadingfrom September of the seeding year to May of the post-by the existing canopy, may have reduced its ability toseeding year (Table 7 vs. Table 6). Increases in seeded-respond to increasing seeding rate, reflecting its lowerspecies occurrence response to seeding rate in Mayresponse compared with orchardgrass. Competitionranged from 65 to 177% of the response observed in theamong plants at high densities further decreases plantprevious September. Reductions in alfalfa occurrenceheight due to reduced availability of resources (Donald,associated with increased seeding rates were similar to1963), further reducing the ability of perennial ryegrassthose observed in September of the seeding year.to show grass component dry matter responses. In-

Annual grasses and broadleaf weeds were signifi-creased red clover seeding rates led to increased legumecantly (P , 0.05) reduced by higher seeding rate at alland reduced grass component dry matter throughsites in which these weeds were present in May of thegreater numbers of red clover plants at high seedingpostseeding year, with the single exception of Arlington-rates, which provided increased competition and shad-Clipped-1996 (Table 7). This result is probably ex-ing of preexisting grasses.

Conversely, the three species that were slowest to plained by a combination of increased seeded-speciesestablish from frost seeding—reed canarygrass, smooth ground cover at high seeding rates between Septemberbromegrass, and timothy—generally did not have signif- and May, and the earlier spring growth of perennialicant responses of grass or legume dry matter contribu- forages vs. annual weeds. Similar principles apply totions to increased seeding rate in September of the seed- suppression of unseeded perennial grass, but due toing year (Fig. 1). Species with low establishment capacity their aggressive nature, similar growth habit, and similarand seedling growth rate apparently have little ability reproductive cycle to the seeded species, unseeded pe-to express short-term responses to increased seeding rennial grasses were reduced at only four of the sevenrate in terms of contributing to sward dry matter. Short- sites, those sites with the highest seeding rate responseterm growth of seedlings for these species is most likely for seeded-species occurrence. Thus, frost seeding offocused on mechanisms that promote long-term survival perennial forages can suppress existing perennial grassesand vegetative reproduction (rhizomes and haplocorms), when establishment conditions are sufficient for frostrather than on producing excessive aboveground dry seeding to be successful (Casler et al., 1999). Suppressedmatter during the establishment year. occurrence of perennial grasses does not appear to be

dependent on their initial occurrence, as noted by thePostseeding Year lack of relationship between mean perennial grass oc-

currence and their response to increased frost-seedingThe log-linear response of seeded-species occurrencerates of the introduced species (Table 7).to seeding rate was significant (P , 0.01) at all seven

Seeded species showed an increased occurrence re-sites (Table 7). Among sites, log-linear responsessponse to seeding rate between September of the seed-showed a sevenfold range of variation and, as observeding year and May of the postseeding year, with thefor September of the seeding year, were positively asso-exception of perennial ryegrass (Fig. 2). This responseciated with mean seeded-species occurrence. At all sites,

the mean seeded-species occurrence increased from was greatest for the three species with the lowest initial


Table 7. Summary statistics for linear regressions of sward component occurrences (%) measured in May of the postseeding year onthe natural logarithm of seeding rate (seeds m22 ) following frost seeding at seven sites, identified by location, harvest management,and year of seeding. Regressions were computed from means over four replicates and six species.



Component/statistic 1995 1995 1995 1996 1996 1996 1996 Overall mean

Seeded speciesMean occurrence, % 44.4 51.8 26.4 7.2 10.7 13.8 29.0 26.2Slope 10.48** 10.72** 6.25** 1.52* 2.92* 3.68** 7.04** 5.94**R2 0.95 0.99 0.94 0.62 0.77 0.86 0.92 0.94

AlfalfaMean occurrence, % 4.9 8.1 35.5 5.3 35.0 29.6 32.4 21.6Slope 20.72** 20.76* 22.31** 0.36 20.98* 21.00* 22.76** 21.17**R2 0.98 0.67 0.86 0.36 0.64 0.65 0.86 0.91

Annual grassesMean occurrence, % 12.3 2.3 6.5 † † † † 7.0Slope 23.68** 20.87** 20.83* † † † † 21.80**R2 0.96 0.85 0.76 † † † † 0.96

Broadleaf weedsMean occurrence, % 28.5 20.2 23.0 68.4 32.5 14.4 25.9 30.4Slope 25.22** 24.82** 22.32* 21.90* 20.81 21.02* 23.95** 22.12**R2 0.70 0.10 0.34 0.25 1.38 0.10 0.24 0.10

Perennial grassesMean occurrence, % 10.3 17.8 8.9 19.3 21.7 42.4 12.8 19.0Slope 20.88* 24.26** 20.80** 0.03 21.09 21.62* 20.30 21.27**R2 0.66 0.91 0.85 0.00 0.32 0.70 0.22 0.86

* Slope is significantly different from zero at P , 0.05.** Slope is significantly different from zero at P , 0.01.† Data were collected before emergence of annual grasses.

occurrence: reed canarygrass, smooth bromegrass, and most likely reflects mortality between September andMay, an effect that was greatest at the highest seedingtimothy, which ranged from 263 to 344% increase in rate

response. Increases in seeding rate responses between rates of perennial ryegrass and red clover. Swards withgreater numbers of established plants of these two rela-September and May were likely due to increased root

development and tillering, which were likely offset by tively nonpersistent species appear to be more suscepti-ble to winter stand losses than swards with fewer num-winter injury to perennial ryegrass.

The seeding-rate response of grass and legume sward bers of established plants.Seeding-rate responses of grass and legume swardcomponent contributions for perennial ryegrass frost

seedings had disappeared by May of the postseeding component contributions were all significant for or-chardgrass, smooth bromegrass, and timothy (Fig. 3).year (Fig. 3). Similarly for red clover, the legume-com-

ponent response was half that observed in September For orchardgrass, the change in response from Septem-ber of the seeding year to May of the postseeding yearof the seeding year and the response of the grass compo-

nent for red clover seedings had disappeared (Fig. 3 vs. reflected a 45% increase in the difference between thegrass and legume component regression coefficients. ByFig. 1). The reduced response for these seeded species

Fig. 2. Seeded-species occurrence in September of the seeding year (open circles ) or May of the postseeding year (closed circles ) followingfrost seeding with six temperate pasture species. Means are over four replicates at each of seven sites. Presence of * or ** indicates significanceof linear regressions of seeded-species occurrence on the logarithm of seeding rate, or significance of the difference between log-linearregression coefficients for September (bSep ) vs. May (bMay ) at P 5 0.05 or 0.01, respectively.


Fig. 3. Grass (closed circles ) and legume (open circles ) contributions to sward dry matter in May of the postseeding year, 14 mo after frostseeding with six temperate pasture species. Means are over four replicates at each of seven sites. Presence of * or ** indicates significanceof linear regressions for grass (G) or legume (L) dry matter on seeding rate, or significance of the difference between linear regressioncoefficients for grass (bG ) vs. legume (bL ) at P 5 0.05 or 0.01, respectively.

May of the postseeding year, orchardgrass had become lower than for any of the other grasses, except perennialryegrass. Aging alfalfa fields frost-seeded to reed ca-the dominant component of swards at the three highest

seeding rates. Thus, mature alfalfa fields can become narygrass may eventually become grass-dominant, butit will require significantly more time than for or-orchardgrass-dominant pastures approximately 1 yr after

frost seeding at rates of $220 seeds m22. chardgrass, smooth bromegrass, and timothy.Forage yield in May of the postseeding year increasedFor smooth bromegrass and timothy, the change in

response from September of the seeding year to May of with seeding rate at three of the seven sites (Table 8).These three sites generally had highest values of mostthe postseeding year reflected a 238 and 228% increase,

respectively, in the difference between the grass and measures of frost-seeding success: seedling density andpercentage establishment (Casler et al., 1999), meanlegume component regression coefficients. These swards

had become grass-dominant, or nearly so, at the two seeded-species occurrence and component contributionto sward dry matter, and log-linear response of seeded-highest seeding rates, 440 seeds m22 or higher. These

seeding rates were considered to be economically ad- species occurrence or sward contributions to increasedseeding rate. At these sites, there was sufficient estab-vantageous for establishment of timothy, but not for

smooth bromegrass, due to its large seed size/seed cost lishment to colonize openings in the alfalfa canopy andto provide competition from the seeded species to sup-ratio (Casler et al., 1999). For smooth bromegrass, eco-

nomical seeding rates of 100 to 200 seeds m22 would press and/or replace the existing vegetation with newspecies. Sites with moderate or poor establishment ofrequire additional time before grass dominance is

achieved. the seeded species did not provide sufficient establish-ment levels to effectively increase forage yield, despiteFinally, for reed canarygrass, a significant seeding-

rate response of the grass component dry matter contri- a log-linear response of seeded-species occurrence toseeding rate.bution was observed in May of the postseeding year

(Fig. 3). However, the rate of response was considerably Red clover had extremely low probability of frost-

Table 8. Mean forage yield in May of the post seeding year for six seeding rates of temperate pasture species following frost seedingat seven sites, identified by location, harvest management, and year of seeding. Means are over four replicates and six species.


Seeding rate Arlington(1) Arlington(2) Lancaster Arlington Arlington Lancaster Lancaster(seeds m22 ) Clipped Clipped Clipped Grazed Clipped Grazed Clippedand statistic 1995 1995 1995 1996 1996 1996 1996 Overall mean

Mg ha21

0 1.56 1.49 1.56 1.57 1.51 1.58 1.12 1.4955 1.49 1.59 1.49 1.59 1.53 1.72 1.14 1.51110 1.58 1.60 1.69 1.81 1.54 1.70 1.24 1.60220 1.78 1.56 1.61 1.49 1.28 1.69 1.17 1.51440 1.75 1.75 1.67 1.67 1.60 1.63 1.19 1.61880 1.82 1.68 1.70 1.66 1.59 1.76 1.25 1.64

Mean 1.66 1.61 1.62 1.63 1.51 1.68 1.19 1.56Slope† 3.25** 1.91* 1.69 0.46 1.22 0.96 1.12** 1.50**R2 0.64 0.49 0.44 0.02 0.12 0.25 0.49 0.63

* Slope significantly different from zero at P , 0.05.** Slope significantly different from zero at P , 0.01.† Units of slope are (Mg ha21 )(104 seeds m22 )21.


Fig. 4. Probability of frost-seeding success (P), as measured by the frequency of sites for which the value of increased May forage yields exceededthe cost of seed, as a function of seeding rate (SR). Regressions were: Perennial ryegrass, P 5 0.951 2 0.144ln(SR), R2 5 0.72, P 5 0.07;Orchardgrass, P 5 0.722 1 3.4 3 1025SR 2 7.9 3 1027SR2, R2 5 0.99, P , 0.01; Reed canarygrass, P 5 0.391 2 0.062ln(SR), R2 5 0.75, P 50.06; and Smooth bromegrass, P 5 1.787 2 0.268ln(SR), R2 5 0.98, P , 0.01.

seeding success (mean probability 5 0.03; Fig. 4), be- For smooth bromegrass and timothy, the energy re-quired to develop rhizomes and haplocorms, respec-cause it generally resulted in decreased sward yields

(Table 5), making it a poor choice to renovate aging tively, appears to limit the ability of these species toproduce new tillers and a high level of aboveground dryalfalfa fields. Reed canarygrass also had a very low mean

probability of short-term success, due to its high seed matter in the seeding year. However, both species hadbecome nearly the dominant sward component in Maycost and low seedling aggressiveness. Timothy had a

uniformly high probability of frost-seeding success of the postseeding year, for the highest seeding rates.Thus, their investment in an underground carbohydrate(mean probability 5 0.80), largely because it resulted

in forage yield increases (Table 5) and its cost per seed storage and vegetative reproductive system may delaytheir rapid establishment from frost seeding, but itis extremely low (Casler et al., 1999). Perennial ryegrass

had moderate probabilities at low seeding rates, largely should prove beneficial for long-term stand develop-ment. Both species showed increased sward foragebecause stand losses due to winter injury resulted in a

lower cost investment. Finally, orchardgrass and smooth yields as early as 14 mo after frost seeding. Reed ca-narygrass showed similar trends to smooth bromegrassbromegrass had high probabilities of frost-seeding suc-

cess at low seeding rates, due to their combination of and timothy, but on a considerably delayed schedule.The length of time required for reed canarygrass tohigh seedling aggressiveness, high tillering capacity, and

low susceptibility to winter injury. become dominant after frost seeding into aging alfalfafields is unclear from this research. While its capacityfor vegetative reproduction was observed within 14 moCONCLUSIONSafter frost seeding, it is possible that other seeding meth-

Frost seeding temperate pasture species into aging or ods, such as no-till drilling, might result in more rapidmature alfalfa stands can increase plant diversity and establishment of reed canarygrass.forage yield and reduce weed pressure. Species differedin their ability to contribute to yield based on their REFERENCESestablishment, growth habit, and winterhardiness. Over

Blazer, R.E., T. Taylor, W. Griffeth, and W. Skrdla. 1956. Seedlingthe 14-mo duration of the seven experiments reportedcompetition in establishing forage plants. Agron. J. 48:1–6.here, frost seeding resulted in successful stand establish- Casler, M.D. 1988. Performance of orchardgrass, smooth bromegrass

ment of five of the six species. Red clover, perennial and ryegrass in binary mixtures with alfalfa. Agron. J. 80:509–514.Casler, M.D., and R.P. Walgenbach. 1990. Ground cover potential ofryegrass, and orchardgrass became successfully estab-

forage grass cultivars in binary mixtures with alfalfa at divergentlished and an important component of swards withinlocations. Crop Sci. 30:825–831.the seeding year. For orchardgrass, aggressive tillering Casler, M.D., D.C. West, and D.J. Undersander. 1999. Establishment

and competitive ability were probably the most impor- of temperate pasture species into alfalfa by frost seeding. Agron.J. 91:916–921.tant factors in its rapid establishment. For red clover

Doll, J.D. 1981. Dandelions in alfalfa don’t affect forage quality.and perennial ryegrass, superior seedling vigor was mostHoards Dairyman 125:691.likely the greatest single factor contributing to this re- Donald, C.M. 1963. Competition among crop and pasture plants. Adv.

sponse. By May of the postseeding year, some loss of Agron. 15:1–118.Evers, G.W. 1985. Forage and nitrogen contributions of arrowleaf andstand was observed at the highest seeding rates for red

subtropical clovers overseeded on bermudagrass and bahiagrass.clover and perennial ryegrass. While red clover wasAgron. J. 77:960–963.very amenable to frost seeding, it was not suitable for Falkner, L.K., and M.D. Casler. 1998. Preference for smooth brome-

renovating aging alfalfa fields, due to reduced forage grass clones is affected by divergent selection for nutritive value.Crop Sci. 38:690–695.yields.

PORTER ET AL.: RESPONSE OF SOYBEAN CYST NEMATODE TO CORN–SOYBEAN ROTATIONS 619

Harris, W. 1974. Competition among pasture plants: V. Effects of Sheaffer, C.C., D.L.Wyse, G.C. Marten, and P.H. Westra. 1990. Thepotential of quackgrass for forage production. J. Prod. Agric.frequency and height of cutting on competition between Agrostis

tenuis and Trifolium repens. N. Z. J. Agric. Res. 17:251–256. 3:256–259.Smith, D., R.J. Bula, and R.P. Walgenbach. 1986. Forage management.Jung, G.A., L.L. Wilson, P.J. LeVan, R.E. Kocher, and R.F. Todd.

1982. Herbage and beef production from ryegrass–alfalfa and or- 5th ed. Kendall Hunt Publ. Co., Dubuque, IA.Sprague, C.C., R.R. Robinson, and A.W. Clyde. 1947. Pasture renova-chardgrass–alfalfa pastures. Agron. J. 74:937–942.

Knight, W.E. 1970. Productivity of crimson and arrowleaf clovers tion: I. Seedbed preparation, seedling establishment, and subse-quent yields. J. Am. Soc. Agron. 39:12–25.grown in a coastal bermudagrass sod. Agron. J. 62:773–755.

Paine, L.K., D.J. Undersander, and M.D. Casler. 1999. Pasture growth, Steel, R.G.D., J.H. Torrie, and D.A. Dickey. 1997. Principles andprocedures of statistics. 2nd ed. McGraw-Hill Book Co., Newproduction, and quality under rotational and continuous grazing

management. J. Prod. Agric. 12:569–577. York, NY.

SOYBEAN

Population Response of Soybean Cyst Nematode to Long Term Corn–SoybeanCropping Sequences in Minnesota

Paul M. Porter, Senyu Y. Chen,* Curt D. Reese, and Lee D. Klossner

ABSTRACT during the 1980s and 1990s. As soybean cropping fre-quency and acreage increased in central and northwest-Soybean cyst nematode [Heterodera glycines Ichinohe] (SCN) canern Minnesota, so too did the presence of SCN (Chen,reduce soybean [Glycine max (L.) Merr.] yields. Rotating soybeanunpublished data, 2000).with a nonhost crop usually reduces SCN populations. Cropping se-

Crop rotation, cultural practices, resistant cultivars,quence experiments initiated in the early 1980s at two Minnesotalocations were monitored in 1996, 1997, and 1998 for changes in SCN and nematicides have been employed to reduce soybeanegg densities. Cropping sequences were: (i) 5-yr consecutive corn (Zea yield suppression caused by SCN (Koenning et al., 1993;mays L.) alternated with 5-yr consecutive soybean, (ii) continuous Riggs and Schuster, 1998; Schmitt, 1991; Young andmonoculture of each crop, (iii) annual alternation of two cultivars Hartwig, 1992; Young, 1998a, 1998b). Rotation withwithin a continuous monoculture of each crop, and (iv) annual rotation nonhosts of SCN such as corn, wheat (Triticum aestivumof each crop. In 1989, SCN was detected in several of the plots at L.), or grain sorghum [Sorghum bicolor (L.) Moench]both locations. By 1996, all cropping sequences had detectable popula-

has been an effective management tactic. Early researchtions of SCN eggs at both locations, regardless of whether the landon crop rotation by Ross (1962) in the southeasternhad been planted to continuous corn since the early 1980s. LowestUSA showed that “two or more years in production ofdensities of SCN eggs were typically found in cropping sequences thata nonhost crop resulted in decreases of the nematode toinvolved continuous corn and where corn had been planted for the

last three or more years, whereas highest levels of SCN eggs were low or undetectable levels with acceptable subsequentfound in cropping sequences that involved continuous soybean and yields of soybean.” Schmitt (1991), however, statedwhere soybean had been planted for the last two or more years. These “even after 3 yr of growing a nonhost, SCN populationresults suggest planting a nonhost to SCN for as long as 5 yr on density increased rapidly on soybean in a single season.”infested land will not eliminate future problems with this pathogen. Based on research in the midwestern USA, Noel (1985)In addition, the results suggest that at least a portion of the crop stated that SCN would not be a problem if soybean wassequence effects on yield has a soil microbiological basis involving grown once every 4 or 5 yr.mechanisms that are specific to a location.

In a field with no known history of soybean produc-tion, Noel and Edwards (1996) determined that it took5 to 6 yr in a continuous soybean (SS) monoculture forthe nematode to increase to yield-damaging densities,Soybean cyst nematode (SCN) was first discoveredwhereas rotation of a susceptible soybean with corn wasin the USA in North Carolina in 1954 (Winsteadeffective in slowing SCN population increases. Theiret al., 1955). It was identified in several midwesterndata indicated that in a 2 yr corn–soybean rotation,states in the 1960s (Miller, 1985) and was first docu-

mented in Minnesota in 1978 (MacDonald et al., 1980).The SCN spread from southern Minnesota northward Abbreviations: DAP, days after planting; Pf, end-of-season egg popu-

lation density of soybean cyst nematode; Pi, early season egg popula-tion density of soybean cyst nematode; Pm, midseason egg populationdensity of soybean cyst nematode; SCN, soybean cyst nematode. InP.M. Porter, Dep. of Agron. and Plant Genet., Univ. of Minnesota,cropping sequences: C1, C2, C3, C4, and C5 are 1st, 2nd, 3rd, 4th,St. Paul, MN 55108; S.Y. Chen and C.D. Reese, Southern Res. andand 5th yr corn following 5 yr of soybean; CC, annual alternation ofOutreach Cent., Waseca, MN 56093; and L.D. Klossner, Southwesttwo cultivars within a continuous monoculture of corn; CC, continuousRes. and Outreach Cent., Lamberton, MN 56152. Minnesota Agric.corn; CS, corn in annual rotation; S1, S2, S3, S4, and S5 are 1st, 2nd,Exp. Stn. Journal Ser. Paper 00-1-13-0158. Received 10 Aug. 2000.3rd, 4th, and 5th yr soybean following 5 yr of corn; SC, soybean*Corresponding author ([email protected]).in annual rotation; SS, annual alternation of two cultivars within acontinuous monoculture of soybean; SS, continuous soybean.Published in Agron. J. 93:619–626 (2001).

frost seeding into aging alfalfa stands: sward dynamics and pasture productivity

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