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PRODUCTION PAPER

Alfalfa Autotoxicity

Effects of Reseeding Delay, Original Stand Age, and Cultivar

^a Dep. of Plant Sci., McGill Univ., Macdonald Campus, 21111 Lakeshore Rd., Ste. Anne-de-Bellevue, QC H9X 3V9, Canada
^b Dep. of Agron. and Plant Genet., Univ. of Minnesota, 1991 Buford Circle, 411 Borlaug Hall, St. Paul, MN 55108-6026
^c Dep. of Soil, Water, and Climate, Univ. of Minnesota, 1991 Buford Circle, 439 Borlaug Hall, St. Paul, MN 55108-6028
^d USDA-ARS, U.S. Dairy Forage Res. Cent. (Minnesota cluster) and Dep. of Soil, Water, and Climate, Univ. of Minnesota, 1991 Buford Circle, St. Paul, MN 55108-6028
^e Southern Res. and Outreach Cent., 35838 120th Street, Waseca, MN 56093-4521
^f Southwest Res. and Outreach Cent., P.O. Box 428, Lamberton, MN 56152

* Corresponding author (sheaf001{at}tc.umn.edu)

Received for publication May 30, 2001.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
Autotoxicity may reduce plant population and productivity of alfalfa (Medicago sativa L.) reseeded into winterkilled alfalfa stands; however, the interaction of important variables such as reseeding delay, stand age, and cultivar with autotoxicity has not been evaluated. We determined the effects of stand age (1-, 2-, or 3-yr-old alfalfa stands), reseeding delay (none or 2-wk delay) after plowing the original stands in May, and cultivar reseeded (‘5262’ or ‘Wrangler’) on populations and productivity of reseeded alfalfa. Oat (Avena sativa L.), corn (Zea mays L.), or tall fescue (Festuca arundinaceae Schreb.) served as controls. There was no consistent evidence for autotoxicity in the reseeding year, as populations and forage yields were similar when seeding followed alfalfa or the control crops, regardless of the age of the previous alfalfa stand or the alfalfa cultivar reseeded. Delaying seeding had inconsistent effects on alfalfa plant population but consistently reduced yields in the seeding year, with yields averaging 4.9 and 6.3 Mg ha^-1 with and without a 2-wk reseeding delay, respectively. In three of six experiments, yields at the first harvest in the year following reseeding averaged 15% lower following alfalfa than a control crop (3.6 and 4.2 Mg ha^-1, respectively), suggesting that alfalfa autotoxic response may be delayed. The lack of consistent evidence of autotoxicity, coupled with yield reductions in the seeding year associated with delayed seeding, suggest that the recommendation to delay alfalfa reseeding 2 wk following plowing of winterkilled stands may be unjustified.

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
ROTATION OF LEGUME and nonlegume crops has been recommended as an effective crop management practice for centuries (Martin and Leonard, 1967). In Upper Midwest rotations, corn is often grown following 2 to 3 yr of alfalfa to utilize the biologically fixed N (Peterson and Russelle, 1991). In addition, continuous production of alfalfa by re-establishment of alfalfa immediately after plowing or by no-till seeding into depleted stands is not recommended because of potential problems due to soil water depletion, plant diseases, and autotoxicity (Martin and Leonard, 1967; Kehr et al., 1983; Miller, 1983; Tesar, 1993). Of these factors, autotoxicity has been most frequently identified as a cause of reseeding failure (Webster et al., 1967; Tesar, 1993; Miller, 1996).

Autotoxicity is an intraspecific type of allelopathy that occurs when a plant releases chemicals harmful to plant growth and development (Miller, 1996). Alfalfa autotoxicity reduces establishment of alfalfa by reducing seed germination and seedling growth on soils where alfalfa was recently grown. Root growth and forage yield may also be decreased (Nelson et al., 1997). Alfalfa plants contain water-soluble compounds that are released from fresh leaf, stem, and crown tissue as well as from seeds, hay, and dried roots (Nielsen et al., 1960; Hall and Henderlong, 1989; Hegde and Miller, 1992, Chung and Miller, 1995b; Miller, 1996); however, based on in vitro assays, herbage is thought to be the most autotoxic plant component. Phenolic compounds, including medicarpin (Dornbos et al., 1990), coumarin (Hegde and Miller, 1992), and chlorogenic acid (Chung et al., 2000), are among those considered phytotoxic; however, autotoxicity is likely caused by an interaction of phytotoxic chemicals (Hegde and Miller, 1992).

Although the direct effect of these compounds on alfalfa development has been demonstrated in greenhouse and laboratory studies, their effects under field conditions are not well elucidated because of the interaction of many factors. For example, Jennings and Nelson (1998) reported that management recommendations could be influenced by soil texture, which might affect the activity water-soluble phytotoxic chemicals, and rainfall patterns, which might affect their movement and concentration. They reported that effects of autotoxic chemicals were greater on a fine sandy loam than on a silty clay loam soil but that the chemicals moved faster through the sandy loam soil.

Stand age may also be an important factor determining the extent of the autotoxic response because plant residues and the autotoxic constituents may accumulate over time (Angler, 1992); however, there have been no studies designed to test stand-age effects. Tesar (1993) inferred from independent studies with 1-, 4-, and 6-yr-old alfalfa stands that stand age had no influence on autotoxicity if there was an adequate interval between plowing and reseeding. He contended that a progressive buildup of autotoxic compounds would not occur in older stands because most of the herbage, which contains the greatest concentration of autotoxic compounds, is harvested. However, his analysis does not consider the potential accumulation of organic compounds due to leaching from leaves; root exudation; turnover and death of small roots; and decaying, dead plants.

Laboratory studies also suggested that alfalfa cultivars and germplasm might differ in susceptibility to autotoxicity (Chung and Miller, 1995a; Chon et al., 2000); however, differences among cultivars in response to autotoxic situations have not been observed under field conditions (Miller, 1983; Cosgrove, 1996).

Although the concept and importance of alfalfa autotoxicity has been intensively debated and researched in the past decades, there is still a lack of comprehensive field studies including multiple environments and management variables, especially in the Upper Midwest. In this region, winter injury often reduces alfalfa stand productivity and necessitates untimely reseeding. Unfortunately, winter injury is often observed in the spring when farmers depending on alfalfa have little flexibility regarding field selection because of herbicide use and fertilization programs for upcoming crops. Consequently, seeding alfalfa immediately following alfalfa is the desired option. For these situations, based on the research of Tesar (1993) and Cosgrove (1996), it is currently recommended for the North-Central region (Undersander et al., 2000) to delay reseeding for 2 wk after plowing an alfalfa stand and delay no-till seeding for 3 wk after herbicide application to kill an old alfalfa stand. Recommendations from other states are to delay reseeding for 1 yr (Miller, 1996). Our objective was to determine the effects of interval between plowing and reseeding, stand age, and reseeded cultivar on alfalfa autotoxicity at three locations differing in soil characteristics and annual rainfall.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
Experiments were conducted at Lamberton (44°15' N, 95°19' W), Rosemount (44°43' N, 93°06' W), and Waseca (44°04' N, 93°31' W), MN, USA. At each location, experiments were repeated in a subsequent year, giving a total of six experiments or environments. The soil at Rosemount was a Waukegan silt loam (fine-silty over sandy-skeletal, mixed mesic Typic Hapludoll); at Waseca, it was a Webster clay loam (fine-loamy, mixed mesic Typic Haplaquoll); and at Lamberton, it was a Normania loam (fine-loamy, mixed, mesic Aquic Hapludoll). The soil pH (6.5–7.0), exchangeable K (>350 kg ha^-1), and Bray-1 P (>40 kg ha^-1) at all locations were considered nonlimiting for alfalfa establishment and growth (Rehm and Schmitt, 1989). In some experiments, temperature and precipitation deviated considerably from the 30-yr average in the seeding year (Table 1). Temperature and precipitation data for each experiment were retrieved from nearby weather-recording stations.

Original stands of 5262 alfalfa were established by broadcast-seeding in a conventionally prepared seedbed at 15 kg ha^-1 in May of 1991, 1992, and 1993 for reseeding in spring 1994 and in 1992, 1993, and 1994 for reseeding in spring 1995. Weeds were controlled using preplant incorporation of 0.5 kg a.i. ha^-1 trifluralin [2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl) benzenamine]. In the seeding year of the original stands, alfalfa was harvested two times at first flower. In the years following seeding, three harvests were made at first flower. According to standard management recommendations to minimize winter injury, the final harvest occurred about 1 September, and regrowth was allowed to accumulate. Oat and corn controls were harvested at maturity in July and October, respectively. The tall fescue control was harvested three times per season at 45-d intervals beginning in June.

To simulate winterkill, regrowth of the original alfalfa stands was treated with a broadcast application of 2,4-D [(2,4-dichlorophenoxy) acetic acid] at 2.0 kg a.i. ha^-1 on 15 October. A similar approach to simulate winterkill was used by Asbil and Coulman (1992). Because night air temperatures after application were frequently <0°C, it is unlikely that this treatment resulted in immediate kill of alfalfa. Treated plants likely stopped normal physiological processes within 7 to 10 d after herbicide treatment and altered winterhardiness. Regardless, by the following spring, all alfalfa plants were dead. At Rosemount, the tall fescue control was killed with a fall application of glyphosate [N-(phosphonomethyl) glycine] at 1.12 kg a.i. ha^-1.

At each location in 1994 and 1995, the soil was chisel-plowed to a 12-cm depth immediately before disking on 1 May. A finishing harrow smoothed and firmed the seedbed before reseeding. Weeds were controlled with preplant application of 0.5 kg a.i. ha^-1 trifluralin. Alfalfa was broadcast-seeded at 15 kg ha^-1. Seeds were inoculated with appropriate rhizobial inoculant and treated with metalaxyl [N-(2,6-dimethylphenyl)-N-(methoxyacetyl) alanine methyl ester]. The first reseeding date (no delay) occurred on about 1 May while the second reseeding date occurred on 15 May, 2 wk after plowing as is currently recommended (Undersander et al., 2000).

Thirty days after each seeding date and in October of the reseeding year, plant populations were determined by counting number of plants within two 0.2-m² quadrats in each plot. In the reseeding year, alfalfa was harvested twice at first flower, with the first harvest occurring about 60 d following emergence; seeding date and climate influenced the harvest dates. No harvest occurred after the first week of September. In the year following reseeding, plots were harvested once at late bud to first flower on about 25 May. At each harvest, a 1- by 3-m area was cut in each plot with a flail harvester to a 5-cm stubble height, and a 500-g subsample was used to determine dry matter content. All samples were dried in a forced-air oven at 60°C for 48 h. During the experiments, plots were sprayed as needed with permethrin [3-phenoxy-phenyl-methyl + cis, trans-3-(2,2-dichloroethenyl)-2,2-dimethylcyclo-propanepropanecarboxylate] for potato leafhopper (Empoasca fabae Harris) control.

Results from each location and year were analyzed separately because of differences in the control species and climatic conditions. Experiments were analyzed as a split-split plot using PROC ANOVA (SAS, 1985). When interactions were significant at P < 0.1, data were reanalyzed for each factor by all levels of the other factor with which it was interacting. Treatments mean comparisons in each experiment were made when F-tests were significant at P < 0.1 using the least significant difference (LSD) test at the P < 0.1 level. This P-level was selected to minimize Type II errors.

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
Alfalfa Populations
Thirty Days after Reseeding
Treatment effects on alfalfa populations 30 d after reseeding were not consistent (Tables 2 and 3). Although reduced plant population has been reported to be an indicator of alfalfa autotoxicity (Miller, 1996), alfalfa populations were similar (P > 0.1) in three of six experiments when alfalfa was reseeded after alfalfa, regardless of stand age, or the control crops. Stand age [0- (nonalfalfa control), 1-, 2-, or 3-yr-old alfalfa] affected plant populations in Lamberton and Rosemount in 1995 and Waseca in 1994. At Rosemount in 1995, populations were lower with 2-yr-old alfalfa stands than other treatments. At Lamberton in 1995 and Waseca in 1994, populations were similar (P > 0.1) following reseeding after the nonalfalfa control and 2- and 3-yr-old alfalfa stands but lower following termination of 1-yr-old alfalfa stands. This may suggest autotoxicity; however, 1-yr-old alfalfa stands generally had the least whole-plant dry matter in the fall before reseeding (data not presented). In addition, there was little correlation across experiments between stand characteristics before reseeding, such as plant populations and amount of forage, and alfalfa populations 30 d after reseeding (data not presented). Tesar (1993) also reported that alfalfa autotoxic response was not correlated with the amount of forage dry matter incorporated before alfalfa reseeding.

Effects of reseeding delay and cultivar were observed in four and three experiments, respectively (Table 2). The response to reseeding delay was inconsistent across experiments, suggesting that environmental conditions before, during, and after reseeding varied, causing subsequent variability in seed germination and seedling development rate. Indeed, variation and deviations from 30-yr average precipitation occurred across environments (Table 1). Finally, plant populations of 5262 were greater than those of Wrangler in three of the six experiments. In previous research in Minnesota, 5262 alfalfa has been shown to have greater yield and persistence than Wrangler, but there is no evidence to suggest that these cultivars differ in seedling establishment (Martin and Sheaffer, 1996).

Fall
Plant populations in October (Tables 2 and 4) were lower than at 30 d after seeding, but treatment effects on alfalfa populations remained inconsistent. There was no effect of stand age on populations in any experiments. Populations following the control crops were similar to those following alfalfa, irrespective of the stand age, suggesting that if autotoxicity was present, its effect on plants was relatively minor and undetectable.

Stand differences between alfalfa cultivars were less in October than at 30 d following seeding. Stands of the two cultivars were similar in five of six experiments. As had occurred 30 d after seeding, populations of 5262 were greater than those of Wrangler at Waseca in 1995.

Alfalfa Yields in Reseeding Year
Stand age affected alfalfa yield in the reseeding year in two of six experiments, but no consistent response was observed that would suggest the occurrence of autotoxicity (Tables 2 and 5). Similar responses were observed at the first harvest (data not presented). At Lamberton in 1995 and Waseca in 1994, when significant stand age effects occurred, total alfalfa yields were similar to or greater following alfalfa of most stand ages than following the nonalfalfa control. In both experiments, alfalfa yields were lower following 1-yr-old alfalfa than following all other treatments. As reported with plant populations, there was no consistent association between yield response and characteristics of killed alfalfa stands (data not presented).

Crossover interactions between reseeding delay or cultivar with previous crop seldom occurred except for the reseeding delay x stand age interaction at Waseca in both years. Again, however, no consistent trend occurred that could be associated with autotoxicity. It is more likely that treatment differences were due to differences in seeding and growing conditions, which resulted from the interaction of several factors, including soil moisture and temperature, that varied between treatments.

Alfalfa Yields in Year after Reseeding
At the first harvest in the year following alfalfa reseeding (Tables 2 and 6), the previous crop had an effect on yields in 1995 and 1996 at Rosemount and in 1996 at Waseca. In these experiments, average yields were 15% lower after alfalfa than after the nonalfalfa control crop, suggesting evidence of delayed autotoxicity. Tesar (1993) reported postreseeding year yield reduction of the same magnitude under similar management conditions. The yield reductions we observed were not associated with effects on stand density, yield in the reseeding year, or stand characteristics of the killed stands. There was no consistent effect due to the age of the killed alfalfa stands on yields 1 yr after reseeding.

These results suggest that the autotoxic response in alfalfa might be subtle and hard to detect in the seeding year but more apparent in the year following reseeding. This delayed response could be the result of changes in root morphology that might result from autotoxicity as reported by Nelson et al. (1997). They showed that chemicals involved in autotoxicity result in a permanent modification of alfalfa root morphology. We did not examine root morphology in our study, and thus were not able to confirm this hypothesis.

	SUMMARY AND CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
There was no consistent evidence among six experiments for autotoxicity in alfalfa in our study where we simulated winterkill by fall herbicide application and incorporated dead residue in spring before planting. Alfalfa plant densities and yields in the reseeding year were relatively unaffected by the previous crop or age of alfalfa stand that preceded reseeding. However, a possible autotoxic response appeared in some environments in spring of the year following reseeding.

It is possible that the lack of a consistent autotoxic response in our study may have been because original alfalfa stands were incorporated by chisel plowing. In some studies reporting alfalfa autotoxicity, alfalfa was no-till seeded into existing stands (e.g., Mueller-Warrant and Koch, 1981; Tesar, 1993; Nelson et al., 1997). Tesar (1993) suggested that autotoxicity may be a greater problem with the absence of plowing as allelochemical compounds in such conditions have direct, early contact with developing alfalfa seed and seedlings. Consistent with our results, Kehr et al. (1983) in Nebraska and Bortnem et al. (1990) in South Dakota also reported no evidence of autotoxicity during the seeding year when original stands were plowed.

Contrary to our findings, Tesar (1993) in Michigan reported autotoxicity in the reseeding year when alfalfa was seeded immediately after spring incorporation of the original alfalfa stands. Differences between his results and ours may be because we used a fall herbicide application to induce winterkill, whereas he plowed or applied glyphosate to spring regrowth of alfalfa that had accumulated from 1.4 to 4.0 Mg ha^-1 herbage. Our approach was successful in killing alfalfa plants as may occur anytime during the winter and early spring in Minnesota. We attempted to simulate actual field situations where plants do not regrow in the spring due to winterkill, in contrast to the approach of Tesar (1993) who killed living plants with significant regrowth in the spring. Because the amount of residue on plants present in the sward might influence the incidence of autotoxicity (Miller, 1996), Tesar (1993) may have established a situation with considerable potential for autotoxicity that is unlikely to occur under field conditions where winterkill was prevalent.

Laboratory studies of Jennings and Nelson (1998) demonstrated that the severity of autotoxicity varies, depending on soil texture and rainfall patterns. In our study, there was no clear association between precipitation and incidence of autotoxicity. However, based on their results, the effects of autotoxic factors might be reduced in the short term in the finer-textured soils of our studies, but their impact may be longer lasting. This could explain why we observed autotoxicity in some experiments in the year following reseeding.

The traditional recommendation to rotate from alfalfa to another crop upon termination of alfalfa stands is still an effective approach to utilize alfalfa N and to minimize potential plant disease and allelopathy. However, in emergency situations, when reseeding without an intervening crop is necessary, our results show a lack of autotoxicity and a yield reduction in the reseeding year when reseeding was delayed. This suggests that the recommendation of delaying alfalfa reseeding in spring 2 wk following incorporation of winterkilled alfalfa may be unsound. Our results also show the need for investigating long-term alfalfa autotoxic response in diverse environments.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
Journal Paper of the Minnesota Agric. Exp. Stn.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 

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