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a Coop. Ext. Serv., Univ. of Arkansas, P.O. Box 391, Little Rock, AR 72203
b Dep. of Agron., 210 Waters Hall, Univ. of Missouri, Columbia, MO 65211
* Corresponding author (jjennings{at}uaex.edu)
Received for publication January 9, 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONSUMMARY AND CONCLUSIONREFERENCES |
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Abbreviations: L1, Location 1 • L2, Location 2 • L3, Location 3
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONSUMMARY AND CONCLUSIONREFERENCES |
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The occurrence of soil-borne pathogens or insect pests may influence the variability in recommended rotation intervals. Alfalfa establishment is affected by fungal pathogens such as Pythium spp., Phytophthora megasperma Drechs., and Sclerotinia trifoliorum Eriks (Graham et al., 1979; Leath et al., 1988). Both S. trifoliorum and P. megasperma contribute to the decline of old alfalfa stands, thus providing ample inoculum to infect a seedling stand (Graham et al., 1979; Frosheiser, 1980; Gray et al., 1988). In Nebraska, anthracnose caused by Colletotrichum trifolii Bain and P. megasperma contributed to significant stand losses in alfalfa reseeded on land previously in alfalfa (Kehr et al., 1983). In Missouri, P. megasperma increased seedling mortality in two alfalfa reseeding experiments (Jennings and Nelson, 1991).
Byers and Bierlein (1984) concluded that problems with pests must be solved before continuous alfalfa planting in Pennsylvania will be attainable. Alfalfa seedling losses to insect pests were greater in Pennsylvania for late-spring than for early spring seedings (Byers et al., 1985; Byers and Templeton, 1988). In Kentucky, stand density of alfalfa seedlings was reduced 40% by pathogenic soil-borne fungi and 44% by soil-borne insects in spring-seeded alfalfa following alfalfa (Godfrey et al., 1986).
The objective of these experiments was to determine the effects of five rotation intervals after killing an old alfalfa stand and of four pesticide treatments applied at planting on establishment, growth, and dry matter yield of alfalfa no till–planted in spring.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONSUMMARY AND CONCLUSIONREFERENCES |
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Each experiment was a randomized complete block design with a split-plot restriction. Rotation intervals were main plots. Pesticide treatments were subplots factorially arranged within rotation intervals. Each interval and pesticide treatment was replicated four times. Rotation intervals were established by killing old alfalfa stands with herbicides in sequence before a common planting date. Intervals between killing the old alfalfa stands and reseeding alfalfa were 18, 12, 6, 0.75, and 0.5 mo. During the rotation interval, plots were left fallow with only occasional mowing to control volunteer weeds, mainly crabgrass [Digitaria sanguinalis (L.) Scop.]. At L3, the planting date was delayed about 1 wk by wet weather conditions resulting in the 0.5- and 0.75-mo rotation interval treatments being 0.6 and 1 mo, respectively. The 18-mo rotation interval was the control treatment in each experiment.
Pesticide treatments were four combinations of seed treatment fungicide (F) and insecticide (I) applied in row (+I+F, +I-F, -I+F, -I-F). Fungicide treatments were no application (control) or the labeled rate of metalaxyl {(R)-[(2,6-dimethylphenyl)-methoxyacetyl-amino]-propionic acid methyl ester} applied at 0.2 g a.i. kg-1 seed. Insecticide treatments were no application (control) or the labeled rate of 15% chlorpyrifos [O,O-diethyl-O-(3,5,6-trichloro-2-pyridinyl) phosphorothioate] granules applied in the row at 1.12 kg a.i. ha-1.
Old alfalfa in the 18-, 12-, and 6-mo rotation interval treatments was sprayed with a tank mix of 2.24 kg a.i. ha-1 glyphosate [N-(phosphonomethyl)glycine] plus 2.24 kg a.i. ha-1 2,4-D [(2,4-dichlorophenoxy)acetic acid]. This treatment successfully killed the old plants. Old alfalfa in the 0.75- and 0.5-mo rotation intervals was sprayed only with 4.48 kg a.i. ha-1 glyphosate to avoid potential soil carryover effects of 2,4-D on alfalfa establishment. Some old plants in the 0.75-mo rotation interval treatment survived the glyphosate treatment and regrew after plots were seeded. In L3, the recovery of old alfalfa plants for the 0.75-mo rotation interval was measured at 5.5% of the previous population.
In April before planting, soil insect pests were sampled at each location in two randomly selected replications of each rotation interval with grain bait traps (Munson et al., 1986) and from soil samples excavated to a depth of 45 cm. Population densities of wireworm (Melanotus spp.) were low at all sites, averaging 0, 0.5, and 0.2 larvae per bait trap for L1, L2, and L3, respectively. No other soil insect pests were found by either sampling method.
Plots at L1 and L2 were planted on 8 May 1992, and those at L3 were planted on 14 May 1993. Plots were 6.1 m long and were seeded with a no-till drill that was 1.62 m wide, with seed planted approximately 1 cm deep in nine rows spaced 18 cm apart. Cody alfalfa was seeded at the rate of 22.4 kg ha-1 at L1 and L2 and at 17.9 kg ha-1 at L3. The higher seeding rate was used in L1 and L2 because of unusually dry weather conditions before planting. Plots were fertilized annually with P, K, and B according to Missouri soil test recommendations for a yield goal of 11 Mg ha-1.
Plant density was measured by manually counting plants in 1-m sections marked in three randomly selected nonborder rows within each plot. Plant density measurements of these row-sections were made soon after establishment and again after the first and last harvests each year. According to this schedule, plant density was determined at approximately 1.5, 3, 6, 12, 18, 24, and 30 mo after planting at each location. To reduce errors in determining plant densities with these nondestructive measurements, plants in row-sections of borders and alleys were occasionally counted and then excavated for actual density determination. This helped train the hand to distinguish between plant crowns.
Dry matter yield was determined by harvesting a 0.9- by 4.3-m strip from the middle of each plot with a sickle bar mower set to leave a 5-cm stubble. Subsamples from each plot were weighed fresh and then dried for 48 h at 60°C in a forced-air oven for dry matter determination. Plots in each experiment were harvested three times during the seeding year (in early August, mid-September, and late October) and five times (about mid-May, mid-June, mid-July, mid-September, and early November) during the second and third years.
Summer annual weeds, mainly crabgrass, and winter annual weeds, mainly chickweed [Stellaria media (L.) Cyrillo], were controlled as needed with sethoxydim {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one], paraquat (1,1'-dimethyl-4,4'-bipyridinium ion), or imazethapyr {2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-ethyl-3-pyridinecarboxylic acid}. Weed infestations were observed to be more prevalent in the 0.5- and 0.75-mo rotation intervals than in the 12- and 18-mo rotation interval treatments.
Alfalfa weevil (Hypera postica Gyll.) was controlled in April of Year 2 and Year 3 by applying chlorpyrifos or carbofuran (2,3-dihydro-2,2-dimethyl-7-benzofuranyl methylcarbamate). To avoid confounding effects of insecticide application with the soil-applied chlorpyrifos treatment, early harvest was used to control potato leafhopper (Empoasca fabae Harris) during the seeding year. In Year 2 and Year 3, either chlorpyrifos or early harvest was used to control potato leafhopper.
Variances were not different among locations, so data were combined. Data were analyzed as a split-split-split plot in time in which the hierarchy of factors from whole plot to the sub-sub-subplot was location, rotation interval, pesticide combination, and time where time was sampling date or year depending on the particular response variable. This kind of analysis is an extension of that for a split plot in time and space as described in Steel and Torrie (1960). Treatment effects were compared using the LSD when the F-test of the analysis of variance exceeded the 0.05 level of probability.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONSUMMARY AND CONCLUSIONREFERENCES |
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Location Effects
The interaction of rotation interval with location was not significant for plant density or dry matter yield even though plant density of the previous stands varied among locations. However, the interaction of location with sampling date was significant for plant density, with locations differing on four of seven sampling dates (Table 1). Ranking of locations for plant density was not consistent among the significant sampling dates. Because the location main effect was not significant, the interaction was probably due to variable rates of alfalfa stand thinning over time among the locations. At each location, however, decline of plant density was most rapid during the growing season of the seeding year (1.5–6 mo after planting), followed by a slower decline during the growing seasons of Year 2 (12–18 mo) and Year 3 (24–30 mo) (Table 1).
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Rotation Interval Effects
The interaction of rotation interval with sampling date was significant for plant density (Fig. 1)
. For presentation, data in Fig. 1 are plant densities, averaged across locations and pesticide treatments for each rotation interval, at all seven sampling dates (1.5–30 mo). Plant disappearance for all rotation intervals was greater during each growing season than during each winter and early spring. By the end of the seeding year (6-mo sampling date), plant densities for the 6-, 12-, and 18-mo rotation intervals ranked significantly higher than the 0.75- and 0.5-mo rotation intervals and remained higher through the 30-mo sampling date (Fig. 1).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONSUMMARY AND CONCLUSIONREFERENCES |
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Fungicide treatment did not improve plant density or yield, which is in contrast to other studies (Godfrey et al., 1986). Lack of response to seed treatment fungicide may be due to low pathogen pressure from the old stands because of good soil drainage at each site and generally dry conditions after planting.
Rotation Interval Effects
The lack of a significant rotation interval x location interaction would suggest that autotoxic effects can occur on new alfalfa planted after alfalfa even if old alfalfa stands have relatively low plant densities. Similar results were reported in New Hampshire where stand densities of only 10 old plants m-2 reduced alfalfa establishment (Mueller-Warrant and Koch, 1981). Other field data (Jennings and Nelson, 2002) indicate that new alfalfa plants spaced closer than 20 cm from an old plant, a density of <8 plants m-2, had a 24% reduction in survival and 60% reduction in yield.
The highest plant densities occurred for the 18-, 12-, and 6-mo rotation intervals (Fig. 1), but highest dry matter yields were measured for the 18-, 12-, and 0.75-mo rotation intervals (Fig. 2). Ranking of rotation intervals for plant density remained consistent after the seeding year, with the 0.5- and 0.75-mo rotation intervals ranking significantly lower than the control, averaging 80.7 and 81.4% of the control at the end of the seeding year, 84.3 and 88% after Year 2, and 85.4 and 85.2% after Year 3, respectively (Fig. 1). Dry matter yield of the 0.5-mo rotation interval ranked 6.8, 9.7, and 6.3% lower in yield than the control for the seeding year, Year 2, and Year 3, respectively. In contrast, yield for the 6-mo rotation interval ranked 10.3 and 7.6% lower than the control for the seeding year and Year 2, respectively, but was not statistically lower in Year 3. Yield of the 0.75-mo rotation interval ranked 6% lower than the control in Year 2 but was not different in the seeding year or in Year 3 (Fig. 2).
These data are consistent with those of others (Miller, 1983; Tesar, 1993). The effect of autotoxicity was greater on plant density than on yield. This was probably due to crown development of plants compensating for yield at lower densities (Volenec et al., 1987; Kephart et al., 1992). Compared with other studies, yield reductions were small in the 0.5- and 6-mo rotation intervals relative to the 18-mo control. For example, alfalfa yield and plant density in Illinois were reduced by 50 and 57%, respectively, when alfalfa was spring-seeded after alfalfa compared with seeding after 1 yr of corn (Zea mays L.) in a soybean [Glycine max (L.) Merr.]–corn–alfalfa rotation (Miller, 1983).
Plant density and yield reductions of the 0.5-mo rotation interval in the current study were similar to those reported for three experiments in Michigan (Tesar, 1993). In the Michigan studies, alfalfa plant density was reduced by 23 to 44% and alfalfa yield by 5 to 17% when no till–planted 14 d after glyphosate application compared with alfalfa planted after fall-plowed corn. In the same studies, when alfalfa was no till–planted 21 d after glyphosate treatment of old alfalfa, plant density of new alfalfa ranged from 97 to 100%, and yield ranged from 98 to 108% of the corn control. In our experiments, the comparable 0.75-mo rotation interval treatment was not significantly lower in yield compared with the 18-mo control due to surviving old plants, but it was significantly lower in plant density. Survival of old plants was not mentioned in the Michigan study.
Effect of Surviving Old Plants
Few, if any, published results from field studies mention the recovery of old plants from herbicide treatment or their contribution to yield. The incidence of regrowth of old plants in the 0.75-mo rotation interval probably occurred because herbicide treatments had to be applied in early April when old plants had about 15 to 20 cm of spring growth. At that height, growth may not have been sufficiently active in the cool weather to facilitate good herbicide uptake. Because of the common planting date for all intervals, plants in the 0.5-mo interval were sprayed a week later and were taller, and few old plants survived. Similar findings were reported in New Hampshire (Mueller-Warrant et al., 1983) where existing old alfalfa was controlled more easily with glyphosate when it was 25 to 30 cm in height than when glyphosate was applied earlier. Survival and growth of old plants may appear beneficial to dry matter yield in the short term, but field observations indicated that this treatment was not desirable for stand persistence. Seedlings near the old plants were stunted and lacked vigor, which is consistent with results of other experiments (Jennings and Nelson, 2002). At the end of the first growing season, we observed that drill rows were much less distinct in the 0.75-mo rotation interval plots in L3 than in the 12- or 18-mo rotation intervals due to poor seedling establishment or greater stand thinning around the old plants. Uncertainty of the recovery of old plants from glyphosate treatment makes the 0.75-mo rotation interval questionable for recommendation.
Dry matter yield was ranked in order of increasing rotation interval in Years 2 and 3 with the exception of the 0.75-mo rotation interval (Fig. 2). Dry matter yield in the 0.75-mo rotation interval was higher than in the 0.5- and 6-mo rotation intervals because yield of the 0.75-mo rotation interval was strongly influenced by old alfalfa plants that survived the glyphosate treatment. These old plants were not removed by hand from the plots because we were simulating production conditions. Plants surviving the herbicide treatment were not counted for L1 and L2, but in L3, an average of 3 old plants m-2 survived in the 0.75-mo rotation interval plots. In contrast, only an occasional plant survived herbicide treatment in the 0.5-mo rotation interval, and none survived in the longer rotation intervals. These old plants occurred mainly between seedling rows where they contributed to dry matter yield but negatively influenced seedling densities in the vicinity, especially in the seeding year. Density of new seedlings in the 0.75-mo rotation interval was significantly lower than for the 18- and 6-mo rotation intervals.
Long-Term Responses
Weed infestation was observed to be more pronounced in both the 0.75- and 0.5-mo rotation intervals compared with longer intervals despite timely herbicide application. This was probably due to slower regrowth observed for alfalfa plants in these rotation intervals, lower plant density, and the noticeably bare spaces around surviving old alfalfa plants.
Poor establishment and growth of alfalfa following alfalfa has been attributed to factors other than autotoxicity. In Nebraska, alfalfa yields were 26, 40, and 35% higher in the seeding year and the first and second years after seeding, respectively, on land with no history of alfalfa compared with the same land reseeded to alfalfa after a 1-yr rotation with soybean (Kehr et al., 1983). This poor growth response for at least 3 yr was attributed to pathogens and subsoil moisture depletion from the previous alfalfa crop. Lack of response to seed treatment fungicide in our experiments suggests that incidence of seedling disease was not a factor. Effects of subsoil moisture depletion by a preceding alfalfa crop has not been studied in Missouri, but it is not likely that this factor contributed to differences in plant density or yield at the three sites in these experiments. Rainfall for Howell County, MO, averages approximately 1150 mm yr-1, with most rainfall events during fall, winter, and spring when subsoil moisture recharge would occur. This conclusion is supported by studies from Alberta, Canada, where low soil moisture status was discounted as a reason for continued poor growth of alfalfa following alfalfa (Webster et al., 1967).
Autoconditioning
Data in Fig. 1 and 2 show that autotoxic effects occurred during the first year and resulted in a ranking of rotation intervals for both plant density (Fig. 1) and dry matter yield (Fig. 2) that continued through Years 2 and 3. Autotoxicity experiments in Michigan and Illinois showed similar results where treatments ranking lower in yield than the controls during the establishment year continued to rank lower in yield than the controls after 2 yr (Miller, 1983; Tesar, 1993). In Nebraska, alfalfa planted after alfalfa continued to rank lower in yield than the control after 4 yr (Kiesselbach et al., 1934). The above studies and results from our experiments suggest that alfalfa that is visibly or subtly affected by autotoxicity during establishment does not completely outgrow the negative effects.
Alfalfa seedlings appear to become autoconditioned by autotoxicity during the establishment period so that plant density and yield remain lower than controls. Thus, autoconditioning is defined as a change in plant morphology due to environmental or chemical factors during establishment that is retained at the population level. The reasons for autoconditioning are not clear, but the effect of autotoxicity on developing alfalfa root systems during establishment may be responsible. Root inhibition caused by autotoxic extracts in petri dish bioassays include decreased taproot growth, decreased root hair formation, and twisted roots (Hegde and Miller, 1992; Chung and Miller, 1995). Hall and Henderlong (1989) and Chon et al. (2000) reported that alfalfa autotoxicity affects alfalfa root growth more than germination. In other experiments, alfalfa extracts reduced both alfalfa root growth and root hair density (Read and Jensen, 1989; Hegde and Miller, 1992). These findings suggest that autotoxicity affects plant development and, thus, potentially long-term growth and yield.
Observations of plants excavated after termination of the current study further suggest that autotoxicity causes a change in plant morphology from a tap-rooted to a more branch-rooted structure that altered production potential of the stand. Plants excavated from the 0.5- and 12-mo interval rotation plots after Year 3 in L1 revealed differences in taproot morphology between the two treatments. Roots in the 0.5-mo rotation interval appeared to have little taproot development but were extensively branched, whereas root systems from the 12-mo rotation interval had less branching but had well-developed, prominent taproots. Further observations indicated that plants in the 0.75-mo rotation interval had root development similar to that in the 0.5-mo rotation interval, but taproot development appeared slightly improved. Plants with well-developed, deep-growing taproots may have greater resistance to moisture stress during dry weather than plants with shallower-growing branched roots, thus improving yield and survival of plants in the two long rotation intervals compared with the 0.75- and 0.5-mo rotation intervals.
Practical Aspects of Autoconditioning
From a practical standpoint, a 6-mo rotation interval would be more desirable than a 12-mo rotation interval for alfalfa producers because it would allow them to kill old stands in the fall and reseed in the spring with minimal interruption in production. Under conditions of this spring-seeded study, plant density in the 6-mo rotation interval was nearly equivalent to that in the 18-mo control, but dry matter yield was lower than in the 12- and 18-mo rotation intervals. These results contrast with those of Mueller-Warrant and Koch (1981) and Tesar (1993), who reported good stands and dry matter yield of alfalfa seeded in spring after a 6-mo rotation interval. However, these results agree with a 2-yr study from Wisconsin (Cosgrove, 1996) where alfalfa was no till–planted into an old alfalfa field after a 36-wk interval. Plant density after alfalfa was 100 to 117% of that after 1 yr of corn (control), but yield was only 58 to 66% of the control. An interval of 9 mo in Wisconsin and 6 mo at each of the three Missouri locations may have been long enough to allow some of the autotoxic chemical to dissipate from the rooting zone so that the less-sensitive responses of germination and emergence were not affected, but enough remained to reduce the more-sensitive seedling root growth, causing autoconditioning and lower dry matter yield of established plants.
The long-term ranking of rotation intervals for plant density and dry matter caused us to carefully evaluate the literature. Nine other experiments on reseeding alfalfa after alfalfa were found that were maintained for more than 1 yr in addition to the three reported here. Dry matter yield was reported for all nine experiments, but plant density was reported for only one (Table 2). In each experiment, treatments which ranked significantly lower than the control treatment during the first year continued to rank lower than the control through the end of the experiment. Average yield reductions for each set of experiments ranged from 8% to at least 52%. Yield reduction was 25% averaged over all 12 experiments. Average plant density reduction of the Illinois experiment and the current study was 23% compared with control treatments. Even discounting the Alberta and Illinois experiments, in which controls and experimental treatments were not located in the same field, the average reduction was 19% for both yield and plant density. These studies strongly support the conclusion that stands affected by autotoxicity during establishment exhibit a long-term autoconditioning response.
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SUMMARY AND CONCLUSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONSUMMARY AND CONCLUSIONREFERENCES |
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONSUMMARY AND CONCLUSIONREFERENCES |
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