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SOIL MANAGEMENT

Suitability of Undersown Sweetclover as a Fallow Replacement in Semiarid Cropping Systems

Agriculture and Agri-Food Canada, Lethbridge Research Centre, P.O. Box 3000, Lethbridge, AB T1J 4B1, Canada

* Corresponding author (blackshaw{at}em.agr.ca)

Received for publication July 21, 2000.

	ABSTRACT

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

 
The negative effects of tilled fallow on soil quality may be reduced by utilizing legume green manure crops. A study was conducted to determine the suitability of biennial sweetclover [Melilotus officinalis (L.) Lam.] as a partial fallow replacement on the northern Great Plains. Sweetclover undersown in field pea (Pisum sativum L.), flax (Linum usitatissimum L.), or oriental mustard [Brassica juncea (L.) Coss.] and killed in June of the subsequent fallow year attained shoot biomass yields of 3110 to 5370 kg ha^-1, depending on the year and companion crop with which it was grown. Living sweetclover plus its residues after being killed provided excellent ground cover to reduce the risk of erosion throughout the 20-mo fallow period. Sweetclover compared to tilled fallow reduced soil water content at the time of seeding spring wheat (Triticum aestivum L.) in 2 of 3 yr. Available soil N in April before seeding spring wheat was 16 to 56 kg ha^-1 greater in sweetclover green manure than in fallow treatments. Wheat yields were 47 to 75% greater in the sweetclover than in the fallow treatments. However, the many positive attributes of sweetclover green manure must be weighed against potential reductions in companion crop yield. Undersown sweetclover reduced flax yield in 1 of 3 yr and mustard and field pea yield in 2 of 3 yr, with yield losses ranging from 10 to 55%. Additional research is needed to determine agronomic practices that would reduce competitive interactions between sweetclover and its companion crop.

	INTRODUCTION

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

 
SUMMERFALLOW remains a common practice to conserve soil moisture and stabilize succeeding crop yields on the Great Plains of Canada and the USA. However, fallow is associated with reduced soil organic matter, increased soil erosion, deterioration in soil tilth, and increased salinization (Larney et al., 1994). The risk of soil loss during fallow can be reduced by replacing some or all of the tillage operations to control weeds with herbicides (Blackshaw and Lindwall, 1995a; Smika, 1990). However, it is often impossible, even with chemical fallow, to maintain sufficient crop residues on the soil surface to protect the land from erosion during a 20-mo fallow (Blackshaw and Lindwall, 1995b). Additionally, reduced tillage fallow does little to solve the problem of declining soil fertility.

Renewed interest in cover crops for annual cropping systems has occurred recently because of their ability to reduce soil erosion, improve soil quality, and maintain or improve crop yield (Power, 1987; Stute and Posner, 1993). Cover crops also may be beneficial in disease, insect, and weed management; thus decreasing the need for pesticides (Vandermeer, 1989).

One potential of cover crop use is as a green manure crop replacing fallow (Pikul et al., 1997; Rice et al., 1993; Schlegel and Havlin, 1997). Legumes are desirable for this purpose because they fix atmospheric N and make it available to succeeding crops (Biederbeck et al., 1996; McGuire et al., 1998; Wallgren and Linden, 1991). Numerous legumes including alfalfa (Medicago sativa L.) (Badaruddin and Meyer, 1990; Townley-Smith et al., 1993; Rice et al., 1993), faba bean (Vicia faba L.) (Townley-Smith et al., 1993), field pea (Pisum sativum L.) (Biederbeck et al., 1993; Brandt, 1996; Wallgren and Linden, 1991; Townley-Smith et al., 1993), lentil (Lens culinaris Medik.) (Biederbeck et al., 1993; Brandt, 1996; Pikul et al., 1997), red clover (Trifolium pratense L.) (Badaruddin and Meyer, 1990; Schlegel and Havlin, 1997; Wallgren and Linden, 1991), and Tangier flatpea (Lathyrus tingitanus L.) (Biederbeck et al., 1993; Townley-Smith et al., 1993; Rice et al., 1993) have been evaluated as partial fallow replacements with varying degrees of success. Problems associated with many of these spring seeded legumes are high seed cost, limited biomass production, and N₂ fixation before being killed to avoid serious depletions in soil water, and poor competitive ability with weeds (Townley-Smith et al., 1993; Unger and Vigil, 1998).

Sweetclover is a biennial, drought-tolerant, winter hardy leguminous forage that is well adapted to the northern Great Plains region (Turkington et al., 1978). Previous studies have indicated that it can be highly productive in terms of biomass production and N₂ fixation needed to minimize erosion and improve soil quality (Schlegel and Havlin, 1997; Sparrow et al., 1993; Stickler and Johnson, 1959). Additionally, there is no need of a separate seeding operation if sweetclover is undersown with a companion crop. However, farmer adoption of green manure sweetclover has been limited because it precludes use of herbicides to control broadleaf weeds in cereal companion crops and it sometimes reduces the amount of available soil water for subsequent crops (Badaruddin and Meyer, 1989; Foster, 1990). There is farmer interest in underseeding sweetclover in broadleaf crops where trifluralin [2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl)benzenamine] could be utilized to control weeds, but little is known about the merits of this agronomic production practice.

A study was initiated to evaluate sweetclover undersown in field pea, flax, or oriental mustard as a partial fallow replacement in dryland cropping systems of the northern Great Plains. A previous paper reported on weed suppression attained with sweetclover and its residues throughout a 20-mo fallow period (Blackshaw et al., 2001). This paper reports on companion crop yields, soil water conservation, plant residue conservation, and soil N accumulation during fallow, and succeeding spring wheat yield.

	MATERIALS AND METHODS

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

 
A study consisting of a series of three experiments, each 3 yr in duration, was conducted at Lethbridge, AB. Establishment years for the three experiments were 1993, 1995, and 1996, respectively. The soil was a Typic Haplustoll soil with a sandy clay loam texture, pH of 7.8, and an organic matter content of 30 g kg^-1 in the Ap horizon. Annual precipitation received at the plot site ranged from 329 to 530 mm during the study (Table 1).

In Year 1, trifluralin at 0.82 kg ha^-1 was applied to the entire plot area and incorporated to a depth of 8 cm with two perpendicular passes of a field cultivar before seeding to control weeds. ‘Trapper’ peas at 100 kg ha^-1, ‘McGregor’ flax at 34 kg ha^-1, and ‘Lethbridge 22A’ oriental mustard at 6 kg ha^-1 were sown in 20-cm rows with a double-disk press drill in early May. While seeding the various crops, the low coumarin sweetclover cultivar Norgold at 9 kg ha^-1 was delivered from a second seed box through hoses to a position in front of the packing wheels, where it was pressed 0.5 to 1 cm deep into the soil. Nitrogen at 8 kg ha^-1 and P₂O₅ at 34 kg ha^-1 were placed with the crop seed at planting. Field pea and sweetclover seed were inoculated with appropriate N₂ fixing Rhizobium sp. Crops were direct-cut at a 20-cm height with a small plot combine at maturity and yields determined. Crop straw after harvest was evenly distributed over respective plots by hand.

In Year 2, sweetclover aboveground dry weight was determined in three 0.25-m² areas in each plot immediately before killing sweetclover.

In Year 3, soil moisture and available N were determined in April by taking one soil core (5 cm diam by 120 cm deep) in the cultivated and hay removal subplots. Each soil core was divided into 0- to 15-, 15- to 30-, 30- to 60-, 60- to 90-, and 90- to 120-cm sections. Soil moisture was determined gravimetrically in all sections and available N (2 M KCl-extractable nitrate and ammonium nitrogen) was determined in the 0- to 15-, 15- to 30-, and 30- to 60-cm sections. Plant residues remaining on the soil surface as an estimate of ground cover to protect the soil from erosion were collected in three 0.25-m² areas in each subplot at seeding and dry weights were determined. Emerged weeds were killed with a mixture of glyphosate [N-(phosphonomethyl)glycine] at 0.48 kg ha^-1 and 2,4-D [(2,4-dichlorophenoxy)acetic acid] ester at 0.46 kg ha^-1 in late April. ‘Katepwa’ spring wheat at 84 kg ha^-1 was sown in 23-cm rows across all treatments in early May. Starter fertilizer containing 25 kg ha^-1 of both N and P₂O₅ was applied to all plots. In-crop weed control was attained by applying the commercial mixture of fenoxaprop-P [(R)-2-[4-[(6-chloro-2-benzoxazoly)oxy]phenoxy]propanoic acid] at 92 g ha^-1, MCPA [(4-chloro-2-methylphenoxy)acetic acid] at 420 g ha^-1, and thifensulfuron [3-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl) amino]carbonyl]amino]sulfonyl]-2-thiophenecarboxylic acid]/tribenuron [2-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)methylamino]carbonyl]amino]sulfonyl]benzoic acid] at 15 g ha^-1 in mid-June. Wheat was harvested with a small plot combine and grain yields were determined.

All data were subjected to ANOVA using appropriate models. Where treatment differences were detected means were compared using Fisher's Protected LSD test at the 5% level of significance (Steel and Torrie, 1980). The appropriate error from the SAS output was used to calculate the LSD value for each variable. All statistical tests were performed using SAS (SAS Inst., 1989).

	RESULTS AND DISCUSSION

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

 
Companion Crop Yield
Undersown sweetclover reduced flax yield in 1 of 3 yr, oriental mustard yield in 2 of 3 yr, and field pea yield in 2 of 3 yr (Table 2). Crop yield reductions due to sweetclover competition ranged from 10 to 26% for mustard and 46 to 49% for pea, depending on the year. In the 1 yr that flax yield was reduced, a 55% yield reduction was recorded.

Sweetclover Biomass Production
Sweetclover shoot biomass, when terminated in June of the second year of the experiment, ranged from 3110 to 5370 kg ha^-1, depending on the year and companion crop with which it was grown (Table 3). These values compare favorably with previous results. Seventeen site-years of data in western Canada indicated that Norgold sweetclover grown without a companion crop and harvested in July of the second year attained an average shoot biomass yield of 5889 kg ha^-1 (Goplen, 1981). Previous multiyear studies in Saskatchewan found that sweetclover harvested in June or July of the second year of growth attained biomass yields ranging from 1065 to 3813 kg ha^-1 when previously undersown in wheat (Foster, 1990), while biomass yields of 1340 to 4700 kg ha^-1 were attained when previously undersown in rapeseed (Brassica rapa L.) (Malik and Waddington, 1988).

Termination of Sweetclover
In a preliminary experiment, disking, cultivating, mowing, and mowing plus removal as hay did not effectively terminate sweetclover growth when applied at the 10 to 20% bloom stage. In the present study, these sweetclover killing treatments were applied at the 60 to 80% bloom stage and, at this developmental stage, all treatments terminated sweetclover growth completely (data not shown). Foster (1990) reported that sweetclover was killed equally well using a moldboard plow, rotavator, tandem-disk, offset-disk, or deep-tillage cultivator from mid-June to mid-July in the second year of its growth.

Crop Residues for Erosion Protection
Crop residues remaining on the soil surface at the end of the fallow period (just before seeding spring wheat) were affected by the interaction of year x sweetclover x termination method of sweetclover (Table 4). Studies have demonstrated that 1000 to 1200 kg ha^-1 of plant residues on the soil surface are required to prevent erosion on a sandy clay loam soil (Anonymous, 1991; PFRA, 2000). At the end of the fallow period nonsweetclover plots had insufficient residues to prevent soil erosion (Table 5), regardless of whether plots were tilled or not. Blackshaw and Lindwall (1995b) reported that even when herbicides replaced tillage to control weeds during fallow it was often difficult to maintain sufficient plant residues, especially with oilseeds and pulses, which have less initial biomass and/or faster rates of stubble decomposition.

In contrast to the crop-only treatments, crop plus sweetclover treatments usually had considerably more surface plant residues than the 1000 to 1200 kg ha^-1 needed to prevent soil erosion (Table 5). The notable exception was the hay removal treatment in 1998, where only 770 kg ha^-1 of surface residues remained. Surface crop residues were lower in the disked and hay removal treatments than in the mowed or cultivated treatments in both 1995 and 1997. Nevertheless, residue amounts in the disked and hay removal treatments was about double the minimum amount required to prevent soil erosion. In all years, income could be attained during fallow by removing sweetclover as hay without leaving the soil susceptible to erosion.

Soil Water Content
Soil cores taken to a depth of 120 cm before seeding spring wheat indicated that soil water content was affected by the interaction of year and sweetclover (Table 4), being 1 to 2 percentage points lower in sweetclover compared with no sweetclover treatments during the fallow period in 2 of 3 yr (Fig. 1). However, the mean soil water content during 3 yr in the upper 15 cm of soil was 18 and 17%, respectively, for the sweetclover and no sweetclover treatments (data not shown). More surface residues in the sweetclover treatments may have facilitated greater snow trap and water infiltration, and less evaporation, than in the nonsweetclover treatments. A study in North Dakota found that tilled fallow conserved 45 mm more water than a green manure fallow where sweetclover was planted in April and killed in September (Badaruddin and Meyer, 1989). Termination date of sweetclover is an important factor affecting the soil water status for the succeeding crop.

View larger version (20K): [in this window] [in a new window]   Fig. 1. Percent soil moisture by weight in the top 120 cm of soil in April before seeding spring wheat.

Soil Nitrogen
Soil-available N determined at the end of the fallow period was affected by the interaction of year and sweetclover (Table 4). Sweetclover compared with nonsweetclover treatments contained an extra 56, 16, and 34 kg ha^-1 of N in the top 60 cm of soil before seeding spring wheat in 1995, 1997, and 1998, respectively (Fig. 2). A 3-yr study in Saskatchewan similarly reported that incorporated sweetclover residues increased the amount of available soil N at seeding of wheat the following spring by 26 to 75 kg ha^-1 (Foster, 1990). Spratt et al. (1975) reported that tilled fallow and sweetclover killed in July resulted in similar levels of available soil N at the time of seeding the succeeding spring wheat crop. In North Dakota, Badaruddin and Meyer (1990) reported that available soil N in April was 43 kg ha^-1 greater where sweetclover compared with wheat was grown the previous year. However, available N in their sweetclover plots was 30 kg ha^-1 less than that of tilled fallow. In that study, sweetclover growth was not terminated until late September, thus limiting the amount of time for N mineralization to occur before planting wheat in April.

View larger version (18K): [in this window] [in a new window]   Fig. 2. Available N in the top 60 cm of soil in April before seeding spring wheat.

Nitrogen fixation by annual legumes is limited if they are terminated after 6 to 8 wk of growth to avoid excessive soil water use. A study in northern Alberta found that the annual legumes Tangier flatpea, lentil, and alfalfa grown as green manure crops only fixed 16 kg ha^-1 or less of N (Rice et al., 1993). Biederbeck et al. (1996) reported N₂ fixation amounts in southern Saskatchewan of 18, 16, 49, and 40 kg ha^-1 for lentil, Tangier flatpea, chickling vetch, and field pea, respectively, when growth was terminated in late July. Another study reported that N₂ fixed by field pea, lentil, faba bean, Tangier flatpea, and annual alfalfa were only 40, 15, 40, 24, and 4 kg ha^-1, respectively (Townley-Smith et al., 1993).

Spring Wheat Yield
Wheat yield was affected by the interactions of year x sweetclover and sweetclover x termination method of sweetclover (Table 4). Wheat yields were higher after sweetclover than nonsweetclover treatments in all 3 yr (Fig. 3). Wheat yield increases in the sweetclover compared with nonsweetclover treatments were 47, 48, and 75% in 1995, 1997, and 1998, respectively. Meyer (1987) found that spring wheat yield was about 10% greater after green manure sweetclover than after tilled fallow, regardless of whether wheat was fertilized with 56 kg ha^-1 N or not. Spring wheat yields in Manitoba were similar following either tilled fallow or sweetclover harvested as hay the previous July (Spratt et al., 1975). Badaruddin and Meyer (1990) found that spring wheat yields were similar following tilled fallow and green manure sweetclover, but that 1000-kernel weight was greater in sweetclover than fallow treatments.

View larger version (20K): [in this window] [in a new window]   Fig. 3. Effect of sweetclover green manure and conventional fallow on succeeding spring wheat yield.

In the nonsweetclover treatments, wheat yields averaged over the 3 yr were greater in the disked and cultivated plots than in the mowed or hay removal plots (Fig. 4). This may be due to greater N mineralization in the tilled treatments and/or due to greater weed control attained in those plots (Blackshaw et al., 2001). Wheat yields in the sweetclover treatments were similar across all plant killing methods.

View larger version (21K): [in this window] [in a new window]   Fig. 4. Effect of sweetclover and termination method of sweetclover on succeeding spring wheat yield averaged over 3 yr.

	SUMMARY AND CONCLUSIONS

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

 
Biennial sweetclover exhibits many advantages over annual legumes as a green manure fallow replacement crop. The living crop provides ground cover to protect the soil from erosion during the first fall and winter and, after growth was terminated in late June, sufficient residues persisted for the remainder of the fallow period to reduce the risk of soil erosion.

Excessive depletion of soil water is often cited as the main reason for the reluctance to include legumes as green manures in cereal rotations (Brown, 1964; Zentner et al., 1996). However, in the current study, soil water available at the time of seeding spring wheat was not markedly lower in sweetclover than in the nonsweetclover treatments. Termination of sweetclover growth in June allows more time for soil water recharge than often occurs with annual legumes that are killed later in the growing season. It should be noted that precipitation received in May and June for each of the 3 yr of spring wheat production was considerably greater than the long-term mean (Table 1). Thus, in years of more normal rainfall, wheat yields may be more negatively affected by the initial lower soil water content in the sweetclover than in the fallow treatments.

Although N₂ fixation by sweetclover was not measured in this study, biennial sweetclover has the potential to fix more N₂ than many annual legumes especially if these green manure crops are terminated early in the growing season to avoid soil water depletion. Our study found that sweetclover, compared with tilled fallow treatments, had 16 to 56 kg ha^-1 more available N at seeding of the succeeding spring wheat crop. This contrasts with results of several studies on annual legumes where available N for the succeeding crop was similar or less in green manure than in tilled fallow treatments (Brandt, 1996; Pikul et al., 1997). Termination of sweetclover growth in June allows for increased time for N mineralization.

Spring wheat yields were 47 to 75% greater following green manure sweetclover than conventional fallow, at least partially attributable to the increased levels of available N. Green manure legume crops can supply the necessary N required for subsequent cereal crops without added fertilizer (Baldock and Musgrave, 1980; Griffin et al., 2000; Groya and Sheaffer, 1985).

The benefits of sweetclover in terms of soil erosion protection, N₂ fixation, and higher yields of succeeding wheat must be weighed against the lower companion crop yields often noted in the study. Further research is required to determine agronomic practices that may reduce the competitive effects of undersown sweetclover with companion crops and to assess the suitability of other companion crops that would be less affected by sweetclover.

	ACKNOWLEDGMENTS

 
This research was partially funded by the Alberta Agricultural Research Institute. We thank T. Entz for statistical advice and S. Torgunrud for preparing the graphs.

	NOTES

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

 
LRC Contribution no. 3870050.

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

 

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