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Published in Agron J 91:622-630 (1999)
© 1999 American Society of Agronomy
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Agronomy Journal 91:622-630 (1999)
© 1999 American Society of Agronomy

SOIL MANAGEMENT

Plant-Available Nitrogen Supply as Affected by Method and Timing of Alfalfa Termination

Ramona M. Mohra, Martin H. Entzb, H.Henry Janzenc and William J. Bulliedb

a Agric. & Agri-Food Canada, Brandon Res. Ctr., Box 1000A, R.R. #3, Brandon, MB, Canada R7A 5Y3
b Plant Science Dep., Univ. of Manitoba, Winnipeg, MB, Canada R3T 2N2
c Agric. & Agri-Food Canada, Lethbridge Res. Ctr., Box 3000 Main, Lethbridge, AB, Canada T1J 4B1

rmohr{at}em.agr.ca

Received for publication April 22, 1997.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 Summary and conclusions
 REFERENCES
 
Herbicide application may provide an alternative to intensive tillage for the termination of alfalfa stands, but might alter N release and N availability to subsequent crops. Our objective was to determine, under field conditions, the effect of timing and method of termination on the pattern of N release from perennial alfalfa, and on N uptake and yield of subsequent wheat crops. Four field experiments were initiated on perennial alfalfa (Medicago sativa L.) in southern Manitoba in 1992 and 1993. A factorial of three methods (herbicide, tillage, herbicide + tillage) and two times of termination (early summer, after first alfalfa cut, and late summer, after second alfalfa cut) was arranged in a randomized complete block design. A spring-applied herbicide treatment was also included. Spring wheat (Triticum aestivum L.) was established after alfalfa termination. Soil NO-3 content, plant N uptake, and yield were then monitored for one to two years. In three of four experiments, plant-available N in the spring after termination was higher in tilled treatments than in treatments receiving only herbicides. Regardless of method, plant-available N in the spring after termination was reduced when termination was delayed from early to late season. Despite the lower short-term plant-available N supply in early- and late-summer herbicide treatments, wheat yields in herbicide treatments were similar to or greater than those in tillage treatments. Differences in the N content among treatments diminished with time; by the fall of the second growing season after termination, differences in the cumulative available N supply were no longer evident. These results suggest that termination of alfalfa with herbicides may improve the synchrony between N release and N demand of a subsequent spring wheat crop, thereby improving N use efficiency.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 Summary and conclusions
 REFERENCES
 
CONCERNS regarding agricultural sustainability, soil and environmental quality, and energy conservation have renewed interest in the use of legumes in cropping systems. Legumes provide substantial amounts of plant-available N to subsequent crops, and their presence may reduce weed populations, reduce the incidence and/or severity of plant disease, and improve soil physical conditions (Mazurak et al., 1955; Toogood and Lynch, 1959; Harvey and McNevin, 1990; Osunlaja, 1990; Zentner et al., 1990; Entz et al., 1995).

One strategy for increasing the proportion of arable land deriving benefits from legumes, without increasing the total legume acreage, is to reduce the duration of alfalfa stands. Currently, the average duration of pure alfalfa or alfalfa–grass stands in Manitoba and Saskatchewan is 6.5 years (Entz et al., 1995). This exceeds the economic optimum of 4 to 5 years reported by Jeffrey et al. (1993) and the duration required to attain maximum N benefits (Hoyt and Hennig, 1971; Heichel et al., 1984). However, successful adoption of shorter-term alfalfa stands requires management practices that effectively terminate alfalfa, but maximize the benefits provided to subsequent crops.

Intensive tillage, the usual method of terminating alfalfa, leaves the soil prone to erosion and moisture loss. An alternative is to apply herbicides, either in the previous summer or in the spring immediately before establishment of a subsequent crop, and leave residue standing on the soil surface. Broad-spectrum herbicides allow reliable, cost-effective termination of alfalfa, and simultaneously control weeds. In addition, standing surface residues reduce soil erosion and moisture loss, and may increase soil moisture by trapping snow (Bullied and Entz, 1999).

Although using herbicides in place of tillage has many apparent benefits, herbicide termination might affect the pattern of N release from alfalfa residue and the availability of this N to subsequent crops. If the rate of N mineralization is too slow, crop yield and quality may be adversely affected (Huntington et al., 1985; Westermann and Crothers, 1993). Conversely, if alternative termination techniques accelerate N release, accumulations of excess inorganic N may be prone to leaching or denitrification (Robbins and Carter, 1980; Firestone, 1982). Leaching losses of legume-derived N may occur even under dryland conditions in semiarid areas (Campbell et al., 1984, 1994).

Few studies have directly compared N release from alfalfa terminated by chemical means with that from tilled alfalfa. Under controlled conditions, incorporation of alfalfa residues resulted in more rapid N release from alfalfa residues and a larger short-term supply of plant-available N than where residues were retained on the soil surface (Mohr et al., 1998a, 1998c). In field experiments, termination of a green manure crop by herbicide application resulted in a 22% lower grain yield in a subsequent wheat crop than termination by tillage (Biederbeck and Slinkard, 1988). The observed yield reduction was attributed to delayed decomposition of herbicide-treated residues, which may have reduced the available nutrient supply. In contrast, the response to N fertilizer by corn (Zea mays L.) established after alfalfa was not affected by tillage (Triplett et al., 1979; Levin et al., 1987).

Our objective was to determine the effect of timing and method of termination on the pattern of N release from perennial alfalfa under field conditions, and on N uptake and yield of subsequent wheat crops. This was accomplished by measuring soil NO-3 accumulation, soil mineralizable-N content, and N uptake by subsequent spring wheat crops for up to two years after alfalfa termination.


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 Summary and conclusions
 REFERENCES
 
Four field experiments were conducted in southern Manitoba from 1992 to 1994 (Tables 1 and 2) . Experiments were initiated in 1992 on alfalfa stands at Glenlea (Glenlea-92) and Carman (Carman-92) and were monitored during the 1993 and 1994 growing seasons. Two additional experiments were initiated in 1993 on alfalfa stands at Winnipeg (Winnipeg-93) and Carman (Carman-93) and were monitored from time of establishment through the 1994 growing season. Uniform, pure stands of alfalfa were used at Carman (cv. Arrow), Glenlea-92 (cv. Beaver), and Winnipeg-93 (cv. OAC Minto). All were hay-type cultivars, developed for maximum yield for hay production. At termination, alfalfa stands were two (Carman-92), three (Carman-93, Winnipeg-93), or six (Glenlea-92) years old. Soil physical and chemical characteristics for each field site (Table 1) were determined on a composite sample of soil collected in the spring after alfalfa termination.


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  		Table 1 Physical and chemical characteristics of soils (0–15 cm depth) at Manitoba field sites established in 1992 and 1993

 

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  		Table 2 Mean air temperature and cumulative precipitation for selected Manitoba field sites

 
Treatments consisted of a factorial combination of three termination methods (herbicide application, tillage, or herbicide application followed by tillage) applied at two times during the growing season (early summer, after the first alfalfa cut, or late summer, after the second alfalfa cut). In an additional treatment, herbicide was applied in spring immediately prior to establishment of a spring wheat crop. All seven termination treatments were included in experiments initiated in 1992, but at Carman-93, herbicide + tillage treatments were omitted, and at Winnipeg-93, only late-season herbicide and late-season tillage treatments were included. In all experiments, treatments were arranged in a randomized complete block design with four replicates and a plot size of 5.5 by 16 m (Winnipeg-93) or 12 by 12 m (all other sites).

Field Techniques
At all sites, the first alfalfa harvest was in mid-June to early July, and the second harvest was in late July to mid-August. For both harvests, alfalfa was cut while in bloom, baled, and removed from the plot area.

In plots designated for early termination, treatments were applied approximately 3 to 4 wk after the first alfalfa cut, once alfalfa had regrown sufficiently to allow herbicide termination. Alfalfa in plots designated for late termination was allowed to regrow after the first cut, and treatments were applied approximately 3 to 4 wk after the second alfalfa cut. Based on random square-meter samples harvested inside the plot area before termination, the amount of top growth (dry matter basis) present within several days of termination averaged about 2 Mg ha-1 in experiments established in 1993. Alfalfa dry matter yields were not determined before termination in experiments established in 1992.

In 1992, herbicide treatments consisted of one application of 5 L ha-1 glyphosate [N-(phosphonomethyl)glycine] (late-season treatment) or two consecutive glyphosate applications, at rates of 5 and 2.5 L ha-1, respectively (early-season treatment). In 1993, a tank-mix of 1.85 L ha-1 glyphosate, 1.25 L ha-1 dicamba (3,6-dichloro-2-methoxybenzoic acid) and 1 L ha-1 2,4-D [(2,4-dichlorophenoxy)acetic acid] was applied once (late-season treatment at Carman-93) or twice (early-season termination at Carman-93; late-season termination at Winnipeg-93). Early-season tillage treatments consisted of repeated tillage during the growing season using a combination of chisel plough (four passes in 1992, two in 1993), tandem disk (two passes in 1992, four in 1993), and harrows (one pass in 1992 and 1993). Late-summer tillage treatments consisted of a combination of chisel plough (one pass in 1992, two in 1993), tandem disk (three passes in 1992, four in 1993), and harrows (one pass in 1992, two in 1993). The exception was Winnipeg-93, where tillage treatments were rototilled three times during the year of termination. Herbicide + tillage treatments consisted of an initial application of 2.5 L ha-1 glyphosate, followed by tillage with a tandem disk (two passes) and harrows (one pass).

At all sites, spring wheat (cv. Katepwa) was seeded with a zero-till offset disk drill during the first growing season after alfalfa termination. In herbicide treatments, wheat was seeded directly into untilled soil. Tillage and herbicide + tillage treatments were cultivated, harrowed, and packed in the spring before seeding, except at Winnipeg-93, where wheat was seeded directly without spring tillage. After harvest, wheat straw was baled and removed, but soil was not tilled. At sites initiated in 1992, a second Katepwa wheat crop was established in 1994, by seeding directly into untilled soil.

Triple superphosphate at a rate of 13 kg P ha-1 was applied in the seedrow in all years. Where required, based on soil testing, S and K were broadcast in the spring as K2SO4. No N fertilizers were applied.

Sampling Procedures and Analytical Techniques
Soil samples to a depth of 60 or 120 cm (in increments of 0–15, 15–30, 30–60, 60–90, and 90–120 cm) were collected periodically throughout the study. Each year, samples (to 120 cm) were collected in the spring shortly before or immediately after seeding, and in the fall after wheat harvest. During the first growing season of wheat, soil samples (to 60 cm) were collected about 6 wk after crop emergence. Soil samples consisted of a composite of three to six cores from each plot.

Soil samples were usually air-dried at room temperature immediately after collection; in a few cases, samples were frozen briefly before air drying. Air-dry soil samples were ground (<2 mm) using a rotating sieve. Plant residues that did not pass through the sieve were discarded. For the spring sampling, soil samples were weighed prior to grinding, to determine bulk density.

Soil inorganic N was extracted with 2 M KCl, and the concentration of NO-3 and NH+4 in the extract was determined by an automated colorimetric procedure (Keeney and Nelson, 1982). Mineralizable N was determined for surface soil samples (0–15 cm) collected during the first spring after alfalfa termination with a procedure similar to that described by Bremer et al. (1994). A 50-g subsample of soil was moistened to about 80% of field capacity and placed in a sealed 1-L glass jar containing a CO2 trap (10 mL of 2 M NaOH). Samples were incubated at 25°C for 6 wk. Jars were aerated and the NaOH traps replaced at 1, 2, 4, and 6 wk. At the end of the incubation period, soil samples were air-dried and analyzed for inorganic N as described previously (Keeney and Nelson, 1982) to estimate mineralizable N.

Plant tissue samples were taken at about 2-wk intervals throughout the growing season beginning approximately 3 wk after seeding the initial wheat crop. The second wheat crop established after alfalfa termination was sampled only at the soft dough stage and at crop maturity. At all sites, plant tissue samples consisted of wheat top growth from a 1-m length of six adjacent rows ({approx}0.9 m2); wheat was cut approximately 2 cm above the soil surface and immediately air-dried using a forced-air system held at room temperature.

To determine grain yields at crop maturity, wheat was harvested with a small plot combine from a 10- to 19-m2 area within each plot. To determine straw yields, wheat was hand-harvested from 1-m lengths of six adjacent rows as described previously. In 1993, straw yields were estimated from the difference between the total dry matter yield of hand-harvested samples and the grain yield of combine-harvested samples. In 1994, hand-harvested samples were threshed and the straw yield measured directly. Several plots at Glenlea-92 could not be harvested by combine due to uneven crop growth resulting from poor emergence; 1994 grain yields were therefore based on the grain yield of hand-harvested samples taken from areas with good emergence.

Air-dried plant samples were oven dried (65°C), weighed, and ground to pass a 2-mm sieve using a Wiley mill. A subsample of this coarsely ground plant material was more finely ground with a Cyclone sample mill (Udy Corp., Fort Collins, CO), and analyzed for total N by an automated combustion technique (Carlo Erba, Milan, Italy).

Data were subjected to an analysis of variance for a 3 x 2 (Glenlea-92; Carman-92) or 2 x 2 (Carman-93) factorial experiment arranged in a randomized complete block design using the Proc GLM procedure (SAS Inst., 1985) in order to determine the effect of two factors: termination method and time of termination. A least significant difference procedure was also conducted to determine the effect of treatment using the PROC GLM procedure. To determine the effect of spring-applied herbicide treatments, data were reanalyzed by two-way analysis of variance for a randomized complete block design and by single degree of freedom contrasts using the Proc GLM procedure. Probability levels of P <= 0.05 were considered statistically significant.


   Results and discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
Summary and conclusions
 REFERENCES
 
Soil Nitrate Distribution
High concentrations of NO-3 were evident in the soil profile, particularly in surface horizons, in the spring after alfalfa termination (Fig. 1) . At all sites except Winnipeg-93 (data not shown), both timing and method of alfalfa termination significantly influenced soil NO-3 concentration.



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  		Fig. 1 Soil NO-3–N concentrations at Glenlea-92, Carman-92, and Carman-93 during the first spring after alfalfa termination as influenced by method and time of termination. Significant effects of method (M), time (T), and method x time interactions (MT) are shown. *,**,*** Significant at the 0.05, 0.01, and 0.001 probability levels, respectively

 
Tillage increased the NO-3 concentration in surface soils (0–30 cm) in the first spring after alfalfa termination, but had variable effects on subsurface soil NO-3 concentrations (Fig. 1). At Carman-92, no effects of termination method on soil NO-3 concentrations were evident below the 30-cm depth. However, in the coarser-textured soil at Carman-93, tillage increased NO-3 concentrations to a depth of 120 cm, suggesting that downward movement of NO-3 may have occurred. In contrast, in subsurface soils at Glenlea-92 (30–90 cm), early-season herbicide application resulted in a slightly higher soil NO-3 concentrations than tillage. One possible explanation is that larger fall soil moisture reserves (Bullied and Entz, 1999) combined with biopores in the untilled herbicide system promoted downward movement of water and NO-3 through the fine-textured soil (Blackwell et al., 1990). Regardless of method, a delay in termination often reduced soil NO-3 concentrations.

Soil Nitrate Accumulations
Tillage, alone or in combination with herbicide application, increased total profile NO-3 accumulations in the first spring after alfalfa termination. Both at Carman-92 and at Glenlea-92, tillage resulted in soil NO-3 contents more than 20 kg NO-3–N ha-1 greater than that in herbicide treatments (Fig. 2) . The largest differences among termination methods was at Carman-93 (Fig. 2), where spring soil NO-3 accumulations were 62 and 84 kg ha-1 in late and early herbicide treatments, respectively, compared with 125 and 196 kg ha-1 in late and early tillage treatments. These substantial differences among termination methods were already evident at Carman-93 in the previous fall.



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  		Fig. 2 Soil NO3–N accumulations at Glenlea-92, Carman-92, and Carman-93 following alfalfa termination as influenced by method and time of termination. Samples were collected in May, June to July, and September to early October in 1993, and in May and August 1994. Significant effects of method (M), time (T), and method x time interactions (MT) are shown. *,**,*** Significant at the 0.05, 0.01, and 0.001 probability levels, respectively. For Glenlea-92, the LSD for the effect of method is 9, {approx}8, and {approx}11 for spring 1993, mid-1993, and spring 1994 samplings, respectively. For Carman-92, the LSD for the effect of method is 21 for the spring 1993 sampling and 5 for the mid-1993 sampling

 
Similar results have been reported for green manure crops: accumulations of soil inorganic N were lower under winter green manure crops terminated by herbicide application than in those terminated by tillage (Sarrantonio and Scott, 1988). Greater accumulations of inorganic N under the tilled system may be due to higher soil temperatures (Mitchell and Teel, 1977) and or improved soil moisture which may promote mineralization (Sarrantonio and Scott, 1988). In part, our observations may simply reflect greater N release from incorporated than from surficial residues (McKay et al., 1952; Wilson and Hargrove, 1986; Varco et al., 1993). Results of controlled-environment studies suggest that differences in N release under herbicide-terminated and tillage-terminated systems are primarily a function of residue placement, rather than of termination method per se (Mohr et al., 1998a, 1998c); N mineralization of soil incorporated residues is enhanced with greater exposure of residues to soil microbial populations (Cogle et al., 1987). In addition, incorporation may reduce volatile N losses from alfalfa residues, and this may contribute somewhat to a larger available N supply (Janzen and McGinn, 1991; Mohr et al., 1998b).

Regardless of termination method, delaying termination until after the second alfalfa harvest substantially reduced soil NO-3 accumulations measured the next spring. The same trend was evident for early and late herbicide treatments at Carman-93 although differences were not statistically significant. Presumably, lower soil NO-3 accumulations in the delayed treatments result from the shorter decomposition period. Differences between early and late termination by the same method ranged from 13 kg NO-3–N ha-1 (herbicide treatments at Glenlea-92) to 71 kg NO-3–N ha-1 (herbicide treatments at Carman-92, tillage treatments at Carman-93).

The effect of further delaying herbicide application until the next spring on spring soil NO-3 accumulations was measured only at Carman-93. At that site, delaying herbicide termination until spring decreased soil NO-3 relative to herbicide termination early in the previous season, but produced soil NO-3 contents similar to herbicide termination late in the previous season (Fig. 2). The differences in soil NO-3 accumulations observed among treatments were still evident at midsummer, but N uptake by the growing wheat crop had depleted soil NO-3 reserves at all sites (Fig. 2).

Differences among termination treatments diminished with time. In both experiments at Carman, effects of termination treatment on soil NO-3 accumulations were no longer evident after harvest of the initial wheat crop. In contrast, at Glenlea-92, effects of both time and method of termination persisted until the second spring after alfalfa termination, perhaps because of different soil or environmental conditions. Even at Glenlea-92, however, effects of termination treatment on soil NO-3 accumulations were no longer evident after harvest of the second wheat crop (Fig. 2).

At Winnipeg-93, soil NO-3 accumulations were comparable to those observed at other sites and followed similar trends over time (data not shown). However, termination method had no effect on soil NO-3 accumulations, perhaps because later termination (second week in September) and cool conditions delayed mineralization in both treatments.

Mineralizable Nitrogen
The mineralizable N content of surface soils in the first spring after alfalfa termination, as measured in a laboratory incubation, was not influenced by time or method of alfalfa termination at most sites (Table 3) . At Carman-93, however, early termination decreased the mineralizable N content in tilled treatments.


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  		Table 3 Mineralizable N concentration, based on a laboratory incubation procedure, in surface soil (0–15 cm) in the spring following alfalfa termination

 
Although laboratory-determined mineralizable N revealed minimal differences among termination treatments, net N mineralization in the field during the growing season (measured as the change in available N between spring and fall) varied considerably among treatments (Table 4) . For example, at Glenlea-92, the amount of N mineralized during the 1993 growing season was highest in herbicide treatments, intermediate in tillage treatments, and lowest in herbicide + tillage treatments. At Carman-92, the amount of N mineralized during the 1993 growing season was higher in late-terminated than early-terminated treatments regardless of termination method. At Carman-93, mineralized N during the 1994 growing season was higher in herbicide than tilled treatments, but was not affected by termination time. Therefore, treatments in which N mineralization was initially delayed, such as the herbicide and late-terminated treatments, often had higher rates of N mineralization during the first growing season following alfalfa termination.


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  		Table 4 Plant-available N mineralized in the 0- to 60-cm soil profile (calculated as the difference in the available N supply between spring and fall of a growing season) following alfalfa termination as influenced by method and time of termination

 
Differences between field and laboratory determinations suggest that most treatment effects were related to residue decomposition and not to differences in soil mineralizable N. Laboratory incubations (which did not include N mineralized from alfalfa residues) showed minimal treatment effects; in the field, where residues were present, differences were much larger. Results of a subsequent 15N study also showed no difference in the mineralizable N content of herbicide and tillage treatments following alfalfa termination when residues were excluded (Mohr et al., 1998a).

Available Nitrogen Supply
Substantial amounts of plant available N (calculated as the sum of soil NO-3 accumulations to 60 cm and N removed in the grain and straw of subsequent wheat crops) were released following alfalfa termination (Fig. 3) . By the fall of the first growing season after alfalfa termination, average plant-available N release at the four sites ranged from 107 to 211 kg N ha-1 (Fig. 3). Cumulative plant-available N by the end of the second growing season was as high as 256 kg N ha-1 (Fig. 3). These values are in agreement with those reported in the literature, which range from 20 to 383 kg N ha-1 (Baldock and Musgrave, 1980; Boawn et al., 1963; Bowren et al., 1969; Groya and Sheaffer, 1985; Hesterman et al., 1986; Westcott et al., 1995).



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  		Fig. 3 Cumulative available N supply (calculated as the sum of N present as soil NO-3–N to 60 cm and N removed in the grain and straw of wheat) at Glenlea-92, Carman-92, and Carman-93 following the termination of perennial alfalfa stands as influenced by method and time of termination. Significant effects of method (M), time (T), and method x time interactions (MT) are shown. *,**,*** Significant at the 0.05, 0.01, and 0.001 probability levels, respectively. For Glenlea-92, the LSD for the effect of method is 9, {approx}14, 17, and {approx}17 for spring 1993, mid-1993, fall 1993, and spring 1994 samplings, respectively. For Carman-92, the LSD for the effect of method is 21 for the spring 1993 sampling and 9 for the mid-1993 sampling

 
Effects of method and time of termination on available N were particularly pronounced shortly after alfalfa termination (Fig. 3). In the first spring after alfalfa termination (1993), tillage produced more plant-available N than herbicide application, and, regardless of termination method, a delay in termination until after the second alfalfa cut reduced plant-available N. At Glenlea-92, the effect of treatment was evident throughout the 1993 growing season. At Carman-92, however, differences among treatments diminished rapidly and were no longer evident by the fall of 1993.

The effect of termination treatments appeared to persist when N release was delayed. At Carman-92, available N increased rapidly until the fall of 1993, then increased slowly. At Glenlea-92, however, available N increased slowly and steadily throughout 1993 and 1994. More rapid N release at Carman-92 may have been due, in part, to higher growing-season temperatures in 1993 (Table 2). The combination of cooler air temperatures and a moist, fine-textured soil at Glenlea-92 may have resulted in cooler soil temperatures, which are less conducive to N mineralization.

Termination treatments at Carman-93 also had a strong influence on the short-term plant-available N supply, but this effect diminished with time (Fig. 3). By the fall of 1994, differences between methods were no longer evident, but the available N supply remained higher in early-terminated treatments. For herbicide treatments, a further delay in termination until the next spring reduced the available N supply by 65 and 56 kg N ha-1, compared with termination early and late in the preceding year. At Winnipeg-93, termination method had no effect on the available N supply.

Nitrogen Uptake by Wheat
For all treatments, total N uptake by wheat increased rapidly until approximately 30 d before crop maturity, then increased at a markedly slower rate or declined (data not shown). Total N uptake by wheat at crop maturity was not consistently related to spring soil NO-3 content; only at Carman-92, in 1994, was there a significant relationship (R2 = 0.51).

At all sites except Winnipeg-93, N concentrations in wheat top growth prior to filling were sufficient (20–30 g kg-1) for maximum yield according to guidelines developed for Manitoba (Manitoba Provincial Soil Testing Laboratory, 1982, unpublished data). At Winnipeg-93, N concentrations in wheat tissue were marginal (15–20 g kg-1), but no visible symptoms of N deficiency were evident.

Method and time of termination did not have a consistent effect on N uptake by the first wheat crop established after alfalfa. At Glenlea-92, N uptake by wheat in early-terminated treatments were the same for all methods, but in late-terminated treatments, they were smaller in herbicide and herbicide + tillage treatments than in tillage treatments (Fig. 4) . At crop maturity, effects of delaying termination were no longer evident; however, N uptake by wheat remained higher in tillage than in herbicide + tillage treatments.



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  		Fig. 4 Nitrogen uptake by wheat at Glenlea-92, Carman-92, and Carman-93 during the initial growing season (midseason and crop maturity) following alfalfa termination by various termination treatments. Significant effects of method (M), time (T), and method x time interactions (MT) are shown. *,**,*** Significant at the 0.05, 0.01, and 0.001 probability levels, respectively. For Glenlea-92 at crop maturity, the LSD for the effect of method is 15

 
In contrast, termination method had no effect on N uptake by wheat at Carman-92, Carman-93, or Winnipeg-93. However, delaying termination reduced N uptake by wheat at midseason at Carman-92 and at crop maturity at Carman-93. At both Carman sites and at Glenlea-92, further delaying herbicide application until the following spring reduced N uptake by wheat both at midseason and at crop maturity.

Grain Yield and Nitrogen Concentration
Grain yield of the initial wheat crop established after alfalfa termination varied considerably among termination treatments at sites established in 1992. At Glenlea-92, yields in herbicide and tilled treatments were similar, despite lower spring soil NO-3 in herbicide treatments; however, herbicide + tillage treatments produced lower yields (Table 5) . At Carman-92, yields were highest in herbicide treatments, intermediate in herbicide + tillage treatments, and lowest in tillage treatments, although spring soil NO-3 was 28 kg ha-1 higher in tilled than in herbicide treatments (Table 5). Higher yields in herbicide than in tillage treatments cannot be attributed to differences in N nutrition because wheat accumulated similar amounts of N regardless of termination method (Fig. 4).


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  		Table 5 Grain yield and grain N concentration of spring wheat crops established during the first growing season following alfalfa termination

 
Delaying termination until after the second alfalfa harvest did not affect grain yield at Carman-92 and Glenlea-92 (Table 5), but a further delay in herbicide application until the next spring reduced yields at both sites, perhaps because of reduced N uptake (Fig. 4). Termination treatments had no effect on yields at Glenlea-92 or Carman-92 during the second growing season (1994) after alfalfa termination (data not shown).

Although termination method strongly influenced yield at Glenlea-92 and Carman-92, it had no effect on the yield of wheat at Carman-93 (Table 5) or Winnipeg-93 (data not shown). At Carman-93, however, a delay in termination until after the second alfalfa cut decreased yield. Yields were reduced even more by delaying herbicide application until the next spring. These yield reductions may be attributable in part to low N availability, as evident from reduced N uptake in these treatments (Fig. 4).

Besides N availability, a second factor that may influence the yield of crops following alfalfa is the availability of soil moisture. Results of a concurrent study assessing the impact of method and time of alfalfa termination on available soil moisture showed increased surface (0–30 cm) soil moisture in herbicide treatments in the spring following alfalfa termination at all sites except Carman-92; time of termination appeared to have relatively minor effects on surface soil moisture (Bullied and Entz, 1999). However, at all sites, precipitation during the first growing season after alfalfa termination was sufficient to allow wheat establishment and growth (Carman-92: 457 mm; Glenlea-92: 520 mm; Carman-93: 243 mm; Winnipeg-93: 443 mm). Moreover, in some cases, high levels of growing season precipitation may have masked early-season soil moisture differences among treatments, and thus potential yield differences.

Previous studies have also shown an effect of termination technique on yield of subsequent wheat crops. In field studies in Saskatchewan, the grain yield of wheat following a legume green manure was 22% less when green manure was terminated by herbicide application instead of tillage (Biederbeck and Slinkard, 1988).

Termination treatment affected grain N concentration at two sites (Table 5). At Carman-92 in 1993, the grain N concentration was higher in tillage treatments than in herbicide treatments and, regardless of termination method, the grain N concentration was significantly higher in early-termination treatments. At Glenlea-92, in 1994, a delay in herbicide application until the spring immediately before wheat establishment resulted in a higher grain N concentration than herbicide termination the previous growing season (data not shown). Although termination technique influenced grain N concentration in some cases, observed differences among treatments were usually equivalent to <1% protein, and therefore may not be highly significant agronomically.


   Summary and conclusions
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Summary and conclusions
REFERENCES
 
Method and timing of termination strongly affected the pattern of N release from alfalfa residues. Compared with intensive tillage, which has traditionally been used to terminate established alfalfa, herbicide application without tillage delayed N release from alfalfa residues and reduced the short-term plant-available N supply. Under the conditions of this experiment, delayed N release from the residues of herbicide-terminated alfalfa appeared to improve the synchrony between N release from alfalfa residues and N needs of a subsequent spring wheat crop. Although herbicide treatments had less spring soil NO-3 than tilled treatments, mineralization during the growing season appeared to provide enough N that grain yield was not compromised.

Although a delay in alfalfa termination until after the second alfalfa cut generally reduced soil NO-3 accumulations, grain yields were reduced at only one site. However, further delaying herbicide application until the spring immediately prior to wheat establishment delayed N release from alfalfa residues and significantly reduced grain yields in 3 of 5 site-years.

At all sites, the effects of method and time of alfalfa termination on N dynamics and grain production were greatest in the initial year following alfalfa termination. By the second growing season following alfalfa termination (i.e., the second wheat crop), differences among termination treatments had largely disappeared, although significant amounts of plant-available N were present.

In summary, these results demonstrate that herbicide application may be a viable alternative to tillage for the termination of established alfalfa stands. Herbicide application allowed sufficient mineralization to meet the N needs of a subsequent wheat crop, but reduced the potential for N losses by reducing the size of the soil inorganic N pool. Although the suitability of this practice may depend on soil and environmental conditions, herbicide application may improve synchrony between N release from alfalfa residues and N needs of subsequent spring wheat, thereby improving N use efficiency and reducing N losses.SAS Institute 1985


   ACKNOWLEDGMENTS
 
We thank Clarence Gilbertson, Mary-Lynn Muhly, and Keith Bamford for technical assistance. Partial funding for this project, in the form of a fellowship for R.M. Mohr, was provided by the Canadian Wheat Board.


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




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