We conducted an on-farm study with north-central Montana no-till wheat producers using legume green manures (LGM) as a summer fallow alternative. Soil water, nitrogen and wheat yield response following LGM and fallow treatment were evaluated, and collaborating producers were interviewed. Wheat yields following LGM treatment were less than following fallow, as was grain protein when wheat was fertilized with N; LGMs conversely increased grain protein when wheat was unfertilized. Wheat yield depressions were attributed to LGM’s temporarily limiting soil N available to subsequent wheat. Producers interviewed noted that seed costs and lack of immediate wheat yield benefits were formidable challenges to LGM adoption.
Summer fallow practice has been a common management strategy employed over the past century to store soil water and stabilize wheat yields in dryland northern Great Plains (NGP) cropping systems. Summer fallow serves this purpose well, but it also can precipitate net declines in soil organic matter, saline seeps (Nielsen and Calderon, 2011) and nitrate leaching (Campbell et al., 2006). While summer fallow acreage has steadily declined since the early 1970s in much of the northern Great Plains (Carlyle, 1997; Tanaka et al., 2010), north-central Montana represents a region where summer fallow practice remains steadfast, with ~80% of cropland remaining in a summer fallow–wheat rotation (USDA-FSA, 2010). Half-season legume green fallow crops (legume green manure, hay or forage) used to replace summer fallow may be a water-conservative way to intensify north-central Montana agroecosystems (Miller et al., 2006) while concomitantly reducing N fertilizer dependence and increasing soil and water conservation. Past plot-scale studies established that green fallow crops can threaten soil water available to subsequent wheat (Army and Hide, 1959; Brown, 1964; Biederbeck and Bouman, 1994; Aase et al., 1996; Brandt, 1996; Zentner et al., 1996), but that this risk is reduced if green fallow crops are terminated early before full bloom stage and no-tillage practices are used (Zentner et al., 2004; Miller et al., 2006). Nonetheless, legume green fallow crop adoption is currently negligible in north-central Montana.
We conducted a participatory research project with five no-till wheat producers trialing legume green manures (LGMs) as a summer fallow replacement in north-central Montana. This study has an original focus on treatments exclusively using no-till management, early legume termination and participatory research at the field-scale. Our objectives were to assess the agronomic viability of using no-till, early-terminated LGMs to replace summer fallow under farmer management and at the field-scale, and to better understand LGM adoption from the producer’s perspective. Soil water use, soil N levels and wheat yield response following LGM and summer fallow treatments were assessed. Producers were interviewed upon completion of the project to elucidate potential challenges to replacing summer fallow with a LGM crop.
1) Quantify the effects of LGMs on soil N, soil water use and the effect they have on the following wheat crop compared to summer fallow.
2) Assess how no-till management and early termination may affect LGM management.
3) Assess LGM management over regionally varied sites and management practices by different producers.
4) Conduct a marginal economic assessment of LGM management compared to standard summer fallow under no-till management.
5) Assess producer initiative to continue LGM management based on results of the study.
We initially recruited six producers based in north-central Montana for this project with the help of the Great Falls, MT NRCS office. Each producer provided one experimental field site. One producer’s site was omitted from the final assessment due to soil-residual herbicide that restricted LGM growth. Therefore, five producers and their field sites were included in the final assessment, referred to as Big Sandy, Box Elder, Joplin, Sunburst and Oilmont, based on proximity to locations in north-central Montana.
Collaborating producers were required to:
1) plant an annual legume green manure (pea or lentil),
2) maintain an adjacent summer fallow control treatment,
3) terminate their legume green manure at first flower stage,
4) use no-till practices exclusively,
5) provide researchers with their management information, and
6) to communicate and coordinate with researchers for timely data collection.
All other management decisions (crop variety, row spacing, fertilizer levels etc.) were delegated to producers. Producers established treatments at field sites as a summer fallow strip within a LGM field or a LGM strip within a fallow field; treatment transects ranged from ~550 to 1300 m in length. A replicated measurement paired t-test experimental design was used, with 6 to 12 paired sampling points per site, depending on the length of the treatment transects. Two producers also elected to add alternative treatments of legumes as summer fallow replacements; at one site, a pea crop was hayed and at another site a pea crop was 1) taken to seed and 2) a pea green manure was foraged by grazing cattle. Results of these additional treatments are presented separately from the LGM vs. summer fallow results. Details of site-specific characteristics are provided in Table 1, crop management details are provided in Table 2, and site precipitation and temperature data are provided in Table 3. Field site examples during the LGM and wheat phases are illustrated in Figures 1a and b, respectively.
Soil water and nitrate levels were determined to 0.9 m (in 0.3 m segments) at LGM termination and at wheat seeding. Soil potentially mineralizable nitrogen (PMN) levels were determined to 0.3 m at wheat seeding. One-m2 biomass samples were taken from each sampling point and used to determine LGM and subsequent wheat yield measurements. Soil water levels were determined by measuring gravimetric water content and converting to units of Equivalent Depth (mm) for reporting. Soil nitrate levels were determined by KCl extraction and Cd reduction with a flow injection analyzer. PMN levels were determined by ammonium mineralized from soils during a 7-d anaerobic incubation at 40° C.
Results were statistically analyzed both by site and across all sites. Paired t-tests were used to determine treatment effects at individual sites. A mixed-effects model was used to determine mean treatment effects across all sites, where treatment was considered a fixed effect and site and paired sample point nested within site was considered a random effect. Treatment effects were considered significant at P ? 0.10.
- Table 1. Orientation, soil, and experimental characteristics of five experimental sites in north-central! Montana.
- Table 2. Crop management details and sampling dates for five experimental sites in north-central Montana.
- Table 3. Precipitation and average temperatures for five experimental sites in north-central Montana, from April 2009 to August 2010.
- Figure 1. Example of field site during LGM stage (a) and subsequent wheat stage (b), and no-till LGM residue breakdown stages in September, 2009 (c) and the following August, 2010 (d).
Wheat yields following LGM treatments were generally diminished compared to wheat following summer fallow (Table 4). At individual sites, treatment effects on yield were not detected; across all sites though, a mean yield depression of 0.36 Mg ha-1 (3.7 bu ac-1) was detected (P = 0.04) from LGM treatment compared to fallow. Grain protein was not uniformly affected by LGM treatment, but instead depended on the producer’s decision to fertilize their wheat crop with N or not. When wheat was fertilized with N (at 3 of 5 sites), LGM treatment caused a mean protein depression of 9 g kg-1 compared to fallow (P = 0.01). When wheat was unfertilized (at 2 of 5 sites), LGMs caused a mean 5 g kg-1 increase in grain protein (P = 0.08).
Contrary to most conclusions of past studies, we theorize that wheat yield depressions in LGM treatments compared to fallow were not caused by soil water availability being limited to subsequent wheat. Soil water use by LGMs appeared to be limited by first-flower stage LGM termination. Soil water levels to 0.9 m following LGMs in this study ranged from 29 to 46 mm less than fallow at LGM termination, and from 0 to 39 mm at wheat seeding (Table 5). Most soil water depletion was limited to the 0.6 m depth. Soil water depletion in LGM compared to fallow in our study was below the 50 mm soil water use threshold set by Aase et al. (1996) for LGMs in order to not compromise soil water available to subsequent wheat. In Saskatchewan, full-bloom terminated LGMs have been reported to cause ~50-60 mm less soil water than fallow when measured soon after LGM termination (Townley-Smith et al., 1993; Biederbeck and Bouman, 1994). Soil water used by full-bloom terminated LGMs can be replenished by spring (Biederbeck and Bouman, 1994), but Zentner et al. (2004) reported that soil water following full-bloom terminated lentil averaged 53 mm less than fallow at spring wheat seeding, over six years in Saskatchewan. Soil water deficits from LGMs in our study were low even though precipitation during the 2009 LGM growing season was 86 to 117 mm (57 to 78%) below average at 4 of 5 sites; at one site LGM water use soil water levels even recovered to levels equal to fallow from a single rainfall event. Also, despite the soil water depletions from LGMs we measured at wheat seeding, precipitation during the 2010 wheat year was 73 to 150 mm (33 to 66%) above average at all sites. This suggests that that any differences in soil water between LGM and fallow treatments wheat may have been effectively eliminated. Additionally, there was also no correlation between wheat yields and soil water levels at wheat seeding (P = 0.35).
We believe that soil nitrogen availability to wheat was temporarily limited in LGM treatments. Soil nitrate levels following LGM treatments were lower than following fallow at both LGM termination and at wheat seeding (Table 6). At wheat seeding, mean soil nitrate in LGM soils to 0.9 m was 19 kg ha-1 (34%) below fallow values; conversely, mean potentially mineralizable nitrogen (PMN) in LGM soils to 0.3 m was 9 kg ha-1 higher than fallow values. These results are in agreement with conclusions by Pikul et al. (1997) and suggest that soil N used by LGMs had not sufficiently mineralized back to plant available forms by wheat seeding, limiting soil N available to subsequent wheat compared to fallow, despite any N fixation by LGMs. Our soil nitrate values at wheat seeding also correlated with wheat yields (r = 0.56, P < 0.01). Dry conditions during the 2009 LGM year likely reduced mineralization potential in surface soils dried by LGMs compared to fallow soils. Using our data and an equation by Cassman and Munns (1980), we estimated that N mineralization potential in fallow soils to 0.3 m in midsummer 2009 could have been ~14 kg ha-1 every 14 d while mineralization in LGM soils would have been negligible. Also, LGM treatments had lower mean wheat grain N yields and higher mean grain density than fallow treatments, and chlorosis was observed in early spring in the LGM treatments at the Box Elder site; each of these factors are indicative of more limited soil N conditions in LGM treatments than in fallow.
Dry conditions in 2009 also likely led to low LGM shoot biomass N contributions to soils at all sites, which ranged from 15 to 44 kg ha-1 (Table 7). A meta-analysis by Tonitto et al. (2006) indicated that when cover crops replacing fallows contained <110 kg ha-1 shoot biomass N, yields of subsequent crops were generally reduced by ~15%. This suggests that even if there is a net gain in N returned to soils in LGM residues from N fixation, net gains must be sufficient enough to overcome residue N that may be sequestered into soil organic N pools. Indeed, studies in the NGP have shown that recovery of LGM biomass N by subsequent crops can be low compared to amounts that may be retained in soils (Janzen et al., 1990; Bremer and van Kessel, 1992). No tillage treatment may also delay recovery of crop residue N until after several years of surface crop residue accumulation (Schoenau and Campbell, 1996; Triplett and Dick, 2008). Examples of LGM crop residues remaining on the soil surface after 3 and 14 months of decomposition in this study are illustrated in Figure 1 c and d, respectively.
Others have concluded that legume N sources in semiarid systems may not precipitate immediate N responses from subsequent crops but rather cause gradual buildups of soil N supplying capacities over time (Ladd et al., 1981; Janzen et al., 1990). Long-term studies in Saskatchewan (Zentner et al., 2004) and northeastern Montana (Allen et al., 2011) substantiate this conclusion; they illustrated that LGMs may need up to three rotations to begin to reliably offset N fertilizer additions and to enhance yields of subsequent wheat crops compared to fallow–wheat rotations. An unfertilized pea green manure-wheat rotation in Bozeman, MT (Miller et al., unpublished data) also recently produced wheat yields and grain protein levels equal to a fallow-wheat rotation fertilized with 139 kg ha-1 of N, after four rotation cycles (eight year).
Analysis of a pea for hay (“Hayed”) treatment at Box Elder and the cattle-grazed pea green manure (“Grazed LGM”) and pea for seed (“Seed Peas”) alternative treatments at Sunburst revealed no distinct advantage over LGM treatments. At Box Elder, pea hay yields were low (Table 8). Wheat yields in the pea for hay treatment were lower than following LGM (Table 9). We lack a clear explanation for this in our soil water and soil N data (Table 10 and 11), but it is suspected that pea water and nitrogen use may have been slightly, but critically, greater in the pea for hay treatment than in the LGM treatment. Pea hay was cut at first flower (anthesis) stage along with termination of the LGM crop, but pea stubble continued to grow for up to a week after termination of the LGM crop; the pea stubble was not herbicide killed until after pea hay had dried sufficiently for bailing. Pea stubble growth accounted for 0.57 Mg ha-1 biomass growth than in LGM treatments (see Table 7 and Table 8). The pea for hay treatment therefore must have used more resources than the LGM crop, even though we did not detect differences in our soil water or nitrogen measurements (Tables 10 and 11). Removal of pea hay biomass is not suspected to have caused an effect in the short-term (Miller et al., 2006).
At Sunburst, yields following the grazed LGM and un-grazed LGM treatments did not differ; nor were the grazed and un-grazed fallow controls (see Tables 4 and 10); therefore no grazing effect is suspected. Yields following the pea-for-seed treatment and its corresponding fallow control showed no difference (Table 9), but yields for both of these treatments were suspiciously lower than wheat yields of the section of the field site that contained the grazed and un-grazed LGM and their fallow controls (~0.40 to 0.67 Mg ha-1 lower, see tables 4 and 9). Soil water and N measurements in this treatment (Table 10 and 11) were unhelpful in elucidating why, for instance, yields following grazed LGM and grazed fallow differed, but there were no differences in yields following the pea-for-seed treatment and its fallow control (Table 9). It is possible that other confounding factors could have been present on the portion of the field containing the pea-for seed treatment and its fallow control that restricted wheat yields across both treatments. It is therefore difficult to conclude if there was any real advantage in the pea-for-seed treatment compared to fallow, but economic returns would have accrued to the harvested peas.
Aase J.K., Pikul J.L., Prueger J.H., Hatfield J.L. (1996) Lentil water use and fallow water loss in a semiarid climate. Agronomy Journal 88:723-728.
Allen B.L., Pikul J.L., Waddell J.T., Cochran V.L. (2011) Long-term lentil green-manure replacement for fallow in the semiarid northern Great Plains. Agronomy Journal 103:1292-1298. DOI: 10.2134/agronj2010.0410.
Army T.J., Hide J.C. (1959) Effects of green manure crops on dryland wheat production in the Great Plains area of Montana. Agronomy Journal 51:196-198.
Atkinson A. (1907) Canadian Field Peas, Montana Agricultural Experiment Station Bulletin No. 68, Montana State College and Agricultural Experiment Station, Bozeman, MT. pp. 81-90
Biederbeck V.O., Bouman O.T. (1994) Water-use by annual green manure legumes in dryland cropping systems. Agronomy Journal 86:543-549.
Biederbeck V.O., Campbell C.A., Rasiah V., Zentner R.P., Wen G. (1998) Soil quality attributes as influenced by annual legumes used as green manure. Soil Biology & Biochemistry 30:1177-1185.
Biederbeck V.O., Zentner R.P., Campbell C.A. (2005) Soil microbial populations and activities as influenced by legume green fallow in a semiarid climate. Soil Biology & Biochemistry 37:1775-1784.
Brandt S.A. (1996) Alternatives to summerfallow and subsequent wheat and barley yield on a Dark Brown soil. Canadian Journal of Plant Science 76:223-228.
Bremer E., van Kessel C. (1992) Plant-available nitrogen from lentil and wheat residues during a subsequent growing season. Soil Science Society of America Journal 56:1155-1160.
Brown P. (1964) Legumes and Grasses in Dryland Cropping systems in the Northern and Central Great Plains: A Review of the Literature, USDA Miscellaneous Publication No. 952, (USDA) United States Department of Agriculture, Washington, DC. pp. 1-64
Burgess M.H. (in preparation) Energetic and economic analysis of the diversification and intensification of fallow-based cropping systems. PhD Dissertation Chapter, Montana State University, Bozeman, MT
Campbell C.A., Selles F., Zentner R.P., De Jong R., Lemke R., Hamel C. (2006) Nitrate leaching in the semiarid prairie: Effect of cropping frequency, crop type, and fertilizer after 37 years. Canadian Journal of Soil Science 86:701-710.
Carlyle W.J. (1997) The decline of summerfallow on the Canadian Prairies. Canadian Geographer 41:267-280.
Cassman K.G., Munns D.N. (1980) Nitrogen mineralization as affected by soil moisture, temperature, and depth. Soil Science Society of America Journal 44:1233-1237.
Janzen H.H., Bole J.B., Biederbeck V.O., Slinkard A.E. (1990) Fate of N applied as green manure or ammonium fertilizer to soil subsequently cropped with spring wheat at three sites in Western Canada. Canadian Journal of Soil Science 70:313-323.
Ladd J.N., Oades J.M., Amato M. (1981) Distribution and recovery of nitrogen from legume residues decomposing in soils sown to wheat in the field. Soil Biology & Biochemistry 13:251-256. DOI: 10.1016/0038-0717(81)90058-4.
Miller P.R. (unpublished data), Montana State University, Bozman, MT
Miller P.R., Engel R.E., Holmes J.A. (2006) Cropping sequence effect of pea and pea management on spring wheat in the northern Great Plains. Agronomy Journal 98:1610-1619. DOI: 10.2134/agronj2005.0302.
Nielsen D.C., Calderon F.J. (2011) Fallow Effects on Soil, in: J.L. Hatfield and T.J. Sauer (Eds.), Soil Management : Building a Stable Base for Agriculture, American Society of Agronomy ; Soil Science Society of America, Madison, WI. pp. 287-300.
O’Dea J.K. (2011) Greening Summer Fallow: Agronomic and Edaphic Implications of Legumes in Dryland Wheat Agroecosystems. M.S Thesis, Department of Land Resources and Environmental Sciences, Montana State University, Bozeman, MT
Pikul J.L., Aase J.K., Cochran V.L. (1997) Lentil green manure as fallow replacement in the semiarid northern Great Plains. Agronomy Journal 89:867-874.
Schoenau J.J., Campbell C.A. (1996) Impact of crop residues on nutrient availability in conservation tillage systems. Canadian Journal of Plant Science 76:621-626.
Tanaka D.L., Lyon D.J., Miller P.R., Merrill S.D., McConkey B. (2010) Soil and water conservation advances in the semiarid northern Great Plains, in: T.M. Zobeck and W.F. Schillinger (Eds.), Soil and Water Conservation Advances in the United States: SSSA Special Publication No. 60, Soil Science Society of America, Madison, WI. pp. 81-102.
Tonitto C., David M.B., Drinkwater L.E. (2006) Replacing bare fallows with cover crops in fertilizer-intensive cropping systems: A meta-analysis of crop yield and N dynamics. Agriculture Ecosystems & Environment 112:58-72.
Townley-Smith L., Slinkard A.E., Bailey L.D., Biederbeck V.O., Rice W.A. (1993) Productivity, water use and nitrogen fixation of annual-legume green-manure crops in the Dark Brown soil zone of Saskatchewan. Canadian Journal of Plant Science 73:139-148.
Triplett G.B., Dick W.A. (2008) No-tillage crop production: A revolution in agriculture! Agronomy Journal 100:S153-S165. DOI: 10.2134/agronj2007.0005c.
USDA-FSA.(2010) Lori Margheim, Farm Service Agency, Bozeman, MT
Zentner R.P., Campbell C.A., Biederbeck V.O., Selles F. (1996) Indianhead black lentil as green manure for wheat rotations in the Brown soil zone. Canadian Journal of Plant Science 76:417-422.
Zentner R.P., Campbell C.A., Biederbeck V.O., Selles F., Lemke R.,
Jefferson P.G., Gan Y. (2004) Long-term assessment of management of an annual legume green manure crop for fallow replacement in the Brown soil zone. Canadian Journal of Plant Science 84:11-22.
- Table 4. Treatment means, standard error of the difference in means, and two-sided p-values for wheat yield component responses across five sites in north-central Montana.
- Table 7. Treatment means and standard error of the means for LGM shoot biomass estimates across five sites in northcentral Montana.
- Table 11. Alternative summer fallow replacement treatment means for soil NO3-N (mg kg- 1) and potentially mineralizable nitrogen (PMN) post harvest and wheat seeding for specified soil depths at two sites in north-central Montana.
- Table 8. Mean biomass yields of harvested and crop residue portions from alternative summer fallow replacements across two sites in north-central Montana.
- Table 9. Means of yield components, for alternative summer fallow replacement treatments at two sites in north-central Montana.
- Table 5. Treatment means, standard error of the difference between means, and two-sided p-values for Equivalent Depth total soil water content (mm) at LGM termination, and wheat seeding for specified soil depths across five sites in north-central Montana.
- Table 6. Treatment means, standard error of the difference in means, and two-sided p-values for soil NO3-N and Potentially Mineralizable N (PMN) content (kg ha-1), measured at LGM termination and wheat seeding for specified soil depths across five sites in north-central Montana.
- Table 10. Alternative summer fallow replacement treatment means for Equivalent Depth total soil water content (mm) at harvest, and wheat seeding for specified soil depths across two sites in north-central Montana.
Educational & Outreach Activities
• O’Dea J.K., Miller, P.R., Jones C.A. “Greening summer fallow with no-till legume green manures: an on-farm assessment in Montana.” Prepared for submission to The Journal of Soil and Water Conservation (High Wire Press).
• O’Dea, J. K. “Greening Summer Fallow: Agronomic and Edaphic Implications of Legumes in Dryland Wheat Agroecosystems” M.S. Thesis. Department of Land Resources and Environmental Sciences, Montana State University, Bozeman, MT.
• O’Dea J.K., Miller, P.R., Jones C.A., Zabinski, C.A. “Greening the Fallow Season: Scaling up Legume-Wheat Research.” LRES 500 Seminar Series, Montana State University, Bozeman, MT. Mar 13, 2010. Oral presentation, Q & A, 25 people, 50 min.
• O’Dea J.K. On-site field demonstration in Joplin, MT. Sage Creek Watershed Group’s Annual Field day, Liberty County, MT. July 11, 2011. Oral Presentation, ~40 farmers, industry representatives, researchers, and community members, 15 min.
• O’Dea J.K., Miller P.R., Jones C.A., Burgess M.H. “Greening Summerfallow: On-Farm Evaluation of Legume Green Fallow Rotations.” ASA/CSSA/SSSA International Annual Meeting, Long Beach, CA, Oct 31-Nov. 3, 2010. Poster presentation, ~3000 conference attendees.
• Miller P.R., Jones C.A., Burgess M.H., O’Dea J.K., McCauley A.M. “Pulse Studies in Montana.” Montana NRCS Soil Health program, Billings, MT. Feb 16, 2011. Oral presentation, 50 people. 1 hr.
• O’Dea J.K. “Greening the Fallow Season: Scaling up Legume-Wheat Research.” in LRES 351: Nutrient Cycling. Montana State University, Bozeman, MT. Mar. 3, 2011. Lecture, 45 students, 35 min.
• Jones C.A., O’Dea J.K., and Miller P.R. 2011. “Research results on cover crops and urea volatilization.” Glacier County NRCS New Trends in Agriculture Winter Seminar, Shelby, MT. Dec 6, 2011. Oral Presentation, 50 min.
• O’Dea, J. K., “Greening Summer Fallow: Agronomic and Edaphic Implications of Legumes in Dryland Wheat Agroecosystems” M.S. Thesis defense, Montana State University, Bozeman, MT. Oral Presentation. ~25 people, 1 hr.
• O’Dea J.K., Miller P.R. “Managing on-farm energy budgets with legume green manure.” NRCS CIG Grant Final Report.
• O’Dea J.K. “NRCS/Western SARE Project on Legume Green Manures for North-Central Montana No-till Wheat Production: Report to producer collaborators.”
• A ~6 minute film documenting producer experiences of adopting LGM crops as summer fallow replacements.
-Producer interviews have been video-recorded, editing still pending.
• Montana State University Extension Fertilizer Fact Sheet.
– Planned two-page summary of this study.
• Montana State University Extension guide to legume green manure practices.
– Planned guide for producers, which this study will contribute data to.
Measureable impacts/outcomes from this study will ultimately be future assessments of the adoption of LGM or other legume green fallow crops replacing summer fallow. Our study indicated that replacement of summer fallow with a legume crop may have consequential impacts on soil and water conservation and N economy. Additional analyses of our data indicated that replacing summer fallow with a LGM crop led to 260 and 25 and kg ha-1 greater residue C and N, respectively, returned to soils (Table 12); this also corresponds to 0.58 Mg ha-1 greater crop residue left to protect surface soils in LGM treatments (see Figure 1c). This indicates the potential for LGM crops to help mitigate soil organic matter otherwise vulnerable to net declines from summer fallow treatment; this has been likewise supported by other studies investigating LGM effects on soil quality (Biederbeck et al., 1998; Biederbeck et al., 2005; O’Dea, 2011). Greater residue N returns to soils also indicate the potential for these crops to increase the N supplying power of soils and lead to a reduced need for N fertilizer inputs; these N fertilizer savings may become substantial after several rotations (Zentner et al., 2004; Allen et al., 2011). O’Dea (2011) observed that after eight years, mineralizable N levels to 0.15 m in legume green fallow-wheat rotations may be as much as 50% greater than in summer fallow-wheat rotations. Greater available N use efficiency (NUE) was also observed in LGM treatments in this study compared to fallow (Table 12) and may indicate improved soil-crop N synchrony through incremental release of N from LGM residues over the wheat growing season. Reduced needs for fertilizer N inputs and improved soil-crop N synchrony may help reduce chances of N losses to the environment through leaching and gaseous pathways.
- Table 12. Treatment means, standard error of the difference in means, and two-sided p-values for conservation parameters across five sites in north-central Montana.
- Figure 1. Example of field site during LGM stage (a) and subsequent wheat stage (b), and no-till LGM residue breakdown stages in September, 2009 (c) and the following August, 2010 (d).
A very basic economic assessment of LGM-wheat vs. summer fallow-wheat management was an objective of this study. Economic viability of LGMs during the adoption period observed in our study did not appear to be favorable compared to summer fallow management. Since LGMs caused wheat yield reductions and any grain protein increases observed from LGMs were inconsequential, we estimate that economic losses in wheat yield from LGMs were approximately $48/ha ($19/ac) for winter wheat and $57/ha ($23/ac) for spring wheat. Two of five producers eliminated a mid-summer herbicide application in their LGM treatment, but seed costs for planting an LGM were high enough that it is unlikely this savings would substantially offset for operation costs of managing a LGM compared to fallow. Seed costs in our study for pea green manure ranged from $52 to $74/ha ($21 to $30/ac) while lentil green manure seed cost $26/ha ($11/ac). It should be noted though that self-sourcing seed or purchasing low-grade seed for LGMs (feed grade or extra No. 3 lentil) might substantially reduce seeding costs.
Even though these economic results are potentially discouraging to LGM adoption, economics of longer-term LGM-wheat rotations should be considered. Zentner et al. (2004) importantly illustrated in a Saskatchewan study that a lentil green manure-wheat-wheat rotation became more economical than a fallow-wheat-wheat rotation after three appearances of lentil green manure in rotation. A recent study in Bozeman, MT indicated that a pea green manure-wheat rotation cost approximately $25/ha ($10/acre) more to manage than a chem fallow-wheat rotation (Burgess et al., in preparation) but also was capable of saving ~$84/ha ($34/ac) in N fertilizer costs after four green manure appearances in rotation. In order for LGM or other legume green fallow adoption potentials to be increased, these practices may need to be viewed and promoted as an investment leading to long-term economic and conservation benefits rather than a practice that immediately replaces the role of N fertilizer.
An assessment of farmers’ initiative to continue LGM practices was a major objective of this study. Four of five producers claimed they would continue to experiment with LGMs. Nonetheless, pertinent concerns amongst producers included:
1) LGM seed cost;
2) planting winter wheat crops into LGM-dried soils;
3) timing spring field operations so that LGMs can be planted early;
4) resisting the temptation to let LGMs grow past first-flower for more biomass accumulation; and
5) absent wheat yield responses.
The last four issues may be rectified through developing appropriate expectations from LGMs and adapting operation routines, but seed cost may be the most formidable barrier to LGM adoption. Concerns about seed costs for LGMs date back to the earliest reports of trials with LGMs in this region (Atkinson, 1907). Using cheaper, lower-grade pea and lentil seed (i.e. feed grade or Extra No. 3 lentils) than used in this study or growing small acreages of pulse crops for seed may be more economical ways to procure seed for LGM cropping. The latter strategy was endorsed by four of the five producers interviewed, and some recommended that other producers interested in the practice start with small acreages to safeguard against potential economic failures. This approach may furthermore help producers unfamiliar with pulse cropping, or wary of re-cropping with pulses, get accustomed to pulse crop management and rotational effects. All producers indicated an interest for future research to better clarify long-term benefits of LGMs in rotation (including economic assessments) and to better understand benefits of LGMs vs. pulse cropping for grain. Alternatively, growing peas or lentils as harvested grain crops has generated high profitability recently in Montana, and that may prove to be the most economical way to engage legumes in wheat-based rotations.
Areas needing additional study
This study illustrated that soil water needed for subsequent wheat crops can be conserved at the field-scale and under farmer management when LGMs are terminated early and are treated as no-till crops. While soil water was conserved sufficiently for subsequent wheat, N availability was at least temporarily limited following LGMs. This may be a phenomenon unique to very dry years, causing low LGM biomass N contributions to soils and lowered mineralization potential in LGM-dried soils, but no-till practices may also generally delay recovery of N in LGM residues by subsequent crops. The economic viability of LGMs during the adoption period is questionable, and may depend on 1) how economically LGM seed is procured, 2) how immediately rotational wheat yield benefits manifest, and 3) whether longer-term rotational LGM benefits can offset any early investment costs of adopting LGMs that may incur. Considering conservation benefits that may result from replacing summer fallow LGMs, federal/state conservation programs may be helpful if they can help subsidize startup costs for farmers adopting LGMs.
Further study is therefore needed to understand longer-term benefits of LGMs in dryland wheat rotations, especially when managed with no-till practices, and when applicable to very dry regions, as early-terminated LGMs. Understanding rotational N benefits from LGMs on a temporal scale, coupled with longer-term economic assessments will importantly clarify whether LGM-wheat rotations can be economically sustainable. As sustainable summer fallow replacements, LGMs should be capable of mitigating legacies of degraded soils and other negative side effects from summer fallow practices. Further study is needed to understand the extent to which LGMs may help conserve/improve cropland soils, and whether they may help reduce chances of nitrate leaching and saline seeps. Understanding whether LGMs can precipitate other rotational benefits such as improved pathogen, insect or weed control should also be explored.