Greening summer fallow with legume green manures: On-farm assessment in north-central Montana

Abstract

Replacing summer fallow practices with annual legumes as green manures (LGMs) may increase the sustainability of northern Great Plains wheat (Triticum aestivum L.) systems. Viability hinges on soil water use management and realizing biologically fixed nitrogen (N) benefits. Plot-scale research has shown that managing LGMs with first-flower stage termination and no-till practices conserves soil water and that rotational N benefits can increase wheat grain quality. Nonetheless, farmer adoption of LGMs has been negligible. To better understand this practice and its regional adoption potential, we conducted a participatory on-farm assessment of no-till LGM versus summer fallow-wheat rotations in north-central Montana. Soil water and nitrate (NO 3 ) levels to 0.9 m (3 ft), potentially mineralizable N (PMN) to 0.3 m (1 ft), wheat yields, conservation potential, and producer adoption challenges were assessed at five farmer-managed, field-scale sites. Compared to fallow, LGM treatment diminished mean wheat yield by 6% (0.24 Mg ha -1 [3.7 bu ac -1 ]), diminished grain protein by 9 g kg -1 when wheat was fertilized with N (p = 0.01), and increased grain protein by 5 g kg -1 when wheat was unfertilized (p = 0.08). Small soil water depletions in LGM treatments below fallow at wheat seeding (17%; 30 mm [1.2 in]) and near-record high rainfall during the wheat growing season (280 to 380 mm [11 to 14 in]) suggest that LGMs likely did not limit soil water available to wheat in this study. Soil NO 3 levels following LGMs were 29% to 56% less than summer fallow at wheat seeding, and conversely, greater PMN was detected in LGM treatments at 3 of 5 sites. We theorize that N mineralization from LGMs was insubstantial by wheat seeding due to dry soil conditions and low LGM biomass N contributions, consequently affecting wheat yield potential due to limited early season soil N availability.LGMs increased average use efficiency of available N by 24% during the wheat year and increased total residue carbon (C) and N returned to soils by 260 and 26 kg ha -1 (232 and 23 lb ac -1 ), respectively, after two years. Our results illustrated that farmers viably managed LGM soil water use with early termination and no-till practices but that LGM adoption may be hindered by a lack of immediate wheat yield or protein benefits from legume-N and seed costs for LGMs. Appropriate incentives, management strategies, and yield benefit expectations (short versus long term) should be fostered to increase the adoption potential of this N-economizing soil and water conservation strategy.

Pea in Rotation with Wheat Reduced Uncertainty of Economic Returns in Southwest Montana

“…By managing soil water use conservatively and for multiple rotations, lentil green manure in tilled systems increased soil N contributions to spring wheat in northeastern Montana (Pikul et al, 1997;Allen et al, 2011) and was found to be economically profi table in southwestern Saskatchewan (Zentner et al, 2004). Miller et al (2006) showed similar promise with chemically killed pea manure in sequence with spring wheat in a notill system in north central Montana but subsequent cropping sequence work showed that wheat yield and protein responses to pea manure were inconsistent (Miller et al, 2011;O'Dea et al, 2013;Burgess et al, 2014). Th e objective of this study was to compare crop yield and quality parameters and net returns among six 4-yr pea-wheat and wheat-only crop rotations in southwestern Montana.…”

Diversifying crop rotations with pulses enhances system productivity

“…Positive rotational effects of pulses to subsequent cereal or oilseed crops have been well documented in Mediterranean-type climates 52 . However, the findings on the rotational benefits of pulses in the short-season, semiarid northern latitudes have been inconsistent 49,[53][54][55][56] . In particular, it is unclear in the scientific literature whether the beneficial features that are associated with conventional summerfallowing can be retained with improved, pulse-based systems in water scarce environments.…”

Legume, cropping intensity, and N-fertilization effects on soil attributes and processes from an eight-year-old semiarid wheat system

“…This delayed response may be a reality of cool, waterlimited conditions in the NGP leading to low annual legume biomass contributions, slow breakdown of residues, and subsequent slow release of available N (Janzen et al 1990;Bremer and van Kessel 1992;Beckie et al 1997), especially in no-till systems (Schoenau and Campbell 1996;Triplett and Dick 2008). A lack of immediate benefits is potentially discouraging to producers expecting N fertilizer-type responses from legumes (O'Dea et al 2013) while a more reasonable expectation may be a gradual buildup of the soil N pool (Ladd et al 1981;Janzen et al 1990). …”

“…Legumes are assumed to provide N benefits to subsequent wheat, but this is not consistently detected in primarily short-term studies from the NGP (Miller et al 2002;Lupwayi and Kennedy 2007;O'Dea et al 2013). Longer-term studies suggest that N-benefits to wheat may be realized only after multiple years of legumes in rotation (Campbell et al 1992;Zentner et al 2004;Walley et al 2007;Allen et al 2011).…”

“…are promising summer fallow replacements (Miller et al 2002), and pulse crop acreage replacing summer fallow has increased tenfold in the NGP since the early 1990s (Long et al 2014). Where summer fallow practice remains steadfast, partial-season legume green fallow crops (forages or green manure crops) have been proposed to intensify and diversify cropping O'Dea et al 2013).…”