Legume Green Manures in Fallow – Wheat No-till Systems. Do They Work in Montana?.

In the most intransigent fallow – wheat area of Montana (MLRA 52), annual legume green manures (LGMs) may improve soil nitrogen status but farmer adoption remains very low. In several different trials, we compared different legume species, growth habits,  plant populations, tillage management, and long-term LGM effects on wheat N uptake, mainly within no-till systems. Growth, total biomass N yield, and weed competition of pea was superior to lentil. Over three years, pea biomass N averaged 101 kg/ha compared with 45 kg/ha for lentil, while wheat grain N content was 2.8 kg/ha greater following pea. Winter pea, when it survived, provided superior biomass N to spring pea. In one no-till study, winter forage pea averaged 113 kg/ha of biomass N compared with 90 kg/ha for spring forage pea over four site-years; resulting in greater grain N yield (10 kg/ha) by wheat in one of two years. In a field-scale study, seed cost ranging from $35 to $75/ha was identified by farmers as a major barrier to LGM inclusion in cropping systems. Seeding rates below that recommended for seed production reduced LGM biomass and increased weed biomass. Tillage, even once, increased N contribution of LGMs but proved difficult to quantify. Short-term effects of LGMs have been inconsistent, especially for spring-sown legumes. In the 8^th year of a long-term no-till study of 2-yr crop rotations near Bozeman, LGM increased spring soil nitrate-N by 40 kg/ha compared with pea grown for seed. Spring wheat grain N uptake was 24 to 53 kg/ha greater, and net grain N removal was 36 to 121 kg/ha greater for the LGM system than for six other cropping systems This long term effect appeared even more striking than that reported for tilled lentil manure – wheat rotations in two other studies elsewhere in the northern Great Plains.