Effects of grazing two green manure crop types in organic farmingsystems: N supply and productivity of following grain crops
Grazing green manures may improve N availability and productivity in integrated crop–livestock systems. We hypothesized that grazing green manures, compared with standard soil incorporation with tillage, would increase autumn soil profile NO3–N concentrations and grain yields in subsequent years. Three multiyear experiments were carried out for three years between 2009 and 2011 in Manitoba, Canada. For all three experiments, spring-seeded oat (Avena sativa cv. Leggett) and pea/oat (A. sativa/Pisum sativum cv. 40-10) green manure crops were grazed with sheep or left ungrazed in year one. Both treatments were soil incorporated with a tandem disk after grazing. Spring wheat (Triticum aestivum cv. Waskada) and fall rye (Secale cereale cv. Hazlet) test crops were grown in the second and third years, respectively. Biomass production was greater for pea/oat than oat in experiment 1; in experiment 2 pea/oat = oat; and in experiment 3 pea/oat < oat. Grazing utilization of green manure biomass averaged 62% across all treatments and experiments with no utilization differences between crop species. On average, less than 10% of biomass consisted of weeds for both green manures. Soil NO3–N to 120 cm was significantly greater in grazed than in ungrazed plots; however soil P and K were unaffected. Nitrate content was greater in pea/oat mixture plots for all three experiments at 0–120 cm than oat plots. The absence of a significant management × green manure type interaction indicated that both crop types responded similarly to grazing. Greater availability of soil NO3–N after grazing translated into significantly greater crop growth and N uptake in some years, although significant increases in yield of following crops were not observed. Importantly, grazing green manures never negatively affected wheat or rye yield. In conclusion, grazing green manure crops increased N supply for subsequent crop production with no negative yield effects on two subsequent grain crops.