GRAZING GREEN MANURES TO OPTIMIZE NITROGEN SUPPLY ON THE CANADIAN PRAIRIES

Summary

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. Experiments were carried out for three years between 2009 and 2011 in Manitoba, Canada. Seven different green manure crops or mixtures were grazed with sheep or left ungrazed. Hairy vetch, pea/oat mix and oats produced the greatest forage biomass in two out of three years. In 2010, sweet clover produced a similar amount (5813 kg ha-1). Soybean and lentil failed to compete with weeds; containing 30 to 73% weed biomass in all years. Utilization by sheep for all crops ranged from 28% to 86% but the most common range was between 60% and 80%. Soil NO3-N to 120 cm was significantly greater in grazed than in ungrazed plots. Greatest soil NO3-N content (226 kg ha-1) was observed in grazed pea/oat plots in experiment 1 and lowest (44 kg ha-1) was in ungrazed mixture plots in experiment 3. In experiment 1, total profile soil NO3-N level in pea/oat plots was greater than in oat plots. In experiment 2, oat plots contained less NO3-N than pea/oat and mixture plots. In experiment 3, there was significant species and management effect where soil in grazed hairy vetch plots contained the greatest amount of NO3-N. This interaction indicated that increase in soil NO3-N availability is greater when hairy vetch is grazed than grazing of other crops. In situations with low legume biomass productivity, grazing may be used to increase the N benefit from legume green manures to the following crops. Therefore, in addition to single benefit of soil fertility, two benefits were reaped from green manures; potential livestock live weight gain and soil fertility.