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University of Manitoba Faculty of Agricultural and Food Sciences Department of Plant Science

Grazed Green Manures:
Part 2 - The Next Crop

Background

Sheep grazing a soybean green manure, August 2009.

Grazing green manure crops has been suggested as a possible approach to gaining some income from green manure crops while maintaining most of the N benefit to the following crop. Grazing livestock is known to excrete about 80% of the nutrients they ingest; however, little is known about the effects of grazing green manures on nutrient cycling, the agronomic performance of the following crops or the economics of such systems.

Study Objectives

The objectives of this study are:

  • to evaluate the potential of various green manure crops for grazing (Phase 1)
  • to evaluate the effect of grazing green manures on soil nutrient dynamics (Phases 1 and 2)
  • to evaluate the effect of grazing green manures on the yield of the following crop (Phase 2)

Experiment Description

Field trials were established in 2009 at Carman, Manitoba and Oxbow, Saskatchewan. Green manures tested in the Carman experiment were oat (non-legume control), pea/oat mix, black lentil (cv. Indianhead), hairy hetch, soybean, and cowpea. At Oxbow, the green manure crop was peas. Each plot was later split into grazed and ungrazed treatments. Read more about Phase 1 of the experiment in Grazed Green Manures.

In 2010, spring wheat was grown on all plots in both Carman and Oxbow to evaluate the effect of grazing on the nitrogen (N) uptake and yield of the following crop. Data collected from the Carman trial included crop establishment, weed ratings, lodging ratings, crop biomass and N content, and grain yield and N content. Soil samples were collected from selected treatments in spring and after wheat harvest. At Oxbow, wheat yield and N content were measured.

Results: Carman

 

Soil nutrients

Soil nutrient status was assessed at the end of the growing season in grazed and ungrazed treatments of oat and pea/oat. These two treatments were chosen in order to test the effect of a legume vs. non-legume green manure in grazed systems.

Both crop type and grazing had signficant effects on soil N at Carman. When soil N content was broken down by soil sample depth, it was evident that the greatest effect of crop and grazing was in the surface layer of the soil, where N content in the grazed treatments doubled those in the ungrazed treatments and N content in the pea/oat treatments doubled those in the oat treatment (Figure 1). However, these trends were observed weakly in deeper sample depths as well, suggesting that a certain amount of N leaching may have occurred. High levels of available N in the surface soil of the grazed systems, especially with the pea/oat green manure, are at a high risk of leaching before a subsequent crop can use these nutrients. Growing a non-legume catch crop after the green manure would mitigate this potential problem and could also potentially provide some additional late-season grazing.

Grazing did not have a significant effect on soil P or K at Carman.

Figure 1. Effect of green manure crop and grazing on soil N status at four soil depths at Carman in 2009.

Results: Oxbow

Green manure (pea) biomass production in the grazing trial at Oxbow averaged 2376 kg ha-1. Biomass production was lower at Oxbow than at Carman due to moisture limitations at Oxbow.

Soil nutrient status was assessed at the end of the growing season in grazed and ungrazed treatments. Total available N, P, and K were very similar between treatments at Oxbow. We did not see the same effect of grazing on soil N at Oxbow as we did at Carman, where total soil N was much higher in grazed plots than ungrazed plots. When soil N content was broken down by sample depth, there was a small difference between grazed and ungrazed plots in the 0-30 cm depth and the 60-90 cm depth (Figure 2), but these differences were not statistically significant. There was no evidence of N leaching to the 90-120 cm sample depth at Oxbow. The potential for N leaching was likely lower at Oxbow than at Carman due to cool, dry soil conditions at Oxbow. The effect of grazing on soil nutrients may have been smaller at Oxbow than at Carman because of the smaller amount of biomass produced at Oxbow.

Figure 2. Effect of grazing on soil N status at four soil depths at Oxbow in 2009.

Conclusions and Recommendations

  • Sheep are willing to graze a wide variety of green manure crops.
  • Intensive grazing can kill an annual legume green manure crop, offering an alternative to terminating the green manure with tillage.
  • Grazing high-N crops such as legume green manures can result in large amounts of available N in the soil.
  • Growing a catch crop after grazing green manures may be required to prevent leaching of excess N.
  • Future field trials will evaluate the effect of grazing green manures on the following crop.

Copyright and Liability

This page created March 2010.