Grass-Legume Mixture Response to Nitrogen Source and Application Date.

Grazing-based livestock producers may wish to apply N to temperate grass-legume pastures to improve productivity, but legume persistence and associated improvements in forage nutritive value may be negatively impacted.  We determined the influence of N source and application date on annual productivity and legume persistence of an orchardgrass (Dactylis glomerata L.) – red clover (Trifolium pretense L.) mixture.  Nitrogen was applied as ammonium nitrate fertilizer, bedded-pack solid dairy manure, or liquid dairy slurry in late April, mid June, or late August.  All treatments supplied 67 kg N ha^-1 yr^-1, except a 0 N control.  Plots were rotationally grazed to a 10-cm stubble when orchardgrass reached vegetative stage (30 – 35 cm sward height; 3 – 4 leaves) five times during each of two growing seasons.  Persistence of red clover seeded the previous year was measured in October.  Mean annual yield of a mixture receiving no N was 8000 kg DM ha^-1.  Applying any N source in April increased annual forage yield by 500 kg DM ha^-1, but applying N in June or August reduced annual yield 320 to 750 kg DM ha^-1 compared to the control.  When no N was applied, red clover was found in 50% of seeded rows at the end of each growing season, but was found in 20 – 35% of rows where any N source was applied in April, June, or August.  Producers should weigh the benefits of greater yield as a result of N application with reduced legume persistence before applying N to a grass-legume sward.