Increasing legume forage productivity through slurry application: A way to intensify sustainable agriculture?

Legume forages are fundamental in the development of sustainable livestock systems, building soil fertility and providing home-grown protein for ruminant livestock. Legumes derive N from the atmosphere but defoliation reduces N-fixation, reducing yield. Livestock, particularly dairy, systems generate considerable quantities of slurry. There is a lack of knowledge on whether this resource can be used sustainably to improve the dry matter (DM) yield of regularly defoliated legumes. An experiment investigated whether applying slurry to red clover (Trifolium pratense), lucerne (Medicago sativa) or hybrid ryegrass (Lolium hybridicum), managed by frequent cutting, would increase DM and nitrogen (N) yield compared with plots without slurry. Treatments were compared within a randomised block design. Plots were harvested for silage on four occasions. Soil P and K indices were maintained at sufficiency levels throughout. Forage DM yields were higher on all plots treated with slurry than those with zero slurry. Overall, legume forages had higher DM yields than ryegrass, with lucerne having the highest DM yield. DM yields decreased at each cut for all forages (P < 0.001); however forage N content increased after each cut (P < 0.001). Slurry addition increased ryegrass yield at first cut and red clover yield at second cut (P< 0.05) indicating ryegrass needed the nutrient boost to commence growth, whilst red clover needed it to aid recovery after defoliation stress. N concentration differed between forages, although for ryegrass and red clover there was no difference between slurry treatments (P > 0.05) but was higher in lucerne grown without slurry. Applying slurry to legumes under frequent cutting regimes increased total DM yield but not N yield. Overall, findings demonstrate the potential to use slurry as a resource to intensify sustainable agriculture by increasing the DM yields of frequently harvested forage legumes.