Repeated application of anaerobic digestate, undigested cattle slurry and inorganic fertilizer N: Impacts on pasture yield and quality

The growth of on-farm anaerobic digestion (AD) has generated significant quantities of digestate for use as a soil amendment. However, relative to other organic and inorganic fertilizers, only limited field trial data exist on the effect of repeated applications of digestate to temperate grasslands. Here, we compare yields and grass quality (protein and digestibility) from a field trial of a mixed pasture ley (ryegrass and clover), following the application of five different fertilizer types (liquid digestate generated from anaerobically digested slurry, dry fibre digestate, undigested slurry, ammonium nitrate and an NPK compound fertilizer) in comparison with a no-fertilizer control. Application rates were normalized in terms of nitrogen (N) and were added as a split dose, with 100kgN/ha added prior to the first harvest and an additional 50kgN/ha supplied after the first harvest, every year for 3years. Overall, our results showed that applying both forms of digestate or undigested slurry gave grass yields that matched those obtained with one compound inorganic fertilizer, and better than those from a straight N inorganic fertilizer. No differences were found with regard to digestibility or sward N-content (hence calculated protein) between any treatments. Although the trial was conducted at only one site, the results indicate that inorganic fertilizers can potentially be replaced by digestate without compromising grassland productivity.