Agronomic Values of Separated Solid of Anaerobically Digested Cattle Feedlot Manure for Barley Forage Production Under Minimum Tillage.

As the demand for renewable biogas energy continues to climb, digestates from that industry are increasingly used as a nutrient source for crop production. A five-year field experiment was conducted to investigate the agronomic value of anaerobically digested cattle feedlot manure in slurry form (ADM) or as separated solids (SS, liquid fraction removed) from a biogas plant. Under minimum tillage in a semi-arid climate, three types of organic amendments, ADM, SS, and undigested cattle feedlot manure (CM), were applied annually for four years with a fifth year examining the residual effect. Amendments were applied at a rate to supply available N at 100 kg N ha^-1 yr^-1 and 200 kg N ha^-1 yr^-1 (1X and 2X recommended agronomic rates) for barley forage production. An unamended treatment (CK) was also included for comparison. Barley forage yield, amount of N uptake, fraction of amendment-derived N uptake and apparent recovery (AR) from ADM were higher than, and from SS were similar to, CM, reflecting the higher nutrient levels and availability in the ADM amendment. The AR for N was 20 and 28% for 1X and 2X ADM, but only 4 to 8% for CM and SS. In contrast, the amount of P uptake, fraction of amendment-derived P uptake and AR for P were similar among all treatments. The AR for N observed in our study is lower than values reported in the literature, suggesting a higher amendment application rate or greater mineral N supplement is needed for optimal barley forage production under minimum tillage than conventional tillage (amendment incorporation). Similar yields, soil available N and soil test P concentrations between SS and CM suggest they have similar agronomic value. Thus, feedlot manure management strategies developed over the years should also be applicable to managing separated solids from digestate.