Dairy Diet Impacts on Fecal Chemical Properties and Nitrogen Cycling in Soils

Availability of manure nitrogen (N) to crops is mitigated by many factors including manure type and composition. Whereas relationships between dairy diets, milk production, manure N excretion, and urine N losses as ammonia have been documented, very little information exists on how diets impact fecal carbon (C), N content, and partitioning, and how these factors impact fecal N mineralization and plant N uptake after application to soil. Feces from 24 to 63 dairy cows ( Bos taurus ) fed 14 typical diets were incubated aerobically in a sandy loam and two silt loam soils, and soil inorganic N (IN) was determined periodically during a 365‐d period. Feces from 12 of the 14 diets were applied to the same soils and oat ( Avena sativa L.), sorghum ( Sorghum bicolor L. Moench), and sorghum ratoon dry matter (DM) and N uptake were determined over a 155‐d period. Feces from cows fed alfalfa ( Medicago sativa L.) silage (AS)‐based diets generally lead to higher soil IN levels than soils amended with feces from corn ( Zea mays L.) silage (CS)‐based diets, especially in soils amended with feces from CS‐low crude protein (LCP) diets; feces from AS‐based diets increased plant DM and N uptake; after application to a silt loam, feces from high crude protein (HCP) diets resulted in greater soil IN levels than feces from LCP diets; and feces from LCP diets did not impact soil IN but decreased plant DM and N uptake. Carbon to N (C/N) ratios of applied feces were found to be significant predictors of plant DM and N uptake. There appears to be a range of dietary options that satisfy nutritional requirements of high‐producing dairy cows and produce feces having differential effects on soil N mineralization and plant N uptake after application to soil.