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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. |
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