Grazing Cattle Diet Composition Effect on Dung Decomposition and Soil Nutrient Movement.

Rapid nutrient cycling improves forage quality and livestock productivity in pastures. Interseeding legumes may be a strategy to enhance N cycling, but effects of dung excreted from cattle grazing pastures with legumes on dung decomposition rates and soil N cycling have not been studied. Our objective was to evaluate how dung excreted from cattle grazing legume-interseeded, N-fertilized, and unfertilized smooth bromegrass (Bromus inermis Leyss.) pastures affects dung chemical composition, dry matter decomposition, CO₂ flux, and N availability in soil.  Freshly deposited dung was collected by hand from the legume-interseeded, N-fertilized, and unfertilized treatment-pastures, refrigerated, separately homogenized, and placed as pats in a neighboring unfertilized pasture. Each treatment was collected 3, 7, and 30 days after placing the pats in two experimental periods (June and August) in 2014. Soil cores were taken directly below and laterally from dung pats to determine rate of nutrient movement through soil. CO₂ flux from dung was measured for each treatment, as well as a non-dung control. Dung collections coincided with vegetation and diet samples from ruminally-fistulated cattle to examine effects of the pasture treatments on N cycling through the plant-animal-dung-soil complex. Forage quality in all diets was high and comparable in June, but was relatively high in legume-interseeded pastures in August. CO₂ flux did not differ among treatments in June but tended to be greater from dung excreted in legume-interseeded pastures in August. Results from June and August 2015 collections will also be presented.