Nitrogen-15 labeling of dairy feces and urine for nutrient cycling studies

Estimates of the availability of dairy manure nutrients to crops rely on indirect measurements and can vary greatly. More accurate estimates of manure nutrient availability me needed to improve manure management. The objective of this study was to enrich dairy fetes and mine in N-15 to study nutrient flow in the feed-animal-manure-soil and crop-environment continuum. Ammonium sulfate (12.3 or 10 atom % N-15) was applied to soil to enrich alfalfa (Medicago sativa L.) and corn (Zea mays L.) plants during growth. Alfalfa hay contained from 2.386 to 3.980 atom % N-15 in three harvests and corn silage contained 8.162 atom % b N-15. A feed mixture containing 55% alfalfa hav and 45% corn silage (4.026 atom % N-15) was fed to two mature nonlactating cows (Bos taurus) for 36 h. The pattern of N-15 excretion in urine and feces was similar for both cows. The N-15 appeared in urine by 8 h and in feces by 24 h, and peaked by 30 h in urine (1.642 atom % N-15) and by 54 h in feces (2.341 atom % N-15), Enrichment approached basal levels at 132 h after initial feeding for bath urine and fetes Of the total N-15 fed, 60% was recovered: 31% from urine and 29% from feces. Approximately 60 to 70% of the total N excreted in dairy fetes was endogenous N and 30 to 40% was undigested feed N, Combining feces excreted during the 16- to 122-h period after initial feeding of N-15-enriched feed would produce feces having uniformly labeled N components, The various N-15 enrichment levels of urine and feces collected during different times after feeding offer possibilities for studying differential N-15 use in short- and long-term nutrient cycling studies.