Nitrogen‐15 Labeling of Dairy Feces and Urine for Nutrient Cycling Studies

Abstract

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 are needed to improve manure management. The objective of this study was to enrich dairy feces and urine in 15N to study nutrient flow in the feed–animal–manure–soil and crop–environment continuum. Ammonium sulfate (12.3 or 10 atom % 15N) 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 % 15N in three harvests and corn silage contained 8.162 atom % 15N. A feed mixture containing 55% alfalfa hay and 45% corn silage (4.026 atom % 15N) was fed to two mature nonlactating cows (Bos taurus) for 36 h. The pattern of 15N excretion in urine and feces was similar for both cows. The 15N appeared in urine by 8 h and in feces by 24 h, and peaked by 30 h in urine (1.642 atom % 15N) and by 54 h in feces (2.341 atom % 15N). Enrichment approached basal levels at 132 h after initial feeding for both urine and feces. Of the total 15N fed, 60% was recovered: 31% from urine and 29% from feces. Approximately 60 to 70% of the total N excreted in dairy feces 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 15N‐enriched feed would produce feces having uniformly labeled N components. The various 15N‐enrichment levels of urine and feces collected during different times after feeding offer possibilities for studying differential 15N use in short‐ and long‐term nutrient cycling studies.