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MANURE MANAGEMENT

Differential Nitrogen-15 Labeling of Dairy Manure Components for Nitrogen Cycling Studies

^a USDA-ARS, Dairy Forage Res. Cent., 1925 Linden Drive West, Madison, WI 53706
^b Dep. of Dairy and Anim. Sci., Pennsylvania State Univ., 324 William L. Henning Bldg., University Park, PA 16802
^c Dep. of Soil Sci., Univ. of Wisconsin–Madison, 1525 Observatory Drive, Madison, WI 53706

^* Corresponding author (jmpowel2{at}wisc.edu).

Received for publication October 7, 2002. Current estimates of dairy manure nitrogen availability to crops are based on indirect measures and vary greatly. The objective of this study was to differentially label dairy manure N components with the stable isotope ¹⁵N for direct measurement of manure N cycling in soils and availability to crops. Dairy urine and fecal N components (microbial and undigested feed N) were differentially labeled by feeding either ¹⁵N-enriched forage or urea to mature dry dairy cows (Bos taurus). Nitrogen-15–enriched ammonium sulfate was used to label alfalfa (Medicago sativa L.) hay and corn (Zea mays L.) silage. These ¹⁵N-enriched forages or either single or multiple doses of ¹⁵N-enriched urea were fed for 2 to 3 d, and feces and urine were collected separately for 8 d after the initiation of ¹⁵N feeding. For both labeling techniques, ¹⁵N appeared first in urine followed by fecal microbial and undigested feed N. For the forage method, the proportionate combination of feces excreted before and after peak ¹⁵N excretion levels would achieve uniform labeling of fecal N components. For the urea method, no undigested feed N in feces was labeled since ¹⁵N-enriched forage N was not fed. The choice of which labeling method to use depends on the intended use of labeled manure. Manure enriched using the forage method and high levels of manure ¹⁵N enrichments should be used for long-term manure N cycling studies. Manure enriched using the urea method and lower ¹⁵N enrichments could be used in shorter-term studies.

Abbreviations: NDF, neutral detergent fiber • NDIN, neutral detergent insoluble nitrogen

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