ANIMAL WASTE MANAGEMENT Nitrogen-15 Labeling of Dairy Feces and Urine for Nutrient Cycling Studies

of factors that make manure an undependable source of plant nutrients, including differences in soil fertility Estimates of the availability of dairy manure nutrients to crops levels where manure application experiments have been rely on indirect measurements and can vary greatly. More accurate estimates of manure nutrient availability are needed to improve maconducted, inherent shortcomings of the fertilizernure management. The objective of this study was to enrich dairy equivalent approach for estimating nutrient availability feces and urine in 15N to study nutrient flow in the feed–animal– (Harmsen and Moraghan, 1988), and other problems. manure–soil and crop–environment continuum. Ammonium sulfate For example, the N and P contents (mean 6 1 SD) of (12.3 or 10 atom % 15N) was applied to soil to enrich alfalfa (Medicago liquid dairy manure (3.4 6 1.32 and 1.6 6 1.08 g L21) sativa L.) and corn (Zea mays L.) plants during growth. Alfalfa hay and of solid dairy manure (5.0 6 1.5 and 3.0 6 1.5 g kg21) contained from 2.386 to 3.980 atom % 15N in three harvests and corn are highly variable (Combs, 1991). Nutrient release from silage contained 8.162 atom % 15N. A feed mixture containing 55% manure is also highly variable. Using the fertilizer equivalfalfa hay and 45% corn silage (4.026 atom % 15N) was fed to two alent approach, from 12 to 63% of dairy manure N and mature nonlactating cows (Bos taurus ) for 36 h. The pattern of 15N from 12 to 89% of dairy manure P have been estimated 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 to be taken up by corn during the first growing season urine (1.642 atom % 15N) and by 54 h in feces (2.341 atom % 15N). after application (Motavalli et al., 1989; Klausner et al., Enrichment approached basal levels at 132 h after initial feeding for 1994). Nutrient availability in the second and subseboth urine and feces. Of the total 15N fed, 60% was recovered: 31% quent years can be even more difficult to predict. from urine and 29% from feces. Approximately 60 to 70% of the The stable isotope 15N has been used extensively to total N excreted in dairy feces was endogenous N and 30 to 40% was evaluate the availability of fertilizer N to crops (Hauck undigested feed N. Combining feces excreted during the 16to and Bremner, 1976; Menzel and Smith, 1984), but its 122-h period after initial feeding of 15N-enriched feed would produce use in nutrient cycling studies involving animal manure feces having uniformly labeled N components. The various 15N-enrichhas been limited (Sørensen et al., 1994). This has been ment levels of urine and feces collected during different times after due in part to the high cost of 15N and the large quantities feeding offer possibilities for studying differential 15N use in shortand long-term nutrient cycling studies. needed to enrich sufficient forage for feeding and labeling feces and urine. Also, the homogeneous 15N labeling of manure N components must be assured. Ruminant fecal N consists of both endogenous N and N remaining T economic value of animal manure depends on in undigested feed. Endogenous N is composed of N its ability to provide nutrients to crops. The timely contained in microbial products and microorganisms delivery and application of known amounts of manure from the rumen, the intestine, and the hind gut, as well nutrients to specific fields form the primary basis for as the N originating from the digestive tract itself. A proper manure management. Estimates of manure nudisproportionate labeling of fecal N components may trient availability to crops, otherwise known as nutrient lead to great error in determining the rate and extent credits, are currently single N, P, and K values given for of manure 15N mineralization in soils (Sørensen et al., the type of manure applied (solid or liquid) and method 1994). Our objective in this experiment was to study of application (incorporated or not). Nutrient credits the 15N-enrichment pattern of feces and urine from nonare adjusted to account for multiple years of manure lactating dairy cows that were fed 15N-enriched alfalfa application, residual nutrient availability, and other hay and corn silage. The relative 15N enrichment of undisuch factors. gested feed N and endogenous N in feces was also Although it has been shown that proper manure manstudied. agement can be profitable through reduced fertilizer costs, many farmers do not credit the nutrients conMATERIALS AND METHODS tained in manure (Nowak et al., 1997). For example, in areas where manure has been land-spread, many farmCorn (NK hybrid N1500) and alfalfa (Cenex cv. Trailblazer) ers continue to apply fertilizers in sufficient quantities plants were enriched in N at the University of Wisconsin for attaining desired crop yield. The lack of manure Hancock Research Station (44879 N, 898329 W) on a Plainfield sand (mixed, mesic Typic Udipsamments) during the 1997 nutrient crediting by farmers may be due to a number cropping season. The plot for corn (32 700 plants ha2) was fertilized with N, P and K at rates of approximately 6, 5, and USDA-ARS Dairy Forage Res. Ctr., 1925 Linden Dr. West, Madison, WI 53706. Received 3 Sept. 1998. *Corresponding author (jmpowel2 Abbreviations: FNUE, fertilizer N use efficiency; NDF, [email protected]). gent fiber; NDIN, neutral-detergent insoluble N; NDSN, neutraldetergent soluble N; PVC, polyvinyl chloride. Published in Agron. J. 91:814–818 (1999).