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

proach, 12 to 63% of dairy manure N may be taken up by corn during the first growing season after application Current estimates of dairy manure nitrogen availability to crops (Motavalli et al., 1989; Klausner et al., 1994). Nutrient are based on indirect measures and vary greatly. The objective of this study was to differentially label dairy manure N components with the availability in the second and subsequent years can be stable isotope 15N for direct measurement of manure N cycling in more difficult to predict. The difference method assumes soils and availability to crops. Dairy urine and fecal N components that the difference in total nutrient uptake between (microbial and undigested feed N) were differentially labeled by feedmanure-amended and nonamended plots are attributed ing either 15N-enriched forage or urea to mature dry dairy cows (Bos to the addition of manure. The fertilizer equivalent aptaurus). Nitrogen-15–enriched ammonium sulfate was used to label proach compares crop N uptake in manureand fertilalfalfa (Medicago sativa L.) hay and corn (Zea mays L.) silage. These izer-amended plots (Klausner and Guest, 1981; Harm15N-enriched forages or either single or multiple doses of 15N-enriched sen and Moraghan, 1988; Motavalli et al., 1989; Muñoz urea were fed for 2 to 3 d, and feces and urine were collected separately et al., 2004). The fertilizer equivalent of manure is the for 8 d after the initiation of 15N feeding. For both labeling techniques, amount of fertilizer N required to achieve the same yield 15N appeared first in urine followed by fecal microbial and undigested feed N. For the forage method, the proportionate combination of and N uptake achieved with manure. Both the differfeces excreted before and after peak 15N excretion levels would achieve ence method and the fertilizer equivalent approach asuniform labeling of fecal N components. For the urea method, no undisume that crop N uptake in the manure-amended, fertilgested feed N in feces was labeled since 15N-enriched forage N was izer-amended, and control plots are accomplished with not fed. The choice of which labeling method to use depends on the the same efficiencies. However, whereas approximately intended use of labeled manure. Manure enriched using the forage half (or more) of manure N is organically bound and method and high levels of manure 15N enrichments should be used for must be mineralized by soil microbes before becoming long-term manure N cycling studies. Manure enriched using the urea available for crop uptake, fertilizer N is more water solumethod and lower 15N enrichments could be used in shorter-term ble and potentially more readily available for crop studies. uptake. The stable isotope 15N has been used extensively to evaluate the availability of fertilizer N to crops. The use O the past 25 yr, the fertilizer value of dairy of 15N to determine the availability of manure N to crops manure has become less appreciated as the use of has been studied using two approaches: (i) postexcretion inexpensive, high-analysis and custom-blend fertilizers 15N enrichment of the NH4 pool or (2) 15N enrichment have become widespread. On many dairy farms, manure of feedstuffs, which are then fed to ruminant livestock. has become an undesirable by-product of milk producWhereas approximately 50 to 60% of the N in slurry is tion, and most connotations of its intrinsic fertilizer in the NH4 form (Dittert et al., 1998), semisolid dairy value have been replaced with an animal waste mentalmanure, the most important manure type on Wisconsin ity. For example, when calculating fertilizer application dairy farms (Jackson-Smith et al., 1997), typically conrates for field crops, many dairy farmers do not credit tains much lower amounts of NH4 and higher amounts the nutrients they applied in the form of manure (Nowak of organically bound N. The chemical composition and et al., 1997). This may be due to many factors, including mineralization of organic N in dairy manure are not a perception that manure is an undependable nutrient well understood. More accurate estimates of manure N source. availability to crops are needed if we are to expect Manure nutrient availability to crops is estimated by farmers to improve manure management. indirect methods and varies widely. For example, using Only 20 to 30% of the N (protein) fed to a dairy the difference method and the fertilizer equivalent apcow is converted into milk, with the remaining excreted about equally in urine and feces (Castillo et al., 2000; J.M. Powell, USDA-ARS, Dairy Forage Res. Cent., 1925 Linden Broderick, 2003). Fecal N can be divided into two pools: Drive West, Madison, WI 53706; Z. Wu, Dep. of Dairy and Anim. (i) endogenous N consisting of microbial products and Sci., Pennsylvania State Univ., 324 William L. Henning Bldg., Univermicroorganisms from the rumen, the intestine, and the sity Park, PA 16802; and K. Kelling, P. Cusick, and G. Muñoz, Dep. hind gut, and the N originating from the digestive tract of Soil Sci., Univ. of Wisconsin–Madison, 1525 Observatory Drive, Madison, WI 53706. Received 7 Oct. 2002. *Corresponding author itself; and (ii) undigested feed N (Mason and Frederik([email protected]). sen, 1979). Rumen microbial products and other endogPublished in Agron. J. 96:433–441 (2004).  American Society of Agronomy Abbreviations: NDF, neutral detergent fiber; NDIN, neutral detergent insoluble nitrogen. 677 S. Segoe Rd., Madison, WI 53711 USA