Dairy Diet Impacts on Fecal Chemical Properties and Nitrogen Cycling in Soils

Availability of manure nitrogen (N) to crops is mitigated by many factors including manure type and composition. Whereas relationships between dairy diets, milk production, manure N excretion, and urine N losses as ammonia have been documented, very little information exists on how diets impact fecal carbon (C), N content, and partitioning, and how these factors impact fecal N mineralization and plant N uptake after application to soil. Feces from 24 to 63 dairy cows (Bos taurus) fed 14 typical diets were incubated aerobically in a sandy loam and two silt loam soils, and soil inorganic N (IN) was determined periodically during a 365-d period. Feces from 12 of the 14 diets were applied to the same soils and oat (Avena sativa L.), sorghum (Sorghum bicolor L. Moench), and sorghum ratoon dry matter (DM) and N uptake were determined over a 155-d period. Feces from cows fed alfalfa (Medicago sativa L.) silage (AS)-based diets generally lead to higher soil IN levels than soils amended with feces from corn (Zea mays L.) silage (CS)-based diets, especially in soils amended with feces from CS-low crude protein (LCP) diets; feces from AS-based diets increased plant DM and N uptake; after application to a silt loam, feces from high crude protein (HCP) diets resulted in greater soil IN levels than feces from LCP diets; and feces from LCP diets did not impact soil IN but decreased plant DM and N uptake. Carbon to N (C/N) ratios of applied feces were found to be significant predictors of plant DM and N uptake. There appears to be a range of dietary options that satisfy nutritional requirements of high-producing dairy cows and produce feces having differential effects on soil N mineralization and plant N uptake after application to soil. DAIRY COWS and replacement heifers in the Midwest and Northeast regions of the USA are fed primarily homegrown feed from crop rotations comprising alfalfa, corn, oats and soybean (Glycine max L. Merr.), and protein and mineral supplements are purchased to balance dairy rations. To remain economically viable, many farms are increasing herd size and replacing alfalfa with CS in crop rotations. Much alfalfa protein is degraded in the silo and rumen, impairing protein utilization and milk production unless more costly rumen undegraded protein supplements are fed. Such supplementation increases the cost of milk production, manure N excretion, and N loss from farmland. Substitution of most dietary alfalfa with CS and soybeans reduces excessive protein breakdown in the rumen, manure N excretion by livestock, and the cost of milk production. Dairy cows utilize feed N much more efficiently than many other ruminant livestock, but under current feeding practices, only 20 to 30% of the crude protein (CP) (N 3 6.25) fed to dairy cows is secreted in milk. An average cow annually producing 8200 kg of milk also excretes 21000 kg of manure containing about 110 kg N (Van Horn et al., 1996). Under current common feeding practices for lactating cows fed in confinement in the Midwest and Northeast USA, N not secreted in milk is excreted about equally in feces and urine, but this can be affected dramatically by diet (Broderick, 2003; Wattiaux and Karg, 2004a). Fecal N can be divided into two general pools: (1) endogenous N consisting of microbial products and microorganisms from the rumen, the intestine, and the hind gut, and the N originating from the digestive tract itself; and (2) undigested feed N (Mason and Frederiksen, 1979). For ruminant livestock, fecal N partitioning can be influenced highly by diet (Somda et al., 1995). Whereas rumen microbial products and other endogenous, organic N forms in feces make a significant contribution to crop N requirements the year following manure application, undigested feed N in feces mineralizes slowly in soil, and is therefore unavailable to plants over the short-term (Sørensen et al., 1994; Somda et al., 1995; Powell et al., 1999). After having already passed through the digestive track of livestock, the undigested feed N component of feces is relatively recalcitrant in soils. Much recent knowledge has been generated about relationships between dietary phosphorus (P), cow performance, the amount and form of P excreted in manure, and its environmental impacts (Satter et al., 2005). Relatively little is known, however, about relationships between the type and amount of forage and CP fed to dairy cows, manure N composition, and cycling in soils. In an effort to examine relationship between dairy feed and manure N mineralization, Van Kessel and Reeves (2002) incubated unfed dairy diet components with soil and tracked soil IN. In a study with sheep (Ovis aries L.), however, Powell et al. (1999) found that unfed diet components and feces derived from the same feed components have very different physical and chemical characteristics, which after application to soil, impact plant yield and N uptake. In a more recent study, Sørensen et al. (2003) determined positive relationships between dietary CP, and negative relationships between dietary neutral detergent fiber (NDF) and mineralization of N contained in slurry from dairy cows fed various diets. These dairy diets fed in Europe, however, comprised forages and supplements which differ considerably from what is fed to dairy cows in confinement operations of the USA. To test an overall hypothesis that diets impact J.M. Powell, G.A. Broderick, andM.D. Casler, USDA-ARSU.S. Dairy Forage Research Center, Madison, WI 53706; M.A. Wattiaux, Dep. Dairy Science, Univ. of Wisconsin, Madison, WI 53706; V.R. Moreira, Louisiana State Univ. Agric. Center Southeast Research Station, Franklinton, LA70438.Received 30Aug. 2005. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 70:786–794 (2006). Soil Fertility & Plant Nutrition, Nutrient Management & Soil & Plant Analysis, Landscape Management doi:10.2136/sssaj2005.0286 a Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: AH, alfalfa hay; AS, alfalfa silage; C/N, carbon to nitrogen ratio; CS, corn silage; CP, crude protein; DM, dry matter; F, fiber; HCP, high crude protein; HF, high fiber; IN, inorganic nitrogen; LCP, low crude protein; LF, low fiber; MF, medium fiber; NDF, neutral detergent fiber. R e p ro d u c e d fr o m S o il S c ie n c e S o c ie ty o f A m e ri c a J o u rn a l. P u b lis h e d b y S o il S c ie n c e S o c ie ty o f A m e ri c a . A ll c o p y ri g h ts re s e rv e d . 786 Published online March 29, 2006