Transformation of Nitrogen Contained in Alfalfa Silage, Corn Silage, Corn Grain and Soybean Meal into Milk, Manure and Recycled Back to Corn.

To better understand the transformative nature of feed nitrogen (N) on confinement dairy farms (cows fed stored feed in barns), a series of cow, laboratory and field experiments was undertaken to quantify the relative amounts of N contained in individual ration components that are secreted in milk, excreted in urine and feces, taken up by crops after manure application to soil, and lost as ammonia (NH₃) and nitrous oxide (N₂O) from barns and soils.  Alfalfa silage (AS), corn silage (CS), corn grain (CG) and soybeans (SB) were enriched in the field with the stable isotope ¹⁵N. Each ¹⁵N-enriched component was then fed individually to mid-lactation cows as part of a total mixed ration (TMR). Highest fertilizer ¹⁵N incorporation (atom % ¹⁵N) was achieved with CG (2.040) and CS (1.857), and lowest with SB (1.385) and AS (0.730) due to ¹⁵N dilution by the atmospherically-fixed N by these legumes.  The percent consumed feed N secreted in milk for AS, CS, CG and solvent-extracted soybean meal (SBM) was approximately 20.2, 19.2, 21.2 and 25.5 (P=0.178), respectively; feed N excreted in urine was 33.7, 26.2, 32.4, and 34.7 (P=0.024), respectively; and feed N excreted in feces was 46.5, 49.6, 47.7 and 40.9 (P=0.127), respectively. The percent of field-applied manure N from AS, CS, CG and SBM taken by corn (harvested as silage) was 33.4, 30.5, 34.7 and 38.1 (P=0.074). Feces and urine from the consumed ¹⁵N enriched AS, CS, CG and SBM ration components will be applied to laboratory emission chambers that simulate barn floors and field soil surfaces, and ¹⁵NH₃, ¹⁵NH₄ ¹⁵NO₃ and ¹⁵N₂O will be measured.