Nitrogen Use Efficiency in Global Dairy Production Systems.

The anticipated increases in global demand for food, especially for animal products, necessitate an urgent search for practices that enhance nitrogen use efficiency (NUE) and reduce environmental nitrogen (N) loss from agricultural production. The objectives of this study were (1) to determine feed-milk-manure N relationships for the global dairy herd, (2) to evaluate how well regional (Africa, Central-South America, Asia, and Europe-North America-Oceania) and production system (confinement, grazing, hybrid confinement-grazing, and hybrid grazing-confinement) modelled estimates of these relationships correspond to actual measurements made under experimental and commercial farm conditions, and (3) to evaluate how well determinations of NUE by lactating cows and N excreted in manure (Nex) correspond to Intergovernmental Panel on Climate Change (IPCC) Tier 1 estimates of these parameters. Data on dairy cattle populations, feed and milk production from 144 countries were used in a Life Cycle Assessment (LCA) model to determine dry matter intake (DMI), N intake (NI), the percent of NI secreted as milk N (NUE-milk), the percent of NI used by the whole-herd (NUE-herd), and Nex.  On a global basis, an average lactating  cow weighs approximately 420 kg; per animal unit (AU=1000 kg live weight) daily DMI and NI are 21.0 kg and 477 g, respectively; annual milk production is 5345 kg/AU; and NUE-milk and NUE-herd are 16.0 and 15.6%, respectively. Approximately 60% of global lactating cows have NUE-milk of <10% and these low efficient cows account for 11% of the milk production. Approximately 12% of global lactating cows have NUE-milk of between 21 to 25%, and these cows account for 56% of the global milk production.  Estimates of NUE-milk and Nex corresponded well to measurements under experimental and commercial farm conditions.  Study determinations of Nex were 30 to 50% lower than IPCC Tier 1 values of Nex for Africa, Middle East, Latin America and Asia. The apparent accuracy of LCA model calculations of Nex should enhance regional, production system and global determinations of dairy Nex loss during collection, storage, and land application, and the amounts of Nex actually recycled through crops and pastures.