Feed-milk-manure nitrogen relationships in global dairy production systems

Nitrogen (N) inputs from fertilizer, biologically-fixed N, feed, and animal manure sustain productive agriculture. Agricultural systems are limited however in their ability to incorporate N into products, and environmental N losses may become local, regional and global concerns. The anticipated increases in global demand for food, especially for animal products, necessitate an urgent search for practices that enhance N use efficiency (NUE) and reduce environmental N loss. The objectives of this study were to determine feed–milk–manure N relationships for the global dairy herd, and to evaluate how well regional and production system determinations of these relationships correspond to measurements made under experimental conditions and on commercial dairy farms. Data on dairy cattle populations, feed and milk production from 142 countries were used in a Life Cycle Assessment (LCA) model to determine dry matter intake (DMI), N intake (NI), the percentage of NI secreted as milk N (NUE-milk), the percent of NI used by the whole-herd (NUE-herd), and manure N excretion (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 5000 kg/AU; and NUE-milk and NUE-herd are 16.0% and 15.6%, respectively. Approximately 37% of global lactating cows have NUE-milk of <10% and these low efficient cows account for 10% of the milk production and 33% of Nex globally. Approximately 30% of global lactating cows have NUE-milk between 21% and 25% and these cows account for 53% of the milk production and 35% of Nex globally. Estimates of NUE-milk and Nex corresponded well to measurements under experimental and commercial farm conditions. Study determinations of Nex were however 3–50% 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.