Factors explaining nitrogen use efficiency in pasture-based dairy systems in Southern Uruguay

Abstract

Nitrogen (N) Use Efficiency (NUE) is a critical indicator of environmental sustainability in dairy systems. This study evaluates NUE across 50 pasture-based dairy farms in southern Uruguay, encompassing a total herd of 9,683 animals, monitored monthly over one year as part of the MejorLeche project (supported by ANDE /MEF-2019). Cow NUE was defined as the ratio between N exported via milk production and dietary N inputs. Two levels of analysis were conducted: (1) the full farm dataset, and (2) a subset of efficient farms, defined as those with NUE>0.25. For the efficient farms, a filtered dataset was used to calculate and compare the average values of key variables: dairy herd size (number of cows), milk yield (kg fat-corrected milk/cow/day and per hectare), stocking rate (cows/ha), estimated dry matter intake (kg/cow/day) across feed types (pasture, conserved forages, and concentrates), and milk composition (fat, protein and urea). Results indicate that efficient farms did not necessarily rely on higher N input levels, but instead optimized N conversion into milk. High NUE was associated with greater productivity both per cow and per hectare, diets with lower protein content, and a more favourable nitrogen output-to-input ratio. Compared to farms with NUE<0.25, the efficient group had 43% larger herds, consumed 19% more concentrate per cow, and produced 29% more milk per cow and 56% more milk per hectare, without increasing milk protein content. These findings highlight the importance of feeding strategies in determining NUE. Higher NUE values appear to reflect better alignment between nutrient supply and animal demand. Therefore, NUE serves as a robust indicator -sensitive to productivity and N intake variations- to simultaneously assess productivity and environmental sustainability. It can be used to guide technical strategies toward more efficient and environmentally friendly dairy systems in the region.