Strategic grazing management and nitrous oxide fluxes from pasture soils in tropical dairy systems.

Greenhouse gases emissions are considered one of the most important environmental issues of dairy farming systems. Nitrous oxide (NO) has particular importance owing to its global warming potential and stratospheric ozone depletion. The objective of this study was to investigate the influence of two rotational grazing strategies characterized by two pre-grazing targets (95% and maximum canopy light interception; LI and LI, respectively) on milk production efficiency and NO fluxes from soil in a tropical dairy farming system based on elephant grass (Pennisetum purpureum Schum. cv. Cameroon). Results indicated that LI pre-grazing target provided more frequent defoliations than LI. Water-filled pore space, soil and chamber temperatures were affected by sampling periods (P and P). There was a significant pre-grazing target treatment × sampling period interaction effect on soil NH concentration, which was most likely associated with urinary-N discharge. During P, there was a greater urinary-N discharge for LI than LI (26.3 vs. 20.9 kg of urinary-N/paddock) caused by higher stocking rate, which resulted in greater NO fluxes for LI. Inversely, during P, the soil NH and NO fluxes were greater for LI than LI. During this period, the greater urinary-N discharge (46.8 vs. 44.8 kg of urinary-N/paddock) was likely associated with longer stocking period for LI relative to LI since both treatments had similar stocking rate. Converting hourly NO fluxes to daily basis and relating to milk production efficiency, LI was 40% more efficient than LI (0.34 vs. 0.57 g N-NO/kg milk·ha). In addition, LI pre-grazing target decreased urea-N loading per milk production by 34%. Strategic grazing management represented by the LI pre-grazing target allows for intensification of tropical pasture-based dairy systems, enhanced milk production efficiency and decreased N-NO emission intensity.