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 (N2O) 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; LI95% and LIMax, respectively) on milk production efficiency and N2O fluxes from soil in a tropical dairy farming system based on elephant grass (Pennisetum purpureum Schum. cv. Cameroon). Results indicated that LI95% pre-grazing target provided more frequent defoliations than LIMax. Water-filled pore space, soil and chamber temperatures were affected by sampling periods (P1 and P2). There was a significant pre-grazing target treatment × sampling period interaction effect on soil NH4+ concentration, which was most likely associated with urinary-N discharge. During P1, there was a greater urinary-N discharge for LI95% than LIMax (26.3 vs. 20.9 kg of urinary-N/paddock) caused by higher stocking rate, which resulted in greater N2O fluxes for LI95%. Inversely, during P2, the soil NH4+ and N2O fluxes were greater for LIMax than LI95%. 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 LIMax relative to LI95%, since both treatments had similar stocking rate. Converting hourly N2O fluxes to daily basis and relating to milk production efficiency, LI95% was 40% more efficient than LIMax (0.34 vs. 0.57 g N-N2O/kg milk·ha). In addition, LI95% pre-grazing target decreased urea-N loading per milk production by 34%. Strategic grazing management represented by the LI95% pre-grazing target allows for intensification of tropical pasture-based dairy systems, enhanced milk production efficiency and decreased N-N2O emission intensity.