Improving greenhouse gas emissions intensities of subtropical and tropical beef farming systems using Leucaena leucocephala

Leucaena leucocephala (leucaena) is a perennial legume shrub of subtropical regions that has forage characteristics favourable for livestock production, often delivering ruminant liveweight gains that are superior to most other forage systems. Recent work suggests that leucaena mitigates ruminant enteric methane emissions, implying that the shrub may also reduce greenhouse gas (GHG) emissions at the whole farm level. However, the high crude protein content of leucaena relative to endemic grasses can increase livestock urine nitrogen concentration and may increase soil nitrous oxide emissions, potentially offsetting benefits of enteric methane mitigation.

Here we examine the effects of leucaena on emissions, production and profitability at the whole farm level by modelling a property in northern Australia, assuming enterprises that specialise in cattle breeding and finishing. To contrast leucaena with a baseline property with Rhodes grass, we modelled three equivalent leucaena scenarios by matching (1) annual average stocking rate, (2) liveweight turnoff or (3) net farm emissions with that of the baseline, assuming all animals had access to leucaena.

To maintain average annual stocking rate or liveweight turnoff, scenarios 1 and 2 carried 5% and 12% less cattle than the baseline because animals on leucaena grew faster and had greater liveweight. In contrast, the number of animals carried and liveweight turnoff in scenario 3 increased by 15% and 31% relative to the baseline, respectively, due to enteric methane abatement and greater liveweight gains. Grazing of leucaena increased soil nitrous oxide emissions by more than 38% in all scenarios, but this did not substantially offset net emissions abatement because nitrous oxide constituted a far smaller proportion of emissions than did methane (< 10% and > 90%, respectively). In all scenarios, emissions intensity (net farm emissions per unit liveweight sold) caused by grazing leucaena was reduced by more than 23% relative to baseline emissions intensities.

This work shows that while income from the Carbon Farming Initiative associated with grazing leucaena is small, leucaena has significant potential to increase both animal production and gross margin, whilst reducing emissions intensity. Provided net farm emissions are maintained or reduced, these results suggest that leucaena is conducive to sustainable intensification of beef production in subtropical grazing systems.