Integrating livestock production with crops and saline fish ponds to reduce greenhouse gas emissions

Abstract

Extensive grazing systems in the tropics have large greenhouse gas (GHG) emissions. Here an integrated cropping, livestock farming and saline pond aquaculture system is described which also reduces GHG emissions in tropical farming. The system was developed in the 1980s in the central Philippines and is re-evaluated in terms of its potential to reduce emissions from livestock production, particularly in wet and wet-dry tropical regions. The estimated cumulative annual GHG emission savings amount to 12.9 tonne CO2-e per head of cattle exiting the integrated feedlot system after 300 days. Use of silage from crop by-products and implementation of a feed-lot programme enabled immediate and significant reductions of GHG emissions over extensive cattle grazing. Integration with a saline animal waste processing pond allowed production of natural feed products, cyanobacteria algal mat for use in aquaculture, as crop fertiliser and soil amendment or in the silage manufacture process. Fish production from ponds receiving the harvested algal mat resulted in an annual average of 3300 kg/ha compared to an average 700 kg/ha in inorganic fertilised ponds. The unitised ratios for the integrated system were respectively: 1 ha sugarcane: 4 head of cattle: 0.13 ha saline waste processing pond. Stoichiometrically, bio-processing of waste material in saline ponds provides a novel solution to GHG emissions from feedlots. It uses a multi-step process dominated by cyanobacteria mats in which all the captured liquid and solid waste is passed through fermentation and a mixed aerobic and anaerobic photosynthesis/photosynthesis and mixed aerobic/anaerobic sulphate reduction pathway to use organic wastes and also reduce GHG emissions. Sulphate is reduced and sequestered as iron sulphide eliminating the potential for harmful sulphide emissions. Calcium carbonate precipitation in the cyanobacteria mats results in significant sequestration of carbon. The process produces organic feed matter with a three-fold increase in nitrogen levels arising from nitrogen fixation in the treatment pond.