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GREENHOUSE GAS EMISSIONS AND CARBON STORAGE IN PASTURES

The emission of greenhouse gases by livestock has been in the spotlight for about a decade now. Activists have criticized livestock farming, particularly cattle farming, as being a major contributing factor in the accumulation of methane and carbon dioxide in the atmosphere. This is not denied, but in South Africa the contribution compared to some other countries is rather modest. In terms of mitigation, there are two avenues that dairy farmers can follow to limit the carbon footprint: 1) by way of increasing carbon sequestration or storage in pastures and soil, and 2) by limiting emissions. In two studies (referenced below), the potential is shown.  

On the south-eastern seaboard milk production is primarily from cultivated pastures. Before 1990 monoculture pastures were conventionally established with deep tillage, resulting in deterioration of soil quality and loss of organic carbon. Since the late-nineties minimum tillage practices were introduced, including the successful pasture management system of kikuyu over-sown with ryegrass. This has improved soil quality and carbon has been drastically sequestered. In an experiment on soil analysis from Swellendam to Humansdorp, soils from the kikuyu-ryegrass management system, shallow tilled and deep tilled (conventional) recorded carbon contents of 50.3kgC/m3, 54.3kgC/m3 and 34.6kgC/m3 respectively. This represents an improvement of 50% in soil carbon stocks above conventional tilling.

Pastures established with minimum tillage including the kikuyu-ryegrass management system now comprise 70-80% of commercial dairy farms between Swellendam and Humansdorp. Crude estimates indicate that the region is about 60 000ha and carries about 240 000 dairy cattle (normal herd composition). The improvement in soil carbon stocks on the 60 000ha amounts to 10.4 million ton carbon, whereas the methane emission of the 240 000 dairy cattle amounts to 0.6 million ton carbon dioxide equivalent per year. The calculation indicates the potential if soil carbon storage can be improved by management that emphasizes limited disturbance to the soil.

Since the kikuyu-ryegrass pasture management system is a highly successful system in the south-eastern seaboard as discussed, the methane emissions of cows are measured and compared with the International Panel for Climate Change (IPCC) guidelines. The cows are supplemented with different levels of concentrates on pasture. Preliminary results suggest that the methane emission is 7% lower than the IPCC guidelines and methane emissions can be reduced further by strategic concentrate supplementation. If the 240 000 dairy cattle is about correct, the methane emission of the area is in fact 0.6 million ton carbon dioxide equivalent minus 7%, which is equal to 0.56 million.

Bottom line: Although there is no direct relationship between emission and storage, the amount of carbon that can be stored by sensible pasture management is much more than what is emitted to the atmosphere. Storage can be further increased if dairy farmers plant trees on unutilised areas, whereas emissions will be reduced with savings in energy and fuel use.

References:

Swanepoel, P. A., Habig, J., du Preez, C.C., Snyman, H.A. & Botha, P.R., 2016. Tillage effects, soil quality and production potential of kikuyu–ryegrass pastures in South Africa. Grass and Forage Science, doi: 10.1111/gfs.12241.

Van Wyngaard, J., 2016. How to reduce on farm enteric methane production. In: Milk Production from planted pasture, Information Day: Outeniqua Research Farm, 19 October 2016, George.