MEASURED VS PREDICTED ENTERIC METHANE EMISSIONS OF PASTURE-BASED DAIRY COWS

The Department of Environmental Affairs (DEA) developed a Mitigation, Reporting and Verification strategy in 2016 which provided a baseline for greenhouse gas emissions (GHG) for the Agriculture, Forestry and Other Land use (AFOLU) sector. The current livestock baseline emissions used by the DEA have been derived from data sources from 2011 to 2013 that were largely based on Intergovernmental Panel on Climate Change (IPCC) Tier 1 and 2 guidelines.  This data may not accurately reflect the South African (SA) situation as they are based on estimates from international models. Thus, for SA it is preferable to directly measure GHG emissions data for incorporation into the baseline document. In this presentation, therefore, the objective of the authors referenced below was to compare directly measured methane emissions from rumen fermentation (enteric) with the existing SA baseline enteric methane emissions for lactating, pasture-based dairy cows.

In their experiment, 36 multi-lactation Jersey cows were equally allocated to three treatments that differed in dairy concentrate feeding level: 0, 3.6 and 7.2 kg dry matter (DM) per cow per day. The cows grazed perennial ryegrass pasture allocated at about 12 kg DM per cow per day. Enteric methane was measured in all cows with the sulphur hexafluoride tracer gas technique. Otherwise, cow and feed parameters were used as input data as explained in the international Tier 2 equations.      

The DM digestibilities (DMD) of the grazed pasture and fed concentrate were 87% and 93%, respectively. The measured methane yield across treatments were not significantly different at 282, 335 and 281 g methane per cow per day for cows fed 0, 3.6 and 7.2 kg DM of concentrate, respectively. The predicted total DM intake was 11.3, 13.4 and 14.5 kg per cow per day respectively, using pasture disc meter measurements, and the predicted methane yield was 280, 311 and 326 g methane per cow per day for cows fed 0, 3.6 and 7.2 kg DM of concentrate, respectively. The results showed that the baseline data underestimated the measured methane emissions by 0.9% for zero concentrate and by 7.8% for 3.6% concentrate, but overestimated measured methane emissions by 13.8% for 7.2 kg DM of concentrate. 

The latter result is substantial as it suggests that with the widely used practice of concentrate feeding on pasture in the South-eastern Cape, the DEA methane emissions baseline for lactating, pasture-based dairy cows is not a true reflection. It is therefore advised that the SA baseline emissions should be updated with results from this study.