ESTIMATES OF GREENHOUSE GAS EMISSIONS ON PASTURE-BASED DAIRY FARMS.

The study was conducted employing a system dynamics model which has been developed to assist dairy farmers to calculate and monitor the impact of environmental indicators. Farm information was collected from six dairy farms on the south-eastern seaboard varying in cow-in-milk numbers between 260 and 706 and milk productions between 4600 and 8000kg FPCM per year.

The important greenhouse gas (GHG) emissions estimated in the model were methane (CH₄), nitrous oxide (N₂O) and direct carbon dioxide (CO₂) emissions which add up to the total GHG emissions. The totals per farm are shown in Figure 1. The results showed comprehensive differences between farms which, as expected, are well correlated with animal numbers but surprisingly not milk production. For example, milk production of Farms 1 and 2 was similar but the GHG of Farm 1 was only about 50% of that of Farm 2, and Farm 3 had the highest milk production but the third lowest GHG emissions.

Figure 1:

Since milk is the product the consumer buys, it makes sense to also compare emissions per unit of milk between farms. The CO₂ equivalent (eq) emissions per kg FPCM are therefore shown in Figure 2. Methane’s global warming potential (GWP) in this instance was accepted as 8 times that of CO₂ according to the latest information instead of the previously accepted 28-32 times, whereas the GWP of N₂O was accepted as 310 as proposed by the IPCC. The results show that Farm 3 and Farm 6 had the highest emissions per kg FPCM. Farm 3 had the biggest cows and Farm 6 used the highest amounts of chemical N fertiliser. Farm 2 had the largest direct emissions mainly due to electricity usage for irrigation and from diesel usage.

Figure 2: