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:

Interestingly, emissions per ha demonstrate a very different pattern than emissions per kg FPCM. For example, Farm 1 showed the highest GHG emissions per ha (Figure 3), whereas although Farm 6 had the highest level of chemical N fertiliser application, due to other efficient strategies it didn’t show the highest emissions per ha. In addition, Farm 6 was also the biggest farm for home grown feed production, which are beneficial compared to bought-in feeds. The large variation in results suggest that opportunities exist for higher efficiency with pasture growth, yield, type of pasture and fertiliser management to enable emission reduction strategies and increased efficiencies.

Figure 3:

In conclusion: The systems dynamic model appears effective in providing dairy farmers with a tool to (1) effectively calculate on farm GHG emissions, and (2) show areas where farmers can zoom in to decrease emissions, one being level and efficiency of production which is negatively correlated with GHG emissions.