Dairy farming is responsible for a significant release of GHGs from various aspects along the production process. Globally the dairy sector contributes 4% of total anthropogenic GHG emissions. Most of these emissions are from the biological processes that underpin the daily rhythms of the cow, such as feeding and dunging and are inherent in the production of milk. However, as with most complex biological processes, there are a range of factors that influence the scale of these emissions and many of them are open to management changes and improvements.
The most significant emission is from 'enteric fermentation' from the cows themselves as the micro-flora in their rumens breakdown the forage, with the subsequent release of methane (CH4) which is then emitted out by the cow. If the dairy industry is to meet the growing global demand for dairy products, ways to minimise greenhouse gas emissions per unit of product will become increasingly important.
Improving milk production through livestock feeding and genetics, has been advocated as a promising approach for reducing GHG emissions from dairy production systems. This recently has been investigated in a study published in Livestock Science from the Scottish Rural University College. The study investigated the emissions intensity output of high producing dairy systems. It compared the results for cattle fed a high forage and low forage diet, and within each group compared cattle with high genetic merit (top 5% of UK genetics for milk fat and protein) with control animals. Data was analysed using Life Cycle Analysis to evaluate the effects of the rations on the whole emissions picture from the farm (so including effect on emissions from manure, fuel used to grow the crops for the diets and nitrous oxide emissions from soil).
What they did
Animals were placed into two feeding groups, high forage and low forage. The high forage group were fed a TMR ration, of which 75% was made up from home grown forage crops, and the remainder from concentrates (that were bought in). The high forage group were also grazed outside on ryegrass when conditions allowed (through March to November).
Animals in the low forage group were fully housed all year round and fed a TMR ration which was made up with 45% forage and 55% purchased concentrate feeds.
Within each forage group, there were two contrasting sets of animals in terms of genetics. Control animals were bred to be of average genetic merit for milk fat and protein production, and select animals represented the top 5% of UK genetic merit.
What they found out
This was a long term experiment that looked at emissions from the whole farm system. There was huge amounts of data that was all crunched together by some very clever people and the main results are below.
If you want to read the full paper (and fully digest the stats!) then follow this link.
The most GHG efficient system was defined as having the lowest emissions intensity per unit of product. The low forage diet with the selected genetic merit cattle was the most GHG efficient system. The high forage diet control cows had the highest emissions intensity. Looking at the split between the three most greenhouse gases, methane contributed the highest to the overall GWP (global warming potential), comprising 51-52% of the total, and on-farm CO₂ emissions making the lowest contribution in all systems.
What does this mean?
This research then suggests that there is potential to reduce the GWP per unit of milk yield of a typical conventional dairy system by up to 24%. By improving the genetic merit on its own, a reduction of 9% could be possible. Genetic improvements obviously take time through breeding and could realistically take several years to return results. Long term results from this study however found that the higher genetic merit delivered an 18% increase in milk yield and contributed significantly to lowering overall emissions intensity.
When looking at dietary strategies, the results suggest that switching to the low forage regime holds potential to reduce GWP by up to 16% per unit of milk production.
The results in this study agree with the findings of previous studies who found that improving milk yield of the herd would significantly reduce enteric methane emissions and overall emissions per unit of milk. Furthermore results of this study confirm that implementing a low forage regime reduced the GWP per kg of milk production irrespective of the cow’s genetic merit. It is important to remember however that when looking at the whole farm system, the low forage regime is much more sensitive to the by-products market than home grown forages.
For more information on strategies to reduce methane emissions from cattle, please visit the dairy section of the Toolkit.
Source: Ross, S.A et al., Effect of cattle genotype and feeding regime on greenhouse gas emissions intensity in high producting dairy cows. Livestock Science (2014), http://dx.doi.org/10.1016/j.livsci.2014.09.011