How to Allocate Greenhouse Gas Emissions from Dairy Production

The dairy sector is urged to reduce negative environmental impacts. But dairy is not only milk: depending on the production system and cow breed, relevant amounts of meat as by-product result. How can emissions thus be allocated to milk and meat?

Dairy cattle produce a substantial proportion of global greenhouse gases from agricultural activities. Over half of these are caused by methane emissions from enteric fermentation (microbial digestion in the rumen). The dairy sector is thus urged to take measures to reduce greenhouse gas emissions. However, not all dairy cattle emissions are allocated to milk, as there is always meat associated.  Depending on the region, surplus calves and culled dairy cows sum up to more than 90% of all beef produced. The question thus arises, what proportion of the emissions have to be allocated to milk and its by-product meat respectively.  Different approaches are applied in lifecycle assessment to allocate emissions to main- and by-products. The most commonly used and dairy industry standard is the biophysical allocation approach. It allocates emissions from dairy farms by the same ratio the energy contained in the feeds was required for lactation (milk) and body mass growth (meat).  

A new allocation approach

The industry standard proposed by the international dairy federation (IDF) was empirically developed based on a US dataset mainly covering very specialized dairy farms with low proportions of meat as by-product. The proportion of the by-product meat to the milk output is considered beef-to-milk-ratio (BMR). The training dataset only covered low BMR from intensive dairy production systems, whereas systems which larger proportions of meat output were neglected.  

We demonstrate that farms with larger BMR (such as production systems with dual-purpose breeds in Switzerland) are not sufficiently represented when applying the standard allocation approach. Based on an international dataset we propose a developed biophysical allocation approach that is suited to adequately represent farms with larger BMR.  

The choice of the allocation approach further impacts the assessment of the effectiveness of greenhouse gas mitigation measures in dairy production. Some measures will reduce emissions from both, milk and meat, similarly. However, some measures affect the BMR and their quantification relies strongly on the choice of the allocation method.

For instance, increased longevity of dairy cows is considered to be an effective approach. By increasing the productive lifespan of a dairy cow, the number of replacement heifers can be reduced.  Emissions associated with the «unproductive» rearing of young stock are thus reduced, too. However, longer living cows also means less culled cows and thus less meat output as by-product from milk production. When the standard biophysical allocation approach proposed by the IDF is applied, an additional lactation would not reduce greenhouse gas emissions. Contrary, if the newly developed approach is used, greenhouse gas emissions per kg of milk are slightly reduced, if cows were to be used for an additional lactation.