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Discipline: carbon footprint; Keywords: methane, nitrous oxide, volatile organic compounds, ammonia, LCA, emissions per unit of production.
This question was discussed in an Invited Review by S.E. Place and F.M. Mitloehner of the University of California in the US, in a paper entitled: Contemporary environmental issues: A review of the dairy industry’s role in climate change and air quality and the potential of mitigation through improved production efficiency, published in the Journal of Dairy Science, Volume 93 of 2010, pages 3407 to 3416.
The dairy industry contributes to the greenhouse gas (GHG) associated climate change by its emissions of methane (CH4) and nitrous oxide (NO2) and to air quality by particulate matter, volatile organic compounds and ammonia (NH3). Methane originates mainly from rumen fermentation as a byproduct constituting an energy loss to the cow, whereas NO2 is emitted primarily from stacked manure. The concern regarding air quality is because pollution affects human and animal respiratory health, ecosystem health and visibility. The dairy industry contributes to air pollution by emissions from cattle, cropping systems, feed management and waste.
In order to limit these emissions or mitigating their effects one has to calculate the extent thereof, and that is not that easy because of the complexity and variation in dairy production systems. Worldwide so-called life cycle assessments (LCA) are used to capture the “cradle-to-grave” GHG and air quality emissions from both inputs and outputs over the entire dairy chain, which is comprehensive, but the only way to allow comparison between systems is to calculate the emissions per unit of production (say per kilogram of 3.5% fat-corrected milk [FCM]). Any input in relation to output is of course a measure of efficiency which allows one then to evaluate the effects of reduction of these emissions on primary indicators affecting efficiency in dairy operations. This is the approach adopted by the authors.
Production efficiency in the dairy industry in this context can be defined as minimizing the amount of inputs (example feeds, fossil fuels) and outputs (example NH3, GHG) to produce a given quantity of milk. Improvement can come from minimizing waste, maximizing a dairy cow’s milk production and maximizing the proportion of her life spent in peak milk production without sacrificing animal health and well-being. To a degree, when milk production per cow is improved, the life-cycle emissions of dairy production decrease per unit of milk. This results in a number of ways. In the first instance the maintenance costs of cows are diluted: in cows that produce more milk the proportion of total consumed feedstuffs going toward maintenance energy is reduced. Secondly, more milk per cow can decrease the total lactating herd size needed to produce a certain amount of milk; to explain by way of research results: historical advances in genetics, nutrition and management of dairy farms in the US allowed dairy production in 2007 to emit 43% of the CH4 and 56% of the N2O that were emitted in 1944 to produce one billion kilogram of milk. Heifer management poses a challenge. Replacement heifers are an important part of the life-cycle emissions of a kilogram of FCM. Before calving, heifers consume inputs and produce both GHG and air pollutants without contributing to the production of milk.
The way to address this is to intensify heifer raising to lower age at first calving, without compromising first-lactation milk yield. This requires excellent health and growth of young calves, in addition to paying specific attention to post-weaning feeding and management. Overall, replacement strategies, reproduction and dry cow management should aim to maximize the number of cows in milk and the number of lactations per cow, because that will support the lowest life-cycle emissions per kilogram of FCM on the farm. Maximizing the number of lactations per cow and reducing inter-calving periods (emissions without milk) requires excellent herd health and management which in our dairy industry is cause for concern. A final way of reducing emissions is by way of nutrition. Through feed composition, so-called precision feeding and by adding specific additives one can reduce both enteric and waste emissions. Effective feeding programs can change the microbial population and increase their efficiency, thereby limiting energy loss to CH4, as well as limiting nitrogen loss through excretion which will reduce N2O emissions from waste.
Bottom line: Maximizing milk per cow per lactation and across her productive life is the most efficient way of reducing GHG and air quality emissions per kilogram of milk produced, and this is irrespective of production system used.