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The need to adopt management practices and technologies that improve productive efficiency to meet increasing product demand while minimizing the environmental impact of dairy production is becoming more and more important. The environmental impact of a system can be evaluated by assessing the global warming potential (GWP) in carbon dioxide equivalent (CO2-eq.) emissions and resource use (e.g. land use) associated with production. The land use associated with a dairy system, amongst others, reflects the cow's requirement for food.
In previous research it was found that a high-milk-producing system with good health and fertility status offers scope for minimizing methane (a major greenhouse gas) emissions per kilogram of milk by reducing the number of herd replacements retained and the calving interval length, and increasing the average daily milk yield of the herd. However, because of breeding largely for increased milk production in Holstein-Friesian dairy cows over the last 25 years, genetic merit for fitness traits such as health and fertility has declined. In particular, more days from calving to conception, conception failure, calving assistance, abortion and mastitis have become risk factors. Consequently, by taking fertility and health parameters into account it was found that the emissions of methane have risen by 11% per herd compared with 1995 levels in the UK.
This issue has been taken further by Dr Bell and co-workers in a study which was reported in the Journal of Dairy Science, Volume 94 of 2011, pages 3662 to 3678, with the title: The effect of improving cow productivity, fertility, and longevity on the global warming potential of dairy systems.
In their study they compared the environmental impact of a range of dairy production systems in terms of their GWP and associated land use, and explored the efficacy of reducing the impact. Models were developed from data of a long-term genetic line × feeding system experiment. Holstein-Friesian cows were selected to represent the UK average for milk fat plus protein production as control line or were selected for increased milk fat plus protein production (select line). In addition, these cows either received a low roughage diet (50% roughage) with no grazing or a high roughage (75% roughage) diet with summer grazing. Therefore four treatments were considered. The model approach was used to describe the herd structure, estimate the GWP per year and land required per cow for the four systems, and calculate the herd average using a partial life cycle assessment of greenhouse gas emissions. The CO2-eq. emissions were expressed per kilogram of energy-corrected milk (ECM) and per hectare of land use, as well as land required per kilogram of ECM. The effects of a phenotypic and genetic improvement on herd feed utilization efficiency, ECM yield, calving interval length, and incidence of involuntary culling were assessed.
The low roughage, no-grazing feeding system with select cows produced the lowest CO2-eq. emissions of 1.1 kg per kg of ECM and land use of 0.65 m2per kg of ECM but the highest CO2-eq. emissions of 16.1 tons per hectare of the production systems studied. Within the herd, an improvement in feed utilization efficiency was the only trait of those studied that significantly reduced the reliance of the farming system on bought-in synthetic fertilizer and concentrate feed, as well as reduced the average CO2-eq. emissions and land use of the herd (both by about 6.5%, of which about 4% is predicted to be associated with selective breeding). Within production systems, reductions in CO2-eq. emissions per kilogram of ECM and CO2-eq. emissions per hectare were also achievable by an improvement in feed utilization. Interestingly, either on the high or low roughage diet, the cows selected for increased milk fat and protein yield (select line) compared to the control line were not compromised by their fertility performance and risk of involuntary culling; therefore, CO2-eq. emissions per kilogram of ECM and per hectare, as well as land use per kilogram of ECM were minimally affected compared with other traits studied. The model, furthermore, predicted that the CO2-eq. emissions per kilogram of ECM of the control line cows would benefit slightly more than the select line animals by further improvements in ECM yield and feed utilization efficiency.
Bottom line: Improvement in efficiency through management, nutrition and selection is key. The authors also commented on scenarios beyond the scope of the study: (1) If a high roughage production system is more economical or health and welfare concerns associated with a non-grazing system dictate that a period at pasture is preferred, then future policy should aim to select dairy genotypes with improved feed utilization efficiency for milk production on a high rather than low roughage diet. (2) Future work should incorporate health, welfare and feed source as impact categories in a whole-system model.