Fifty years of environmental progress for United States dairy farms

Dairy farms in the United States (US) have changed in many ways over the past 50 years. Milk production efficiency has increased greatly with about 30% fewer cows producing about twice the amount of milk today. Other improvements include increases in crop yields, fuel efficiency of farm equipment and efficiency in producing most resources used on farms (electricity, fuel, fertilizer, etc.). These improvements have led to changes in the environmental impact of farms. Through simulation of representative dairy farms in 1971 and 2020, changes in nutrient losses and farmgate life cycle assessments of greenhouse gas (GHG) emissions, fossil energy use, and blue (ground and surface) water use were determined for 6 regions and the US For all environmental metrics studied, intensities expressed per unit of fat and protein corrected milk produced were reduced, but the total impacts over all farms or milk produced increased for 5 of the 13 environmental metrics. Reductions in the impacts of dairy farms in the eastern US were offset by large increases in western regions because of a major increase in cow numbers in the west. The national average intensity of GHG emissions decreased by 42%, which gave just a 14% increase in the total GHG emission of all dairy farms over the 50-yr period. The intensity of fossil energy use decreased by 54% with the total for all farms decreasing 9%. Water use related to milk production decreased in intensity by 28%, but due to the large increase in dairy production in the dry western regions with greater dependence on irrigated feed crops, total blue water use increased 42%. Major pathways of nitrogen loss included ammonia volatilization, leaching, and denitrification, where total ammonia emissions related to US dairy farms increased 29% while leaching losses decreased by 39% with little change in nitrous oxide emission. Simulated nitrogen and phosphorus runoff losses totaled for all dairy farms decreased 27 to 51% through more efficient fertilizer use, reduced tillage, and greater use of cover crops. Emissions of methane and reactive non-methane volatile organic compounds increased 32% and 53%, respectively, due to greater use of long-term manure storage and silage stored in bunkers and piles. Although much progress has been made in improving production efficiency, continued improvement with new strategies and technologies are needed to meet the demand for dairy products while mitigating total environmental impacts, particularly in view of projected climate variability.

Use of critically important antimicrobial classes early in life may adversely impact bacterial resistance profiles during adult years: potential co-selection for plasmid-borne fluoroquinolone and macrolide resistance via extended-spectrum beta-lactam use in dairy cattle

The transfer of antimicrobial resistance genes commonly occurs via vertical and horizontal gene transfer, as such genes are often found on the same mobile genetic element. This occurrence can lead to the co-selection of resistance to antimicrobials without their application. Dairy cattle located in the south-western United States were enrolled in a matched-pair longitudinal study to evaluate the effects of a two-dose ceftiofur treatment for metritis on levels of third-generation cephalosporin resistance among faecal Escherichia coli temporally. Escherichia coli chosen for further investigation were isolated on selective media, harboured extended-spectrum beta-lactam, fluoroquinolone and macrolide resistance genes. This combination has previously been unreported; importantly, it included genes encoding for resistance to antibiotics that can only be used in dairy cattle less than 20 months of age. Fluoroquinolones, macrolides and third and higher generation cephalosporins are considered critically important and highest priority for human medicine by the World Health Organization.