Rotz C A, Beegle D, Bernard J K, Leytem A, Feyereisen G, Hagevoort R, Harrison J, Aksland G, Thoma G
USDA-ARS, Pasture Systems and Watershed Management Research Unit, University Park, PA 16802.
Emeritus, Department of Plant Science, The Pennsylvania State University, University Park, PA 16802.
J Dairy Sci. 2024 Jun;107(6):3651-3668. doi: 10.3168/jds.2023-24185. Epub 2024 Jan 11.
Dairy farms in the United States have changed in many ways over the past 50 yr. Milk production efficiency has increased greatly, with ∼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 (e.g. electricity, fuel, fertilizer). 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 United States. For all environmental metrics studied, intensities expressed per unit of fat- and protein-corrected milk produced were reduced, but the total effects over all farms or milk produced increased for 5 of the 13 environmental metrics. Reductions in the impacts of dairy farms in the eastern United States 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 emissions 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 by 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 that have a greater dependence on irrigated feed crops, total blue water use increased by 42%. Major pathways of nitrogen loss included ammonia volatilization, leaching, and denitrification, where total ammonia emissions related to US dairy farms increased by 29%, while leaching losses decreased by 39%, with little change in nitrous oxide emissions. Simulated nitrogen and phosphorus runoff losses totaled for all dairy farms decreased by 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 by 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 improvements with new strategies and technologies are needed to meet the demand for dairy products and mitigate total environmental impacts, particularly in view of projected climate variability.
在过去50年里，美国的奶牛场发生了诸多变化。牛奶生产效率大幅提高，如今奶牛数量减少了约30%，但产奶量却增加了约一倍。其他方面的改进包括农作物产量提高、农场设备的燃料效率提升，以及农场大多数资源（如电力、燃料、化肥）的生产效率提高。这些改进导致了奶牛场对环境影响的变化。通过模拟1971年和2020年具有代表性的奶牛场，确定了6个地区以及美国养分流失的变化情况，以及农场大门生命周期内温室气体（GHG）排放、化石能源使用和蓝色（地表水和地下水）用水情况的评估。对于所研究的所有环境指标，每生产单位脂肪和蛋白质校正奶的强度都有所降低，但在13个环境指标中的5个指标上，所有农场或所产牛奶的总体影响却有所增加。美国东部奶牛场影响的减少被西部地区的大幅增加所抵消，因为西部奶牛数量大幅增加。全国温室气体排放平均强度下降了42%，这使得所有奶牛场在50年期间的温室气体排放总量仅增加了14%。化石能源使用强度下降了54%，所有农场的总量下降了9%。与牛奶生产相关的用水强度下降了28%，但由于干旱的西部地区对灌溉饲料作物依赖更大，奶牛产量大幅增加，蓝色用水总量增加了42%。氮素损失的主要途径包括氨挥发、淋溶和反硝化作用，美国奶牛场的总氨排放量增加了29%，而淋溶损失减少了39%，氧化亚氮排放量变化不大。通过更高效地使用化肥、减少耕作以及更多地使用覆盖作物，所有奶牛场模拟的氮和磷径流损失总量减少了27%至51%。由于更多地使用长期粪便储存以及储存在料仓和堆中的青贮饲料，甲烷和活性非甲烷挥发性有机化合物的排放量分别增加了32%和53%。尽管在提高生产效率方面已经取得了很大进展，但仍需要采用新的策略和技术持续改进，以满足乳制品需求并减轻总体环境影响，特别是考虑到预计的气候变化。
