Review: Genetic selection of high-yielding dairy cattle toward sustainable farming systems in a rapidly changing world.

Progress in the dairy industry has been remarkable as globally milk production has doubled over the past five decades, while the total number of cows have declined dramatically. This has been achieved mainly through the intensification of production systems, direct genetic selection for milk yield and a limited number of related traits, and the use of modern technologies such as artificial insemination and genomic selection. The intensification has been supported by a consistent flow of innovations and technological breakthroughs, although conventional genetic selection still played a major role. In this, animal breeding and genetics has been extensively conceptualized in artificial and standardized environments, where the linear equation: Observed phenotype/performance = Additive genetic merit plus Environmental effects, proved to be highly efficient, especially under controlled environmental conditions and high-input production systems. However, when not accounting for the interactions between genotype and environment (GxE)), successful selection for high-producing animals largely depends on well-controlled environments for the expression of the traits of interest. Indeed, together with genetic selection, the dairy industry has also benefited from major advancements in nutritional practices, precision management, wide adoption of reproductive technologies such as artificial insemination, embryo transfer and sexed semen, and precision health and care management. These advancements are not independent from each other and it is clear that many of them have increased the effectiveness of genetic selection for increased productivity. From this perspective, the high-producing dairy cow is thus more than the simple result of high genetic merit for key biological mechanisms and adequate environmental factors; it also reflects complex positive feedback between these two components that took place during the industrialization and intensification of dairy production.

Genetic selection for increased milk yield has been a key driver of dairy intensification and the development of highly specialized milk production systems, with increasing herd size, and heavily relying on cereals and protein-sources. Locally, the concentration of intensive dairy farms can have a large environmental impact due to the large amounts of waste produced. Thus, there is growing evidence that uncoordinated levels of intensification in high-input dairy production systems are not sustainable. Despite the major signs of progress in productivity, the long-term success of the dairy industry depends on the adoption of more sustainable breeding goals and management practices, especially from an agro-ecological perspective. Current high-producing systems need to be refined with a greater focus on animal health and welfare, environmental efficiency, climatic adaptation, and more preparedness for future challenges through the conservation of a diverse genetic pool. Some breeding programs have recently included several of these traits in the breeding goals, but there is still a need for substantial improvements. The transition toward lower-input (with improved usage of resources) production systems also needs to be favoured. This is required to minimize the environmental footprints of the industry, meet the food demands of a steadily growing population, limited resources and land availability, greater environmental and ethical awareness of animal husbandry practices, demand for higher-quality products produced with lower use of antibiotics, and natural challenges (e.g., new pathogens and diseases, climate change etc.). Non-economic selection dimensions of dairy farm sustainability has mainly concerned animal welfare. However, unfavourable genetic relationships among traits of great relevance to the industry (e.g., milk yield and fertility or welfare) have deteriorated some economically important traits, which has consequently motivated the development of more efficient breeding strategies for increased long-term sustainability. However, to contribute to dairy farm sustainability, genetic selection needs to consider its direct and indirect effects on the multiple sustainability dimensions.

The extent to which the dynamics of genetic specialization are interrelated with dairy farm intensification is key to address the contribution of genetic selection to the development of sustainable production systems. In this context, dairy industry stakeholders should continue seeking alternatives to further increase the profitability and sustainability of the enterprise. Key players such as breeding companies and national genetic evaluation systems will continue refining the selection indexes used in face of emerging threats and opportunities. However, where applicable, there could be a need for greater governmental involvement through policy or legislation to evoke changes in certain directions, especially toward better animal welfare and environmental footprints.