**Abstract**

With increased selection pressure on milk production, many dairy populations are experiencing reduced fertility and disease resistance. Reducing susceptibility to metabolic diseases, such as ketosis, displaced abomasum, retained placenta, metritis, mastitis, and lameness, has long been excluded from genetic improvement programs, due to low heritability of those traits. However, research has shown that using large producer-recorded data, genomic information, and suitable statistical models can result in accurate genomic predictions for metabolic diseases, enabling producers to select animals with improved disease resistance early in life. Improving wellness in dairy herds not only increases economic efficiency of dairy herds, but also improves overall animal welfare as well as product quality and public perception of dairy farming. This chapter describes the development of genomic predictions for wellness traits in Holstein dairy cows in the United States and presents examples of validation of those predictions in commercial dairy populations in the United States and other countries.

**Keywords:** dairy cows, wellness traits, genetic evaluation, genomics, validation

#### **1. Introduction**

Over the last 50 years, genetic selection to improve milk production in dairy herds has been very successful. In many developed countries, the milk production per cow has more than doubled. About half of that progress can be contributed to genetics [1]. Along with increase in production, dairy farming has become more intensive. While the number of dairy farms is decreasing globally, the average herd size is increasing [2]. Selection pressure for higher yields and intensive farming have been linked to reduced welfare and an increased incidence of many common diseases in dairy cows, mostly due to genetic antagonisms between production and health traits [3–5]. Consequently, dairy cows are becoming less "robust," which have negative consequences for the health and fertility of the modern day dairy cow [6, 7].

Profitable dairy cows are productive, fertile, and mostly "invisible"—they do not require extra attention or intervention to maintain their health through all phases of production. Having a larger proportion of mature cows that are productive and healthy during multiple lactations can enhance profitability of dairy operations. To reach their full potential and longevity, animals need to remain healthy from birth until calving, and then stay healthy and structurally sound, in addition to regularly calving and producing milk. Dairy animals that experience adverse health events negatively affect herd profitability through increased culling, veterinary expenses, and labor, as well as monetary losses through reduced milk sales [8]. The costs per case of the common dairy cow diseases were estimated to range from \$181 for ketosis to \$391 for displaced abomasum [9].

Dairy researchers and producers have made progress on providing the best environment for animals to reduce health events through nutrition, management, and housing. Additionally, genetic improvement of health and wellness traits in dairy cows is an attractive option for dairy producers because genetic gains are permanent and cumulative from one generation to another [10].

Genetic evaluation and selection for improved health traits has been lagging compared with selection for production and reproduction in dairy cows due to low heritability of health traits and the lack of centralized recording. Most health events in dairy herds have not been recorded by trained veterinarians, but rather by producers themselves using herd management software. However, research has shown that, given the large amount of data, availability of genomic information, and advanced statistical methodology, it is possible to provide accurate genetic and genomic predictions that producers can use as a tool to improve health and wellness of their herds.

In many countries, including the United States, the most frequently cited reason for not using health data in genetic evaluation of dairy cattle is the lack of a centralized national system to collect health record data. Although most dairy producers record health information of their animals using herd management software, the subjectivity of diagnosis and the user-defined terminology of health events contribute to increased difficulty in using health data in a genetic evaluation due to insufficient accuracy and inconsistency of recording [11]. However, several studies based on large amounts of producer-recorded data have shown that genetic selection for wellness traits can be effective in improving herd health in dairy cattle as long as the recording protocols within a herd are fairly consistent [8, 12, 13].

Genetic evaluation of health traits has a long tradition in countries with routine health data recording. In Scandinavian countries, health traits have been included in breeding programs since the mid-1970s [14]. Currently, over 97% of Norwegian dairy cows are included in the recording system [15, 16]. In other countries, the use of direct health data in genetic evaluation is progressing rapidly. Routine data collection and genetic evaluation for health traits in Germany and Austria started in 2006 [17]. In France, clinical mastitis has been included in routine genetic evaluation since 2010 [18]. In 2014, genetic evaluation for mastitis resistance was introduced for Canadian dairy cows; the evaluation is based on clinical mastitis incidence recorded in the first and second lactation and SCS [19, 20]. In Canada, genetic evaluation for ketosis and displaced abomasum was implemented in December 2016, followed by metritis and retained placenta, hoof health and lameness, and other functional traits in the following years [21].

The advances in molecular genetics and genome sequences have created unprecedented opportunities to select for genetically superior animals and increase the speed of genetic improvement of production, reproduction, and, especially, health traits in farm animals. In March 2016, Zoetis Genetics launched CLARIFIDE Plus, the first commercially available genomic test for wellness traits of dairy cattle. Today, CLARIFIDE Plus provides genomic predictions for 14 health and wellness traits in cows and calves of Holstein and Jersey breeds.

The goals of this chapter are (1) to describe the research leading to the development of genomic predictions for wellness traits mastitis (MAST), metritis (METR), retained placenta (RETP), displaced abomasum (DA), ketosis (KETO), and lameness (LAME) based on large producer-recorded data, genomic information, and

sophisticated statistical methodology and (2) to present examples of studies focused on assessing efficacy of genomic predictions for wellness traits in independent commercial dairy herds in the United States and other countries.
