Invited review: Environmental enrichment of dairy cows and calves in indoor housing.

In recent years, an increasing number of farmers are choosing to keep their cows indoors throughout the year. Indoor housing of cows allows farmers to provide high-yielding individuals with a nutritionally balanced diet fit for their needs, and it has important welfare benefits for both cows and their calves, such as protection from predators, parasites, and exposure to extreme weather conditions. However, it also confronts cows and calves with a wide range of environmental challenges. These include abiotic environmental sources of stress (e.g., exposure to loud and aversive sound) and confinement-specific stressors (e.g., restricted movement and maintenance in abnormal social groups). Cows and calves that live indoors are also faced with the challenge of occupying long periods with a limited range of possible behavioral patterns. Environmental enrichment can improve biological functioning (measured as increased lifetime reproductive success, increased inclusive fitness, or a correlate of these such as improved health), help animals to cope with stressors in their surroundings, reduce frustration, increase the fulfillment of behavioral needs, and promote more positive affective states. Here, we review recent findings on the effect of social, occupational, physical, sensory, and nutritional enrichment on dairy cows and calves, and we assess the appropriateness and practicality of implementing different enrichment practices on commercial dairy farms. Some of the enrichment methods reviewed here may also be applied to those more extensive cattle-raising systems, where similar challenges occur.