# The Housing System Contributes to Udder Health and Milk Composition
## 1 Introduction  
Modern consumers have an interest not only in the quality of milk and dairy products, but also in the welfare of dairy cattle. Consumers demand husbandry practices that promote optimal housing and management conditions (animal welfare) and enable animals to express their innate behavior [1]. Animal welfare is determined mainly by housing conditions and herd management practices, which considerably influence the milk yield and quality, the animals' health and comfort, and the farm's profits [2]. Polish agriculture is characterized by considerable fragmentation, and dairy cattle are kept mainly in tie-stall systems and milked where they are tethered [3]. [PERSON] and [PERSON] [3] calculated the index of technical standards fulfilment (ITSF) to evaluate housing conditions in four barn zones (lying, social, feeding, and milking) and found that the ITSF values were higher in all four zones in the free-stall system than the tie-stall system. An optimal housing system (bam) should facilitate cow handling, cleaning, and management operations to keep animals in good health and to promote their welfare. Both tie-stall and free-stall systems have their strengths and weaknesses in this respect [4]. According to [PERSON] et al. [5], the risk of hoof diseases is higher, whereas the prevalence of mastitis is considerably lower in free-stall than in tie-stall systems. [PERSON] et al. [6] analyzed the prevalence of diseases in Norwegian dairy cattle farms and found that clinical mastitis was more prevalent in tie-stall than in free-stall systems. A study evaluating housing changes in Swedish dairy herds revealed that the transition from a tie-stall to a free-stall system decreased the prevalence of clinical mastitisti and teat injuries [7]. A Romanian study also demonstrated that free-stallhousing systems contributed to the health and welfare of dairy cattle [8]. However, the access to animals, handling operations, veterinary treatments, and individual feeding are easier to perform in tie-stall systems [3]. The compared systems also differ in their milking methods. In tie-stall barns, cows are milked directly in stalls, whereas in free-stall systems the milking operations are performed in milking parlors or at automatic milking stations [6]. High-quality milk and dairy products (such as cheese) are positively correlated with cow health, stall cleanliness, and milking hygiene [9]. Mascitis, an intramammary infection (IMI) caused by pathogenic bacteria, generates the greatest losses in dairy cattle production [10]. Mascitis is classified as clinical if symptoms of mammary dysfunction and defective milk are evident, or as subclinical if no clinical signs are visible. Subclinical mastitis is most challenging because it is asymptomatic, and the quality of milk may deteriorate when milk with high somatic cell counts (SCCs) is included in the bulk tank [11]. Intramammary infections increase treatment costs and lead to premature culling and milk production losses [12]. Udder health is commonly monitored based on SCC [13]. Somatic cells are mainly immune system cells that are involved in the innate immune response. These include lymphocytes, macrophages, polymorphonuclear cells, and some types of epithelial cells [14]. An increase in SCC is indicative of an udder infection, and it leads to a decrease in milk yield and undesirable changes in milk composition [14; 15]. To improve milk quality, many countries have introduced SCC limits, and milk payments are linked with the physicochemical and microbiological parameters of raw milk. Maximum SCC levels differ across countries. In the European Union, milk is deemed to be fit for human consumption when the SCC is below 400 ths mL\({}^{-1}\) cells, but the SCC limit in Brazil is higher, at 600 ths mL\({}^{-1}\) cells [16]. Somatic cells are an important source of endogenous milk proteins, including enzymes.