Social associations in lactating dairy cows housed in a robotic milking system

Dairy cows are gregarious animals that are able to thrive in a stable social group and form long-lasting dyadic relationships. However, in the modern UK commercial dairy industry, cows are commonly regrouped/relocated as part of the management plan, forcing the cows to change social partners regularly. Social bonding in group-housed adult dairy cows and calves have been shown to be associated with spatial-proximity of conspecifics within the pen, with closer individuals establishing stronger relationships, but little is known about the influence of consistent social associations at the AMS on milk production in dairy cattle in a robotic milking system under commercial conditions. This study determined if there were consistent social associations during milking at AMS between cows in a free traffic system using the time between milking events (i.e. time between the next cow arriving and the preceding cow leaving) in the same pen on the same day retrieved from the AMS and whether these associations influence milk yield, fat and protein content. The relative association strengths, accounting for opportunity to associate, were calculated (i.e., the observed number of associations between pairs was divided by the number of days cows shared the same pen together) and used as the measure of social association between the cows. Multiple linear regression models in R were used to assess whether relative mean and maximum association strength scores (mean-centred) influenced average daily milk yield and for each composition trait depending on the parity. Associations among cows at the AMS were much more variable in strength than expected by chance, indicating the presence of consistent associations that might represent social preferences. Multiparous cows had stronger mean social association strength compared to the primiparous cows. Both mean and maximum association strengths were not related to average milk yield but were related to milk fat and protein percentages depending on the parity group. On average, multiparous cows had greater average daily milk yield compared to primiparous cows, however, a significant decrease in milk fat and protein percentage was found in multiparous (older) cows with increased mean association strength which did not occur in the primiparous (younger) cows. In conclusion, while consistent social associations at the AMS may bring benefits for younger cows, older cows may adjust their daily activity budgets to establish these associations, with consequences for their milk composition.