Effects of social hierarchy-related stress on hormonal reactivity and milk production in dairy cows at a commercial dairy complex

Regrouping disrupts established social relationships among dairy cows. Temporary overstocking additionally increases competition for feed, water, and lying space. The aim of this study was to evaluate the hormonal and production responses of cows with different stress-resistance types to social hierarchy-related stress at a commercial dairy complex. Cortisol and adrenaline concentrations were measured in 86 lactating cows before and 12 hours after 36 additional cows were introduced into a 540 m² pen; the cows were classified by stress-resistance type from basal cortisol, and daily milk yield, milk fat, and milk protein were assessed. Low, unstable, and high stress resistance was identified in 13 cows (15.12%), 55 cows (63.95%), and 18 cows (20.93%), respectively. Cortisol increased from 129.86 to 138.23 nmol/L in low-resistance cows, decreased from 72.33 to 68.89 nmol/L in unstable cows, and remained almost unchanged in high-resistance cows, at 47.21 and 48.03 nmol/L. At 12 hours, adrenaline was lower than baseline by 35.13%, 41.00%, and 14.12%, respectively. Daily milk yield decreased by 4.18 kg, or 14.88%, in low-resistance cows and by 1.28 kg, or 4.62%, in unstable cows, whereas it increased by 0.39 kg, or 1.30%, in high-resistance cows. Milk fat decreased by 2.96%, 6.63%, and 0.46%, respectively, whereas milk protein changed by -0.46%, -2.26%, and +3.13%. Correlations between post-challenge cortisol and milk yield were 0.33, 0.18, and 0.45, respectively; all 95% confidence intervals included zero and all p values exceeded 0.05. Low-resistance cows had the greatest risk of production loss and should be managed using low-disruption regrouping procedures and reduced competition for essential resources