Investigating the interaction effects of coat colour and short-term heat-stress on milk traits and metabolic indicators in Holstein Friesian cows.

This research aimed to investigate the interaction effect of coat colour and short-term heat stress (HS) on milk traits and blood parameters in lactating Holstein Friesian cows under a confinement system with controlled outdoor access for 2 h. A total of 660 milk and blood samples were collected between April and September 2021. 55 s-lactation cows (average body weight 637 ± 29 kg) were classified into three groups based on coat colour ratio: 18 white cows (DIM 98 ± 15), 18 black cows (DIM 101 ± 18), and 19 mixed-colour cows (DIM 105 ± 21). The assessment was conducted under two conditions: HS, where the temperature-humidity index (THI) was ≥68 during the 2-h outdoor exposure, and thermoneutral conditions (NHS), where the THI remained <68. No significant interaction between coat colour and HS was observed for milk traits or blood parameters (p ≥ 0.05). However, cows under NHS exhibited significantly higher milk production (42.4 kg) compared to HS cows (40.7 kg; p < 0.05). Milk protein levels were also elevated under NHS (3.29% vs. 3.14%; p < 0.05), and lactose content decreased during HS (5.07% to 5.00%; p < 0.05). Heat stress resulted in a 194% increase in insulin levels, a 7.03 ng/ml rise in prolactin, and a 67% increase in NEFA. Overall, short-term heat stress significantly affected milk traits and some metabolic indicators, but there was no evidence that black cows were more susceptible to HS than white cows under the conditions of this study.