Effect of exposure to heat stress conditions on milk yield and quality of dairy cows grazing on Alpine pasture

Milk yield, gross composition and fatty acid (FA) profile of 14 Aosta Red Pied (ARP) and 14 Aosta Black Pied‐Aosta Chestnut (ABP‐AC) cows were investigated in an Alpine pasture during the two years’ grazing seasons: 2003 (heat stress conditions) and 2004 (thermoneutral conditions). The anomalous adverse weather settings (extreme dryness) occurred in summer 2003 determined a strong worsening in fresh grass quality at pasture. ARP cows suffered a significant drop in dry matter intake (P<0.01), milk production (P<0.01) and fat and protein yields (P<0.01) compared to summer 2004. ABP‐AC cows showed lower milk yields and lower fat and protein yields compared to ARP ones (P<0.001), but they were able to maintain same feed intakes, production performances and fat and protein yields between years. In summer 2003 an improvement of the nutritional properties of milk associated to changes in the FA profile was observed: milk fat showed lower hypercholesterolemic (C12 to C16) and total saturated FA as well as higher total mono‐ and polyunsaturated FA with respect to summer 2004 in both ARP and ABP‐AC cows (P<0.001). ABP‐AC had a significant healthier milk FA composition than ARP in both years of analysis. The differences in milk FA between breeds were more evident in summer 2004, showing that thermoneutral conditions can better emphasize the genetic potential of different cattle breeds.