Abstract
This study aimed to evaluate the effects of chromium yeast (CY) on lactation performance, antioxidant status and immune function, and chromium concentrations in the plasma of mid-lactation dairy cows under heat stress. Twenty-four healthy Chinese Holstein mid-lactation dairy cows with similar milk yield (27.0 ± 1.9 kg/day), parity (2 or 3), and days in milk (DIM, 105 ± 15 d) received the same basal diet containing 0.11 mg Cr/kg of DM. All the animals were divided into 2 blocks according to milk yield (block 1 and block 2 for low- and high-producing cows). Cows of block 1 or block 2 each were randomly allocated to four treatments: a negative control group (without CY supplementation, CON), and groups that received CY at 0.18, 0.36, and 0.54 mg Cr/kg DM, respectively. The experiment lasted 10 wk over a hot summer, including a pre-feeding period of 2 wk. Cows were experiencing heat stress as the average temperature-humidity index (THI) is greater than 72. The averagerespiration rates (RR) and rectal temperature (RT) of the cows in CON group were 81.11 ± 1.42 breath/min and 39.66 ± 0.05 ℃, respectively. The results showed that supplementation with CY reduced the RR and RT of dairy cows under heat stress in both a linear and quadratic manner; the lowest was 75.33 ± 0.76 breath/min and 39.40 ± 0.09 ℃ for the CY-0.36 group (P < 0.05). CY supplementation did not affect milk yield or milk composition, but linearly increased dry matter intake and milk lactose content (P < 0.05). Furthermore, increasing amounts of CY increased the activity of glutathione peroxidase, superoxide dismutase and total antioxidant capacity in serum, but decreased the concentration of malondialdehyde (P < 0.05). Although no differences were observed in the concentrations of IL-6, IL-10, IgA, and IgM, supplementation with CY decreased the concentrations of IL-2, IL-4, and IL-1β, but increased that of IgG (P < 0.05). The plasma Cr concentration increased as the dose of supplemented Cr increased (P < 0.05). In summary, CY supplementation improved the welfare of mid-lactation dairy cows by reducing RT and RR, and increased dry matter intake and milk lactose content. Supplementation with CY improved the antioxidant and immune function in mid-lactation dairy cows. The appropriate dose of CY at 0.36 mg Cr/kg DM is recommended for lactating dairy cows experiencing heat stress during hot summers.
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