The objectives of this research were to investigate the effects of chromium (Cr) on the performance, dry matter intake (DMI), digestibility, blood factors and some physiological responses of heat stressed cows in early lactation. Fifteen Holstein cows (38 ± 12 days in milk) were arranged as randomized complete block design with three dietary chromium treatments of (C) control (no supplemental Cr), (M) 0.05 and (H) 0.10 mg Cr/kg BW0.75 a chromium-methionine. This experiment lasted for 65 days. The average of maximum THI in this experiment was 77.7 units. Supplementation with chromium in M (p ? 0.05) and H (p ? 0.15)groups increased DMI in comparison with control group. Milk yield, fat corrected milk (FCM), milk components concentrations and yields were not affected by treatments, but milk protein ((p ? 0.09) and total solids (p ? 0.11) percentages were higher for M group than cows were not supplemented Cr. Glucose, glucagon, IGF-1, total protein, globulin, BUN, BHBA, TG, cortisol, progesterone concentrations and cortisol/progesterone ratio were not affected by chromium supplementation. Supplementation with chromium (M and H groups) decreased insulin concentration in comparison with control group (p ? 0.01 and p ? 0.05, respectively). Supplementation with 0.05 mg/kgBW 0.75 increased HDL concentration (p ? 0.04) and decreased insulin/glucagon ratio (p ? 0.05) in comparison with control group significantly. It is possible that metabolical and heat stresses increased Cr requirement and 0.05 mg/kgBW 0.75 has supplied this requirement. These observations suggest that there might have been Cr toxicity at our highest dose, although the toxic level of Cr for ruminants is unknown.
