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One hundred and twenty early lactating Holstein dairy cows were assigned to investigate the effect of dietary chromium (Cr) supplementation (0 or 6 mg Cr/head/day from organic preparation) on the productive and reproductive performance as well as on some blood serum parameters under heat stress (35-40 degrees C). Cows received treatment from 3 weeks pre-partum through 12 weeks post-partum. Chromium supplemented diet had lower body weight loss and improved dry matter intake with consequent reduction in the energy balance deficits during the first period after calving when compared with the control group. Chromium supplementation increased (p < 0.05) milk yield by 6.7%, 12.3% and 16.5% at 4, 8 and 12 weeks post-partum, respectively, whereas milk composition and milk to feed ratio were unaffected (p > 0.05) when compared with the control. Moreover, dietary Cr supplementation had no effect (p > 0.05) on blood serum glucose, calcium and phosphorus concentrations. A reduction (p < 0.05) of non-esterified fatty acids at 1 week pre-partum, 2 and 4 weeks post-partum was also observed. Serum insulin concentration increased whereas cortisol concentration decreased, when compared with the control group, throughout the whole experimental period. Also Cr supplementation showed a trend towards improving reproductive performance as indicated by increased percentage of pregnant cows in the first 28 days of breeding. It could be concluded that dietary Cr supplementation at level of 6 mg/head/day may offer a potential protective management practice to lessen the effect of heat stress in dairy cattle.

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