The objective of this study was to evaluate the effect of duodenal infusion of histidine on milk yield, milk composition, and plasma amino acids in lactating dairy cows. In the present experiment, three high-yield lactating Holstein cows fitted with duodenal closed T-shaped cannulas were used. The cows were allocated to two treatments: histidine deficient (control) and histidine supplemented (HIS). The experiment was divided into four periods of 7 d. Each period consisted of a 3-d preliminary period and a 4-d experimental period. A crossover design was used for the experiment. The cows were fed ad libitum a diet consisting of maize silage, lucerne hay, and a supplement mix. Dry matter intake did not differ (P>0.05) between treatments. Milk yield and the yields of protein, casein, and lactose were significantly higher in HIS in comparison with the control. The contents of protein, casein, fat, and lactose were not significantly different (P>0.05). The proportions of α-casein and β-casein did not show significant variations (P>0.05), but the κ-casein proportion was significantly higher (P<0.01) in HIS. The yield of each casein fraction in HIS was significantly higher compared with the control. No effect of duodenal infusion of histidine on the concentration of free amino acids in blood plasma was observed (P>0.05) except for histidine (P=0.012).
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