عنوان مقاله [English]
This research was conducted to assess the effect of substituting corn for barley grain in diet on performance and reproductive characteristics of early lactating, Holstein dairy cattle. A number of fifteen cows in their second lactation (26±11 DIM) with 36.7±5.6 kg milk/d were investigated for 75 day (15 day for adaptation and 60 day for sampling) in a completely randomized design. The iso-energetic and iso-nitrogenous diets contained different levels (80, 160 or 240 g/kg DM) of corn substituted for barley grain. Feed was offered ad libitum as total mixed ration. In the experimental period, daily Dry Matter Intake (DMI), milk yield, body weight blood chemistry parameters, as well as reproductive characteristics were recorded. The DMI, milk yield, milk fat and protein percentage, plasma glucose and insulin concentrations, Services Per Conception(SPC) and Open Days (OD) for the cows fed the diet containing 80 g of corn/kg DM, respectively, were 24.76, 42.07 kg/d, 3.26 and 2.98%, 60.1 mg/dL, 0.409 µg/L, 1.6 and 95.8 d. Daily DMI, milk yield and feed efficiency were recorded as similar among treatments. Increasing the level of corn in the diet did not affect milk fat, protein and lactose, although the percentages of protein and fat were, numerically slightly increased. With increase in the level of corn in the diet, plasma concentration of insulin, numerically, increased, but beta hydroxyl butyrate and cholesterol declined, numerically. Replacing 240 g/kg of corn for barley in the diet caused a numerical decrease of SPC and OD as compared with other treatments. Overall, replacing corn grain for barley in the diet did not significanty affect milk production and composition. On the other hand, starch source did but only slightly affect the blood metabolites and reproductive parameters.