Abstract: The effect of the source of crude protein in isonitrogenuse diets on early lactating cow responses was investigated. Three isonitrogenuse diets (crude protein: 16.7%) with similar rumen degradable protein, but witb different source of protein (Fish Meal (FM) + Canola Meal (CM) as 42.8 + 82.4 g kg- DM (FMlCM) or Soybean Meal (SM) + Cotton Seed Meal (CSM) as 48.8 + 91 g kg- DM (SMlCSMl) or 84 + 42.8 g kg- DM (SMlCSM2» were provided. The diets were fed to 24 early lactating Holstein cows (8 animals per each treatment), 10±3 days in milk and 37.34 kg day- milk yield, for 7 weeks, using a completely randomized design with repeat measures of data in time. Daily Dry Matter Intake (D1v1I), milk yield and milk composition were recorded. Blood glucose and Blood Urea Nitrogen (BUN) were measured in weeks 3 and 6 of the experiment and some blood metabolites and enzymes including insulin, Sennn Glutamic Oxaloacetic Transaminase (SGOT) and Sennn Glutamic Pyrovic Transeaminase (SGPT) were measured at the last week of the experiment. There was no significant effect on DMI (FMlCM ~ 20.32, SMlCSMl ~ 21.04 and SMlCSM2 ~ 20.82, SEM ~ 0.59 kg day- ) (p>0.05). However, the effect of time was significant (linear audquadratic) (p<O.OOOl). Milk yield was similar among treatments. Milk composition (percentage and yield) was similar for all treatments, although time effect on milk protein and fat percentage was significant (linear and quadratic) (p<0.05). Plasma concentration of insulin was similar in cows fed the experimental diets (F1.1ICM = 9.16, S1.1ICSMI = 7.90 and S1.1ICSM2 = 12.41, SEM = 1.34 micro U mL -1). Blood urea nitrogen concentration was not differed among treatments (p>0.05). Sennn glutamic oxaloacetic transaminase was significantly affected by the dietary protein sources (FMlCM~ 74.62, SMlCSMl ~ 91.12 and SMlCSM2 ~ 83.50mg dL - , SEM ~ 6.36) (p<0.05), in4 hafter the morning feeding. It was concluded that protein sources with similar nnnen degradability might not causing significant impact on performance and some blood metabolites in early lactating Holstein cows.

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