Effects of dietary starch and protein levels containing wheat straw as forage source on milk yield and composition of dairy cows

Two trials were conducted to determine the effect of dietary starch and protein levels on production performance and milk composition of dairy cows fed only wheat straw as a source of roughage. Twenty eight Holstein cows (averaged 41±31.5 and 82±24 days in milk, and 30.4±3.49 and 29.0±2.22 kg/d milk yield, for Trial 1 and 2, respectively) in each trial were used in a complete randomized design with a 2 x 2 factorial arrangement of two starch (14 vs. 22% in the Trial 1 and 16 vs. 27% in the Trial 2) and protein (15 vs. 18% in the Trial 1 and 16 vs. 18% in the Trial 2) levels. The TMR consisted of 70:30 and 75:25 concentrate : wheat straw in the Trial 1 and 2, respectively. Both trials were lasted 6 weeks. Dry matter intake was not affected (P> 0.05) by the dietary treatments in both trials. Milk yield increased with increased dietary protein level in the Trial 1 (P< 0.01) and dietary starch level in the Trial 2 (P< 0.01). In both trials, milk urea nitrogen concentrations were affected by dietary protein and starch levels, but there was no interaction effect. Nitrogen efficiency (Milk N / N intake) was reduced by increase in dietary protein level (P< 0.01) in the both trials, but it was improved by increased dietary starch level in the Trial 2 (P< 0.01). In conclusion, the cows fed TMR based on wheat straw responded better to high dietary protein (17%) when the starch level less than 18%. The results, also, revealed that starch level in the diets based on wheat straw should be increased above 27% to maintain milk yield.