MANIPULATION OF MICROBIAL PROTEIN SUPPLY AND RESULTANT PERFORMANCE OFBULLOCKS BY INCREASING STRAW CONTENT IN THE DIET

This study involved two similar experiments. Thirty-four Bulgarian Bro wn bullocks were used in a 2 x 2 factorial design to investigate the e ffect of the quantity of dry matter intake (DMI) and dietary source of protein on microbial protein supply, nitrogen utilization and resulta nt performance. The four isonitrogenous experimental diets consisted o f concentrate plus wheat straw: 50% straw and urea; 22% straw and urea ; 50% straw and sunflower oil meal; and 21% straw and sunflower oil me al. Increasing the level of DMI through higher proportions of straw in the diet with urea tended to improve both the microbial protein suppl y to the small intestine and the average daily gain (ADG). In addition , the length of the fattening period and the amount of concentrate con sumed per kg of live weight gain were reduced. Higher straw levels in the diets with the sunflower oil meal lowered the ADG and increased th e length of the fattening period. In conclusion, it was found that add ing large quantities of wheat straw to fattening diets containing adeq uate amounts of natural protein was not justified.