The amount and source of nitrogen required to optimize microbial protein synthesis has been widely discussed in the literature. However, information available on factors affecting microbial protein synthesis in young heifers fed high concentrate diets is limited. Our objective was to study the effect of CP concentration on ruminal fermentation and urinary excretion of purine derivatives, used as an index of microbial yield, in heifers fed concentrate and barley straw, both ad libitum, from 100 to 230 kg LW.
Ten crossbreed beef heifers (101±4.5 kg LW), of which 6 where ruminally cannulated, were individually penned and randomly offered (115% of previous day consumption) a concentrate (14%, L or 17%, H) and barley straw diet once daily. Concentrates (H and L) were composed of ground corn (32; 38%), barley (27%), cassava (17%) and sunflower meal (5.8%) and soya bean meal included either at 16% or 9%, respectively.