RAW SOYBEANS AND ROASTED SOYBEANS OF DIFFERENT PARTICLE SIZES .2. FATTY-ACID UTILIZATION BY LACTATING COWS

This study was conducted to examine the effects of feeding whole raw s oybeans and roasted soybeans of different particle sizes on utilizatio n by ruminal bacteria and lactating cows. Five cows with ruminal and d uodenal cannulas were used in a 5 x 5 Latin square design. Diets conta ined 55% com silage and 45% concentrate. Treatments were control (Ca s oaps at 4.24% of dietary DM), whole raw soybeans, whole roasted soybea ns, cracked roasted soybeans, or ground roasted soybeans. All soybean diets contained 19.7% soybeans. Source of fat (Ca soaps vs. soybeans) caused more differences in utilization of fatty acids than did process ing of soybeans. Ruminal bacteria harvested from cows fed roasted soyb eans had higher fatty acid content than from cows fed raw soybeans. Di fferent methods for calculation of ruminal biohydrogenation of fatty a cids are discussed. Roasting tended to increase the total tract digest ibility of fatty acids, and roasted soybeans tended to increase polyun saturated fatty acids in milk compared with raw soybeans, primarily be cause of changes in concentration Of C18:2. Change in particle size of roasted soybeans had minor effects on utilization of fatty acids. Dec isions on processing of soybeans need to be based primarily on factors other than fatty acid digestibility.