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 soybeans and roasted soybeans of different particle sizes on utilization by ruminal bacteria and lactating cows. Five cows with ruminal and duodenal cannulas were used in a 5 x 5 Latin square design. Diets contained 55% corn silage and 45% concentrate. Treatments were control (Ca soaps at 4.24% of dietary DM), whole raw soybeans, whole roasted soybeans, 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 processing of soybeans. Ruminal bacteria harvested from cows fed roasted soybeans had higher fatty acid content than from cows fed raw soybeans. Different methods for calculation of ruminal biohydrogenation of fatty acids are discussed. Roasting tended to increase the total tract digestibility of fatty acids, and roasted soybeans tended to increase polyunsaturated fatty acids in milk compared with raw soybeans, primarily because of changes in concentration of C18:2. Change in particle size of roasted soybeans had minor effects on utilization of fatty acids. Decisions on processing of soybeans need to be based primarily on factors other than fatty acid digestibility.