Koenig, K. M. and Beauchemin, K. A. 2011. Optimum extent of barley grain processing and barley silage proportion in feedlot cattle diets: Growth, feed efficiency and fecal characteristics. Can. J. Anim. Sci. 91: 411-422. A study was conducted to evaluate the effects of forage proportion and extent of processing of barley grain on growth, intake, feed conversion efficiency, and fecal characteristics of feedlot cattle. Crossbred steers (120; 407±31 kg) were housed individually and assigned to 10 diets (n=12): two degrees of temper rolling of barley grain [processing index (PI) of 82% (standard) or 87% (coarse)] were combined with five levels of barley silage [3, 6, 9, 12, and 15% of dietary dry matter (DM)]. The PI was determined as the weight of grain after processing expressed as a percentage of the weight before processing. Cattle were slaughtered after 104-109 d on feed, at a final weight of 576 kg, SEM = 5.1. There were very few interactions between grain processing and silage proportion for the variables measured. Grain processing had no effect (P>0.05) on average daily gain (1.59 kg d^-1, SEM = 0.057) or final weight. Feeding PI-87% barley tended to increase dry matter intake (DMI) over the experiment (7.63 vs. 7.34 kg d^-1, SEM = 0.123, P=0.10) compared with feeding PI-82% barley. Higher DMI of cattle fed PI-87% barley corresponded to lower estimated starch digestibility, as a result of increased appearance of whole kernels and grain fragments in feces. Consequently, gain:feed ratio tended (0.210 vs. 0.219, SEM = 0.0035, P=0.06) to decrease by 4% with PI-87% versus PI-82% barley. Similarly, silage proportion had no effect on gain but DMI increased (P=0.05) linearly from 7.19 to 7.75 kg d^-1 (SEM = 0.194) with increasing proportion of silage. Consequently, gain:feed ratio decreased linearly from 0.225 to 0.202 (SEM = 0.0056) with increasing silage proportion. Optimum proportion of barley silage to maximize feed conversion efficiency was 3 to 6% of DM. Decreasing the extent of barley processing or increasing the silage proportion may reduce the risk of acidosis, but feed conversion efficiency is lowered. Formulating diets to reduce the incidence of digestive disorders may decrease the cost of mortalities and treatment of sick animals, thereby improving animal health and welfare, but these costs are unlikely to offset the increased cost of gain in commercial feedlots.