Technical Abstract: Researchers are still in the early stages of understanding how supplementation affects grazing behavior. Conventional nutrition wisdom, including early research with grazing cattle, has been based almost entirely upon stored feeds fed in confinement. In these situations, most dietary “choices” were taken away from the cow with the advent of total mixed rations. With a few exceptions, U.S. studies have not accounted for grazing behavior or diet selection, which is so important to the total dry matter (DM) intake and productivity of grazed animals. Our USDA-ARS facility conducted a continuous culture fermenter study to evaluate how ruminal fermentation was affected by the time of supplementation in relation to a restricted grazing period. Fermenters were “fed” a corn silage supplement (40% of total DM fed) either nine hours or one hour prior to being “fed” a pasture meal. The results showed that N utilization may be improved and N losses to ammonia may be reduced by feeding corn silage nine hours before a before a short and intensive grazing session. Also with the earlier feeding, there may be improved production of volatile fatty acids (namely propionate) that are important energy sources for ruminants. Our USDA-ARS facility also conducted a survey of 13 grazing dairies in New York and Pennsylvania that utilized a total mixed ration with pasture. All farms involved in the survey were supplementing some level of protein, and all were exceeding recommended guidelines for protein intake — some by as much as 80% above National Research Council recommendations. Nine of the farms never analyzed their pastures for nutrient quality, and these farms tended to do the most overfeeding of protein. Such overfeeding creates financial, animal productivity, and environmental issues. A better understanding of why, how, and when cattle graze is essential in trying to improve pasture utilization and the management of livestock. We have only touched the surface in truly developing this understanding.