The overall objective of this thesis research was to determine if replacing switchgrass (SG) hay for wheat straw (WS) in moderate or high starch lactating dairy rations would affect feed intake, milk pro-duction, as well as feeding and sorting behavior. In the first study, mid-lactation cows were fed a mod-erately high starch diet with the direct substitution of either SG hay or WS. It was determined that SG hay depressed dry matter intake (DMI), milk yield, and protein yield, but did not influence milk fat or protein percent. In the second study, mid-lactation cows were fed diets with either SG hay or WS as a direct substitution when transitioned from a low starch to a moderate or high starch diet. It was deter-mined that SG hay inclusion did not affect DMI nor milk yield but did depress milk fat yield and tended to depress milk protein yield. Also, regardless of SG hay or WS inclusion, cows fed the high starch diets had a greater DMI, milk yield, and milk protein yield, but a lower milk fat percent.