Twenty-five multiparous cows in their second trimester of lactation were randomly assigned in a 5 x 5 latin square rotation to five silages: control (N), chopped added (CA), ground added (GA), chopped ensiled (CE), and ground ensiled (GE). Chopped (2-6 cm) or ground (< .5 cm) hay was added to corn green chop at 12.5% of the total forage dry matter before ensiling (CE and GE). Chopped or ground hay was also added to the control silage at the same rate prior to feeding (CA and GA). Silages were supplemented to contain 14% crude protein and 1.67 Mcal/kg on a dry matter basis and fed as a total mixed ration. Hay-containing rations averaged 22% ADF, compared to 20% ADF in the control ration. Each cow received the assigned ration for three weeks and milk samples were collected on days 19 and 20 of each feeding period. Means ± S.E. for N, CA, GA, CE and GE were: percent milk fat 3.07 ± .08, 3.31 ± .09, 3.37 ± .08, 3.28 ± .08; percent total solids 11.7 ± .11, 11.8 ± .12, 11.9 ± .12, 12.02 ± .13, 11.94 ± .12; and dry matter intake per kg metabolic body weight .170 ± .003, .161 ± .003, .163 ± .004, .158 ± .004, .166 ± .004, respectively. Percent milk fat was significantly higher in rations CE, CA, and GE when compared to N. Milk production and percent milk protein did not differ. The results indicate increased milk fat production when chopped hay (added or ensiled) or ground hay (ensiled) is included in corn silage based rations for dairy cattle. Whether chopped or ground, hay added at time of ensiling tended to stimulate milk fat production more than hay added just prior to feeding. The beneficial effect of increased hay fiber length on milk fat production was evident whether hay was ensiled with or added to corn silage. / M.S.