A study was initiated in December, 2005 to determine if adding ionophore to replacement heifer grazing diets improves animal performance and nutrient absorption efficiency, therefore reducing nutrients in excretia.  Further, we evaluated the effects of two annual ryegrass (Lolium multiflorum Lam.) cv. Jumbo, grazing systems [tilled seedbed (TS) vs. over-seeding (OS) into dormant bahiagrass (Paspalum notatum Flugge)] and the possible interaction with our dietary treatments.  Eight 1.33 ha pastures were utilized.  Four pastures were planted with annual ryegrass in December into clean-tilled seedbeds.  Two weeks later, four pastures were over-seeded with annual ryegrass into dormant bahiagrass using a no-till drill.   Six crossbred yearling beef heifers were allocated to each pasture.  All heifers were fed a pelleted concentrate (cornmeal) either with (CMI) or without (CM) ionophore (monensin sodium).  Concentrate was adjusted with animal weight gain and predicted intake each 21 day period to provide 250 mg∙hd^-1d^-1.  The grazing season ended in May for PS fields and June for SS fields.  Due to winter drought, the number of animal grazing days (d ha^-1 yr^-1) was equivalent (76) for both TS and OS treatments.  Forage dry mass was 316 and 131 kg ha^-1 yr^-1 for the TS and OS treatments, respectively.  Average daily animal gain prior to June averaged 2.4 and 3.1 kg d^-1 for the TS and OS treatments, respectively.  Monensin concentrations in heifer excretia ranged from below detection (< 0.05 mg kg^-1) to over 1 mg kg^-1.  Forage crude protein (CP) was not affected by grazing system but it peaked with the March clipping at 18% CP.  Excretia and soil nitrogen were sampled and data will be discussed.