Cover crops can provide many benefits, but the short growing season and variable weather in the northern Great Plains makes it difficult to include them in rainfed cropping systems. Relay intercropping - planting cover crops into standing grain crops - could be a way to successfully grow cover crops in this region. A 3-year study was conducted on the Area 4 SCD Cooperative Research Farm near Mandan, ND USA to determine the best time to intercrop cover crops in corn. Study treatments included a no cover crop control (CNTRL) and cover crops planted at advancing corn growth stages [1st planting = V4 (FIRST); 2nd planting = V6 (SECOND); 3rd planting = V8 (THIRD)]. Corn was planted in mid-May or early-June using locally adapted varieties at a population of 59,280 seeds/ha in 76 cm rows. Interseeded cover crops included a mixture of cereal rye (19.2 kg/ha), spring triticale (3.6 kg/ha), purple top turnip (2.4 kg/ha), and cowpea (21.2 kg/ha). Cover crops were interseeded into standing corn treatments in three 19-cm rows/intererrow. Aboveground cover crop and weed biomass was measured by clipping two representative 0.76 m2 interrow quadrats in each plot immediately before a killing frost. Collected biomass was oven dried and weighed. Cover crop and weed biomass was not separated during collection and processing. Corn grain was harvested by hand from two 1-m row lengths in each plot, oven-dried, threshed, and weighed. Data may be used to better understand cover crop interseeding effects on corn grain yield and aboveground biomass under rainfed conditions within a semiarid continental climate. Applicable USDA soil types include Temvik, Wilton, Grassna, Linton, Mandan, and Williams.