Field experiments were conducted in 1997 and in 1998 to determine the effects of density and time of emergence on Eriochloa villosa growth and seed production in Zea mays. E. villosa was transplanted at four densities (3, 9, 27, and 81 plants m−2) to simulate emergence at four Z. mays growth stages (VE, V2, V5, and V10). Compared to E. villosa plants that emerged with Z. mays plants, total above-ground E. villosa biomass at maturity of plants grown at 3 plants m−2 was reduced by 54, 97, and 99% when emergence was delayed until the V2, V5, and V10 stages of Z. mays, respectively, in 1997. In 1998, total aboveground E. villosa biomass at maturity was reduced by 70, 87, and 99% when emergence was delayed until the V2, V5, and V10 stages of Z. mays, respectively. E. villosa aboveground vegetative biomass per plant at maturity was linearly related to seed production per plant in each year. E. villosa seed production m−2 decreased nonlinearly as density decreased and time of emergence was delayed. Based on estimated model parameters, maximum seed production was 57,100 and 12,700 seeds m−2 in 1997 and 1998, respectively. Within time of emergence, E. villosa density did not affect seed mass per seed, however, seed mass of late-emerging cohorts was less than that of early-emerging cohorts. Time of weed emergence relative to the crop was a very important factor in determining biomass and seed production. Results suggest that late-emerging plants may not be very important to long-term management of E. villosa.