Use of the winter cover crops is an integral component of the conservation systems in cotton (Gossypium hirsutum L.) and corn (Zea mays L.). A field experiment was initiated in 2004 to evaluate weed suppression provided by winter cover crops in a conservation tillage cotton and corn rotation. Rotation for winter cover crops included rye (Secale cereale L.) preceding cotton and clover (Trifolium incarnatum L.) preceding corn. Both covers were planted at five different planting dates with the median date corresponding to the thirty year average first frost. Termination dates in the spring were 4, 3, 2 and 1 week prior to cash crop planting, based on thirty year average soil temperature. Results showed a dramatic decrease in cover biomass production with each week's delay in winter cover crop planting and subsequently had a negative bearing on the cover crop benefits. More than ten times difference in biomass produced by clover was observed when clover was planted on the earliest date and terminated on last date compared to late planting and early termination. Rye produced almost eight times more biomass in the same comparison. Correspondingly, weed biomass was 556 kg/ha in the treatment with least rye biomass, 8 times higher compared to the treatment with greatest rye biomass. Weed biomass sampled in clover were less than in rye even though the difference was only 34 kg/ha in case of clover. Data for the first two years show no significant relationship between cover crop biomass and the cash crop yield.