Cultivated rice yield losses due to red rice infestation vary by cultivar, red rice density, and duration of interference. The competition effects of red rice could be influenced further by emergence characteristics, red rice biotype, and planting time of cultivated rice. We aimed to characterize the emergence of red rice biotypes at different planting dates and evaluate the effect of red rice biotype, rice cultivar, and planting date on cultivated rice yield loss. Field experiments were conducted at the Southeast Research and Extension Center, Rohwer, AR, and at the Arkansas Rice Research and Extension Center, Stuttgart, AR, in the summer of 2005 and 2006. The experimental design was a split-split plot with three or four replications. Planting time, Clearfield^TM (CL) rice cultivar, and red rice biotype were the main plot, subplot, and sub-subplot factors, respectively. There were three planting times from mid-April to mid-May at 2-wk intervals. CL rice cultivars, CL161 and hybrid CLXL8, and 12 red rice biotypes were planted. The emergence rate and coefficient of uniformity of germination differed among some red rice biotypes within a planting time. Planting date affected the emergence characteristics of red rice biotypes. The mean emergence rate of red rice was 0.043 d⁻¹ in the mid-April planting and 0.058 d⁻¹ in the late April planting. For the mid-April planting, 50% of red rice biotypes emerged in 20 ± 2 d compared with 15 ± 2 d for CL rice cultivars. Yield losses due to red rice biotypes generally increased in later planting dates, up to 49%. Yield losses due to interference from red rice biotypes ranged from 14 to 45% and 6 to 35% in CL161 and CLXL8, respectively. Cultivated rice became less competitive with red rice in later plantings, resulting in higher yield losses.

Nomenclature: Red rice, Oryza sativa L. ORYSA; rice, Oryza sativa L. ‘CL161’, ‘CLXL8’