Velvetleaf (Abutilon theophrasti) Seed Populations in Six Continuous-Corn (Zea mays) Fields

To assess velvetleaf ( Abutilon theophrasti Medic.) seed populations and trends in continuous corn ( Zea mays L.), six fields were selected and sampled in August and November of 1977 and 1978 at the 0- to 10- and 10- to 20-cm depths, and at the 20- to 30-cm depth in August of both years. The overall population mean was 51 million viable velvetleaf seeds/ha in the top 20 cm of soil. Means of individual fields ranged from 34 to 88 million seeds/ha. The population mean increased 13 million (28%) from 1977 to 1978. Individual fields had increases ranging from 8 million (14%) to 24 million (112%) seeds/ha. The seed population increased 16 million (37%) from August to November in these 2 yr, and 70% of the seeds were found in the top 10 cm of soil. The 20- to 30-cm depth contained 7% (3 million) of the seed extracted in the August sampling. A comparison of techniques for estimating the number of velvetleaf seeds in a soil sample showed that wet sieving was more efficient and accurate than were soaking and sieving, dry sieving, or direct incubation and germination. Seed immersion in water at 70 C for 5 min increased velvetleaf germination from 11% to 84%, compared to 52% for sulfuric acid, 48% for scarification, and 62% for seed-coat puncture. In the greenhouse, velvetleaf control with atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)- s -triazine] and metribuzin [4-amino-6- ten -butyl-3-(methylthio)- as -triazin-5(4 H )-one] declined as the seed population increased.