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(4H)-one] declined as the seed population increased.

Diagnosing herbicide-resistant weeds as a first step in resistance management and monitoring their nature, distribution, and abundance demands efficient and effective screening tests. This review summarizes and recommends appropriate seed sampling techniques, protocols for screening weeds for resistance to herbicides of different sites of action, interpretation of results, and information given to the grower. Elements common to all screening procedures are reviewed. Choosing appropriate discriminating doses to distinguish between resistant and susceptible weed biotypes is the most important factor in achieving accurate and consistent results. Interpretation of results is also critical because resistant weeds may comprise a small portion of the population in suspected accessions or biotypes.