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Soybean Stand Establishment and Yield as Affected by Herbicides and Cultural Practices¹

When reduced soybean (Glycine max (L.) Merr.) stands occur, herbicides are often blamed for the problem. The object of this research was to determine if metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5 (4H)-one] and/or vernolate (S-propyl dipropylthiocarbamate) herbicides interact with other cultural variables to reduce stands and grain yield. Cultural variables researched in the field included soybean seed vigor, cultivar, planting depth, planting date, and surface versus incorporated herbicide application. In field experiments on a Typic Haploquoll soil herbicides were applied at slightly higher than recommended rates, and soybeans were seeded in 75-cm row widths at a density of 350,000 seed/ha.

In all experiments visual symptoms of herbicide leaf damage occurred, but herbicide treatments had minor effects on stand establishment and grain yield. In contrast, several other variables significantly reduced stands and grain yields. Differences in seedbed condition created by different planting dates altered stand establishment. Seed lots low in seed vigor (as measured by cold test germination) resulted in reduced stands and in some situations reduced grain yields when compared with high vigor seed sources. Compared with a 1.9 cm planting depth, a 3.8 cm planting depth reduced stands in crusted seedbeds. In contrast, the shallow depth caused stand problems in a dry seedbed. Of the cultural variables affecting stand establishment and grain yield, none exhibited a significant interaction with herbicide treatment. In a greenhouse study, extremely high vernolate rates were more detrimental to emergence of low vigor than high vigor seed. Our data suggests that even when visual symptoms of herbicide injury occur, cultural variables such as seedbed conditions, seed quality, and minor differences in planting depth are often the major variables causing stand problems.

Key Words: Seed vigor • Planting depth • Seed quality • Glycine max (L.) Merr.

² Associate professor, Dep. of Agronomy, and research agronomist, SEA-AR, USDA, Univ. of Illinois, Urbana, IL 61801.

Received for publication March 2, 1979.