INFLUENCE OF SEEDING RATE ON BIG BLUESTEM ESTABLISHMENT WITH HERBICIDES

Weeds interfere with establishment of big bluestem (Andropogon gerardi i Vitman var, gerardii Vitman) on rangeland and cropland in the centra l Great Plains of North America. A study was conducted to determine th e influence of seeding rate on establishment of big bluestem when meto lachlor ethylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] and atrazin e hyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine] were applied alon e or in combination. 'Pawnee' big bluestem was planted in late May to early June 1990 and 1991 near Clay Center and Mead, NE, at 110, 220, a nd 440 pure live seed (PLS) m(-2). Metolachlor at 0, 2.2 or 3.3 kg a.i , ha(-1) and atrazine at 2.2 kg a.i. ha(-1) were applied alone or in c ombination before weeds or the seeded big bluestem emerged, Big bluest em stand frequency (the number of times the planted grasses occurred w ithin sampling grids and expressed as a percentage of the total number of grids evaluated) and yield were measured the year after planting t o determine establishment success. Big bluestem stand frequency and yi eld were influenced primarily by the main effects of herbicide and see ding rate. Grass stand frequencies usually increased with increasing s eeding rate. Big bluestem stands were successfully established in thre e of the four environments evaluated when seeded at 110 PLS m(-2), and in all environments when seeded at 220 or 440 PLS m(-2). Big bluestem yields were at least 1.2 Mg ha(-2) greater on areas treated with meto lachlor than on areas not so treated at Clay Center in 1991 and 1992 a nd Mead in 1991, Atrazine increased big bluestem yields by 1.2 and 2.4 Mg ha(-1) at Mead in 1991 and 1992, but had no effect on yields at Cl ay Center, Based on these findings, metolachlor and/or atrazine can be applied preemergence the year of planting to improve yield of big blu estem seeded at a rate as Low as 110 PLS m(-2).