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Influence of Seeding Rate on Big Bluestem Establishment with Herbicides

USDA-ARS and Dep. of Agron., Univ. of Nebraska, Lincoln, NE 68583-0915

^* Corresponding author (rmasters{at}unlinfo.unl.edu).

Weeds interfere with establishment of big bluestem (Andropogon gerardii Vitman vat. gerardii Vitman) on rangeland and cropland in the central Great Plains of North America. A study was conducted to determine the influence of seeding rate on establishment of big bluestem when metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-l-methylethyl)acetamide] and atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-l,3,5-triazine-2,4-diamine] were applied alone 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, and 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 combination before weeds or the seeded big bluestem emerged. Big bluestem stand frequency (the number of times the planted grasses occurred within sampling grids and expressed as a percentage of the total number of grids evaluated) and yield were measured the year after planting to determine establishment success. Big bluestem stand frequency and yield were influenced primarily by the main effects of herbicide and seeding rate. Grass stand frequencies usually increased with increasing seeding rate. Big bluestem stands were successfully established in three of the four environments evaluated when seeded at 110 PLS m^–2, and in all environmenls 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 metolachlor than on areas not so treated at Clay Center in 1991 and 1992 and 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 Clay Center. Based on these findings, metolachlor and/or atrazine can be applied preemergence the year of planting to improve yield of big bluestem seeded at a rate as low as 110 PLS m^–2.

Received for publication December 2, 1996.

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