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Control of Witchgrass in Fields of Seedling Kentucky Bluegrass¹

Competition by witchgrass (Panicum capillare L.) in newly planted fields of Kentucky bluegrass (Poa pratensis L) reduces vigor and stand of bluegrass seedlings and may significantly reduce seed yields the first harvest year. In some years a seeding may be lost. Control of witchgrass during seedbed preparation is impossible because bluegrass is a coolseason grass that should be planted early, particularly in non-irrigated areas, while witchgrass is a warm-season species that emerges 3 to 4 week after bluegrass. Use of selective herbicides is thus the only feasible means of controlling witchgrass.

In a preliminary screening trial of 10 herbicides, only MSMA (monosodium methanearsonate) showed acceptable selectivity. MSMA at rates ranging from 0.6 to 5.6 kg/ha was applied on irrigated Kentucky bluegrass seeding in a Typic Haploxeroll (Garrison gravelly loam) soil. Witchgrass seedling ranged from two to four leaves through six to ten tillers of growth, and bluegrass development was from one to two leaves through five to seven tillers. Three consecutive years of replicated field trials showed that seed yields tended to increase with increasing control of witchgrass. Treatment with 3.4 to 5.6 kg/ha of MSMA when witchgrass had begun rapid growth (more than two tillers) provided excellent control of witchgrass with no injury to Kentucky bluegrass seedling.

Key Words: Seed production • Herbicides • MSMA

² Research agronomists, ARS-USDA, Washington State Univ., Pullman, WA 99164.

Received for publication May 8, 1976.