Introduction

About 100,000 acres of Kentucky bluegrass are produced in the Pacific Northwest. A phase out of burning in Washington and opposition to smoke may reduce acreage of bluegrass. Seed yield declines as stands age, with some cultivar yields declining more rapidly than others. High stand density may be partially responsible for yield decline, especially when post-harvest residue is removed mechanically. Older bluegrass stands may be revitalized during a one year renovation period using herbicides such as glyphosate. No-till planting of annual crops in chemically suppressed bluegrass stands may allow economic return during renovation. Glyphosate rate and bluegrass cultivar response to chemical renovation are not adequately known.

Materials and Methods

Two experiments were established in a five year-old stand of Kentucky bluegrass near Moscow, Idaho to evaluate chemical renovation of Kentucky bluegrass varieties with different rates of glyphosate, and to evaluate the effect of glyphosate rate and bluegrass variety on lentil seed yield. Both experiments were arranged as a strip plot design with four replications. The main plots for experiment 1 were five rates of glyphosate (0.5, 0.75, 1.0, 1.25, and 1.5 lb ai/A), and five bluegrass varieties (Adelphi, Glade, Liberty, South Dakota, and Suffolk) were the sub-plots. Each sub-plot was 4 by 8 ft. The main plots for experiment 2 were two rates of glyphosate (1 and 1.5 lb ai/A), and sixteen bluegrass varieties were the sub-plots. Each sub-plot was 8 by 10 ft. Glyphosate treatments were applied to 1 inch tall bluegrass in both experiments on April 8, 1997 with a CO₂ pressurized backpack sprayer calibrated to deliver 10 gpa at 40 psi. In both experiments, ‘Pardina’ small brown lentil was seeded in rows spaced 7 inches apart at 52 lb/A using a no-till drill on May 14, 1997. Fertilizer (16-20-0) was banded between alternate rows at 150 lb/A during seeding. In experiment 2, 4 inch sod cores were taken from each bluegrass sub-plot 1 week prior to glyphosate application and 2 weeks after application. Cores were transplanted into pots in the greenhouse. After 6 weeks, grass shoots and rhizome sprouts were counted. Lentil was harvested from each sub-plot at maturity with a small plot combine on August 22 (experiment 2) and August 23, 1997 (experiment 1).

Results and Discussion

In experiment 1, lentil seed yield increased with increasing glyphosate rate regardless of bluegrass variety (Table 1). Seed yield of lentil seeded into early maturing bluegrass varieties such as South Dakota was significantly higher than lentil yield seeded into the sod of late maturing varieties such as Glade (Table 2). Early maturing bluegrass varieties were damaged more by glyphosate because they had greater vegetative growth at herbicide application time, and thus were less competitive with the lentil crop (data not shown). Also, the regrowth potential of South Dakota from remaining plants was less than Glade, because it is a less aggressive variety than Glade.

In experiment 2, no bluegrass varieties were affected differently by glyphosate rate (data not shown). Pre-glyphosate rhizome weights were not different between varieties (Table 3). Cheri had the highest pre-glyphosate tiller number and Kenblue had the lowest, while most other varieties had similar tiller numbers. Pre-glyphosate rhizome weights and tillers were not significantly correlated (P = 0.05) with post-glyphosate shoot re-establishment, rhizome sprouts, or lentil seed yield. The number of rhizomes that sprouted after glyphosate application was not affected by glyphosate rate, but varied with variety. Midnight and Glade, late maturing varieties, had the highest number of new rhizome sprouts, while Huntsville and Baron, early maturing varieties, had the lowest. Late maturing varieties sustained less damage at the time of application, and thus had greater root and shoot regeneration. This also was due to genetic differences in aggressivity between early and late maturing varieties. Post-glyphosate rhizome regeneration and lentil seed yield were not significantly correlated (P = 0.05), but grass shoot re-establishment did correlate significantly with lentil yield and had a correlation coefficient of -0.22 (data not shown). Yield was highest from lentil seeded into early maturing bluegrass varieties such as Huntsville and Kenblue, and late maturing, aggressive varieties such as Ram I and Midnight reduced lentil yields.

¹ Values for grass shoots and rhizome sprouts are means of two glyphosate rates (1 and 1.5 lb ai/A).

² Grass shoot re-establishment evaluated after 6 weeks of growth; includes bluegrass and annual grass seedlings.