Thomas G. Chastain

The Willamette Valley has experienced very dry late summer and early fall conditions to date and long-range projections are for more of the same coming in the middle and late parts of fall.  How dry has it been?  Rainfall for the July through September period has been 0.87 inches at Hyslop Farm or 37% of the normal 2.35 inches for the period.  Only 10 years in the past 123 years have been this dry or drier in the Willamette Valley.   In these dry years, rainfall in October has averaged 2.11 inches or 66% of normal.  When it gets this dry in July through September, dry Octobers typically follow.  Thus, no relief from the dry conditions in the near term can be expected given either the forecasts or the historical records observed in past drought periods.

The late summer and early fall period has long been thought to be critical for regrowth of the grass seed crops after harvest and for the following year’s seed yield.  Extremely dry conditions during this period reduce stands and crop regrowth.  In work led by one of my former graduate students, Dr. Theo Velloza, the water available to two cultivars of perennial ryegrass was controlled during August and September.  Total available water through rainfall and irrigation during this period ranged from 0 inches (rain out sheltered plots) to 4.6 inches.  Rainfall for August and September over the past 123 years has averaged 1.98 inches.

Figure 1. Effect of water availability (rainfall and irrigation) on tiller production in Affinity and Buccaneer perennial ryegrass.

Perennial ryegrass tiller production as measured in October was increased proportionally by increasing availability of water in August and September (Fig. 1).  Stand cover was also improved by increasing water availability to the perennial ryegrass seed crop (Fig. 2).  Nevertheless, the number of spikes and seed yield in the two cultivars was not affected by water availability in August and September over a two-year period.

Figure 2. Stand condition of perennial ryegrass irrigated in August and September (top) and in perennial ryegrass receiving no water in August and September (bottom). (T.G. Chastain photo)

In our studies conducted during the 2009-10 and 2010-11 crop years, 5 inches of irrigation was applied in September to perennial ryegrass and tall fescue seed crops.  August and September rainfall was 1.44 inches (73% of normal) in 2009-10 and was 3.12 (158% of normal) for the same period in 2010-11.  Despite dry conditions in 2009-10, seed yield in perennial ryegrass and tall fescue was not affected by the 5 inches of water applied in September and similar results were produced in the wetter 2010-11 trial.

These investigations suggest that while early fall irrigation increases tiller production and may enhance stand persistence under Willamette Valley conditions, there were no beneficial effects of fall irrigation on seed yield in perennial ryegrass and tall fescue even under dry conditions.

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