Thomas G. Chastain

The Willamette Valley does not expect much rain during harvest of grass seed crops.  The production practices for grass seed crops in the region have evolved to take advantage of the dry conditions that are prevalent during harvest.  But when rain falls during the harvest season, questions arise regarding the effects of this late precipitation on seed yield and quality of grass seed crops.

Seed Field
Aerial view of windrow harvested Willamette Valley grass seed field.

For grass seed crops that are at the pre-harvest stage during precipitation events, rain is usually not a problem and even after the crop is cut, the worst consequence is that the crop will take more time to dry down to reach combine seed moisture in the swath.  However, if the delay in drying is too long, grass blades can grow up and through the swath making combining operations difficult.  Tall fescue seems to be particularly problematic with regard to post-swathing leaf growth.  When grass blades grow back up through the swath, some farmers recut the swath to facilitate combine harvesting but some seed losses can be expected in this operation due to shattering.  If the crop is swathed onto wet soil and remains there for an extended period, there can be some losses in yield and quality as a result of the action of fungi and other pests.

The potential for damage to seed crop yield and quality is dependent on the intensity and duration of rainfall.  Excessive rain after the grass seed crop is swathed can be a problem, especially in July and early August (for later maturing crops).  First, the action of the falling rain in some circumstances can be sufficient to cause shatter losses of seed in the swath.  Secondly, if rain is persistent and is accompanied by high humidity, then seed can sprout, thereby reducing the quality and marketability of the seed.  These late season high rainfall events are not very common and that is why the seed industry has few artificial drying facilities in the region.  Pre-harvest sprouting of grass seed was observed in the Willamette Valley in 1983 and again in 1993.  Light rainfall after swathing of the crop is not generally a problem.

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