Growing degree days (GDD) are commonly used to schedule the beginning of spring nitrogen fertilizer applications in grass seed crops. Typically, 200 GDD (base 0 C) from January 1st is used as the opening date of spring nitrogen fertilizer applications.  This number of GDD marks the beginning of the period that the average daily temperature reaches 5 C (41 F) and as a result, grass seed crops resume growth after winter quiescence.

The current (February 13th) GDD  is 197 GDD at Hyslop Farm near Corvallis.  At the current rate of GDD accumulation, 200 GDD should be attained on the 14th.  The long-term average date for attaining 200 GDD is February 14th.

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.

Thomas G. Chastain

Field trials conducted in the Willamette Valley showed that crop water use from April 1st through seed harvest in perennial ryegrass seed crops was 10.5 inches on a medium textured soil (silt loam).  Crop water use in tall fescue during the same period was 10.1 inches on the same soil type.  A perennial ryegrass or tall fescue seed field will need a combination of water stored in the profile over winter and irrigation to meet this water use for best seed yields especially if rainfall is short of this 10.1 to 10.5 inch total.

Spring rainfall at Corvallis averages 5.8 inches, but the crop water use need exceeds 10 inches on a medium textured soil. Our results indicate that a single irrigation (over a few days) of 3.7 inches timed at early flowering (BBCH 60) resulted in a seed yield increase of 16% in perennial ryegrass.  However, the highest perennial ryegrass seed yield increase of 25% was made possible with multiple irrigations (total irrigation water = 6.5 inches) timed between spike emergence (BBCH 50) and peak flowering (BBCH 65).
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Thomas G. Chastain

Growth and development of grass seed crops are progressing at a faster rate than is usual for the Willamette Valley as a result of warm winter and spring temperatures in the 2015-16 crop year (Fig. 1).  The same pattern was observed in the 2014-15 crop year.  These crop years were both much warmer than the average temperatures observed for the region and this is reflected in the growing degree days (base temperature = 5°C or 41°F) accumulated during the crop years.

Figure 1. Growing degree days (GDD) for the crop year through April 21 at Corvallis, Oregon.
Figure 1. Growing degree days (GDD) for the crop year through April 21 at Corvallis, Oregon.


These warm temperatures in the past two crop years are the result of strong El Niño conditions that have been prevalent.  The effect of more growing degree days (GDD) accumulated earlier in the crop year drives development of the crop so that stages of crop development are reached at earlier calendar dates.  In other words, the crops are progressing toward maturity at a faster rate than seed growers might otherwise expect.  Certain management practices like PGR applications and others are taking place earlier in the season as a result.

One aspect of the 2015-16 crop year that is different than in 2014-15 is the high precipitation in the current crop year.  While it has been warm, it has also been wet.  Last year was marked by severe drought conditions.  Looking ahead, scientists are projecting that the El Niño conditions have waned and that there is a possibility of cooler weather in the next crop year.

Thomas G. Chastain

Annual rainfall can be tabulated as calendar year (January –December) or crop year (September – August), but given the seasonal distribution of rainfall in the summer dry, winter wet climate of western Oregon, crop year precipitation is more relevant with regard to crop production and is considered here. The wet cycle in our Mediterranean climate begins in autumn and ends in spring so crop year precipitation better captures the seasonal nature of our region’s rainfall patterns (Fig. 1).

Figure 1.  Monthly precipitation and temperatures for Corvallis Oregon.  (TG Chastain)
Figure 1. Monthly precipitation and temperatures for Corvallis Oregon. (TG Chastain).
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Thomas G. Chastain

The big concern in the region continues to be the persistent drought and high temperatures that have prevailed throughout May and June.  These months are very important for grass seed yields because two important processes take place during the period: pollination and seed filling.

Drought and heat both can adversely affect pollination and subsequent seed filling.  The lack of water as a result of drought means that the movement of sugars into the seed is impeded, and thus, are not available for filling of the seed’s endosperm tissues with starch.  Heat can have direct effects on enzymes that are involved in seed filling.  Consequently, seed yield can be reduced by both reduction in seed weight (light seed) and seed number (aborted seed).

Average temperatures for June (+4.5°F above the long-term average) at Corvallis were the 2nd warmest since 1948 with 1992 having the warmest June in that period.  May was 2.2°F above the average.  Rainfall in May and June was also very low with the combined rainfall the 3rd lowest since the mid 1970s.  The lowest combined May and June rainfall was recorded on 1992.  This year’s drought and heat bears a strong resemblance to 1992.

Seed yields of perennial ryegrass were reduced by 11% in 1992 and tall fescue experienced a 14.5% loss in yield in that year.  By contrast, a large number of seed growers are reporting greater losses in seed yield of these crops this year even though the drought and heat have not been as severe in 2015 as they were in 1992.

For more information on this still developing story, see the following article in the Capital Press:

Grass seed suffers from drought, heat