Thomas G. Chastain

The combination of very dry and warm weather in spring and early summer 2015 is a cause for concern for growers of grass and forage legume seed crops in the Willamette Valley. Moreover, these conditions have accelerated the timing of the harvest of seed crops in the region. One question that has arisen is how will these conditions affect seed yield?

Seed yield is hard to predict with any certainty but what we can do is to examine the effects of similar conditions on seed yield in an analog year – a year that experienced similar drought and heat.

One such year was 1992. That year was characterized by no rainfall in May and record 102⁰F in late June. Rainfall in May and June 1992 was only 1.18 inches compared to the average 3.68 inches for the period. To date rainfall in May and June 2015 has been 1.53 inches but overall temperatures have been warm, and hot temperatures are expected to persist through late June.

How did the conditions of 1992 affect seed yield in that year? An examination of past seed yield records for average grower yields in the Willamette Valley reveals that the drought and heat conditions in 1992 had significant deleterious effects on seed yields. Among grass seed crops, warm and dry conditions in 1992 were in part responsible for an 11% seed yield loss in perennial ryegrass and 14.5% seed yield loss in tall fescue. These results include seed yields for both dryland and irrigated crops so potential losses in seed crops without irrigation might have been greater than the losses reported above. For forage legume seed crops, losses of 16% were observed in red clover seed yield in 1992.

Will the results of the 2015 harvest be similarly affected by the current drought and heat? Past results suggest that grass and legume seed yields will be adversely influenced by recent and current weather conditions, but it is too early to know the exact magnitude and severity of losses in seed yield.

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