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Potential frost impacts for Ohio’s late planted crops

Alexandra Knight

By Alexandra Knight, Pioneer Field Agronomist, Lewis Center, Ohio

The cool, wet spring of 2019 got many farmers off to a slow start in Ohio. The challenging weather conditions carried over from last Fall and continued through May and well into June. The question remains, “Will crops have enough time to will finish out?” Early September was not excessively warm, but the majority of Ohio continues to track 4 to 5 days ahead of the long-term average for heat unit accumulation. This is true of whether your plant date was May 30, June 30 or anywhere in between.

Each day corn planting is delayed past May 1 a decrease of 6.8 GDUs will be required to reach maturity. This means a May 30 planted corn would require approximately 204 GDUs less than it would on April 30. With this in mind, 105-day corn is anticipated to be at blacklayer mid-September when planted May 30, late September when planted June 15 and late October when planted June 30. These anticipated timeframes are with the expectation of weather keeping up with the long-term average and most relevant for central Ohio.

Corn producers — especially in northwest Ohio — are hoping to avoid an early frost to maximize yield and have dry corn to shell. If a cold snap does hit early, the severity of frost and stage of corn will determine the yield impact. In general, a hard-killing frost at an early reproductive stage of corn will be more detrimental to yield. Research would define a light frost event as one in which leaves are killed but not the stalk and shank. A hard-killing frost (29 degree F or less for several hours) at beginning dent (R5) has shown the potential of a 35% to 40% yield hit. Once reaching half milk line (R5.5), the risk of yield reduction decreases. However, a hard frost has shown a 12% to 15% yield decline while a light frost is a 7% to 10% yield hit. A freeze event at R5.75, or three-quarters milk line, has shown a 5% to 6% yield decline and 2% to 4% with a light frost.

Soybeans are photoperiod plants. Meaning plants will initiate flowering and crop maturation in response to longer night lengths. Later planting will therefore result in a decrease in the time spent in vegetative growth as well as reproductive growth. As a general rule, for a three-week delay in planting, soybean harvest will be delayed one week. Soybeans typically take 70 days from R1 to R7. This will be lesser with a later planting date. Soybeans will continue to add nodes and flowers until R5 or beginning seed.

As we get later in the year, the risk of frost obviously becomes greater. What timeline are we looking at for soybeans to finish out and what is the yield penalty if an early frost occurs? From full seed (R6) to maturity we are looking at approximately 25 days remaining until full maturity and seed moisture of 75% to 80%. A frost at this growth stage would lead to a yield loss of 20% to 35%. R6.5, or mid-way between full seed and maturity, there is roughly 16 to 18 days to maturity with pods between green and yellow in color. A frost event occurring at this growth stage would mean a yield hit between 10% and 15%. R7 is the beginning of maturity where one pod on the main stem has reached mature color. At this point, the crop is 8 to 10 days from full maturity. A frost event at this stage would range from 0% to 5% loss. The start of full maturity (R8) will mean that most pods are of mature color but still require 5 to 10 days to reach a harvestable moisture. Beginning R8 will have pods between 25% and 35% moisture.

With much of the state showing late October as the historical date for the first killing freeze, there is still time on the calendar for crops to reach full maturity without cold weather limiting yield.

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