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Area crops need more growing degree days to reach maturity

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Area crops need more growing degree days to reach maturity
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With the late start to the growing season, most of the full season crops grown in our area of the Minnesota will need some warm days and especially warm nights to reach maturity. Extending the frost-free growing season this fall would certainly be helpful.


As of Aug. 4, Growing Degree Day (GDD) accumulation for corn is about 120-140 heat units behind for the Detroit Lakes/Park Rapids area, compared to the average GDD accumulation.

Growing Degree Day accumulation is calculated by adding the high temperature (T Max) for the day (not to exceed 86) to the low temperature (T min) for the day (not below 50 degrees), and then dividing by two and subtracting that total from 50 (T Base). This will give you the GDD accumulation for that particular day. Generally we start to calculate GDD for corn from the first week in the month of May.

If the daytime high is 82 degrees and the night time low is 56 degrees, we would add 82 + 56 = 138 and then divide by 2, which would equal 69, and then subtract 50 from that total for a GDD accumulation of 19 units for that day.

Most 85-88 day 'maturity rated' corn varieties in north central Minnesota need to accumulate somewhere near 2,200 GDDs to reach maturity. As of Aug. 4 in north central Minnesota, we have accumulated 1,200-1,300 GDDs. If we accumulate an average of 18 GDDs per day, it would take approximately 55 days from Aug. 4 for corn to get to 2,200 GDD heat units. So, we will need those warm days and nights and hopefully an extended growing season!

To be completely safe from frost, crops must reach physiological maturity. In most crop species, a hard killing frost after physiological maturity has little, if any, effect on yields. Physiological maturity in various crops has been defined as the point at which maximum dry matter accumulation has occurred in the seed. But crops are not ready for harvest at physiological maturity, since dry down usually takes an additional 10 to 14 days or longer.

Physiological maturity is not an absolute value because it varies among species, varieties and years, but it is rapidly approached when crop seed is below 40 percent in water content. Physiological maturity is usually complete before seed reaches 30 percent moisture.

Determining the physiological maturity of full season row crops grown in Hubbard / Becker County and north central Minnesota:

- Corn -- Physiological maturity of grain corn varies in the northern Corn Belt. Usually about 50 to 55 days after 50 percent of the plants have visible silks, corn is safe from frost. The moisture content may range from 32 to 40 percent at physiological maturity, depending on the hybrid. The cob dries much more slowly than the kernel. When the kernel is 20 to 21 percent moisture, the cob is 36 to 40 percent. Another sign of physiologic maturity in corn is the formation of the "black layer" in the tip section of the corn kernel. Once this black layer develops, little additional dry matter accumulation occurs. Once corn is physiologically mature, the normal drying rate is about 0.5 percent per day -- which suggests about a two-week span between physiological maturity and harvesting.

- Dry Edible Beans -- Physiological maturity of navy beans is reached 95 to 105 days after planting. At least 80 percent of the pods should show yellowing and be mostly ripe, with only 40 percent of the leaves still green in color. Beans within the pods should not show evidence of any green color. Pinto beans reach physiological maturity in 95 to 100 days. At maturity at least 80 percent of the pods show yellowing and are mostly ripe. Only 30 percent of the leaves are still green.

- Soybeans -- Maximum dry matter accumulation of soybean has been reached when all the leaves are yellow and about 60 percent of the leaves have dropped from the plant. Pods are all yellow and more than 50 percent of the lower pods have turned brown. Beans within the pods should have 60 percent moisture, or less, show little evidence of green color, and be shrinking. Soybeans are usually harvested at moisture contents of 14 percent or less.

- Sunflower -- Physiological maturity is generally reached in sunflower when the backs of heads are yellow and bracts are brown. But many new varieties have a stay-green trait and will vary considerably in these characteristics. Seed moisture of 35 percent or less is a fairly accurate indicator of physiological maturity. To determine moisture content, producers should sample seed from average heads in several locations within a field.

Sunflowers will reach physiological maturity in 40 to 50 days after mid-bloom, depending on the hybrid and environmental conditions. After flowering and pollination, sunflower is quite tolerant to light frost (28 to 32 degrees Fahrenheit) -- even prior to physiological maturity.

- Garden Crops such as cantelope, water melons, pumpkins and winter squash could also use some more GDDs to reach maturity and to produce additional yield.

- For more information on crop maturity please contact your seed dealer, agronomist, local Extension Educator or give me a call: Will Yliniemi, Hubbard/Becker County Extension Educator at 1-218-732-3391 or 1-218-846-7328, by cell at 1-218-252-1042, or by e-mail at