NEWS for North Dakotans
Agriculture Communication, North Dakota State University
7 Morrill Hall, Fargo, ND 58105-5665

August 19, 1999

End of Growing Season Brings Frost Questions

Some row crops in North Dakota are developing several weeks behind the five-year average due to late planting. Cool night temperatures and probabilities of early frost have many North Dakota growers concerned. To be completely safe from frost, crops must reach physiological maturity, says a specialist with the North Dakota State University Extension Service.

"In most crop species a hard killing frost after physiological maturity has little, if any, effect on yields," says Duane R. Berglund, extension agronomist at NDSU. "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."

Physiological maturity is not an absolute value because it varies among species, varieties and years, Berglund says, 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.

Berglund offers the following information for determining the physiological maturity of this region's row crops:

• 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. Sunflower 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 F to 32 F)--even prior to physiological maturity.
• Mustard/Canola--Physiological maturity is reached when leaf drop has occurred and the overall field color has changed from green to a yellowish-tan hue. To determine physiological maturity, producers should select pods from the middle of the racemes of several plants in areas representing the average maturity of the field and examine these seeds. When lower pods have 15 to 25 percent of the seeds changing color from green to yellow, purple, or brown, then the canola is at optimum maturity for swathing. Seed moisture should be under 35 to 40 percent at this time and yield should not be affected. Seed color and moisture percent are more important than pod color.
• Lentils--When lentil plants begin to turn yellow and lower pods become brown to yellow, maturity is advanced enough for swathing. Stems or vines may still be green, but leaves are yellow. A majority of the upper pods should be a golden yellow to tan color. Pods will be firm and not juicy when squeezed.
• Buckwheat--Since buckwheat has an indeterminate growth habit, flowers, green seed and mature seed are present on the plant at the same time. If flowering is almost finished, swathing is recommended when 75 percent of the seeds are mature. The stems may still be quite juicy at this stage and will require one to two weeks to cure in the swath. If frost kills the crop, swath it immediately to prevent shattering and lodging.
• Safflower--Safflower is ready to harvest when seed moisture is 8 percent or less. The leaves are dry, brown in color and only slight green remains on the bracts of the latest flower heads. Safflower should be swathed only when excessive green weeds prevent straight combining. The crop can be windrowed to dry out green weeds when seed moisture content is as high as 25 percent and not affect yield or grades.

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Source: Duane R. Berglund (701) 231-8135
Editor: Dean Hulse (701) 231-6136