Most of the corn crop in Iowa has reached full dent stage at this time - as of August 28. "When the ear is initially at full dent - when all of the kernels have just dented - the ear is at one-quarter milk line and the crop has about three weeks to reach the black layer stage. When the black layer forms in the tip of the corn kernel, that's when the crop is safe from frost," says Brian Lang, Iowa State University Extension crop specialist in northeast Iowa.
Silage harvest could begin anytime from one-quarter to one-half milk line. "But the maturity of the corn plant should be double checked by measuring the actual crop moisture with the appropriate equipment," he says. "You can use a Koster Tester or a microwave oven," he says.
Also helpful is an ISU Extension publication, complete with color photos, that shows you how to stage corn for development. Contact your county Extension office or the ISU Extension publications Web site to get a copy.
What about soybean development?
To get an idea of how far along your soybeans are in development, Lang offers the following guidelines:
At the R6 stage of development, one pod in the top four nodes on the main stem is filled. That plant is 27 days from maturity. At the R6.5 stage, all pods in the top four nodes on the main stem are filled with beans so the plant is 18 days from maturity. At the R7 growth stage, one pod on the main stem has mature color, which means the plant is 7 days from maturity. At the R8 growth stage, 100% of the pods have reached full color. That's when the crop has reached maturity.
Full-season soybeans planted timely in the spring and under average weather conditions during the summer will reach beginning R6 stage in the last week of August, says Lang. The plant is considered "frost safe" once it reached R7 stage.
For a complete explanation of stage of development in soybeans, get ISU Extension publication PM-1945, "Soybean Growth and Development."
Give alfalfa a fall rest period
This time of year ISU agronomists get questions from farmers asking about cutting alfalfa in the fall with regard to stage of maturity and time of killing frost.
Typically, it is recommended to not harvest alfalfa during the five to six week period before a killing frost. "You shouldn't harvest it during that time," says Lang. "In northeast Iowa, an alfalfa killing frost is about 24 degrees Fahrenheit and that typically occurs around the third week in October. This means an alfalfa harvest within the window of the second week of September through the second week of October puts the stand at risk of winter injury."
There is no sharp demarcation in this window. "The risk of putting additional stress on the stand tends to get progressively worse as we approach the fourth week in September. And then it becomes progressively less risky as we approach the actual date of the killing frost," he says.
"Once we get this close to the potential killing frost, it tends to be a little easier to harvest just ahead of the killing frost than after it," he adds. "The problem being that October has poor hay drying conditions."
Harvest alfalfa a little ahead of frost?
Harvesting it a little ahead of when the first killing frost occurs is not a problem, says Lang, because the short days and cold weather don't stimulate much alfalfa growth anyway.
Under good fall growing conditions, harvesting in both early September and after frost is possible. This puts more stress on the stand than just taking one harvest, but it can provide a significant amount of excellent quality forage while surviving most winters just fine, he notes.
The "fall" harvest of alfalfa is just one stress factor that will contribute to possible winter injury. Others include variety selection - there are variety differences in winter hardiness and disease resistance. Soil fertility, pest management, wheel traffic, fall soil moisture conditions (wet falls increase risk of winter injury) and leaving some forage cover (stubble height) for insulation into the winter.
"The better you manage these factors, the less stress you put on the alfalfa. Also, the better the alfalfa can tolerate less than ideal fall harvest management," says Lang. "And yet, on occasion, the winter can be just plain nasty - too cold, too warm, too much freezing and thawing - and it can harm the alfalfa no matter what safeguards you take."