It feels and looks that fall is well upon us now, the days and nights are getting cooler and the crops are rapidly maturing. Earliest planted corn in the southern tier counties of northwest IA have reached physiological maturity (PM), as well as many soybean fields. Once corn reaches PM, it is the time to collect samples for the fall corn stalk nitrate test. This test can help determine if the nitrogen needs of the crop were sufficient or over abundant during the growing season. Many soybean fields are now turning color. This normally indicates the field has reached physiological maturity, yet in many area fields I believe there are diseases and nematodes causing premature death and defoliation. Determining maturity in a soybean crop can help to plan harvest activities and avoid unnecessary harvest losses. Alfalfa is preparing itself for winter survival by building energy stores in the root system, do not cut alfalfa during this period if at all possible.

Determining when soybeans are mature (both physiological and harvest maturity) helps to schedule harvest operations and reduces in-field harvest losses. Physiological maturity is when the soybean has accumulated its maximum dry weight. The growth stage classification is R7 - R8.

With an indeterminate soybean, all pods will not reach physiological maturity at the same time. There is a period of about two to three weeks from the time the 1st pod reaches physiological maturity to the last pod reaching physiological maturity. The average seed moisture at the time the pod is physiologically mature is 55%. When all pods on the soybean plant have reached physiological maturity, the average seed moisture for the plant is usually around 44%.

Two main indicators can be used to determine when individual pods reach this stage.

• Pod Color: A short time prior to seed shrinkage, the pods will begin to lose their green color and will take on a yellowish cast. When the pod is completely free of green, the seeds inside have already started to shrink.
• Seed Shrinkage: Once the seed has attained its maximum dry weight and size, it will begin to shrink. Upon shrinking, the seed will become less associated with the white membrane surrounding the seed. Eventually, the membrane will no longer cling to the seed and will stay with the pod wall when split open.

Harvest maturity indicates when all beans on the soybean plant are ready to be harvested. This is usually when the average seed moisture is 13%. Harvest maturity is usually reached about one to two weeks after the entire plant has reached physiological maturity, depending on temperature and humidity. Timely harvest of a mature field will help minimize losses due to shattering and decreased seed quality.

I have been in a number of soybean fields with expanding yellow areas in recent weeks. In some cases, the fields are reaching normal physiological maturity. But in fields that have yellow patches and still have green pods, other factors may be occurring in the field that need to be investigated. A cumulative effect of drought stress, disease (continuing root rot problems, phytophthora stem rot, charcoal rot) and soybean Cyst nematode may be the problem.

Most soybean varieties on the market have some level of resistance to race 1,3 and 4 phytophthora, yet a new race (25) has become established in Iowa soils, which may be causing problems in our area. I have sent plant samples to IA State disease diagnostic lab to determine if race 25 phytophthora may be a problem in our area.

Although you can not do anything about field and disease problems this year, you can attempt to determine the cause of the problem so it can be avoided in following years. When you walk your fields, dig plants and examine the root system (check for root rot and cyst nematodes), examine the lower stalk area (look for dark brown discoloration, indication of phytophthora stem rot), split stems (look for discoloration of the pith, indication of charcoal rot) and collect soil samples for Cyst nematode testing.

When you take your last harvest of alfalfa affects its winter survival and next spring's vigor. Alfalfa needs about six weeks of uninterrupted growth in the fall to become fully winterized. Dr. Barnhart (ISU’s Extension Forage Specialist) recommends that for the best survival of the stand, attempt to take last summer cut by late August or very early September, and let regrowth stand in the field (no late cut or grazing). If you have not fertilized in the summer, topdress any needed potassium in late August or early September.