Heat and dry weather has prevailed across much of the region last week and will continue into most of this week. Heavy rain showers once again covered parts of Calhoun, Sac, Ida, Pocahontas and B.V. counties late last week, while many areas in Carroll, Crawford and Monona counties remain very dry. Moisture is important for both the corn and soybean crops, but the soybeans (at the seed fill stage) are at the most critical stage for moisture deficit. Heat stress may be robbing some yield from the soybean and alfalfa crops. The 3^rd crop alfalfa and pastures could also use a good rain. 2^nd generation European Corn Borer are laying eggs in surrounding cornfields, so we need to be scouting the more immature fields.

R4 Stage (Dough) commonly occurs 24-28 days after silking. Continued starch accumulation in the endosperm has now caused the milky inner fluid to thicken to a pasty consistency. The reduced fluid and increased solids within the kernel at this time produce a doughy consistency. Just prior to R5 kernels along the length of the ear begin to dent or dry on top.

Soybean Development has moved into the seed fill period in many fields. This period is characterized by rapid seed filling and redistribution of dry weight and nutrients within the plant. At initial R5 (the R5 stage is reached when one pod in the uppermost four nodes contains a seed 1/8 inch wide) reproductive development can range from flowers just open to pods containing seeds 8 mm long. Root growth and expansion slows. Therefore it's important that water now be available where the developed roots are -- no longer can the tap and branch roots grow to where the water is. At R5.5 the plant attains its maximum height, node number and leaf area. The nitrogen fixation rate peaks and begins to drop. The seeds at this point will begin a rapid period of dry weight and nutrient accumulation. As development approaches R6, nutrients accumulated in the leaves and petioles begin to be redistributed to the seed.

As mentioned earlier, this is a very critical period to maintain good moisture status in the soil. If irrigation is available, make sure the soil water profile was full at R3 and again at R5. The developing seeds need moisture throughout development.

Over the last two weeks heat stress may be an issue with some crops. High daytime temperatures can be harmful, high nighttime lows can be even more damaging. As long as corn has adequate moisture and does not experience any moisture stress, the heat we’ve experienced the last two weeks should not have any appreciable affect on yield. Due to physiological differences, Soybeans and Alfalfa are much less able to cope with the heat stress. Respiration (energy use) in the plant (night respiration and daytime [photo] respiration) uses up assimilated energy that is gained during the day from photosynthesis. The warmer the plant temperature is, the greater the respiration rate. As more energy is lost through respiration there is less energy to go into seed fill (soybeans), plant growth and root energy storage (alfalfa).

You can think of this process as putting money into a savings account. Your paycheck (photosynthesis) is a set amount and you have fixed and variable costs. A set amount of money is spent on fixed costs (normal plant maintenance) but as your variable costs (plant respiration) go up you put less money in the savings account (soybean seed). At the end of the year, the more variable costs (plant respiration) you have, the less money you’ll have in your savings account (seed or root storage). Ultimately, less energy is available for seed fill, resulting in smaller seed and lower test weight (soybeans) or less plant biomass and root energy storage (alfalfa).

Respiration rate is governed by plant canopy temperature, which is determined by air temperature and moisture status. When plants are under moisture stress, they are less able to cool themselves via transpiration and the temperature of the canopy increases. During the night, leaf stomata close so the plant cannot cool its self. When nighttime temperatures are high (mid 70’s to low 80’s), plant canopy temperature remains high and nighttime respiration burns up even more assimilated energy.

Second generation European Corn Borer moth flight has peaked in most locations. Scouting for egg masses and small larvae in non B.t. fields should be occurring now. Control of 2^nd generation ECB depends on timely detection and insecticide applications. Fields that have green silks and are shedding pollen during the peak period of moth flight are most susceptible to second-generation infestation. When scouting, select a minimum of 50 plants per field, choosing plants from several different parts of the field. Examine the underside of leaves for white borer egg masses. These masses, usually found on leaves in the middle third of the plant (frequently near the midrib), normally hatch in about five days. Each egg develops a black spot just before hatching. Best control is realized when application is timed to the first significant egg hatch and when young larvae are still located in the leaf axils. As the plant approaches blister stage and beyond, potential economic benefits of an insecticide application rapidly decline.

This page last updated on 02/03/04

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