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ISSUE 14  August 2, 2001

 

CORN RESILIENCE AND REPEATED STRESS

Weather, soil and growing conditions all play a part in the development of the corn root and stalk. If the root system does not develop normally or if slow then very rapid stalk growth occurs, problems can erupt and be accentuated by future plant stresses in the season. Factors that interfere with root development are: shallow planting; dry topsoil conditions; soil compaction; seed bed soil settling; wind; excessive rainfall; very rapid plant stalk elongation; very wet soil conditions; high air and soil temperatures; misapplication of herbicides; corn hybrid sensitivity to herbicides (and herbicide-insecticide combos); seed furrow not closed; insect or disease damage on roots; and, herbicide carryover. Factors that interfere with stalk development are: cool soil or air temperatures; dry soil conditions; very wet soil conditions; insect tunneling or disease introduction in the stalk; cold and then sudden high air and soil temperatures; corn hybrid sensitivity to herbicides; and, rapid growth once any stress is muted or eliminated in the corn environment. Breakage of green corn can happen any time after the growing point emerges from the soil, generally from the fifth-leaf stage and beyond. Corn breakage prior to tasseling is usually, at least partially, weather related. Timing and severity of windstorms influence whether corn will upright itself or have greensnap. Before brace roots are established corn must rely on a strong and extensive root system as well as stalk flexibility to bend with the wind. Because pretasseled corn has internodes actively elongating, stalks can become brittle and in fact do lose flexibility over time. Early breakage can occur at the soil line, the point of the most mechanical stress, or can occur higher on the plant, depending on stalk flexibility and leaf weight load. With flexibility and an established root system, however, corn may not break but temporarily lodge. Corn that has temporarily lodged will try to reorient itself upward. Often, however, the stalk may remained slightly bowed due to some continued growth toward light while lodged in a pile of corn stalks and because less light may penetrate down into the canopy to insure equal growth along the stalk internodes still elongating. Fields that do not reorient perfectly from high winds may be more susceptible to late season lodging. Harvest these fields as soon as physiological maturity and the targeted drydown point has occurred so that harvest will have less headaches.

 

ESTIMATE CORN YIELDS IN YOUR FIELDS

With silking just finished on corn that is progressing in a timely fashion in order to mature before the average frost date, estimates of the relative grain yield can be done. First, count the number of harvestable ears per 1/1000th of an acre. With 30-inch rows that is a row length of 17 feet, 5 inches and with 22-inch rows count the number of ears down 23 feet, 9 inches of a row to determine the estimate. Within this ear count area, also count the number of kernel rows on the ears of every fifth plant. Also, count the number of kernels per row on these same ears. Remember to only count kernels that are developing well (do not count kernels on the tip that are less than half the size of those mid-way up the ear). Average the total number of ear rows counted within the area, then separately average the total number of kernels per row within the area. If possible, move to various areas of the field and repeat the above estimations and average your findings. Next use the following formula to determine yield:

[(average ear #) x (average row #) x (average kernel #)] / 90 = bushels per acre

Remember, this estimate of relative grain yield will be underestimated in a year with good grain fill conditions.

 

WEATHER SHEDS LIGHT YELLOW ON SOME SOYBEANS

Some soybean fields are showing pockets of yellow where the high temperatures of the past few weeks may have spotlighted soybeans under earlier stresses. Soybean varieties that are more susceptible to iron chlorosis like symptoms or may have been infected with root disease as a seedling may be showing you signs of these stresses late this season. Several soybean samples have revealed that the recent high air and upper soil surface temperatures are revealing just where early season problems lie in fields. In some cases early infection of root rots on seedlings may not have completely revealed themselves until the recent high temperatures. Daytime conditions that tax soybean energy reserves such as high air temperatures above 86F or so, may pull from mending efforts the soybean plants were undergoing from previous stresses. Other stresses that may also be revealed now in spots of yellow include: poor root development in compacted or very wet areas, poor nodulation due to ample available nitrogen early in plant growth as seen if high water tables bubble up available nitrogen early but later these reserves recede with the moisture, very dry conditions that cause poor root growth or insect/disease problems. Look across fields and map out the regions of problems and consider if these soil areas should be treated differently in future cropping seasons. Also, look at other fields and find out the plant genetics that overcame stresses the best for this year in similar soils.

 

ESTIMATE SOYBEAN YIELDS PRIOR TO HARVEST

Soybean yields can be estimated before harvest once the full pod load is established but is more accurate closer to harvest. Yield estimates on this crop are most accurate within weeks of maturity, namely within no more than three weeks prior to harvest as pods can be dropped under stress conditions. However, check your fields to determine how the crop is progressing from podding up through harvest.

 

WILL RAINS FINISH OFF SEASONAL NEED FOR MOISTURE?

Figuring the need for final moisture requirements can trim several costs for irrigators. Besides saving one to four inches of water and the fuel for pumping for a last irrigation, reviewing needs can prevent harvest delays and even soil compaction. Especially with corn, you want to provide enough moisture through the root zone to carry the crop to maturity while maintaining yields and making use of in-season precipitation reserves. To predict needs for corn or soybeans know the current crop stage, the water needed to maturity and the amount of usable water in the root zone as suggested from the data shown below on water use that was derived from Nebraska tests. Although many cut back on water prior to physiological maturity in crops, under droughty circumstances even this late stress on a last irrigation run can limit yield.

Stages of Corn

Stage

Description

Silking (R1)

Silks visible, pollen shedding, no stress is best

Blister (R2)

Kernels just filling, resemble a white blister

Milk (R3)

Kernels of yellow corn are just turning color, milky white fluid fill

Dough (R4)

Kernel fluid has a pasty texture

Dent (R5)

Kernels are denting, cob may have changed color if red cobbed

Physiological Maturity (R6 or PM)

Kernels are hard, maximum dry weight, no yield loss from frost

 

Stages of Soybeans

Stage

Description

Beginning Bloom (R1)

At least one flower on the plant, no stress is best

Full Bloom (R2)

At least one of the two upper nodes has an open flower and the soybean has about 50% of its full height

Beginning Pod (R3)

A pod on the upper four nodes is 3/16 inch long

Full Pod (R4)

Beginning seed development

Beginning Seed (R5)

Seed is at least 1/8 inch long in one of the pods on the four upper nodes

Full Seed (R6)

The green bean or beginning full seed stage

Beginning Maturity (R7)

One pod on the main stem obtains mature color

Full Maturity (R8 or PM)

95% of the pods have reached mature color and moisture drops to 15% with five or ten days required to drop to 13% for long_term storage


Normal Water Requirements for Corn and Soybeans at Different Stages, Assuming No Available Soil Water

Stage of Corn Growth

Water Use to Maturity
(inches for 100 day corn)

Blister (R2)

10.5

Dough (R4)

7.5

Beginning Dent (R5)

5.0

Full Dent (late R5)

2.5

Physiological Maturity (R6 or PM)

0.0

 

Stage of Soybean Growth

Water Use to Maturity
(inches for Group 1 to 3 soybeans)

Full Pod (R4)

9.0

Beginning Seed (R5)

6.5

Full Seed (R6)

3.5

Full Maturity (R8 or PM)

0.0

Remaining usable water in the root zone (soil profile) can be determined if the allowable water deficit and the current soil water deficit is known. Different soil types will have different water holding capacities. Just consider the moisture available in the upper three feet of soil as 70% of the crop water use is taken from this top half of the root system of corn and soybeans.

Yield Losses to Corn with Extreme Drought or Frost Stress

Corn Growth Stage

Average Days to Maturity

Potential Yield Loss (%)

Dough (R4)

25

41

Beginning Dent (R5)

20

32

Dent (mid-R5)

15

23

Full Dent (late R5)

10

15

Half Milk Line (R5.5)

5

8

Physiological Maturity (R6 or PM)

0

0

Recent rains may have replenished the soil profile to finish off the season. However, check fields that are known to be sandier as these fields may drain water supplies beyond the reach of the crops. Yield can be affected drastically by moisture needs. While soybeans are generally more drought tolerant, corn can have yields cut if stresses such as moisture deficiency or frost truncates plant development.

Denise McWilliams
Crop Production Specialist

dmcwilli@ndsuext.nodak.edu


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