ISSUE 16    August 18, 2005

WHEN TO STOP IRRIGATING

The 2005 cropping season is soon coming to an end and crop growth and development is near the long term averages. The last irrigation of the season may be the most important. To ensure optimum yields, adequate soil moisture must be available to crops until they are physiologically mature. Applying irrigation water beyond maturity often is unnecessary and increases your pumping costs. For management decisions on final irrigation, you will need to know the current moisture condition of your soil and the amount available for crop use. Both soil texture and effective root zone will determine the amount of water that can be stored for crop utilization.

Stage of crop maturity and weather conditions will affect the time period when the crop continues to use water prior to maturity. Know the signs and symptoms of physiological maturity in crops you are irrigating. Both the extra savings of eliminating unnecessary irrigation and peace of mind that the crop is safe from frost are worthwhile.

Some crops, such as corn, can endure an increased soil water deficit as the crop nears maturity, while others, such as potatoes or alfalfa, should continue to be irrigated until harvest, maturity or frost.

Corn: should be irrigated until sufficient soil moisture is available to ensure that the milk layer of the kernel moves down to the tip of the kernel or black-layer formation (physiological maturity). Corn reaches physiological maturity about 55 days after 50 percent of the plants have visible silks. The grain moisture may range from 32 percent to 40 percent at the time, depending on the hybrid. Yellow dent corn usually is well-dented at physiological maturity. Once corn is physiologically mature, the dry-down rate is approximately 0.52 percent moisture loss per day.

Dry edible beans: The last irrigation should be when the first several pod sets are filling, or the irrigation stopped when 50 percent of the leaves are yellowing on the plants. When overwatered, indeterminate varieties (pinto) may continue to vine and set flower with delayed maturity. Navy beans reach physiological maturity when at least 80 percent of the pods show yellowing and re mostly ripe, with 40 percent of the leaves still green in color. Pinto beans are physiologically mature when 80 percent of the pods show yellowing and are mostly ripe, and only 30 percent of the leaves still are green. Beans within pods should not show evidence of any green. Beans that have begun to dry will not need irrigation because they no longer are removing much water from the soil profile.

Soybeans: should be irrigated until sufficient moisture is available to allow full bean development and pod fill. This stage is when leaves are yellowing (75 percent to 80 percent) and all pods are filled, with lower pods just starting to turn brown. At physiological maturity, pods are all yellow and more than 65 percent of the lower pods have turned brown. Beans within pods should have little evidence of green color and should be shrinking. Studies show that yellow pods sprinkled with brown are the best clue of physiological maturity. Usually, if one or two pods show these signs on the upper two or more nodes of the plant, it has reached physiological maturity. Also, soybeans should be tolerant of a killing frost at this time.

Sunflowers: should be irrigated until sufficient moisture is available for the sunflower achenes (seeds) to fill. This is when the backs of the heads turn from a lime green to a yellow-green color and ray petals are completely dried.

Potatoes: will use soil moisture until harvest. Maturation stage begins with canopy senescence as older leaves gradually turn brown and die. Research has shown final irrigation can be used to reduce bruising during the harvesting process. On sandy soils, soil moisture content between 60 and 80 percent of field capacity (40 percent to 20 percent depletion) provides conditions for a desirable soil load into the harvester, with optimum separation of potatoes and soil and a minimum of physical tuber damage. If soil is dry before harvest, a final irrigation should be applied at least one week prior to harvest to raise the soil moisture level and raise the tuber hydration level.

Alfalfa: should be irrigated to maintain active growth until a hard frost stops the growth. Alfalfa going into the winter with adequate soil moisture has a much better chance of little or no winter kill.

Sugarbeets: will use moisture until harvest time. You usually can terminate irrigation seven to 14 days before harvest to allow the soil to dry.

 

FALL FROST AND CROP MATURITY

A killing frost is a temperature which kills the plant tissue. If your crops have frost (low temperature) damage, it is the result of tissue death.

Critical stages and temperatures for various major crops are described as follows:

Wheat, durum and other small grains: Temperatures below 32 degrees will cause sterile spiklets if the plant is in the late boot through the flowering stage. In the milk stage it will create shriveled kernels. Frozen immature spikes will turn white. After mid-dough stage, temperatures as low as 25 degrees will result in bran damage, some kernel shriveling and possible germination reduction. The bran damage will change test weight and probably will be discounted in the market. Grain that may be saved for conditioning to seed should be tested for germination and vigor.

Flax: is most susceptible during flowering and early boll stage. Immature seeds can be killed by temperatures from 28-32 degrees. After flax reaches dough stage it is more resistant to frost.

Sunflower: is most susceptible at bud and flowering. Temperatures of 28 degrees to 30 degrees can result in damaged buds and sterile sections or rings in the flowering head. After pollination and petal wilting and drying (late R-6) sunflower can withstand temperatures as low as 25 degrees F with only minor damage. If 25EF temperatures occur at the bud stage this will often damage stalk tissue below the bud and seeds will not develop. Temperatures in the low 20's are needed to kill and dry down the stalks and the backs of sunflower heads.

Soybean: Leaves are easily damaged by light frosts in the 31 to 32EF degree range. Frost of 30EF or less will cause damage to stems and green pods. Beans that are still green and soft will shrivel. Stalks rapidly turn dark green to brown and will not recover. Beans planted in narrow row spacings tend to have better tolerance to freezing temperatures than wide row planted soybeans or those in spare populations. Beans in pods that have turned yellow will mature normally. Some green beans will turn yellow after 30-40 days of storage.

Pinto & Navy beans: are very sensitive to frost (30-32 degree range). Tops of beans are easily killed and will turn a dark green to black color in a matter of days after a killing frost. Lower developing, earlier pods in the bottom of the canopy may not be affected. Also the vines and stems should be examined to see if any damage has occurred. Top of plants and late green pods or flowers are easily damaged by frost. Green beans will shrivel but should be left in field until dry in order to separate from mature beans.

Corn: usually is damaged by temperatures in 29EF degree range or less. Corn is usually physiologically mature 50-55 days after the 50 percent silking date. Colder temperatures will kill the entire stalk. If only leaves above the ear are frosted by a light frost, then kernel development will continue. If entire stalk, ear shank and leaves are frozen, kernel development will cease and soft shriveled corn will result. If corn is at around 35 percent moisture or if a black layer has formed at the base of the kernel the plant is physiologically mature and kernels will develop normally despite frost. Frosted immature corn is best used for silage or fodder.

Other crops: Most other crops such as buckwheat and proso millet are damaged in flower to milk stage. Buckwheat is reported by the Canadians to be very sensitive to frost prior to the mid-dough stage.

Potato Tops: will turn black but tubers are not usually damaged by light frosts.

Sugarbeets: are very resistant to frost.

Alfalfa: can withstand light damage to tops but if frozen to ground level will not recover and should be harvested as soon as possible.

Remember temperatures of 32EF at weather stations or farmsteads may result in temperatures of 20 to 29EF in low lying areas of fields. Also, time of exposure to freezing temperatures will influence degree of damage done. Usually 2 to 4 hours duration of a critical low temperature will cause damage. Two other factors which may influence critical frost temperatures are soil moisture and wind velocity. When soil moisture is higher, frost injury is somewhat reduced due to heat slowly released from the stored heat in the soil within a plant canopy. Wind movement also helps reduce freezing to some degree and is better than a still cold night with no air movement. Cloudy nights are also better than clear nights.

Duane R. Berglund
NDSU Extension Agronomist
duane.berglund@ndsu.edu

 

PRELIMINARY WINTER WHEAT VARIETY TRAIL RESULTS

Data from the winter wheat variety trials conducted in Prosper and Lisbon during the 2004/05 growing season are summarized in the following table. The experiment at Prosper included varieties only; no fungicide was applied. At Lisbon, the same varieites were grown with and without fungicide. Plots treated with fungicide received half the labeled rate at the 4-5 leaf stage and a full rate of Folicur at flowering. Leaf diseases were a combination of leaf rust and Septoria leaf spot. The low test weights were largely due the presence of scabby kernels; samples were not cleaned in the lab to remove light weight kernels before determining test weight.

Observations from these data-

  • Varieties differed significantly in yield, scab severity and leaf disease development. The shorter varieties (i.e. CDC Falcon, Jagalene, Wendy and Wesley) tended to be the most affected by scab.
  • There was a negative correlation between scab severity and test weight, demonstrating the devastating effect scab can have on kernel development.
  • Fungicides were very effective in controlling leaf diseases. Scab, however, was only partially controlled by fungicides. The best scab control with fungicides was achieved when applied to the most scab resistant varieties, illustrating the benefits of integrating more than one control strategy.
  • Preliminary results of varietal performance of winter wheat at Prosper and Lisbon, 2004-05.

     

    Prosper

     

    Yield

    Test wt

    Variety

    bu/A

    lb/bu

    Arapahoe

    65.2

    50.0

    CDC Buteo

    55.0

    51.0

    CDC Falcon

    61.4

    48.4

    Expedition

    59.2

    47.9

    Harding

    76.3

    51.2

    Jagalene

    45.5

    40.9

    Jerry

    78.0

    54.9

    McClintock

    49.2

    49.1

    Millennium

    71.7

    50.7

    NuSky

    34.5

    42.5

    Ransom

    66.6

    52.5

    Roughrider

    52.0

    53.9

    Wahoo

    55.7

    42.5

    Wendy

    51.0

    45.7

    Wesley

    44.4

    43.1

    Mean

    57.7

    48.3

    LSD 0.05

    12.0

    4.1

     

     

    Lisbon1

     

    Yield

    Test weight

    Variety

    No Fung

    Fung

    No Fung

    Fung

    Arapahoe

    29.2

    46.3

    48.1

    53.0

    CDC Buteo

    37.0

    54.1

    48.0

    52.4

    CDC Falcon

    27.4

    37.1

    44.4

    49.0

    Expedition

    21.7

    37.9

    43.3

    43.7

    Harding

    39.5

    51.2

    47.1

    49.4

    Jagalene

    27.0

    40.9

    36.5

    44.7

    Jerry

    35.5

    52.0

    52.0

    53.3

    McClintock

    34.9

    45.9

    45.7

    51.0

    Millennium

    37.9

    54.8

    49.3

    51.7

    NuSky

    27.7

    45.6

    42.5

    46.6

    Ransom

    35.6

    46.1

    48.1

    50.6

    Roughrider

    27.0

    37.3

    50.2

    52.4

    Wahoo

    24.5

    35.5

    39.5

    41.7

    Wendy

    23.2

    29.6

    41.3

    47.2

    Wesley

    28.1

    39.8

    43.5

    43.0

    Mean

    30.4

    43.6

    45.3

    48.6

    LSD 0.05

    9.7

    9.7

    5.2

    5.2

     

     

    Lisbon1

     

    Scab field severity

    Leaf disease2

    Variety

    No Fung

    Fung

    No Fung

    Fung

    Arapahoe

    24.5

    6.7

    85.0

    8.0

    CDC Buteo

    29.3

    8.5

    62.4

    10.8

    CDC Falcon

    67.9

    26.7

    83.3

    7.3

    Expedition

    49.7

    20.1

    100

    17.2

    Harding

    23.2

    5.6

    63.4

    9.9

    Jagalene

    63.1

    43

    100

    10.7

    Jerry

    16.8

    3.1

    83.8

    15.0

    McClintock

    44.5

    18.1

    28.4

    2.3

    Millennium

    34.5

    17.2

    65

    10.2

    NuSky

    29.6

    7.2

    80.8

    6.0

    Ransom

    19.6

    4.6

    100

    13.9

    Roughrider

    15.8

    3.8

    84.4

    8.8

    Wahoo

    29.2

    12.9

    100

    15.5

    Wendy

    75.4

    33.7

    100

    12.5

    Wesley

    69.8

    46

    100

    12.6

    Mean

    39.5

    17.1

    82.4

    10.7

    LSD 0.05

    9.5

    5.7

    11.1

    NS

    Joel Ransom
    Extension Agronomist - Cereal Crops
    joel.ransom@ndsu.edu


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