ISSUE  11   JULY 15, 1999

ESTIMATING YIELD POTENTIAL OF WHEAT/DURUM

I have received several requests on how to estimate yield in wheat, following is information to use when estimating yield.

Estimating yield in wheat, or any crop, is done based on the components that compose yield and only determines potential; crop yield is determined after harvest. Small errors in counting the components that contribute to yield can result in large errors in the yield estimate.

Yield in small grains is the sum of three components: i) the number of heads in a unit area; ii) the number of kernels that are produced on a head; and iii) the weight of each of those kernels. Kernel weight can not be determined until harvest so a historical average must be used.

Formula:

row spacing (inches)

* Conversion factor that incorporates area, kernel weight and volume.

Using the formula:

1. Determine the number of heads in three feet of row. Small heads with two or three kernels should not be counted; they will result in biased high yield estimate.

2. Determine the number of spikelets per head. This should be an average of six or more randomly selected heads. Top and bottom spikelets contribute little to overall yield and should not be counted (Figure 1).

3. Determine the average number of kernels per spikelet. Hand thrash the heads used to determine spikelet number and divide total kernels by total spikelets for an average kernel per spikelet number. Small errors in this number result unrealistically high yield estimates; consequently, using a fixed number that accurately reflects long term yield trends best. Usually 2.3 gives the most accurate results, or 2.1 when the crop has been stressed.

4. Finally determine the drill row width. When unknown simply measure the distance between several rows of plants and use the average. Most double disc drills are set at 6, 7, or 8 inch row spacings. Air seeders place seed in bands that can range from three to five inches wide; in this case the band width plus distance between bands is used. Measure several rows from the left side of the band to the left side to determine the width.

Figure 1. Wheat spike, arrows indicate which spikelets do not contribute significantly to yield and should not be included in number of spikelets per head when calculating yield.

As illustrated yield estimates are subjective. The correlation between an estimate and the final yield is related to how close head and kernel counts are to the real numbers and crop development during the remainder of the growing season. Keep in mind that no field is uniform and the yield potential varies tremendously within a single field. To get accurate counts the process should be repeated several times, not less than eight per field.

Michael D. Peel
Small Grains Extension Agronomist
mpeel@ndsuext.nodak.edu

WHEN WILL MY SOYBEANS BE GREEN AGAIN?

Recent conditions in soybean fields have, in some cases, resulted in very yellow fields. Many different causes or a single stress may be contributing to the change of hue in your plants. Start by checking the variety planted and the soil pH. Iron chlorosis is showing up in many of the fields, especially those planted to varieties with poor iron chlorosis ratings and on soils above a pH of 7.5. Yield can be lessened as much as one bushel per acre for every one percent change in pH above 7.5 on susceptible varieties. And, the iron chlorosis in the Valley can be accentuated with soil wetness that increases bicarbonate solubility in soils and may elevate soluble salt levels, further stressing the soybeans.

Any herbicide application made right around the first trifoliate leaf stage on soybeans may also contribute to the stress on the soybeans if the plants were already laboring with iron chlorosis, varying temperatures, wet feet, insects, weed competition, disease, soil compaction or other previous damage such as hail or minor root pruning due to early, cool soils or excess preemerge or preplant herbicide applications. Generally, yellowing quickly disappears from soybeans; however, this year the problem seems to have come early and is lingering. Apical growth on the plant should now begin to green and continue if the plant will have good recovery. And, green color all over the plant should return within two to three weeks unless continued stresses further slow plant development.

IS RHIZOC ROOSTING IN YOUR SOYBEANS?

Rhizoctonia diseases can show up in crops as damping-off, root or stem decay or even leaf or bud blight. Early postemergence soybean damping-off can reduce stands as much as 50% and later yield losses can be up to 40%. The causal fungus, Rhizoctonia, survives easily in the soil (up to three months in dry culture, 9 months in liquid culture) and it also has excellent saprophyicability on a very wide range of host plants which include most field crops, vegetables, ornamentals and fruits.

Growth of this disease depends on nutrient supply, soil moisture, temperature and pH as well as competition from other soil microorganisms. A few cases of some rhizoc have been seen recently on soybeans as plants show lesions at the base of the stem, just below the soil line. These cases have been very difficult to separate out from several of the other stresses that have recently occurred on soybean plants in the field.

The lesions of rhizoc are brown, dark brown or reddish with this initial infection possibly developing into a reddish brown cortical decay above the soil level on the plant.

Carefully observe the plants and the root system to determine the problem as the stem lesions can easily be confused with hail damage or with dinitroaniline herbicide damage to the roots (due to the poorer root proliferation). Nodulation on the plant may be reduced and yields can be lessened. Preventative controls currently are the best solution. Use of fungicide seed protectants at planting (although foliar systemic appear promising during better crop pricing), maintenance of good soil drainage and the use of less susceptible cultivars is encouraged.

Denise A. McWilliams
Extension Crop Production Specialist
dmcwilli@ndsuext.nodak.edu