NDSU Crop and Pest Report

Plant Science


ISSUE 3  May 20, 2004

MINIMUM STANDS-SOYBEANS

In last weeks Crop and Pest report in the article entitled "Minimum Stands for Crops" it was suggested that for soybean stands to be 40-75,000 plants/A as a minimum. After reviewing some research literature and hail loss tables the minimum stand for soybean is suggested to be around 75,000 which would be approximately a 50% stand loss. If one uses the "hula hoop" method of determining stand counts, then one would need a minimum of 1.7 plants per square foot. Even with a 50% stand loss, yield reduction would be somewhere between 10-20%. This agrees with studies in Minnesota, Iowa and reported hail research. However, soybean stands will not always be uniform or in even stands at lower populations. These uneven gaps between plants will lower yields depending on how many gaps and distance of spacings. Soybeans do compensate for lower populations by setting more pods per plant and filling more seeds per pod.

Branching development also increases thus allowing more pods per plant. The lower branches may break before or during harvest thus increasing potential for greater harvest losses. Extra care and efforts during harvesting can reduce this problem. How about seeding into patchy or thin stands? In cases of poor stand establishment, replanting alongside the established seedlings to patch up or thicken the existing stand seldom improves yields. Repair planting often leads to timing difficulties with weed control and harvest date. When dealing with unacceptably low stands, significant yield improvement will be best achieved when the original soybean stand was so poor that it needs to be destroyed and a new stand re-established. There is still time to do that in North Dakota this year.

 

PLANTING DATES-BEANS, FLAX AND SUNFLOWER

Flax

Seeding in April to early May. Yield decreases approximately 2 bu/week for each delay in seeding. More severe yield decline occurs when planting after May 20 in northern North Dakota. Early June planted flax was 8 bu/A less than early May planted flax at Minot averaged over 9 years. These data suggest that all flax should be planted between late April to May 15 to ensure maximum yield potential. Good seedbed preparation, adequate shallow seed placement to moisture, weed control and fertility management also will contribute to high flax yields. Yield goals of 30+ bu/A are not unrealistic for well managed flax production.

Soybeans

Some soybeans already planted are now slow to emerge and may result in poor stands due to seasonably low soil temperatures so far this season. With soybeans, yield losses due to late planting of adapted varieties in the Red River Valley are usually negligible until May 20. Depending on weather, possible yield loss can be approximately 13 percent from May 26 until June 9. Some late planting losses can be near 25 percent and soybeans planted as late as mid June can have a 40 percent or more yield loss compared to more timely May plantings.

Dry Edible Beans

May 12-31 is the planting window for dry edible beans in North Dakota and Minnesota. Beans will not germinate if planted in soils cooler than 51-52 F. Soil temperatures of 55 F or higher is most ideal for rapid germination and emergence. Plant shallow if planting early. One to two inches deep is ideal under most conditions.

Sunflower

May 15 to 25 in most years is the most ideal time to plant sunflower in North Dakota. For more northern areas (North of highway 2) growers should consider May 10_20, while south of that line May 20_30 is most appropriate for full season hybrids. Generally its suggested that confectionary and NuSun hybrids be planted earlier than the conventional oilseed hybrids. Early planting of both confectionary and NuSun will aid in improved quality. When adverse spring planting weather forces northern location growers to plant into early June, then early maturing hybrids should be selected. Many of these early maturing hybrids can be planted up until June 10. Later plantings than June 10 would be risky in most seasons.

 

CANOLA STANDS (HOOP METHOD)

Some canola is emerging, some canola has been frosted and killed, with some reported to have erratic stands. The question comes up, what is an adequate stand in canola? Seeding rates of 5 to 6 pounds per acre should result in 450,000 to 650,000 seeds dropped per acre depending on seed size and numbers per pound. An ideal stand of canola seedlings should be 500,000 plants per acre or 11 to 12 plants per square foot. We don’t always reach this ideal goal. What is the "minimum" stand of canola population required to still obtain decent potential yield? Research results from Canada and U.S. have shown that 4 plants per square foot is the minimum stand to still expect good yields. Weed competition however becomes minimal with reduced stands. The canola plant has a way of compensating with additional branching and a thicker stem to support the extra branching when growing under a reduced or lower plant population.

An easy method to determine the stand count in canola is using the "hula hoop" method. Use a hula hoop in a drilled or solid seeded canola field. Randomly toss the hula hoop in 10 different areas of the field and make counts within the hoop.

Using the table below, multiply your average counts by the multiplication factor which corresponds to the size of the hoop being used. The product answer equals the plant population per acre. This method will also work with solid seeded soybean in row spacings of 8 inches or less.

Hoop Diameter

Multiplication Factor

30 inches
32 inches
34 inches
36 inches
38 inches
8,900
7,800
6,900
6,200
5,500

Example: 36 canola plants in 32 inch diameter hoop equals: 36 x 7800 = "280,800 plants/A" or 6.5 plants per square foot.

Canola plants per "hoop count" to equal the minimum 4 plants per square foot.

Hoop Diameter

Number of plants to equal 4 per sq. ft.

30 inches
32 inches
34 inches
36 inches
38 inches

19.6
22.3
25.2
28.1
31.7

After you have counted and determined the canola stand, you can investigate further to see if any additional canola seedlings will or can emerge. Dig or scratch below the soil crust and see if any old or new seedlings have a chance to emerge. Once all the information is assembled then one can judge to leave a field or replant to another crop. Reseeding to canola is getting late and rather risky for this 2004 season.

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

 

MORE INFORMATION ON FROST DAMAGE IN CEREALS

Two short articles on frost damage to crops were published in last week’s Crop and Pest Report. Nevertheless, given the pervasiveness and severity of the sub-freezing temperatures we experienced last week in North Dakota, I believe some additional information on frost damage to cereals is in order. Numerous factors affect how sub-freezing temperatures impact a crop; temperature alone does not always accurately predict damage.

Factors affecting frost damage

Some of the main factors that influence frost damage in cereal crops are: crop tolerance, growth stage, moisture content of the soil, duration of the sub-freezing temperature, location in the field, and environmental conditions before the occurrence of the sub-freezing temperature.

Crop

Cereal crops are generally considered to be frost tolerant early in their growth cycle. There is considerable variation for tolerance between crops, however. In general terms for the cereals grown in ND (and provided they are at the same stage of development), tolerance to freezing temperatures can be ranked in the following order: winter rye (most tolerant to frost) > winter wheat > oats > barley > wheat > corn (least tolerant).

Growth stage

One reason that cereals tolerate frost early in the season relative to broadleaf crops, is that their growing points remain below the surface of the soil for several weeks after emergence and are therefore protected from the extremes in the temperature of the air above. In small grains the growing point extends above the soil’s surface at about the 6 leaf stage or just prior to jointing and in corn at about the 5 leaf stage. Fortunately, all of the spring sown cereals in the state were still relatively young and their growing points were below the soil’s surface during last week’s cold weather. Winter wheat, however, has started to joint in parts of the state and is potentially more susceptible to frost damage than its spring sown counterpart. I would recommend that you carefully inspect growing points in your winter wheat if you had temperatures below 25 degrees for any extended period of time. The growing point is more susceptible to damage than leaf tissue during jointing. A damaged growing point will appear brownish or water soaked. A dead leaf may appear in the whorl if the growing point has been damaged.

Prolonged (or very low) sub-freezing temperatures can kill growing points of cereal crops even if they are below the surface of the soil. Corn is the most sensitive and temperatures of less than 28 degrees can be lethal.

Environmental factors

Temperatures change more slowly in wetter soils than in dryer soils. Therefore, there is more risk of low temperatures killing plants if your soils are also dry.

Plants are capable of hardening themselves against freezing and plants that had previously been exposed to near freezing temperatures are more likely to tolerate lower sub-freezing temperatures than those that have not. Most of the cereal crops were probably at least partially hardened before the arrival of the coldest weather last week and this may explain why there is less reported damage to cereals than was anticipated.

Cold air is heavier than warm air, so cold air will flow towards the lowest parts of your field, making frost damage more severe in these areas. When assessing frost damage, I would suggest that you visit the lowest spots of your farm first.

Inspecting for damage

Foliage that has been damaged by frost will initially appear yellow and within a few days turn black usually towards the tips of the leaf first. If the growing point was not damaged, after 3 to 5 days of reasonably warm weather, new growth should appear from the whorl. If this occurs, then you can be assured that the growing point was not killed. Loss of leaf tissue at this early stage will have little if any effect on yield. For corn that has not yet emerged, I would suggest that you dig up a few seeds to check for germination and to see if the emerging seedling is viable. Seeds that had not germinated will not be adversely affected, but emerging seedlings could be killed if soil temperatures were very cold. I am happy to report that my colleagues have informed me that emerging corn seedlings are rarely killed by cold soil temperatures, even in ND!

As mentioned above, if you have winter wheat that was beginning to joint, I would also advise you to check the health of its growing point, even if you did not have excessive damage to the foliage.

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


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