Issue 103
Prairie Grains

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Prairie Grains is the official publication of the Minnesota Association of Wheat Growers, North Dakota Grain Growers Association, Montana Grain Growers Association and South Dakota Wheat, Inc.

Copyright Prairie Grains Magazine
January 2010

Soybean Variety Selection

By Hans Kandel, Ted Helms, and R. Jay Goos.

Selection

Soybean variety selection should be based on maturity, yield, seed quality, lodging, iron deficiency chlorosis tolerance and disease reaction. Later-maturing varieties tend to yield more than early-maturing varieties when evaluated at the same location. After determining a suitable maturity for the farm, comparing yields of varieties that are of similar maturity is important. Although later maturity increases yield potential, later- maturing cultivars are more risky to grow than earlier-maturing varieties because an early fall frost may kill a late-maturing variety before the beans have completely filled in the pods, which will reduce yield greatly.

Soybean Maturity

Soybeans respond to day length and heat units, so the actual calendar date a variety will mature is highly influenced by latitude location because each variety has a narrow range of north to south adaptation. Soybean yield and quality are affected if a season-ending freeze occurs before a variety reaches physiological maturity. In 2009 a number of varieties entered at the Langdon locations were not mature when the killing frost took place and yields generally were low for those varieties. Dates of maturity are listed in the performance tables and indicate when varieties were physiologically mature. Usually harvest can commence approximately seven to 14 days after the soybean crop is physiologically mature. Relative maturity ratings also are provided for many of the varieties entered in the trials at various locations. Relative maturity ratings for private varieties were provided by the companies entering the variety in the trial.

Varieties of maturity groups 00 (double zero), 0 (zero) and 1(one) are suitable for eastern North Dakota and northwestern Minnesota. Maturity group 00 is very early and primarily grown in the northern Red River Valley and the north-central area of North Dakota. Maturity group 0 is adapted to Traill, Cass, Richland, Barnes, Sargent and Richland counties and other counties with similar latitudes. Maturity group 1 is primarily suitable for southern areas. These maturity groups are further subdivided. For example, a 0.1 maturity group is an early group 0 variety and a 0.9 is a late maturity group 0 variety.

The best way to select a high-yielding variety is to use data averaged across several locations and years. Because weather conditions are unknown in advance, averaging across several years’ data will identify a variety that likely will yield well across different weather conditions. Selecting a variety that has performed well in dry and moist conditions is the best way to pinpoint a variety that does relatively well, regardless of weather fluctuations.

Phytophthora

Phytophthora root rot is the number one disease problem of soybeans in North Dakota. Phytophthora root rot tends to be more of a problem in the Red River Valley and on poorly drained, heavy soils, but the disease can cause significant stand reduction and yield loss in other areas when conditions are favorable. Most varieties have phytophthora root rot-resistance genes. Each gene for resistance confers resistance to a different race (or races) of phytophthora. For example, a gene that may confer resistance to Race 3 may not confer resistance to Race 4, and vice versa. According to a survey of phytophthora races done by NDSU’s soybean pathologist, Dr. Berlin Nelson, Races 3 and 4 are most common in North Dakota. However, numerous other races are found in the state. Based on these findings, resistance genes RPS 6 and RPS 1K (commonly called the K gene) are the most likely genes to provide resistance against the races common in North Dakota. Although selection of RPS 6 or RPS 1K does not guarantee control, selection of one of these two resistance genes will maximize the likelihood of some protection against phytophthora root rot.

White Mold

There are genetic differences among varieties for tolerance to white mold. Varieties that are less susceptible to white mold should be grown on fields where white mold has a past history of causing problems. The same pathogen causing white mold in soybeans causes white mold in other crops (dry beans, sunflowers, peas, canola, etc.); therefore, recent white mold problems in any crop in that field should be noted.

Iron Deficiency Chlorosis

Iron deficiency chlorosis (IDC) is a major problem in the eastern part of ND and NW MN. Iron chlorosis symptoms might be present during the two to seven trifoliolate leaf stages. Plants tend to recover and start to turn green again during the flowering and pod-filling stages. However, IDC during the early vegetative stages can reduce yield severely. Some varieties are more tolerant to IDC than others. For high pH soils with known IDC problems, select an iron chlorosis-tolerant variety of suitable maturity that is high yielding. The 2001-09 variety IDC scores are posted at www.yellowsoybeans.com.

2009 IDC Observations

Field studies were set out at five locations in 2009, to measure the resistance of over 300 soybean varieties to iron chlorosis. Two sites in Richland County were lost to excess moisture after the plots had emerged. The remaining three sites were located near Ayr, Hunter, and Leonard, ND. The sites ranged in pH from 8.0 to 8.4, salinity (EC) from 0.4 to 0.9 mmho/cm, and CaCO3 contents ranging from about 3 to 15 %. Each plot consisted of a hill, where eight seeds were planted in a hill, thinned soon after emergence to three plants. Prior research has shown that the results given by three plants per hill are about the same as given by soybeans planted in 30-inch rows at a normal plant population. The experimental design was a randomized complete block with four replications at each site. There were separate trials for Roundup Ready and conventional varieties at each site.

Visual ratings were made on a 1 to 5 scale, with 1 representing no chlorosis and 5 the most severe chlorosis. Ratings were taken at the 2-3 trifoliolate and 5-6 trifoliolate stages. As far as we know, this is the most comprehensive evaluation available to farmers.

The year 2009 was a “bad chlorosis year.” The fall and winter of 2008-2009 were exceptionally wet, and resulted in historic flooding in the southern Red River Valley. The early part of the growing season also turned wetter than normal, especially in Richland County. About 130 varieties were entered in both 2008 and 2009. In general, the results for 2009 were similar to the results obtained in 2008.

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1 Remarks 1= Good iron-chlorosis resistance, 2 = Sensitive to iron-chlorosis on high pH soils, 3 = Plant early, 4 = Resistant to races 1-4 of phytophthora root rot, 5 = Resistant to races 1, 2 and 3 of phytophthora root rot, 6 = Susceptible to phytophthora root rot, 7 = Roundup Ready variety, 8 = Natto bean, 9 = Tofu bean.

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The “Chlorosis Report Card”

Last year, we introduced the concept of the chlorosis “report card.” We received many favorable comments about this system. We still report the numerical results (2.2, 3.1, etc.), but a disadvantage of this method is that chlorosis scores, like yields, go up and down from year to year. Thus, a score of ‘2.5’ in one year may not mean the same thing as a rating of ‘2.5’ in another year. Thus, we are experimenting with a second way of interpreting the data, “The Chlorosis Report Card.”

The Chlorosis Report Card gives a variety a letter grade, from A to D-, representing how a variety did compared to the other offerings in the marketplace. The range in the scores of commercial varieties is divided into eleven categories (A, A-, B+, B, B-, C+, C, C-, D+, D, D-), and the varieties placed into these eleven categories. In other words, the most resistant commercial variety defined the top end of the “A” range, and the most susceptible commercial variety defined the bottom of the ‘D-‘ range. We did not give a variety an ‘A+’ grade, because we have never seen a variety immune to chlorosis. We did not give out any ‘F’ grades either, as no commercial variety has proven to be as susceptible as the two most susceptible varieties identified (T203 and Pride B216).

For chlorosis-prone land, a good interpretation of this system would be:

1. Flee the D’s! These varieties are not adapted for chlorosis-prone land.

2. You can do better than a C! Many varieties exist with better chlorosis resistance.

3. A grade of B or B- is a good grade, especially for fields that tend to have slight to moderate chlorosis for a couple weeks and recover, or where chlorosis is present some years and not others. By going with a B, B-, or stronger, you are selecting from the top 25% of the varieties on the market with regards to chlorosis resistance.

4. For the fields with significant chlorosis problems, consider a variety with a grade of A, A-, or B+, consistent with your other objectives, such as maturity. This represents about the top 5-10% of the varieties with regards to chlorosis resistance.

5. Taking on new land? Be careful. If a farmer is buying or renting new land, the soil test shows calcium carbonate in the topsoil, pH values above 7.6, and the farmer is not sure of the chlorosis history of the field, the farmer should probably be conservative, and go with a variety with higher levels of resistance.

6. Chlorosis ratings are not perfect. Chlorosis is a variable problem within a field, and chlorosis ratings are affected by experimental error. Ratings do vary from year to year. Consistency of performance is important for selecting varieties for fields with severe chlorosis problems. A variety with a proven performance across more than one year’s trials is a safer bet for chlorosis-prone land than a new variety with a limited track record.

7. No chlorosis? Then these charts aren’t for you! Not every production area, not every field, has chlorosis. Chlorosis is generally only seen on poorly-drained fields, or fields with lime in the topsoil. The letter grades listed here deal only with chlorosis. It is often observed that a variety may have a chlorosis grade of C or D but be an excellent choice for a field with no chlorosis problems.

Soybean Cyst Nematode

The soybean cyst nematode (SCN), Heterodera glycines, is a small parasitic roundworm that attacks the roots of soybeans. SCN is found and verified in Cass and Richland counties of North Dakota. There are unverified reports that SCN is also found in fields in adjacent counties.

SCN causes yield losses in infested fields. Crop rotation and resistance are the most important management practices growers must use to control the disease. Growers may want to consider testing their soils for SCN. If a nematode problem is in the field, only resistant soybean varieties should be planted. For performance of adapted SCN-resistant soybean varieties, see page 28 in this magazine.

General information about the tables

Variety trial data from all NDSU Research Extension Centers for all crops can be found at www.ag.ndsu.edu/varietytrials. The agronomic data presented in this publication are from replicated research plots using experimental designs that enable the use of statistical analysis.

The LSD (least significant difference) numbers beneath the columns in tables are derived from the statistical analyses and only apply to the numbers in the column in which they appear. If the difference between two varieties exceeds the LSD value, it means that with 95 percent probability, the higher-yielding variety has a significant yield advantage. If the difference between two varieties is less than the LSD