Using Soybean Variety Choice to Alter Management Practices
Choosing a Maturity Group (MG) of soybean from which to select varieties is based on latitude, intended planting date, timing and length of the typical drought period during the growing season for dryland production systems, length of the irrigation season for irrigated production systems, and projected harvest schedule. With the advent of the Early Soybean Production System (ESPS) in the Midsouth, earlier-maturing varieties are now widely used in the region.
The increasing occurrence of herbicide-resistant (HR) weeds, and glyphosate-resistant (GR) weeds in particular, has created a veritable plethora of weed control issues for soybean producers to deal with. This has led to potentially reverting to tillage for weed control, as well as the application of preemergence (PRE) and non-glyphosate postemergence (POST) herbicides to HR soybean varieties. This in turn has increased the potential cost of soybean production. An option that is presently less common is the use of conventional varieties (non-GMO) with conventional weed control herbicides.
Another strategy for weed control in this era of HR weeds is modifying soybean canopy closure to prevent or lessen competition from weeds. Generally, this involves the proven strategy of planting in narrow rows, and increasing seeding rates. This latter management modification will necessarily increase the cost of production when using high-cost, patent-protected varieties. However, this will not result in as dramatic an increase in production cost when using conventional varieties because of their much lower seed cost. Thus, using higher seeding rates for these varieties is deemed economically promising.
A recent article in Crop, Forage, and Turfgrass Management entitled “Economic implications of soybean maturity group, herbicide program, and irrigation requirement” by Wegerer, Popp, Hu, and Purcell at the Univ. of Arkansas presents results from research that was conducted in Arkansas to provide insight into how using early MG soybean varieties coupled with increased seeding rates will affect irrigation requirement, herbicide use for weed control, and subsequent economic returns.
Specific objectives of the research were to determine; 1) effect of varied seeding rates across MG on profitability; 2) how earlier canopy closure resulting from high plant populations affects weed control; 3) how using earlier-maturing MG varieties will affect irrigation costs/savings; and 4) how results from 1-3 will be affected by seed and irrigation costs, and soybean price. One of the premises in objectives 1 and 2 is that seed of non-patent-protected GR varieties and conventional varieties will be cheaper.
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Based on the above objectives, specific treatments and their levels were: |
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MG |
II, III, IV |
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Seeding rate |
90, 150, and 240 thousand seeds per acre |
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Herbicide program
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1) GR soybean with glyphosate applied POST; 2) Conventional soybean with PRE and POST non-glyphosate herbicides; 3) Conventional soybean at highest seeding rate with PRE herbicide only. |
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Locations |
Fayetteville (35°5' N), Keiser (35°40' N), and Pine Tree (35°7' N) |
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Planting date was mid-May each year, and seed were drill-planted in 7.5-in-wide rows. All treatments were irrigated. Partial returns to production were defined by treatment-specific revenue less relevant input costs for seed and herbicides, and irrigation costs. A 10-yr average price of $10.38/bu for soybeans was used. A minimum price of $8.01/bu and a maximum price of $11.73/bu was used for sensitivity analysis of output price. |
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A summary of results follow.
● Total weed control cost/acre was lowest with herbicide program 1 ($16.66/acre) and highest with herbicide program 2 ($57.95/acre). Weed control cost for herbicide program 3 was $25.01 (treatment with highest seeding rate and no POST herbicides).
● As expected, maximum yields for MG IV varieties were attained at lower profit-maximizing plant populations (average of ~145,000 plants/acre) than for MG III (average of ~180,000 plants/acre) and MG II (average of ~185,000 plants/acre) varieties across all locations and years.
● Yield potential was highest for MG IV varieties, followed by MG III and MG II varieties at all locations and years.
● Ratings taken at harvest indicated that weed control was better in MG III and MG IV systems (good to excellent) than in MG II systems (fair to good). Weed control using MG IV varieties was good to excellent in all herbicide programs across years and locations.
● Irrigation water applied to MG II, MG III, and MG IV varieties averaged 7.3, 9.5, and 11.9 acre-inches, respectively, across locations and years.
● Average partial returns as defined above were highest for MG IV varieties ($580/acre) than for MG III ($509/acre) and MG II ($461/acre) varieties across all locations and years.
● Conventional soybean varieties in all MG’s grown with the highest seeding rate and receiving only PRE herbicides (treatment 3 above) provided the highest partial returns.
● Comparison of the estimated partial returns across years and locations showed the combination of MG IV varieties and the highest seeding rate was superior. This is attributed to yield increase at the higher plant populations, no POST herbicide applications, and no early weed pressure as a result of early canopy closure with the high seeding rates.
● The economic analysis revealed that a producer could earn about $45/acre more by growing a conventional MG IV variety at a high seeding rate than by growing a GR MG IV variety. This advantage is realized over a large range of price and cost conditions.
The results from this research show that higher plant populations that are affordable with lower seed cost of conventional or non-patent-protected varieties may be a soybean production alternative worth considering.
Following are logical questions that arise from this research.
Would a mid-April planting date have changed the results regarding the irrigation amounts that were required or applied?
Would the differences among MG’s have been the same if planting date had been in mid-April instead of mid-May?
Would the results, especially regarding the high plant populations, have been different if the studies had been conducted in a dryland or nonirrigated setting?
Composed by Larry G. Heatherly, Apr. 2016, larryheatherly@bellsouth.net