Inoculants containing Bradyrhizobium japonicum are available for soybean production but may not be necessary in fields where soybean previously has been produced.
Researchers from several Midwestern states wanted to determine yield response and probability of an economic return from inoculants in fields with a recent history of soybean production. Fifty-one inoculant products were evaluated in experiments conducted in Indiana, Iowa, Minnesota, Nebraska, and Wisconsin between 2000 and 2008. Inoculant products were similar and did not produce a yield response relative to an untreated control different from zero at 63 environments.
Probability for a break-even economic return at a soybean sale price of $0.33 kg–1 was 59% for Nebraska, 36% for Wisconsin, 25% for Minnesota, 25% for Indiana, and 4% for Iowa. Attaining a return on investment of 67 kg ha–1 (a 2:1 return) reduced success to 11, 2, 1, 7, and 0.2%, for the five states, respectively. Data from this range of environments and products indicate that application of an inoculant offers limited success for either a yield increase or improved economic return on soils where soybean has previously been grown in the upper Midwest.
Soybean is the most widely planted legume in the United States, with >30 million ha planted in 2008. Seed of current U.S. soybean cultivars contains approximately 42% protein and 20% oil on a dry-weight basis. Because of its seed protein concentration, soybean has a large nitrogen demand, often exceeding >200 kg ha–1. Total nitrogen accumulation for soybean is supplied by two sources: (i) a symbiotic relationship with nitrogen-fixing rhizobia bacteria, Bradyrhizobia japonicum, that convert atmospheric nitrogen gas (N2) to plant-available nitrogen in exchange for photosynthetically derived energy from the plant; and (ii) from the residual soil nitrogen pool
It is has been estimated that nitrogen supplied by nitrogen fixation was in the range from 49 to 67% of the total nitrogen required. A survey of farmers in Indiana indicated that 18% used an inoculant while a sample of farmers in Wisconsin indicated that 85% use an inoculant. Current recommendations for states of the upper Midwest are to use an inoculant if fields have no history of soybean production in the past three to five years, a soil pH <6.0, sandy soil, low organic matter, or been flooded for more than a week.
Previous work suggests that fields planted to soybean for the first time require inoculation to ensure the occurrence of symbiotic nitrogen fixation and to encourage establishment of rhizobia populations.
A study in Midwestern U.S. grass pasture fields found the ability of soybean to form nodules decreased with the length of time since the last cultivation with soybean. Fields with a previous history of soybean typically have a large population density of rhizobia that range from 4.8 to 6.0 log10 cells g–1 of soil in Iowa and 5.5 log10 cells g–1 of soil in Minnesota. Response to inoculation decreases exponentially as the population density of rhizobia increases in the soil between 0 and 2 log10 cells g–1 of soil. It has been documented that in soils with a rhizobia population density ≥1000 cells g–1 of soil, the infection rate of rhizobia from inoculants is very small.
Today, most fields in the Midwest have experienced soybean in the rotation, likely increasing the population density of rhizobia. Several studies have shown no yield response to inoculation where soybean previously had been grown. Recently, a 2008 study found that inoculants increased soybean yield in six of 14 site-years in fields that had been in soybean rotation, with an average yield increase of 85.6 kg ha–1. In contrast, inoculants increased yield 995 kg ha–1 in fields in Michigan where soybean had never been produced and 126 kg ha–1 in a field in Iowa where soybean had not been produced during the previous 15 years Improved inoculant technology coupled with low-cost products, ease of application, and increased commodity costs have many growers reconsidering the use of inoculants.
Extension agronomists at land-grant universities in the United States continue to test soybean inoculants each year as part of their outreach programs. Information from these trials is often presented at extension meetings but not summarized and published in scientific journals. This paper is the first attempt to quantify the regional impact of inoculants on soybean yield and the probability of return on investment. Different environments represented common soil types and climates for a majority of the U.S. soybean production region. No states included in this analysis gave a high probability of a yield response or positive return on investment. Based on these data, the widespread use of soybean inoculants in fields with a history of soybean is not recommended, regardless of price or ease of application.
Adapted from Probability of Yield Response to Inoculants in Fields with a History of Soybean by J. L. De Bruin, P. Pedersen, S. P. Conley, J. M. Gaska, S. L. Naeve, J. E. Kurle, R. W. Elmore, L. J. Giesler and L. J. Abendroth. Published online 30 December 2009 in Crop Sci 50:265-272 (2010) © 2010 Crop Science Society of America, 677 S. Segoe Rd., Madison, WI 53711 USA