Using a liquid inoculant on soybean seed and a granular inoculant in the seed row has become standard practice for growers planting beans on virgin fields.
This is intended to quickly establish a population of rhizobia bacteria in soil on which beans have never been planted.
Some agronomists say one inoculant — on the seed — is usually sufficient once a field has a history of soybeans.
However, BASF Canada suggests that growers may want to use two inoculants every year. Company field data and plot trials show a two to three bushel per acre yield benefit from double inoculation.
“We’ve seen trials that probably on average, with the extra inoculant in furrow along side what they’re putting on the seed-placed stuff, probably an average two bushel increase because of the extra bugs in the soil,” said Russell Trischuk, technical marketing specialist for functional crop care and biologicals with BASF.
Trischuk said the yield difference can be significantly higher, particularly on flooded land and fields on which soybeans are planted for the first time.
“We do have some trials that have seen six or seven (bu. per acre).”
Trishuck said 25 percent of prairie soybean growers apply two inoculants. Some farmers do it for insurance to ensure a healthy rhizobia population, but most are chasing additional yield.
“By having more rhizobia present … the more nodules that are produced and ultimately the more nitrogen they can provide to the plant.”
With the majority of the soybeans being grown in southern Manitoba, there might agro-climactic issues that differ for prairie growers outside that region.
Trischuk said double-inoculating soybeans is unique to the northern Plains. Other soybean growing areas of North America, such as Iowa, use on-seed inoculant or none at all.
“If you were to draw a big circle … around the southern portions of Manitoba and Saskatchewan and those northern states along the border, that’s about the only place where double inoculation is required.”
Trischuk said extreme winter temperatures and saturated fields can potentially destroy rhizobia in the soil.
“Soils that have flooded, soils that have had some above average stress. If it’s an extra cold winter … you could see a more dramatic reduction in population.”
Garry Hnatowich of the Irrigation Crop Diversification Corporation in Outlook, Sask., has data that validates the value of double inoculation.
Results from 2014 field tests show a two to three bu. per acre increase from double inoculation versus on-seed liquid.
“However, this was on virgin soybean ground. I suspect that the effect of double inoculation will decline the longer soybeans are in rotation,” he said.
“I’m uncertain as to how many times soybeans need to be on a field before this occurs.”
Internal BASF research may show a yield boost, but Manitoba Pulse Growers Association data is less convincing.
Last year, the association’s On-Farm Network conducted side by side field trials at seven eastern Manitoba sites, comparing double inoculants to a single, on-seed inoculant. The fields in the trial had a previous history of soybeans of at least two years.
A report released in November summarized the 2014 results:
- The trials showed an average .2 bu. per acre yield disadvantage for the strips with seed applied and in-furrow inoculant. None of the trials showed a statistical significance in favour of using both inoculants at a 95 percent confidence interval.
- Soybean yields ranged from 20 to 53 bu. per acre across the trials. The yield difference between any two treatments ranged from –1.5 to .8 bu. per acre.
- Nodulation counts were done at the V3 and R2 stage. Nodulation counts across all the trials were high and showed no difference between the two treatments.
- The cost of granular inoculant is $11 per acre, assuming five pounds per acre of granular. The cost of in-furrow liquid inoculant is $3 per acre.
- The strip trials showed no economic return for using in-furrow inoculant.
- The 2013 and 2014 trials show an economic advantage from on-seed and in-furrow inoculants three out of 17 times (18 percent).
Ron Tone of Tone Ag Consulting in St. Pierre, Man., said one more year of trial data is needed to clearly determine if double inoculants provide a yield boost.
Gregory Endres, a North Dakota State University agronomist, tells soybean growers in his state to use an on-seed inoculant every year but that double inoculants are unnecessary.
“Most other NDSU specialists say the same,” he said.
“If people handle their liquid or peat based inoculant correctly … our data is indicating no yield advantage with the granular with the liquid on fields that have a prior soybean (history).”
Endres said it’s important to double inoculate virgin land and it may be needed on flooded fields.
“(But) I don’t think cold weather has anything to do with the bacteria population in the soil.”
Endres said North Dakota soybean growers are more interested in a different inoculant question: is an on-seed inoculant necessary every year or is it possible to skip inoculation?
“You’ll find some people at NDSU who say it’s not needed, as long as you have soybeans grown within two or three years of each other.”
“I think it’s a good idea. If you have a liquid formulation or peat based, they’re very inexpensive … (and) inoculants continue to get better.