The August Bean Report from the Manitoba Pulse and Soybean Growers (MPSG) suggested one of the reasons that some growers were seeing yellowing on their soybean crops could have been due to a nitrogen deficiency because of inadequate nodulation, with the possibility that dry conditions earlier in the season may have reduced the efficacy of inoculant.
“We have had a few reports of nitrogen deficiency in soybeans over the past three dry seasons (including this one),” says MPSG production specialist, Cassandra Tkachuk. “It appeared as yellowing and nitrogen deficiency in the early “R” stages and was sometimes related to liquid inoculant failure.”
There are other reasons for yellowing in soybeans such as iron deficiency chlorosis (IDC), salinity and disease like root rot, but there could still be a lesson to be learned this year about inoculating soybeans under dry conditions.
Once farmers notice the yellowing, and have eliminated other reasons, they could still make a rescue application of nitrogen but only up to the R2 or R3 stages. “Liquid nitrogen should be directed below the crop to prevent fertilizer burn or granular N broadcasted on,” says Tkachuk.
What can farmers do to combat this issue in future dry springs? Should they consider double inoculation — something that is usually recommended for first year soybean fields?
Small plot trials by MPSG from 2014 to 2016 compared 14 inoculant products, formulations, rates and combinations across a range of locations and field histories in Manitoba. Four of the site-years tested had a history of soybeans and five of the site-years had no history of soybeans.
Inoculant treatments increased number of nodules per plant by 20, yield by 15 bu/acre and protein by 4.8 per cent compared to the uninoculated control. On fields with a history of soybean, there was no difference in yield, number of nodules or seed protein between inoculant treatments and the uninoculated control. Regardless of field history, under optimal seeding and plant establishment conditions, inoculant product, rate or combination did not effect nodule numbers per plant or seed yield.
MPSG suggests there could be several possible explanations for the lack of response to double inoculation in this trial, and recommends using a double inoculation strategy on fields with a limited history of soybeans.
“The research has shown that double inoculation mainly benefits first-time soybean production, then single inoculation can be considered in subsequent years following our single inoculation checklist,” says Tkachuk, who acknowledges that it is sometimes hard for farmers to drop to a single inoculation.
“It’s a legitimate fear because if one inoculant fails you have the other to fall back on, but I encourage farmers to test this on their own fields,” says Tkachuk.
No inoculant at all?
“Information coming from other soybean growing regions like the United States or Ontario has suggested that farmers do not need to double inoculate their soybeans every year and that in some cases inoculant use can be avoided in a given year,” says Tkachuk.
Tkachuk says more research is needed to answer questions like “should I rotationally inoculate?”
“Maybe it’s not a case of going away from it completely, but after how many years should you use the product, how many years can we get away with not using one?” she says. “We don’t know the answers to any of those questions yet.”
Tkachuk recommends that growers test their inoculation strategy each year in their own fields, checking nodulation at the late V-stages to R1 (early flower) stage. “You want at least 10 healthy nodules per plant,” she says.
Healthy nodules appear pinkish-red when split open. If there are no nodules present on the plant roots, and the crop looks yellowish-green, growers should consider a rescue nitrogen application at the R2 (full flower) to R3 (early pod) stage, after confirming with a plant tissue test.
An interesting question for soybean growers is how long the rhizobium survive in the soil, and how effective different strains are at forming a symbiotic relationship with soybeans to fix nitrogen. Research is currently being done at the University of Manitoba by Dr. Ivan Oresnik to try and answer these questions.
“Results only go up to a four-year rotation, so we don’t have numbers for rhizobium bacteria populations beyond four years,” says Tkachuk. “We can say that they have seen a decline in the bacteria population after four years, but the values for unit populations that they have found have not been quantified in terms of what that means for the next crop’s nodulation capability.”
Matt Pfarr, technical manager for Lallemand Plant Care says continued use of inoculant can be beneficial, especially in longer rotations, where there might be three or four year between pulse crops.
“What happens over time is that if you don’t use a fresh inoculant, you have native rhizobia, but they select for survival, whereas when you buy from an inoculant company, they’re all clones selected for high performance and efficiency when they nodulate and produce nitrogen for the plant,” says Pfarr. Additionally, inoculants often have other technologies included to benefit early nodulation and plant growth.”