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Hold the nitrogen when planting soybeans

Hold the nitrogen when planting soybeans

Prairies not a high yield environment for soybeans, so it’s important to maximize the amount of nitrogen

Manitoba soybean growers at a CropConnect conference once asked if a nitrogen application at planting would help the crop in the province’s short-season climate.

University of Manitoba crop researchers, Navneet Brar and Yvonne Lawley, designed a study to examine if starter nitrogen can increase soybean growth, yield, grain protein and oil.

“Sometimes you do projects because you have a good idea and sometimes you have projects because you need to show that a bad idea is a bad idea,” Lawley said.

“Nitrogen fertilizer is expensive and one of the reasons why soybean has become such a popular crop is because you don’t put nitrogen fertilizer down and that saves a lot of money.”

The study used treatments with starter nitrogen fertilizer rates of 0, 17, 34, 50, 67, and 84 kilograms of urea per hectare, which was broadcast and incorporated before planting a maturity group 00 soybean variety.

Biomass, yield, grain protein, oil, and biological nitrogen fixation were then examined for each treatment.

In two out of three years, a starter nitrogen application did not improve soybean yield.

The study also found that applying nitrogen during planting does not improve biomass, grain protein, or oil content in soybeans.

Brar said nitrogen fixation by the soybeans was reduced in two of the three years relative to the control.

“If you apply nitrogen fertilizer, even in low amounts, you are going to trade it off with the biological nitrogen fixation,” Brar said.

“With our low-yielding environment. It is more important that we get more benefit from the fixing bacteria.”

The study has implications for selecting the best place to grow soybeans.

“High nitrogen fields are probably better suited to other higher nitrogen demanding crops,” Brar said.

“It’s not that the soybeans won’t grow in higher nitrogen soils, but you’re definitely better off utilizing that nitrogen with crops that are going to respond to that nitrogen.”

The study was designed so that it provided insights into soybeans agronomy and into the biology of what’s going on in the plants.

It measured biomass, nodule counts, relative ureide, which is a measure of biological nitrogen fixation, plant nitrogen, and a soil nitrate levels.

The combination of these measurements helped the researchers quantify how much nitrogen soybeans actually fix on the Prairies.

“In the control part, we found out that during the R2 stage it ranged from 34 to 53 kilograms of nitrogen fixed (per hectare), to 93, 94 kg of nitrogen fixed (per hectare) at R5,” Brar said.

The study’s findings line up with the results of a 2017 study commissioned by the Saskatchewan Pulse Crop Development Board.

Chris Holzapfel of The Indian Head Agricultural Research Foundation led the study and he does not recommend the use of starter nitrogen in soybeans.

“In short, there was really no benefit to any of the starter nitrogen applications in particular. When we see nitrogen fertilization have some merit is in the event of a nodulation failure, and in our case this would have been when we did not dual inoculate,” Holzapfel said.

“I would really only look at nitrogen fertilization as a kind of a last-ditch effort to rescue the crop from a nodulation failure. You know, whether that’s due to poor environmental conditions or bad inoculants or not enough inoculants,” Holzapfel said.

For more information on the University of Manitoba study, go to: https://bit.ly/36sg4np

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