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Updating spring wheat N strategies

Yields of 80 bushels per acre or more call for new strategies.

May 26, 2019
By Bruce Barker

The average total N supply for high yielding wheat was two pounds of nitrogen per bushel, which is less than the current recommendation of 2.5 pounds of nitrogen. Photo by Bruce Barker.

The average wheat yield in Manitoba has come a long way since the 1970s when yields were around 30 bushels per acre (bu/ac). Today, wheat yields sometimes top 100. Provincial fertilizer guidelines haven’t kept up, though. The Manitoba Soil Fertility Guide provides nitrogen (N) recommendations for spring wheat yields up to 50 bu/ac based on University of Manitoba (U of M) research conducted in the 1970s.

“The standard recommendation for spring wheat is 2.5 pounds of N per bushel target yield. At those recommendations, an 80-bushel wheat crop would require 200 pounds of soil and fertilizer N,” says Don Flaten, professor of soil science at the University of Manitoba.

Flaten and Amy Mangin, also with the department of plant science at the U of M, conducted research in 2016 and 2017 to see if those recommendations from the 1970s still held up with today’s high-yielding wheat varieties.

“There is a large financial risk for wheat growers when using those kinds of rates, as well as agronomic and environmental risk such as lodging, leaching, or nitrous oxide emissions,” Mangin says. “High yields can also mean that protein content may be below the 13.5 per cent threshold for protein premiums without high enough N fertility.”

Field trials for spring wheat N treatments

Eight site-years of field trials were conducted with high yielding wheat varieties AAC Brandon (Canadian Western Red Spring class, CWRS) and Prosper (Canadian Northern Hard Red class, CNHR). High-intensity Gold-level experiments were conducted at Carman and Brunkild during both seasons (four site-years), and Silver-level experiments were conducted at Melita in both seasons, Carberry in 2016 and Grosse Isle in 2017 (four site-years).

Nitrogen treatments included zero, 50, 80, 110, 140, 170 and 200 pounds per acre at Gold-level sites and zero to 170 pounds per acre at Silver-level sites. Nitrogen was midrow banded at planting as conventional urea at Gold-level sites. At Silver sites, N was applied by hand broadcast shortly after seeding as Agrotain-treated urea.

A blanket application of seed-placed MAP (11-52-0) was applied at 40 lbs. P2O5 per acre across all treatments at each site. Seeding rate targeted 250 plants per square meter. Herbicides were applied as required. Twinline fungicide (200 mL/ac) was applied at flag leaf for leaf disease control, and Caramba (400 mL/ac) was applied at anthesis for Fusarium head blight control at all sites.

Prosper consistently out-yielded AAC Brandon, while AAC Brandon consistently had higher grain protein content across all N rates. AAC Brandon yields ranged up to an exceptionally high yield of 114 bushels per acre at Brunkild, Man., in 2017 and Prosper up to 129 bushels per acre at the same location. At Gold-level sites, the minimum fertilizer N rate for maximum yield for Prosper was 110 pounds of N per acre and 140 pounds per acre for AAC Brandon. Pre-seeding residual soil nitrate N on the plots was 40 to 47 pounds N per acre across the four site years.

Economic optimum rates were determined using a five-year average price for urea of $0.43 per pound of N and wheat prices in southern Manitoba on Jan. 8, 2017. Fertilizer rates that hit maximum yield were not always the most economic. For example, at Melita, Man., in 2016, Mangin says that the maximum yield was hit with 110 pounds of fertilizer N plus 43 pounds of soil N, but the maximum economic yield was with 80 pounds of fertilizer N per acre.

For the best economic yield, the total N supply (soil test NO3-N plus fertilizer N) varied from 1.5 to 2.3 lbs. N/bu at Silver-level sites and 1.7 to 3.0 lbs. N/bu at Gold-level sites.

“Excluding the hail-damaged site at Carman 2016, the average total N supply at the optimum yield and protein content was two pounds of nitrogen per bushel, which is less than the current recommendation of 2.5 pounds of nitrogen,” Mangin says.

Investigating 4R principles

In addition to the right rate of nutrients, the three other pillars of 4R nutrient planning are timing, placement and source. The research also looked into these variables. In-season N application timing compared a base rate of 80 pounds per acre N at planting with an additional 30 or 60 pounds N applied at stem elongation or flag leaf timing as broadcast Agrotain-treated urea.

Mangin says in-season application is a strategy used to try to match fertilizer uptake with crop demand in the hopes of improving fertilizer use efficiency and minimize risk of losses. She says that there weren’t any decreases in yield from delaying a portion of N application to stem elongation or flag leaf stages compared to equivalent N rates applied entirely at planting.

“Stem elongation and flag leaf split applications yielded at least as well as applications at planting,” Mangin says.

At Gold-level sites, grain protein content increased with stem elongation split applications, compared to when N was applied entirely at planting. Flag leaf split applications consistently increased grain protein content compared to equivalent rates of N applied at planting and stem elongation split applications (0.3 to 0.7 per cent).

“These results indicate that it is possible in Manitoba growing conditions to delay a portion of total N applied into the growing season without detrimental effects to final grain yield or protein,” Mangin says. “Additionally, applying N as late as the flag leaf stage may allow for increased grain protein content compared to when N is applied entirely at or near planting.”

Rain is a good thing

Mangin cautions that rainfall after application is necessary for split applications to be successful. In the two years of this study, a rainfall event of at least five millimetres occurred within one week of N application. Also, the split application utilized Agrotain-treated urea to minimize volatilization losses.

“Rainfall after application is likely a major contributing factor to the success of the midseason N applications in this study. If we hadn’t had the rainfall, we might have had different results,” Mangin says.

Other research supports the need for rainfall shortly after a split application in order to maintain or increase yield and protein content compared to all fertilizer applied at planting. Research published by Chris Holzapfel at Indian Head, Sask. in 2007 evaluated delayed N fertilizer application in Saskatchewan for canola and spring wheat. The research found no effect on spring wheat grain protein content, but measured reduced yield in one of three years due to very dry conditions following the midseason application.

Post-flowering nitrogen applications

Post-anthesis (post-flowering) N application of 30 pounds N per acre was also applied in addition to a base rate of 80 pounds that had been applied at planting. The post-anthesis N applications were foliar-applied using flat fan herbicide nozzles as diluted UAN (diluted 50:50 with water to 14 per cent N solution) at both Gold and Silver sites, while Gold sites had an additional treatment using dissolved urea solution (nine per cent N solution). Post-anthesis, foliar applications of N are typically used as a method to increase grain protein content rather than yield due to the application timing being so late in the season.

At Gold-level sites, yield with the base rate of 80 pounds N was similar to the 80 plus 30 pounds at post-anthesis split. However, at Silver-level sites, yield was decreased by 3.3 bu/ac by post-anthesis application compared to the base rate. At Gold-level sites, post-anthesis applications increased protein content by 1.8 per cent and at Silver sites 1.1 per cent compared the base rate of N applied at planting.

“We saw higher protein and yield with the urea solution. It appeared to be easier on the crop. Urea solution did result in lower levels of leaf burn compared to UAN, but it was applied at a slightly higher water volume, which could have contributed to crop safety,” Mangin says, adding while post-anthesis foliar N application did increase protein content, the strategy is difficult to implement because it is hard to predict if protein content would have been high enough for protein premiums if adequate N fertility had been applied at seeding. Additionally, post-anthesis applications may still not help the protein content meet premium levels, depending on environmental growing conditions.

Gold-level trials also had additional treatments to examine ESN blended with conventional urea applied as midrow bands at seeding. Two rates of ESN:Urea blends were tested, a suboptimal rate of 80 lbs. N/ac (50:50, ESN:Urea) and a higher rate of 140 lbs. N/ac (100:40, ESN:Urea).

ESN blends produced grain yield, protein, N uptake, and N removals that were similar to those for conventional urea when applied midrow banded at seeding. Mangin says conditions at seeding were generally dry, and that if conditions had been wetter and more favorable for early season losses, there may have been an advantage to using ESN fertilizer.

Flaten says that the research provides agronomists and farmers with additional strategies to help meet 4R nitrogen management. Along with identifying right rate, source, timing and placement strategies for high yield wheat, the research also shows that N fertilizer use efficiency has also improved.

“At two pounds of N per bushel of wheat produced, compared to the former rate of 2.5 pounds, we see that along with an increase in yield potential comes an increase in nitrogen use efficiency,” Flaten says.