Features Agronomy Fertility and Nutrients
Nitrogen fertilization of winter wheat: alternative sources and methods

Different rates of N fertilizers, as well as different forms, continue to be studied.

Research results evaluating nitrogen (N) sources and placement methods at planting in the early fall for winter wheat production show that controlled release urea (CRU) can be either seed-row or side-band placed while regular urea performs better when side-banded. These N fertilizer methods are considered as feasible replacements to the previously recommended practice of applying the majority of N as ammonium nitrate (AN) in early spring.

A formerly recommended N fertilization practice for winter wheat production was to apply no N or a low rate of N fertilizer at planting, then broadcast the remainder of N required as AN in early spring (Fowler, 2002). Due to the recent removal of AN as a generally available commercial product in the Northern Great Plains region, alternative strategies are required for N fertilization of winter wheat.

One strategy is to replace AN with urea as the N source and apply it in the early spring, but this can sometimes result in unwanted losses due to ammonia volatilization when urea is hydrolyzed on the surface from the action of urease enzyme present in soil and crop residues. Another method is to apply the N at the time of planting in the early fall. It can be placed in the seed-row, but this can result in excessive ammonia toxicity damage to germinating seed when normally required N rates are used. An alternative method to allow use of urea at planting is to side-band the N away from the seed-row. Yet another option is to replace regular urea with CRU in the seed-row at planting. The objective of this study was to evaluate the effectiveness of seed-row and side-banded urea and CRU at the time of winter wheat planting.

Outlining the research
Field experiments were conducted at three locations in 2002-03, 2003-04, and 2004-05 (Table 1). In the Canadian and US taxonomic systems, soils at the Bow Island site were Brown Chernozemic and Aridic Borolls. At the Lethbridge, Magrath, and Spring Coulee sites, they were Dark Brown Chernozemic and Typic Borolls. Locations had been no-till seeded for a period of at least three years prior to study initiation. The Bow Island location was in a fallow-spring wheat rotation, while other locations were continuously cropped. The experiment evaluated different options of applying N fertilizer at seeding in mid-September. The experiment consisted of three treatment factors: 1) N placement: seed-placed vs. side-banded, 2) fertilizer type: 20-day CRU (ESN), 40-day CRU, and conventional urea, and 3) N rate: zerio, 27, 54, 80 and 107 lbs of N per acre. The CRU products had polymer coatings that provide a gradual release of all urea within 20 days or 40 days when immersed in water at 73 degrees F (about 23 degrees C) (Agrium Inc., Calgary, Alberta). Plots were arranged in a split-plot design with three blocks, with N placement as main plot treatment and fertilizer type and N rate as sub-plot treatments.
Soil samples were obtained just before seeding. Five soil cores (two-inch diameter per site) were combined for sample depths of zero to six inches, six to 12 inches, 12 to 24 inches and 24 to 36 inches. Samples were air-dried and ground to pass a 0.08-inch sieve. All samples were analyzed for extractable NO3 -N, P, K and SO4-S. Soil pH and electrical conductivity were determined in 2:1 water extracts.

All figures, tables and photos courtesy of Dr. Ross McKenzie and Dr. Tom Jensen.

Winter wheat cultivar AC Bellatrix was no-till planted with a small-plot seeder equipped with Stealth openers (Flexi-Coil, Saskatoon, Saskatchewan). Seedbed utilization was approximately 10 percent. Plots were seeded at a target plant density of 23 plants per square foot based on germination counts (approximately 95 percent) and an assumed seedling mortality of 15 percent. Each plot contained eight rows of winter wheat at a row spacing of seven inches. The outer rows of each plot were separated by a distance of 21 inches. All plots received 19 lbs of P2O5 per acre as triple superphosphate applied with the seed. Weed control was achieved with recommended herbicides.

Plant density was determined in the last week of October and at the two- to three-leaf stage in the spring. Plant counts were determined in an area of 3.9 square feet in each plot.

Plots were trimmed to a length of 23 feet prior to harvest and whole plots (107 square feet) were harvested with a small-plot grain combine. Grain protein concentration was determined using near infrared spectroscopy. Grain N yield was estimated assuming a protein to N ratio of 5.7. All yields and concentrations are reported on a 14 percent moisture basis.

Effect of seed-placed fertilizer type and application rate on stand density of winter wheat at the two- to three-leaf stage in the spring. Nine-site average is expressed relative to the average stand density of side-banded treatments, which were unaffected by fertilizer type or application rate (standard deviation = 9).
Effect of fertilizer placement, type and application rate on the increase in grain yield relative to unfertilized checks (all sites average, standard deviation of 8 bu/ac).

Results of the research
When N fertilizer was side-banded, plant densities were unaffected by fertilizer type or rate of N application. When N fertilizer was seed-placed, plant densities were reduced by urea at most sites when application rates exceeded 27 or 54 lbs of N per acre, but were unaffected by application of the two CRU types (Figure 1). Use of CRU was highly effective for reducing seedling damage to winter wheat caused by seed-row application of urea. Current recommendations for maximum safe rates of seed-placed urea for cereals range from 0 to 36 lbs of N per acre, depending on soil texture, row spacing, seedbed utilization and moisture conditions (Karamanos et al., 2004). The recommended maximum safe rate of urea for our equipment and soil types is approximately 27 lbs of N per acre under good moisture conditions, with a substantial reduction recommended under dry soil conditions. These recommendations were valid for most locations in our study. Stand density declined significantly at most locations when the rate of seed-placed urea was increased from 27 to 54 lbs of N per acre (Figure 1). In contrast, application of CRU did not reduce stand density at application rates up to 107 lbs of N per acre, even when applied under dry conditions (Figure 1).

At almost all sites, N fertilizer provided a highly significant increase in grain yield, protein concentration, and N uptake (Table 2). The average increase was 21 percent for grain yield, 13 percent for grain protein concentration, and 36 percent for grain N uptake.

Application of CRU significantly increased grain yield compared to seed-placed urea, but did not significantly increase grain yield compared to side-banded urea in our study at N rates greater than 54 lbs of N per acre (Figure 2). Earlier studies often found that CRU products reduced grain yields because N release was incomplete or too slow to meet crop growth demands (Malhi and Nyborg, 1992; Delgado and Mosier, 1996). A study with a similar product (40-day release CRU) at 11 locations in Alberta and Saskatchewan found that CRU did not increase grain yield of spring wheat compared to side-banded urea, but increased grain protein concentrations at two of 11 locations and increased N fertilizer use efficiency by an average of four percent (Haderlein et al., 2001). Increased N availability during later growth is an effective means to improve grain protein concentration of cereals (Wuest and Cassman, 1992). However, the release of N from the CRU used in our study was rapid enough to increase crop yield but may be too rapid to ensure increased grain protein concentration for winter wheat.

Fall application of CRU, either seed-placed or side-banded, and urea side-banded, were effective means of supplying N for winter wheat in Southern Alberta. Seed-placed regular urea reduced plant stand density and yields at N rates greater than 54 lbs of N per acre. Application of CRU did not reduce stand density when seed-placed at rates as high as 107 lbs of N per acre, although further study is required to confirm the safety of these rates under conditions less favorable for plant survival. Grain yield, protein concentration and N uptake were similar for seed-row, and side-banded CRU and side-banded urea.

*Reprinted from Better Crops with Plant Food, with permission of International Plant Nutrition Institute (IPNI).