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Agronomy update: Optimizing nitrogen fertility for hybrid canola

Hybrid varieties of canola are widely grown on the Prairies, but their yield response to N fertilizer has not been recently updated, especially as newer, higher yielding hybrids are introduced to the marketplace. A research study was conducted across Canada to investigate hybrid canola yield response and N use efficiency (NUE) to N fertilization and to determine site-specific economic optimum N rates (EONR).

A four-year field trial led by Bao-luo Ma at the Ottawa Research and Development Centre, Agriculture and Agri-Food Canada, was conducted at each site from 2018 through 2022. The sites were at Beaverlodge and Olds, Alta., Scott, Swift Current and Melfort, Sask., Brandon and Carman, Man., and Ottawa, Ont.

Site-specific canola hybrids were used across years, including 6074RR, PV581GC, PV533G, L233P, L255PC and L252. They were seeded to ensure a minimum plant density of seven plants per square foot (80 plants/m 2). Preceding crops varied and included oat, wheat, canola, barley or corn.

Eight fertilizer treatments were compared with urea (46-0-0) applied at:

1. Pre-plant 45 lb. N/ac. (50 kg N/ha)
2. Pre-plant 90 lb. N/ac. (100 kg N/ha)
3. Pre-plant 134 lb. N/ac. (150 kg N/ha)
4. Pre-plant 178 lb. N/ac. (200 kg N/ha)
5. Split 45 lb. N/ac. (50 kg N/ha) at pre-plant plus 45 lb. N/ac. (50 kg N/ha; N50 +50) topdressed at the 4–6 leaf stage
6. Split 45 lb. N/ac. (50 kg N/ha) at pre-plant plus 90 lb. N/ac. (100 kg N/ha; N50 +100) topdressed at the 4–6 leaf stage
7. Split 45 lb. N/ac. (50 kg N/ha) at pre-plant plus 134 lb. N/ac. (150 kg N/ha; N50 +150) topdressed at the 4–6 leaf stage
8. Unfertilized control treatment (N0)

 Composite soil samples were collected prior to planting at each site in each year. Phosphorus (P), potassium (K) and sulfur (S) were applied and incorporated as required. These applications added an additional 15.6 lb. N/ac. (17.5 kg N/ha).

Canola yield significantly increased by up to 40 per cent with increasing N rate up to 134 lb./ac. pre-plant application. The estimated maximum yield at the 139 lb. N pre-plant rate (156 kg N/ha) was slightly higher but not significantly different than the 134 lb. N rate. Further increases in N application may result in lower yields due to factors other than N application limiting yield development.

Across years, the highest average yield was at the Olds site in the Black soil zone at 41 bu./ac. (2,323 kg/ha), and the lowest average yield was at Swift Current at 18 bu./ac. (1,011 kg/ha) due to severe drought and uneven rainfall. The highest yield of 65 bu./ac. (3,644 kg/ha) was at Beaverlodge in 2019.

Nitrogen application method – all pre-plant or split – did not significantly affect canola yield. However, there were some individual site-year cases where differences were observed. At eight of 32 site-year cases, the split application strategy produced similar to slightly higher canola yield than pre-plant-only N.

For example, at Ottawa in 2021, the split-N treatment produced 19 per cent higher yields than the equivalent preplant-only application under normal rainfall and slightly cooler conditions. At Melfort in 2022, the opposite was observed with the pre-plant applications yielding higher than split applications under environmental conditions of sufficient rainfall leading up to topdressing N followed by moderate drought stress.

Nitrogen fertilizer rate significantly impacted NUE. The highest NUE was at pre-plant 45 lb. N, and the lowest at pre-plant 178 lb. N, and this trend occurred at 25 of 32 site-year cases. Nitrogen use efficiency decreased from 13 to 6.6 lb. yield/lb. N input (six to 2.8 kg yield/kg N) as N fertilizer rate increased from 45 to 178 lb. N/ac. Overall, the application method did not affect NUE.

Analysis of the data found that the number of heat-stress days (i.e., days with temperature above 29.5°C) and heat-induced thermal accumulation four weeks before and after flowering directly impacted canola response to N fertilization and EONR.

The economic optimum nitrogen rate (EONR) was calculated taking into consideration the yield and N rate responses and the price ratio of N fertilizer to canola seed. The EONR was impacted by heat-stress days and thermal accumulation.

The EONR varied by site and was estimated at 75 to 90 lb. N/ac. (85 to 100 kg N/ha) in the low-yielding Brown soil zone, 130 to 148 lb. N/ac. (146 to 166 kg N/ha) in the Black soil zone and 125 lb. N/ac. (140 kg N/ha) in Ontario.

The researchers suggest that a split-N method be considered as a strategy to help adjust topdressing levels to account for early-season weather conditions. This could help achieve the goals of increasing canola productivity and reducing greenhouse gas emissions from fertilizer use. However, significant rainfall after topdressing N is required and the energy and time costs must also be considered. 

Bruce Barker divides his time between CanadianAgronomist.ca and as Western Field Editor for Top Crop Manager. CanadianAgronomist.ca translates research into agronomic knowledge that agronomists and farmers can use to grow better crops. Read the full Research Insight at CanadianAgronomist.ca.