NUE improving more slowly than yield

KEY RESULT:

Total plant nitrogen uptake (i.e. in the leaves, stems, straw and seed) of the old rapeseed and canola varieties was actually just as good as modern hybrids, they just don’t have as much seed to pack the nitrogen into as the high-yielding hybrids. Modern canola hybrids do have superior ability to utilize ammonium fertilizer.

PROJECT TITLE, PRINCIPAL INVESTIGATOR:

“Making of a more sustainable canola: using genetic diversity to improve nitrogen use efficiency,” Sally Vail, Agriculture and Agri-Food Canada Saskatoon

FUNDING:

AgriScience Program (Canola Cluster) under the Canadian Agricultural Partnership

PUBLISHED ARTICLES:

Read the full report on the Canola Research Hub.

Researchers are using a historic series of rapeseed and canola varieties, along with the new Nested Association Mapping resource, to compare how a diverse collection of B. napus lines perform under various nitrogen-related circumstances. By comparing the genetics of lines that perform well and lines that perform poorly, researchers can identify traits that improve nitrogen use efficiency (NUE) in canola.

This project included an extensive field study that focused on comparing performance and nitrogen (N) uptake for 35 B. napus lines developed over a 70-year period. Canola growth parameters were contrasted at low N rates (60 kg/ha of N) or high N rates (120 kg N/ha) over two years – 2019 and 2020 – and two field sites in Saskatchewan.

Researchers calculated NUE using current popular equations, and added nitrogen harvest index (NHI).

Results

Generally NUpE, NUtE, and NUEyld could not detect differences
in canola grown in each year (2019 or 2020), where NUEcr and
NHI could.

All five metrics could detect differences in N treatment, with the greater NUE values seen with the low rate treatment. Low N treatments showed higher NUE because NUE is a calculation of yield or “N in yield” divided by a measure of supplied N (soil, available, fertilizer, or in the shoot). With the low N, yield relative to N rate is often higher, giving a higher NUE. However, this low rate is insufficient for best yield or the highest level of profit. Understanding NUE differences under low vs. high N rates and between canola varieties or lines is fundamental for developing a strategy to improve NUE.

NUE is useful for comparing varieties across the same management regime or at one environment. When comparing diverse soils, N management practices or years (environments), the amount of available N, amount of water, and amount of growth have to be qualified.

When comparing varieties by decade, yield and “N in yield,” results show an improvement for the most modern canola lines. This might be because of the introduction of high-yielding hybrid varieties. A gradual improvement for NUEyld and NUEcr can be observed, as their calculations are dependent to yield and N in yield. Interestingly, the NUpE almost remained same for the oldest and newest decade group (with slight ups and downs in middle), meaning no observed progress over time was reported for this trait.

What to make of all these NUE metrics?

Use NUtE and NHI to gauge performance of moving within-plant N into yield. Of these, NHI should be more useful if lines have diverse seed oil, carbohydrate and protein concentrations. Use NUpE to gauge performance of N uptake ability into the shoot, and in studies where plants are not grown to reproduction. NUEcr may be useful in studies that have a wide range of soil N availability and only access to seed yield samples, providing that researchers stay with one crop type and do not use this metric to assess a wide range of crop types (cereals, legumes, oil crops).

Another interesting discovery. Relative to older open-pollinated lines, modern canola hybrids appear to be collectively better able to utilize ammonium as their sole N source. Selection for yield in canola may have resulted in this change.