Nitrogen residue in leftover seed crop - Ask Extension
Hi - can you provide any ideas on where to find info for nitrogen leftover in fields from perennial grass (rye, fescue) and annual seed (clover, beets...
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Nitrogen residue in leftover seed crop #924210
Asked February 03, 2026, 6:50 PM EST
Hi - can you provide any ideas on where to find info for nitrogen leftover in fields from perennial grass (rye, fescue) and annual seed (clover, beets, mustard)? I'm looking for something that would give me an idea of typical pounds per acre as nitrogen in the Willamette Valley. Thanks in advance!
Lane County Oregon
Expert Response
This OSU Extension publication on post-harvest residue management for grass seed crops provides information about the amount of nitrogen and other nutrients in grass seed straw:
https://extension.oregonstate.edu/catalog/pub/em-9051-postharvest-residue-management-grass-seed-production-western-oregon
Tall fescue and perennial ryegrass straw contain about 55 lb of N per acre after grass seed harvest. Annual ryegrass straw contains 40 lb of N per acre.
I haven't been able to find data for beets or mustard, but these are grown on relatively few acres. This paper looked at (perennial) white clover crops in New Zealand:
https://www.frontiersin.org/journals/agronomy/articles/10.3389/fagro.2025.1708560/full
The authors found that after seed harvest the "combine offal residue-N" was 125 kg per hectare (111 lb N per acre). They also report N amounts in above ground biomass, and track the N through the system for the following year.
A few things to keep in mind:
Annual crops tend to move a lot of their nutrients into the seed as they mature and die. These plants are not trying to stay alive for another year, they give their seeds a good start by filling them with nutrients. This means that the N content of a vegetable crop is not representative of N left in the field after that vegetable species is harvested for seed.
What happens to crop residue N after harvest depends on other characteristics of that crop residue, especially the carbon to nitrogen ratio.
https://extension.oregonstate.edu/catalog/pub/em-9051-postharvest-residue-management-grass-seed-production-western-oregon
Tall fescue and perennial ryegrass straw contain about 55 lb of N per acre after grass seed harvest. Annual ryegrass straw contains 40 lb of N per acre.
I haven't been able to find data for beets or mustard, but these are grown on relatively few acres. This paper looked at (perennial) white clover crops in New Zealand:
https://www.frontiersin.org/journals/agronomy/articles/10.3389/fagro.2025.1708560/full
The authors found that after seed harvest the "combine offal residue-N" was 125 kg per hectare (111 lb N per acre). They also report N amounts in above ground biomass, and track the N through the system for the following year.
A few things to keep in mind:
Annual crops tend to move a lot of their nutrients into the seed as they mature and die. These plants are not trying to stay alive for another year, they give their seeds a good start by filling them with nutrients. This means that the N content of a vegetable crop is not representative of N left in the field after that vegetable species is harvested for seed.
What happens to crop residue N after harvest depends on other characteristics of that crop residue, especially the carbon to nitrogen ratio.
