Cereal Rye Before Corn: Should You Adjust Your Nitrogen Rate?

Well, not as straightforward. It depends on the biomass and when you terminated it.

If rye biomass reaches 1,000 lb/A, consider adding starter nitrogen and/or sulfur applied near the row at planting. Rye can tie up N when there's a lot of biomass, and depending on the conditions, you might see an N penalty almost immediately.

Research at Iowa State showed that adding starter N is a management practice that can offset the negative corn yield effects of a rye cover crop. Overall, I think there's about a 2% yield drag in Iowa following rye. However, research has shown that in a rye cover crop system before corn with early rye termination, starter N — in either tilled or no-till systems — can offset potential negative effects of the cover crop. A higher starter rate (30 lb N/acre) could be especially useful if you intend to apply the majority of fertilizer N as a sidedress.

That said, this comes with a caveat: this work is still being researched. Other studies in Iowa have shown that 50 lb N/acre as a starter following rye increased early-season N uptake but had an inconsistent impact on corn yields.

Terminate when rye is less than 8 inches tall to minimize the risk of N immobilization and reduced corn yield potential. Annual ryegrass terminated by a fall frost, or brassica mixes, are less risky for N tie-up.

As far as a future N credit goes, there's often 20–60 lb of N per acre in aboveground biomass. Vetsch noted that 60% or more of that N could become available within 10 days. This is based on research at Iowa State (Sawyer et al. 2017) showing that 60–77% of the nitrogen in cereal rye was released 105 days after spring termination.

Disease pressure is another factor to consider.

Key takeaways:

• Terminate cereal rye before it reaches 8 inches tall or ~1,000 lb/A biomass to minimize N immobilization risk.
• Consider 30 lb N/acre starter at planting, especially if you're applying the majority of N as sidedress.
• Expect ~2% yield drag on average in Iowa following rye, with high variability across studies.
• Aboveground biomass typically contains 20–60 lb N/acre; 60–77% releases within ~105 days of spring termination (Sawyer et al. 2017).
• Annual ryegrass (winter-killed) and brassica mixes carry lower N tie-up risk than cereal rye.
• Account for disease pressure in the rye-to-corn transition.