Cereal Rye Cover Crop Rates, Cold Germination, and Termination

Corn and soybean harvest often ends after the soil has gone cold, and most cover crop species stop germinating in it. Cereal rye is the exception: it germinates at soil temperatures as low as 34 degrees F [1].

Why cereal rye germinates in soil that stops other cover crops

Germination runs on a temperature threshold, not on a calendar date. Iowa NRCS lists cereal rye at a germination temperature of 34 degrees, with winter survival expected and about 18,160 seeds per pound [3].

That threshold is what buys the late window. The same guide sets the latest seeding dates for winter cereal rye between October 31 and November 5 across the northwest and central areas of the state [3].

Seeding depth matters more in cold ground than in warm. Place the seed a quarter inch to two inches deep, into moisture rather than into dry dust [3]. SARE puts the same ceiling on it. Rye is more sensitive to depth than other cereals, so plant no deeper than two inches [2]. Rye seeded that late tillers very little before dormancy, so it comes out of winter as a thin stand that still holds the surface against wind and water.

How to set the seeding rate by goal: cover, weed suppression, and forage

The rate follows the job, and Penn State Extension separates cover crop seeding rates for cereal rye by goal:

• 45 to 90 pounds per acre where the target is weed suppression and erosion cover [4]
• 30 to 60 pounds per acre where the plan is to plant green into standing rye [4]
• 2 bushels per acre where the rye is grown for forage, and 2.5 bushels for a later seeding [4]

The general cover crop range that Penn State Extension gives for cereal rye is 30 to 120 pounds per acre, which is wide because the goals inside it are that different [1].

A seed lot matters as much as the number on the tag: a 2 bushel per acre rate, about 112 pounds, can deliver anywhere from 1.3 to 2.3 million germinable seeds per acre depending on seed size [4].

How to adjust the rate for drilled, broadcast, and aerial seed

A drill places every seed at depth, so it needs the least seed. SARE gives 60 to 120 pounds per acre drilled into a prepared seedbed, and 90 to 160 pounds per acre broadcast and lightly disked or cultipacked [2].

Broadcast seed that lands on residue and never reaches moisture is simply lost, which is why the rate climbs. If you are broadcasting late in the fall and the scale and budget allow, that same source goes as high as 300 or 350 pounds per acre to secure an adequate stand [2]. At those rates the seed bill is the limit. Price the acre against a bulk quote before the order goes in.

Aerial interseeding into standing corn or soybeans is a broadcast job flown rather than driven, so it takes the top of the range. Penn State Extension suggests raising the rate to 150 pounds per acre when seeding into standing soybeans [1].

Iowa NRCS lists a single 45 pound per acre rate for winter cereal rye whether it is drilled or broadcast, which reads as a floor for erosion cover rather than a rate for biomass [3].

What rye allelopathy suppresses, and what the biomass actually does

Allelopathy is the release of plant chemicals that hold back germination in neighboring species. Rye has it, and it is often oversold. Penn State Extension reports that the effect is short lived, subsides within about a week of termination, and likely plays only a small role in weed suppression [1].

The residue mat does most of the work, and the mat is a function of biomass. Iowa State University Extension cites published work putting the requirement at least 9,000 pounds per acre of biomass to consistently reduce weed establishment [6].

That is a high bar for a late seeding. In the same Iowa work an October planting produced less than 500 pounds per acre regardless of rate, while a September planting produced about 3,200 pounds per acre [6]. At that level the result was a 15 percent cut in lambsquarters emergence, and waterhemp densities more than doubled [6].

When to plant green and when to terminate early ahead of corn

Planting green means putting the cash crop into living rye and terminating afterward. Penn State Extension reports that planting green into high rye biomass, above 9,000 pounds per acre of dry matter, reduces weeds more reliably than rye killed two weeks ahead of planting [1].

The same page states the tradeoff in front of corn. Standing rye can cut the number of weeds in the corn row. It can also cause etiolation, a leggy stretch in the young corn, along with reduced corn yield [1].

Ahead of soybeans that tradeoff is easier to accept. Ahead of corn, an early termination and a clean seedbed remain the lower risk choice on most acres.

How the carbon to nitrogen ratio ties up nitrogen at high biomass

Carbon to nitrogen ratio is the number that decides whether residue releases nitrogen or borrows it. Penn State Extension tracks rye from 14:1 as a young plant, to 20:1 at mid boot, to 40:1 at anthesis, the flowering stage [1].

Once rye reaches the reproductive stage and a ratio above 40 to 1, free soil nitrogen is not available quickly to the next crop, which often leaves corn short of what it needs [1].

Two moves close that gap. Terminating at boot rather than at flowering keeps the ratio lower, and Penn State Extension recommends a high starter nitrogen rate or nitrogen applied early in the season [1].

Guide to termination timing, crop insurance rules, and the herbicide label

USDA NRCS defines cover crop termination as a practice that historically and under reasonable circumstances results in the end of the cover crop’s growth [5]. That happens by natural causes such as frost, or by management: chemical application, crimping, rolling, tillage, grazing, or cutting [5].

For crops planted in the 2020 crop year and later, insurance attaches at the time the insured crop is planted, and cover crop management is reviewed under Risk Management Agency Good Farming Practice rules [5].

Two limits still apply on insured acres. A cover crop may be hayed, grazed, or harvested as silage unless crop insurance policy provisions prohibit it, and it cannot be harvested for grain or seed [5]. On acreage insured under the summerfallow practice, terminate the rye no later than June 1 preceding the insured crop [5]. That acreage also cannot be hayed, grazed, or otherwise harvested [5].

Chemical termination has its own conditions: glyphosate needs actively growing rye, so a hard cold snap before spraying is a reason to wait [1]. Penn State Extension gives a rate of 0.75 to 1.5 pounds acid equivalent per acre with appropriate adjuvants, and the label governs [1].

Roll crimping is the tool for rye taller than about 18 inches, and rolled before anthesis it should be followed by a burndown spray [1].

Pick the rate from the goal, get seed on the ground while the calendar still allows it, and settle the termination plan before the planter moves. If you are seeding cereal rye behind corn or soybeans this fall, check current cereal rye availability at Nature’s Seed and order by the bag or ask for a bulk quote.

Sources

[1] Pennsylvania State University Extension. Cereal Rye as a Cover Crop, extension.psu.edu/cereal-rye-as-a-cover-crop.

[2] Sustainable Agriculture Research and Education. Cereal Rye, in Managing Cover Crops Profitably, sare.org/publications/managing-cover-crops-profitably.

[3] USDA Natural Resources Conservation Service. Iowa Cover Crop Quick Reference Guide, Northwest Area, nrcs.usda.gov/NW_CC_QuickReference.pdf.

[4] Pennsylvania State University Extension. Cover Crop Seeding Rates, extension.psu.edu/cover-crop-seeding-rates.

[5] USDA Natural Resources Conservation Service. NRCS Cover Crop Termination Guidelines Version 4, nrcs.usda.gov/cover-crops-termination-guidelines.

[6] Iowa State University Extension. Will a Cereal Rye Cover Crop Suppress Your Weeds, crops.extension.iastate.edu/will-cereal-rye-cover-crop-suppress-your-weeds.