Seed Production Newsletter - May 12, 2026

Perennial ryegrass growing degree days (GDD)

Perennial ryegrass GDD’s will be tracked during the 2026 growing season with comparisons to the previous eight years. The accumulation of GDDs will begin after the snow has melted from the perennial ryegrass fields and continue through swathing. A base temperature of 32 °F will be used for perennial ryegrass (T-Base = 32 °F). 

• Year to date GDD = 276 (Table 1)
• GDD last week (May 4-10 = 45 or 6.4/day; long term average = 104 or 14.9/day
• GDD forecast for the next 10 days = 203 or 20.3/day
• Average GDD accumulation for second week of May = 124 or 17.7/day
• The 10-day forecast suggests warmer than average temps for the second week of May. Projected GDD is 20.3/day compared to the long-term average of 17.7/day 

General crop condition

In the spring of 2026, April 21 was the first day of recorded 40 degree soil temperature at the Magnusson Research Farm. Since that date, soil temperatures have been on a roller coaster! Soil temps of mid-40s was recorded on April 22 & 23, followed by temps in the high 30’s on April 29. The soil temp was back in the mid-40’s on May 3rd but was back in the high 30’s on May 5 & 6. The soil temp was in the mid-40’s again on May 8 - 10. One of the factors that can be attributed to the fluctuations in soil temperatures is the low temperatures at night. In a two week period from April 27 to May 10, freezing temperatures were recorded on 11 of 14 days and 8 days had recorded temps in the mid-to-high 20’s. The short term forecast points to an average daily high temps in the 60’s and nightly lows in the 40’s. Look for soil temps to increase this week which will stimulate emergence of weeds and promote the growth and development of perennial ryegrass.

If the forecasted temperatures for this week are accurate, soil temps should move into the 50’s will allow a better assessment of perennial ryegrass winter survival. 

Crop management

When should spring nitrogen applications be made in perennial ryegrass? Ideally, after the frost is out, the ground is firm enough to support application equipment, and green ryegrass rows are visible. In the environmental conditions of northern MN, perennial ryegrass will go through three distinct phases in the uptake and utilization of nitrogen. These three phases:

• Phase one: Slow nitrogen uptake, up to 700 GDD
• Phase two: Rapid nitrogen uptake, 700 to 1,300 GDD
• Phase three: Nitrogen redistribution, 1,300 GDD to physiological maturity

The GDD accumulation and perennial ryegrass critical growth stages in northern Minnesota, are listed below (Table 2). This data is averaged over years, locations and planting dates of ryegrass fields grown in the environmental conditions near Roseau, MN. These ryegrass plant stages are a benchmark to help in the scheduling of various field operations and monitoring pest infestations throughout the growing season.

As of May 10, the year-to-date accumulation of GDD was 276. The new 10 day forecast projects 203 GDD by May 20 for a projected year-to-date GDD total of 479. Spring seeded ryegrass will be in the tillering stage for the next couple of weeks. To maximize perennial ryegrass seed production nitrogen fertilizer must be in the rooting zone prior to the accumulation of 700 GDD.  

Pest management

With the forecasted improvement in temperatures, average daily high temps in the 60’s and nightly lows in the 40’s look for emergence of the cool season annual weeds and increased growth rates of perennial and winter annual weeds. If a broadleaf weed control application was NOT applied last fall now would be a good time to scout for winter annual and perennial weeds. Winter annual weeds will soon be bolting, and dandelions will be in full flower. High populations of these weeds will require broadleaf treatment to reduce the competition and prevent seed set of these winter annual weeds. 

 Next week’s newsletter will be released on May 19.