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 = 1,283 (Table 1)
- GDD last week June 8-14 = 211 or 30.1/day; long term average = 186 or 26.6/day
- GDD forecast for the next 10 days = 265 or 26.5/day
- Average GDD accumulation for third week of June = 197 or 28.1/day
- The 10-day forecast suggests close to the average temperatures for the middle of June. Projected GDD is 29.9/day compared to the long-term average of 28.1/day
| Year | 2026 | 2025 | 2024 | 2023 | 2022 | 2021 | 2020 | 2019 | 2018 | 2026 vs. 2025 |
|---|---|---|---|---|---|---|---|---|---|---|
| March | 0 | 0 | 0 | 0 | 0 | 131 | 30 | 0 | 0 | 0 |
| April | 191 | 229 | 296 | 93 | 95 | 236 | 183 | 211 | 184 | -38 |
| May | 619 | 812 | 653 | 959 | 649 | 640 | 600 | 548 | 815 | -193 |
| June* | 473 | 932 | 859 | 1,064 | 959 | 1,007 | 995 | 919 | 1,007 | N/A |
| July | N/A | 1,040 | 1,119 | 985 | 1,104 | 1,174 | 1,179 | 1,067 | 1,100 | N/A |
| Total | N/A | 3,013 | 2,927 | 3,101 | 2,807 | 3,188 | 2,987 | 2,745 | 3,106 | N/A |
General crop condition
The thunderstorm activity last week and the associated cold front resulted in moderation of the temperatures of the last couple weeks. Two weeks ago, the average accumulation of GDD’s was 37.4/day, compared to last week of 30.1/day. The short term weather forecast points to cooler weather which should be positive for perennial ryegrass growth and development. Spring seeded ryegrass is now in the early heading stage of growth. After heading, the next critical stage in ryegrass growth and development is pollen shed which will begin after plants are fully headed on the main stem.
Crop management
In many spring seeded perennial ryegrass fields, the main stem of perennial ryegrass is heading with heads on the tillers soon to follow. To maximize ryegrass seed production, it is important to protect the entire seedhead, after pollination and full head extension, from leaf rust. What plant parts contribute to seed weight? Research conducted by Tretheway et.al., evaluated seed weight (thousand seed weight) produced from various parts of the perennial ryegrass plant at full head extension. Various plant structures were covered during the seed filling period. The results of this research indicate that the lowest seed weight was from plants that had the entire ryegrass seed head covered. Disease infestations like leaf and stem rust on the seed head will reduce the photosynthetic area. This research highlights the importance of timing a fungicide to protect the ryegrass seedhead from full head extension through physiological maturity. One of the conclusions of this research was that over 60% of the energy required to produce ryegrass seeds came from the seedhead and associated plant structures (spikelet’s, glumes, and peduncle).
| Treatment | Thousand seed weight (grams) |
|---|---|
| Stems wrapped | 2.95 |
| Heads wrapped | 2.55 |
| Flag leaf removed | 3.06 |
| Flag leaf attached | 3.10 |
| Unlodged tiller | 3.10 |
| LSD (0.05) | 0.28 |
Pest management
For the week ending June 12, 149 armyworm moths were captured from five locations (average of 4.3/day) in northern MN. Individual trap counts ranged from a low of 19 to a high of 42. The highest moth capture was in areas with recent rain and/or thunderstorm activity. Why do we see various sizes of armyworm larvae? Thus far in 2026 we had had three flights of armyworm moth. The first armyworm moth capture was May 22 and with the warm temps in late May and early June has resulted in egg hatch and armyworm larvae in the first to third instar. Flights last week will still be in the egg stage. Remember armyworm larvae pass through 6 instar stages. It takes about 8-10 days for the eggs to hatch (eggs tolerant to insecticides) and about 3 to 4 weeks for the armyworm caterpillar to pass through 6 instar stages. Data from Guppy indicates that caterpillar instar stages 1-4 account for only 6% of the feeding damage, instar stage 5 (up to 3/4-inch-long worms) approximately 15% and instar stage 6 (up to 1.25 inch) approximately 80% of the feeding damage. More on armyworms in next week’s newsletter.
Summer grass seed field tour
The annual grass seed summer tour is scheduled for June 24 at the U of MN Magnusson Research Farm. The field tour will begin at 5:00 p.m. Directions to the U of MN Magnusson Research Farm. From the intersection of MN Hwy 11 and 310, proceed north on MN 310 for two miles, turn left (west) on Roseau County 16, and proceed west for approximately three miles. The U of MN Research Farm is located on the north side of Roseau County 16. Perennial ryegrass field trials include: preemergence herbicides, fertility, postemergence herbicides, growth regulators, and a perennial ryegrass variety trial.
Seed Certification
With the perennial ryegrass fields now beginning the heading stage of growth is a good reminder that many ryegrass grass seed fields may require an isolation strip in the certification process. Kris Folland, Field Supervisor with the Minnesota Crop Improvement Association (MCIA) is a good contact. Reach out to Kris at 218-791-2156 if you have questions or concerns.
Next week’s newsletter will be released on June 23.