Perennial ryegrass growing degree days (GDD)
Perennial ryegrass GDDs will be tracked during the 2025 growing season with comparisons to the previous seven 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 GDD model.
- Year-to-date GDD = 1,457 (Table 1)
- GDD last week (June 9-15) = 187 or 26.7/day; long-term average = 186 or 26.6/day
- GDD forecast for the next 10 days = 321 or 32.1/day
- Average GDD accumulation for the fourth week of June = 212 or 30.3/day
- The 10-day forecast suggests above average temperatures for the fourth week of June. Projected GDD is 32.1/day compared to the long-term average of 30.3/day
| Year | 2025 | 2024 | 2023 | 2022 | 2021 | 2020 | 2019 | 2018 | 2025 vs. 2024 |
|---|---|---|---|---|---|---|---|---|---|
| March | 0 | 0 | 0 | 0 | 131 | 30 | 0 | 0 | 0 |
| April | 229 | 296 | 93 | 95 | 236 | 183 | 211 | 184 | -67 |
| May | 812 | 653 | 959 | 649 | 640 | 600 | 548 | 815 | +159 |
| June | 416* | 859 | 1,064 | 959 | 1,007 | 995 | 919 | 1,007 | N/A |
| July | N/A | 1,119 | 985 | 1,104 | 1,174 | 1,179 | 1,067 | 1,100 | N/A |
| Total | N/A | 2,927 | 3,101 | 2,807 | 3,188 | 2,987 | 2,745 | 3,106 | N/A |
General crop condition
Perennial ryegrass fields in the 2025 production year are showing a wide range in ryegrass growth and development. Perennial ryegrass fields with healthy crowns this spring will be shedding pollen this week, while other fields are just beginning to produce a seedhead on the main stem. Several factors contribute to these variable ryegrass stand including snow cover, soil moisture, ponding water, and low temperatures.
Dry weather last fall and spring with limited snowfall and freezing temperatures after snow cover were contributing factors to winterkill. In addition, dry conditions coupled with hot temperatures in early May reduced ryegrass tillering capacity. In a 5-day period from May 11-15 the average daily high temperatures were over 91 degrees F compared to the long term average of 63 degrees F.
Crop management
Many grass seed fields require an isolation strip in the certification process. Kris Folland is the local Field Supervisor with the Minnesota Crop Improvement Association (MCIA). If you have questions or concerns, please contact your grass seed agronomist, seed conditioner or Kris with MCIA (218-791-2156).
The USDA-ARS tracks rust development and movement from the Gulf of Mexico to the northern plain states. Crown rust has been observed at low levels in southern MN. Buckthorn is an alternate host for rust that infects wheat and other grasses. Common barberry is an alternate host for leaf rusts.
In many fields, the main stem of perennial ryegrass is heading with the tillers soon to follow. To maximize ryegrass seed production, it is important that the entire seedhead is protected with a fungicide. Perennial ryegrass, treated with a growth regulator, produce plants with an upright growth habit which leads to more efficient pollination compared to plants without a growth regulator. How about 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. Ryegrass plant parts 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.
| Treatment | Thousand seed weight (g) |
|---|---|
| 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 |
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).
Pest management
Armyworm moth capture continue to be low in 2025. Last week moth numbers ranged from 0 to 3 with a seven trap average of 1.7/trap.
Crown or leaf and stem rust has not been observed at the U of MN Magnusson Research Farm. Field scouting will continue in these areas not treated with a fungicide to determine when leaf rust infections are observed in perennial ryegrass in the 2025 season. Crown rust pustules are orange in color while leaf and stem rust are red in color. In northern MN conditions we typically can see Crown rust after 1,500 and leaf and stem rust after the accumulation of 1,900 GDD.
Next week’s newsletter will be released on June 24.