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 = 1,072 (Table 1)
- GDD last week June 1-7 = 262 or 37.4/day; long term average = 177 or 25.3/day
- GDD forecast for the next 10 days = 299 or 29.9/day
- Average GDD accumulation for second week of June = 186 or 26.6/day
- The 10-day forecast suggests warmer than average temperatures for the middle of June. Projected GDD is 29.9/day compared to the long-term average of 26.6/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* | 262 | 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
Last week brought temperatures more like the middle of August than the first week of June. The average GDD accumulation last week was 37.4/day, which is well above the average of 26.6 for the first week of June. Spring seeded ryegrass is now in the early heading stage of growth. After the 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. Perennial ryegrass typically will shed pollen in mid-morning, and these pollen clouds almost look like dust from gavel roads. The growth and development of late summer seeded ryegrass fields is behind that of spring seeded ryegrass.
With the perennial ryegrass fields now beginning the heading stage of growth, which 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.
Crop management
Perennial ryegrass plants should be a deep green color. If areas if the field are yellow in color or plants have stunted growth patterns may indicate a nutrient deficiency. If the plants are exhibiting a yellow color these plants may be deficient in nitrogen, sulfur, or both. If detected early, there is still time for rescue treatment. To troubleshoot for suspected nutrient deficiency one method is to collect paired plant and soil samples from the area with poor plant growth and an adjacent area with good plant growth. A single plant sample from the poor area is seldom enough information to accurately identify the nutrient deficiency. A soil sample from the same area is needed to determine if the soil nutrient supply is truly lacking or if reduced plant nutrient uptake is caused by another factor (e.g., soil saturation, soil compaction, cool temperature, disease).
Plant and soil samples should be collected within a week of symptom development to identify the nutrient deficiency and to have enough time for a rescue fertilizer application if possible. Plant samples collected after this window may be suspect as other issues develop and confound the results.
Now that ryegrass has entered the heading stage, growth regulator timing will be right around the corner. See last week’s newsletter for guidelines for growth regulators in perennial ryegrass seed production.
Pest management
In the last couple of weeks low level jet streams and south winds have carried army worm moths and other pests (leafhoppers, leaf rust) to northern Minnesota. For the week ending June 5, 129 moths were captured from five traps in perennial ryegrass fields, for an average of 3.7 moths/day. Last week the average from 4 trap sites was 5.6/day. This week individual trap counts ranged from a low of 6 to a high of 47. The highest moth capture was in areas with recent rain and/or thunderstorm activity.
Reports aecia of Puccinia cornonata on buckthorn has been detected in Steel County (SE MN) on May 29. This pathogen is responsible for crown rust in oats and perennial ryegrass. We have yet to observe rust in perennial ryegrass in northern MN. However, with the recent identification of aecia in SE MN and the recent strong southerly winds, spores may have been deposited in northern MN last week. Field scouting will determine when rust occurs in the perennial ryegrass growing regions of northern MN.
Summer grass seed field tour - June 24
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. At 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.
Dr. Nancy Ehlke, U of MN grass seed breeder will provide an update on the U of MN ryegrass breeding program. Dr. Eric Watkins will highlight research with hard fescues. Donn Vellekson will lead a tour of the perennial ryegrass field trials to include: preemergence herbicides, fertility, postemergence herbicides, growth regulators, and a perennial ryegrass variety trial.
Next week’s newsletter will be released on June 16.