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 = 534 (Table 1)
- GDD last week May 18-24 = 130 or 18.6/day; long term average = 151 or 21.6/day
- GDD forecast for the next 10 days = 375 or 37.5/day
- Average GDD accumulation for end of May = 175 or 25/day
- The 10-day forecast suggests warmer than average temperatures for the fourth week of May. Projected GDD is 37.5/day compared to the long-term average of 25/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* | 343 | 812 | 653 | 959 | 649 | 640 | 600 | 548 | 815 | N/A |
| June | N/A | 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 current 10-day forecast suggests daily high temperatures that are more common for July than late May. The temperature roller coaster in May continues as the first couple weeks of May were much colder than average and the last week will be well above the long term average. If the current forecast holds for the next 10 days it will accumulate more GDDs than in the previous three weeks in May.
The new 10-day forecast points to elevated daily low temperatures (lows in the high 50’s) and daily high temperatures in the low 80’s. If this forecast holds true expect a rapid growth phase for both crops and weeds. Area ryegrass fields are in the tillering stage of growth and will soon transition to the early jointing stage. As ryegrass plants begin to joint, plants will exhibit more of a vertical growth pattern, and the ryegrass plants will begin to overtop the wheat stubble.
Crop management
Perennial ryegrass production fields that exhibit thin line growth or gaps in the ryegrass stand would benefit from a soil applied herbicide to provide extended weed control when these pre-herbicides are added to the standard broadleaf treatment. A tank mix of dicamba plus 2,4-D is a standard broadleaf herbicide treatment in perennial ryegrass seed production fields. Several preemergence herbicides have been documented to provide extended weed control when added to a standard dicamba + 2,4-D broadleaf treatment. Callisto, Dual, Nortron and Prowl H2O are soil active herbicides that could help control weeds that emerge later in the season. Talk to your agronomist or seed conditioner for local experience. In addition, the U of MN has over 20 years of weed control research in perennial ryegrass.
Many winter annual weeds are bolting, clovers are growing well, and dandelions are flowering. Cool season annual weeds emerging include wild buckwheat, wild mustard, wild oats, smartweeds and common lambsquarters. Field scouting will determine the infestation levels of winter annual weeds. Now would be a time to get these fields sprayed before bolting and flowering is complete in these winter annual weeds. A second application for broadleaf weed control may be necessary depending upon the level of infestation of warm season broadleaf weeds.
If nitrogen fertilizer has been applied and is in the root zone, ryegrass plants should be a deep green color. Perennial ryegrass plants that are not tillering, or showing a pale green color could indicate nutrient stress. If spring fertilizer has yet to be applied now is the time to get that scheduled and applied. If spring nitrogen has been applied, but plants remain yellow with non-vigorous growth, applied nitrogen may not be in the root zone, some of the applied nitrogen may have been lost, or could indicate a sulfur deficiency.
Pest management
The first significant armyworm flights into the perennial ryegrass growing region of northern MN occurred on May 22 & 23. In four Scentry wing traps, a total of 61 moths were captured over two days. The armyworm’s rate of development depends on temperature. Armyworm eggs have been reported to hatch in as few as three days at 84 °F and an average of four days at 77 °F. Over several days, a single female has been observed to produce over 1,800 eggs. In the northern part of its range, the armyworm life cycle from egg to adult takes 35–60 days. Multiple generations, usually two to three in Minnesota, are produced until cold temperatures or environmental conditions kill the insects, triggering a southward migration of adult moths. More of the armyworm life cycle in next week’s newsletter.
Aster leafhoppers have been observed in high numbers in perennial ryegrass fields in the last week. I am not aware of an economic threshold for aster leafhoppers in grass seed crops. However, if the leafhoppers move to canola fields they can transmit aster yellows. If my memory is correct, 2012 and 2023 were two years with significant aster yellows in canola.
Mark Your Calendar - Annual Grass Seed Summer Field Day
The annual Grass Seed Field Day held at the U of MN Magnusson Research Farm is scheduled for June 24. Additional details will follow in future newsletters.
Next week’s newsletter will be released on June 2.