At this point in the season, it is important to make sure that crops planted in June and early July will have enough heat accumulation, measured in Growing Degree Days (GDDs), to reach maturity and provide adequate yields. This year we have seen fewer GDDs than average, and a wet spring has delayed corn planting. However, it may not be too late to plant corn for silage in some locations.
From May 1st to June 11th, 224 GDDs (with a base temperature of 50°F and a maximum temperature of 86°F for corn) have accumulated in Alburgh VT, which is 108 less than the 30-year normal of 332 GDDs, and 152 less than the 15 year average of 376. Lower accumulations of GDDs have occurred before, with the lowest on record being 207 GDDs from May 1st to June 11th in Alburgh, VT, and the highest being 553 GDDS.
For the year to date, from January 1st to June 11th, 240 GDDs have accumulation total, 135 less than the 30-year normal of 375.
The accompanying figure at the end of this document shows this year’s accumulated GDDs for corn in Alburgh, VT, starting on May 1st, in comparison to the 30-year normal and period of record.
One option to ensure an adequate corn yield is to plant a silage variety with a lower relative maturity (RM). Corn varieties will higher relative maturities, like 110-115 RM, will require 2700-2900 GDDs. By planting corn with a lower RM, you can harvest a crop that requires closer to 2000 GDDs. Silage corn with 90-95 RM will take approximately 2200-2300 GDDs to maturity, 85-90 RM will take approximately 2000-2200 GDDs, and 80-85 RM will take approximately 1700-2000 GDDs. After June 10th it is too late to plant most corn varieties.
Suggested planting dates in Wisconsin recommend planting corn for silage with 85-90 RM around June 10th, 80-85 RM around June 20th, and can be planted as late as July 1st. The table below shows projected GDDs that will likely accumulate from June 14th to the end of October in different locations around Vermont, calculated from the average frost dates and average monthly highs and lows from usclimatedata.com.
| Town | Average frost date | Projected GDDs June 14th-October 31st |
| Newport | Sept 21-30 | 1413-1448 |
| St. Johnsbury | Sept 21-30 | 1501-1582 |
| Rutland | Sept 21-30 | 1467-1534 |
| Burlington | Oct 1-10 | 1772-1878 |
| Salisbury | Sept 21-30 | 1727-1816 |
| Randolph | Sept 11-20 | 1327-1381 |
In the warmer regions of Vermont such as Addison and Chittenden counties, you may be able to successfully plant and harvest silage corn with 80-85 RM varieties. In cooler regions, it is too late to plant corn that will reach relative maturity.
If you are outside Addison and Chittenden counties, and your silage corn is still not in the ground, or if you are in Addison and Chittenden and will not be able to plant in the next week or so, you will want to consider your other options, such as planting a cover crop. If you have crop insurance, consider what the latest planting date you can plant without affecting your coverage. When considering whether to plant soybean instead of corn, take into account whether any nitrogen was applied to the field for a corn crop, as corn will use more of the applied nitrogen than soybeans. If nitrogen amendments have been applied, corn will make the best economic use of the nutrients already applied. The application of corn herbicides will also limit what can be planted next. Brassica cover crops for forage can be planted in July, and can be planted with cereal cover crops. Brassicas will remain a good grazing forage into November as they are frost tolerant. Winter cereals and legumes can be planted in August. For more information on summer alternative forages, see Northwest Crop and Soils’ Use of “Alternative Forages” on Certified Organic Dairy Farms in the Northeast (PDF) fact sheet at https://www.uvm.edu/sites/default/files/media/FAFOAlternativeForages.pdf.
References
Climate Smart Farming Growing Degree Day Calculator, Cornell University.
Grubinger, V. (2015). Scheduling Sweet Corn Plantings. University of Vermont Extensions Vegetable and Berry Program. https://www.uvm.edu/vtvegandberry/factsheets/SchedulingSweetCorn.html
Lauer, J. (2013). How Late Can I Plant Corn? Field Crops 28: 421-111.
usclimatedata.com
