Many forage growers actually prefer late summer seedings, primarily due to reduced insect and weed pressures.

Usually summer seedings of forages are in small grain stubble fields or in fields where a spring establishment has failed.

Most alfalfa growers are aware of the allelopathic effect of old alfalfa stands on a new seeding. Research has found that old alfalfa crowns and roots exude a toxin that inhibits the germination and development of new seedings.

This is one reason why alfalfa producers rotate their stands for at least one growing season prior to a new seeding.

Another reason for rotating is the buildup of alfalfa diseases in an older stand and the opportunity to utilize the nitrogen produced by the alfalfa crop by a succeeding corn crop in a rotation.

However, the short interval from spring to mid-summer and the fact that the stand is considered a failure allows for a summer seeding to remedy a failed stand.

There are a few risks associated with summer seedings. The primary concern is moisture stress during germination and seedling development.

Avoid seeding when soil moisture levels are low in the upper 3 to 6 inches. Watch short-term weather forecasts and be prepared. No-till establishment will minimize soil moisture losses.

Post-establishment weed control challenges will need to be monitored. Summer annual weeds will not become well established.

However, winter annual weeds including chickweed or purple deadnettle and many others, will germinate in September and October and can present significant problems if not controlled.

In small-grain stubble fields, volunteer small grains can present another weed control issue. There are many options for weed control available. Treat this year, when weeds are small. Controls next spring will be less effective.

Early to mid-August is the preferred time to sow alfalfa in south-central Pennsylvania to ensure optimum establishment and production next spring.

However, later seedings do occur with some stands sown as late as mid-September. Work at Penn State by Dr. Marvin Hall, forage specialist, has taken a look at optimum summer seeding dates.

Dr. Hall’s work has shown that delays in forage seeding results in a reduction in plant height in late November, a reduction in the amount of the field that is occupied by the alfalfa (plant vigor) and a lower dry matter (DM) yield the next growing season for delays after August 1 in central Pennsylvania.

He concluded that delaying planting by two weeks after August 1 can result in a loss in production the following year by approximately 0.5 ton per acre.

By delaying four weeks (late August) to plant alfalfa the yield reduction was 1 ton per acre the following production year.

Dr. Hall also compared summer planting dates for three grass species; orchardgrass, ryegrass and reed canarygrass.

His results with grasses indicate that late August seedings are preferred for orchardgrass and ryegrasses; however, reed canarygrass yields were higher with the earliest seeding dates, and planting canarygrass after September 5 resulted in poor plant vigor and lower DM yields the following spring.

Do not harvest late summer seedings this fall. By not harvesting, root reserves will be maximized this fall, resulting in improved winter survival and rapid plant development next spring. Next spring, manage the stand as you would your other established stands.

In summary, planting forage crops during August is preferred, but these crops can survive through the winter if planted a few weeks after this period.

However, delaying planting will result in a reduction in plant vigor and lower yields as a result of this delay in establishment. Planting after late September can result in severely weakened stands next spring.  FG

—Excerpts from Penn State Field Crop News, Vol. 8 No. 21