Seeding Perennial Forages in Late Summer

By Mark Sulc 

The month of August provides a window of opportunity for establishing perennial forage stands or filling in seedings made this spring that have gaps. The primary risk with late summer forage seedings is having sufficient moisture for seed germination and plant establishment. The decision to plant or not will have to be made for each individual field, considering soil moisture status and the rainfall forecast. Rainfall and adequate soil moisture in the few weeks immediately after seeding is the primary factor affecting successful establishment.

No-till seeding in August is an excellent choice to conserve soil moisture for seed germination. Make sure that the field surface is relatively level and smooth if you plan to no-till, because you will have to live with any field roughness for several years of harvesting operations.

Sclerotinia crown and stem rot is a concern with no-till seedings of alfalfa in late summer where clover has been present in the past. This pathogen causes white mold on alfalfa seedlings and infects plants later during the cool rainy spells in late October and November. Early versus late August plantings dramatically improve the alfalfa's ability to resist the infection. Late August seedings are very susceptible to this disease, with mid-August plantings being intermediate.

In a no-till situation, minimize competition from existing weeds by applying glyphosate burndown before planting. If herbicide-resistant weeds are present, such as marestail, creates a very difficult situation with no effective control options in no-till management, so conventional tillage for seedbed prep is probably a better choice in those situations.  

For conventional tillage seeding, prepare a firm seedbed to ensure good seed-to-soil contact. Be aware that too much tillage depletes soil moisture and increases the risk of soil crusting. Follow the "footprint guide" that soil should be firm enough for a footprint to sink no deeper than one-half inch.  Tilled seedbeds do not need a pre-plant herbicide. 

Patching in new 2021 spring seedings with gaps is possible this late summer, even for alfalfa. Autotoxicity will not be a limiting factor yet in alfalfa seedings made this spring. Alfalfa plants that are less than a year old will not release enough of those compounds into the surrounding soil that are toxic to new seedlings of alfalfa. So, this summer is the last opportunity to try to “patch-in” alfalfa in thin areas of alfalfa stands seeded this spring.

Grassy weeds are probably present in the thin areas of those new spring seedings, so consider applying a grass herbicide as soon as possible. If broadleaf weeds are present, effective herbicide options are much more limited, because most broadleaf herbicides labeled for use in alfalfa are only effective when the weeds are quite small. Before applying a herbicide check its label for pre-plant time intervals that may be required. Use only herbicides with little or no time interval between application and seeding forages. Do take a cutting in early August and then immediately drill seed into the thin areas. Try to time drilling the seed when you see some rain in the forecast, especially if the soil is dry.

The following steps improve the chances for stand establishment success regardless of what type of seeding you are making:

• Soil fertility and pH: The recommended soil pH for alfalfa is 6.5 to 6.8. Forage grasses and clovers should have a pH of 6.0 or above. The optimal soil phosphorus level for forage legumes is 30 to 50 ppm Mehlich-3 and for grasses 20 to 30 ppm Mehlich-3. The optimal soil potassium level is 120 to 170 ppm for most of our soils.
   
• Check herbicide history of field. A summary table of herbicide rotation intervals for alfalfa and clovers is available at  http://go.osu.edu/herbrotationintervals.  Forage grasses are not included in that table, so check the labels of any herbicides applied to the field in the last 2 years for any restrictions that might exist.
   
• Seed selection: Be sure to use high quality seed of adapted varieties and use fresh inoculum of the proper Rhizobium bacteria for legume seeds. “Common” seed (variety not stated) is usually lower yielding and not as persistent, and from our trials the savings in seed cost is lost within the first year or two through lower forage yields.
   
• Planting date: Planting of alfalfa and other legumes should be completed between late July and mid-August in Northern Ohio and between early and late August in Southern Ohio. Most cool-season perennial grasses can be planted a little later. Check the Ohio Agronomy Guide for specific guidelines (see http://go.osu.edu/forage-seeding-dates).
   
• Planter calibration: If coated seed is used, be aware that coatings can account for up to one-third of the weight of the seed. This affects the number of seeds planted in planters set to plant seed on a weight basis. Seed coatings can also dramatically alter how the seed flows through the drill, so calibrate the drill or planter with the seed to be planted.
   
• Seed placement: The recommended seeding depth for forages is one-quarter to one-half inch deep. It is better to err on the side of planting shallow rather than too deep.

Do not harvest a new perennial forage stand this fall. The ONLY exception to this rule is perennial and Italian ryegrass plantings.  Mow or harvest those grasses to a two and a half to three-inch stubble in late November to improve winter survival.  Do NOT cut any other species in the fall, especially legumes.

Scout your new forage seeding this fall on a regular basis. Post-emergence herbicide options exist for alfalfa that control late summer and fall emerging winter annual broadleaf weeds. A mid- to late fall application of Butyrac (2,4-DB), bromoxynil, Pursuit or Raptor are the primary herbicide options for winter annual broadleaf weeds. Fall application is much more effective than a spring application for control of these weeds especially if wild radish/wild turnip are in the weed mix.  Pursuit and Raptor can control winter annual grasses in the fall in pure legume stands but not in a mixed alfalfa/grass planting.  Consult the 2021 Ohio, Indiana, Illinois Weed Control Guide and always read the specific product label for guidelines on timing and rates before applying any product (https://extensionpubs.osu.edu/2020-weed-control-guide-for-ohio-indiana-and-illinois-pdf/).