Seeding Perennial Forages in Late Summer

By Mark Sulc

August is the second good window of opportunity of the year for establishing perennial forage stands (spring being the first good planting time). August is also the ideal time for filling in gaps in seedings made this spring. The primary risk with late summer forage seedings is having sufficient moisture for seed germination and good plant establishment before cold weather arrives. 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 forage establishment.

No-till Seedings

No-till seeding is an excellent choice to conserve soil moisture for seed germination in late summer. 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 multiple years of harvesting operations. No-till into wheat stubble would be an excellent option.

Sclerotinia crown and stem rot is a concern with no-till seedings of alfalfa or red clover in late summer in fields with a recent history of clover. This pathogen causes white mold on alfalfa seedlings and infects plants during the cool rainy spells in late October and November. Early August plantings dramatically improve the alfalfa's ability to resist or tolerate the infection. Late August or early September 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. Herbicide-resistant weeds, such as marestail, create a very difficult situation and there are no effective control options in no-till management, so conventional tillage for seedbed prep is probably a better choice in those situations.  

Conventional Tillage Seedings

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 usually do not need a pre-plant herbicide. 

Patching Spring Seedings

Where gaps exist in seedings made this spring, it is possible to drill in seed now, even in alfalfa. Autotoxicity will not be a limiting factor in alfalfa seedings made this spring. Alfalfa plants that are less than a year old do not release enough autotoxic compounds into the surrounding soil to harm 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. By next spring, autotoxicity will be a concern.

Grassy weeds are probably present in thin or weak areas of new spring seedings. As soon as possible, consider applying a grass herbicide to pure legume stands or a broadleaf herbicide if needed in pure grass stands. If broadleaf weeds are present in legume stands or mixed grass-legume stands, effective herbicide options are much more limited, because most broadleaf herbicides labeled for use in alfalfa or other legume forages are only effective when the weeds are quite small. Before applying a herbicide, check the label for pre-plant time intervals that may be required. Use only herbicides with little or no time interval between application and seeding forages. 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 for forage grass seedings.
   
• 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 and don’t depend on planter calibration charts There is an excellent video on calibrating drills available at https://forages.osu.edu/video.
   
• 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 stubble height of two and a half to three-inches in late November to improve winter survival.  Do NOT cut any other forage species in the fall, especially legumes.

Scout your new forage seeding this fall on a regular basis. Post-emergence herbicide options exist for alfalfa to 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 cannot be used in a mixed alfalfa/grass planting.  Consult the 2022 Weed Contro Guide for Ohio, Indiana, and Illinois (https://extensionpubs.osu.edu/crops/field-crops/) and always read the specific product label for guidelines on timing and rates before applying any product.