Richard Taylor, Extension Agronomy Specialist; rtaylor@udel.edu
Many growers like to overseed with legumes into their pasture and hay fields by using a method called frost-crack seeding or frost seeding. This method offers a relatively inexpensive means of introducing legumes into pastures or hay fields.
For a quick review, let’s look at the seven steps to take when considering a frost-crack seeding of forage legumes.
Step 1. Evaluate your soil fertility and soil pH status either by reviewing past soil test reports or taking a soil sample of the chosen field in the fall prior to the seeding. Make corrections in pH by liming the fall or spring before overseeding, as well as making corrections in the phosphorus (P) and potassium (K) status. You should avoid nitrogen (N) fertilizer additions in the fall prior to frost seeding the field, as fall applied N will be picked up and stored by the grasses (and weeds) present and will stimulate serious competition for the legume seedlings the next spring.
If you’ve waited until the spring of the frost crack seeding as many of us do, then use your old soil tests to determine the field’s fertility status and take this into account when you are making your species selection in Step 3 below.
Step 2. Provide seedlings with more sunlight and less competition as well as make it easier to get soil to seed contact. By this I mean that when possible the fall before a frost-crack seeding, you should graze or mow the field very close to stress the grass present to make it less competitive the following spring. This activity can be repeated just before overseeding to maximize soil exposure to the seed and to freezing and thawing temperatures.
Step 3. Select the correct species for your situation. In the mid-Atlantic region, we have three primary clover species from which to select. For fields that are generally wetter or lower in soil pH, alsike clover may be the best choice, unless the ultimate consumer of the forage will be horses, which can have reactions to this clover. All-around, white or ladino clover seems to respond best to this method of seeding especially under good soil fertility levels. Red clover is another species that responds well to frost-crack seeding but it is a taller growing species, but like alsike clover it is a short-lived perennial.
If you decide to base your selection on the grazing animal species you have, then for horses I would choose white clover. You will need to keep the seedling rate lower since we recommend no more than about 20 percent white clover in a horse pasture, even though this limits the effectiveness of the legume in providing N for the companion grass crop. For beef, all three species are suitable but for small ruminants where close grazing occurs, white clover is again probably the best choice.
For hay production fields where some legume contributed N is desired to boost grass yields and lower N fertilizer costs, the choice is more problematic. The tall growing species, red clover and alsike clover, have certain limitations. Red clover is more difficult to dry and because of the fine hairs that are on stems, petioles, and leaves it can make for dusty hay. Alsike clover is not suitable for the horse hay market since some horses develop alsike clover poisoning which shows up as photosensitivity causing the animal to sunburn easily. I’ve seen a vigorous tall growing ladino-type of white clover used in hay but its contribution to yield is limited to leaves and petioles since the stem stays on the soil surface. Ladino-grass hay for second and third cuttings can be very good although producers often are disappointed in the amount of legume in first cutting hay.
Finally on species selection, many of the species that contain quantities of condensed tannins that are thought to be useful in small ruminant parasite control are very difficult to establish using the frost-crack seeding method. The legumes in this category such as Birdsfoot trefoil and Sericea lespedeza are suited for more conventional seeding methods.
Step 4. Inoculate the seed before planting. Although the probability is very high that white, red, and alsike cover inoculating bacteria are present in all pasture and hay fields, buying preinoculated seed or inoculant for the seed is a good habit to get into. If preinoculated seed is past its sell-by date, you should add more inoculant before seeding. Also when you buy the inoculant, check the label to be sure that you are within the expiration date on the package. Inoculant consists of live bacteria so protect its viability by keeping it cool and out of sunlight. If the soil pH is below 6, then consider adding molybdenum (Mo) to the seed before planting. The rate for seed treatment ranges from 1/8 oz/acre to a maximum of 1 oz/acre using ammonium molybdate (54% Mo) or molybdenum trioxide (66% Mo) [but please note that sodium molybdate (39% Mo) is toxic to the rhizobia bacteria]. Depending on your source of Mo and seeding rate, you will need to calculate the amount of the Mo product to add per 100 lbs of seed.
Step 5. Calibrate your seeding method and equipment to be sure that you are putting on the correct amount of seed. Making a pass over a parking area or tarp that’s been placed on the ground is a good way to check both the width of the application pass and the density of seeds per square foot. Careful attention to this detail will pay extra dividends later in the season. This is especially true for broadcast spreaders or cyclone spreaders that fling the seed outward. Although clover seed is light it is fairly dense and may not travel as far as you expect.
Step 6. Frost seed at the correct time. Do not frost seed so early that the seed sits on frozen soil where heavy rainfall can move it off site. Also, do not frost seed on snow covered soil since rapid snow melt can again move the seed off-site. Instead seed in very early spring once the soil has at least begun to thaw, daytime temperatures are enough above freezing that the surface of the soil will thaw, and nighttime temperature are below freezing. You will need a number of weeks of this type of weather (at least off and on) to work the seed into the soil. You can also help in this process by allowing grazing animals access to the pasture or by running over hay fields with the tractor and mower. In addition to pressing the seed into the soil, you will also help reduce competition against the legume seedlings as they emerge and try to establish themselves.
Step 7—The Final Step. Essentially by returning to Step 2, your goal again is to control spring vegetation growth to encourage enough sunlight, nutrients, and water reaching the legume seedlings that they can effectively compete and establish themselves. Grazing can again help at this step but you will need to manage the grazing intensity closely as well as frequently so that you prevent the animals from grazing the tender young leaves of the new legume plants. As soon as you notice animals feeding on the new legumes or the legume reaches a height that will tempt the animals, remove them and change over to mowing. Once the plants have 6 to 8 trifoliate leaves or reach a height of 3 to 4 inches, the legume should be able to compete with the grass and weeds present in the pasture or hay field. Do not apply N-containing fertilizers since this will stimulate grass growth and suppress the nitrogen fixing ability of the legumes. Use grazing or hay harvest management techniques and fertilizer (lime, P, and K) management to favor the legume species you frost seeded. A rotational grazing system or hay cut system designed for the legume seeded can help ensure a longer lasting stand.
To summarize: If you are considering frost-crack seeding to thicken or improve your pastures, then a keep in mind the dos and don’ts that apply to this practice.
Do’s:
- Plan ahead and obtain the needed seed and rhizobia inoculant before it’s time to seed.
- Use either fresh (with an unexpired expiration date) rhizobia inoculant on the legume seed or use lime-coated pre-inoculated seed that is not more than 9 months since it was inoculated and lime coated. If older than 9 months, new inoculant should be applied just prior to seeding using a commercial sticking agent.
- Choose the legume species or variety best adapted to your location and grazing style.
- Frost-crack seed at the end of winter or in early spring when daytime temperatures are above freezing and nighttime temperatures fall below freezing so that the surface soil goes through freezing and thawing cycles that will help work the small seeds into the top few millimeters of soil.
- Closely mow the area or heavily graze the area before seeding to remove any excess vegetation and maximize sunlight penetration to the soil surface.
- After seedling emergence in the spring, watch the new seedlings closely and mow the pasture to the height of the legume if the pasture grasses begin to significantly shade the new seedlings.
Don’ts:
- Do not attempt frost-crack seedings with large seeded legumes and grasses. Although small seeded grasses such as timothy and very vigorous grasses such as ryegrass and festulolium have been frost seeded successfully according to producer reports, results have been very variable and probably would not justify the seed expense.
- Do not mix inoculated seed with fertilizer since many fertilizers will cause salt injury to the rhizobia bacteria and often kill the bacteria due to osmotic pressure.
- Do not broadcast seed on snow covered pastures.
- Do not broadcast seed just prior to a predicted or expected snow, ice, or rain storm since the risk of seed loss is great if runoff occurs.
- Do not mow or graze below the height of the newly establishing seedlings until the anchoring root system is well established and sufficient top growth has occurred to sustain the seedlings.
Also consider using a no-till drill to incorporate or improve a legume component in the pastures. No-till legume seedings are more frequently successful than frost-crack seedings. Some producers report additional recruitment of new legume seedlings in the second spring following an initial frost-crack seeding. Many legumes contain a significant proportion of hard seed that is broken during the following winter period permitting the seed to germinate the second spring following frost-crack seeding.
Regardless of your method, your goal should be to maintain at least 70% soil coverage in your pastures and hay fields to ensure you protect your soil resource, maintain pasture productivity, and reduce the incursion of weed species.