Updated: Aug 18
Summer seeding alfalfa and alfalfa-grass mixtures can be a good way to establish new fields so that full season yields can be harvested the following year. It is typically done after winter wheat or spring cereals are harvested, and also during years when alfalfa winterkill necessitates the quick establishment of new stands. Summer seeding of alfalfa forage mixtures can be a viable alternative to spring seeding.
The most reliable time to seed forages is usually in the spring. With an April or early-May seeding, moisture is usually adequate and the legumes are well established for winter survival. However, spring seeding is not always possible due to a number of factors, including wet field conditions in the spring. Also, with spring seeded stands, yields are not at their maximum until the year following establishment. Fields seeded during the summer can be treated as established stands the next year with a full yield potential.
The biggest risk of summer seeding forage is lack of moisture after seeding. Do not use companion crops with summer seedings. They compete too strongly for available soil moisture and will reduce stand establishment. Summer seedings present a higher risk on heavier soils. The following are some points to consider regarding successful summer seeding of forages:
There are two critical risks associated with summer seeding alfalfa.
Seeding too early in the summer increases the risk of hot, dry conditions during germination and seedling development. Summer seedings fail if seeds germinate and then starve for moisture.
Seeding too late increases the chance of freeze-up before the alfalfa seedlings are adequately established. Seeding must be done early enough so that the alfalfa can accumulate sufficient root reserves to survive the winter.
Alfalfa needs about 6 weeks of growth after germination to survive the winter, and will generally survive if a crown develops before a killing frost. When exactly that frost will occur is unknown. Although weather and soil condition vary each year, this means seeding dates based on historical weather data have been suggested as follows:
> 2900 CHU areas – August 10th – 20th
2500 – 2900 CHU areas – August 1st – 10th
< 2500 CHU areas- July 20th – 30th.
Seedings up to a couple of week later than these dates are sometimes successful, but are at some risk in drought conditions or when a killing frost occurs early.
Lack of moisture for adequate germination is always a risk. If soil conditions are extremely dry and no rain is in the forecast, abandon plans for summer seeding.
Most grasses can usually be successfully seeded up to 2 to 3 weeks later than alfalfa. Sufficient growth is desired to prevent erosion. Birdsfoot trefoil and reed canarygrass have slow seedling development, so summer seedings of these species are rarely successful.
Fertility and pH
Rapid growth and establishment of alfalfa seedlings is important. Apply the required fertilizer and lime according to a soil test. If soil phosphorus levels are low, band applying a starter phosphate fertilizer in-furrow will encourage rapid root growth and establishment. For soils low in pH, lime should be added at least a year in advance to allow for reaction in the soil.
Seed-to-soil contact is important for germination, particularly in dry summer conditions. A loose, lumpy seedbed dries out quickly. A fine seedbed can be more difficult to prepare in August on clay loam soils, compared to loams, sandy loams and silt loams. Soil should be firm enough at planting for a footprint to sink no deeper than 1 cm (3/8 inch). Packing before seeding can help. Seed shallow (¼ inch) into a firm seedbed. Always pack the soil after seeding. Usually no adjustment is needed in seeding rate.
Summer seedings of alfalfa are more prone to heaving in late-winter and early-spring, especially if root development was limited due to slow germination or cool fall weather. Avoid summer seeding on heavier soils that have a history of alfalfa heaving.
Winter annual weeds, such as pennycress and shepherd’s purse, can be a common problem. It is not unusual to have to spray summer seedings. If warranted, a low rate of 2,4-DB can be used for broadleaf weed control, but be sure that the alfalfa is in the 1st to 3rd trifoliate stage. Caution must be used to avoid delaying growth due to the herbicide effect. Refer to OMAFRA Publication 75 “Guide To Weed Control”. If these weeds are not controlled, they will compete with forage establishment and show up in the first cut next spring, but should not be a problem after that. Annuals such as lambs quarters and pigweed will compete with forage establishment and then be killed by fall frosts. Annual grasses are usually not as big a problem. Perennials, such as quackgrass, need to be controlled before seeding.
Seeding forage after winter wheat is harvested can be a good opportunity if the wheat is harvested early enough, but competition from volunteer wheat can be a serious problem. This is especially true if a lot of light grain went through the combine, such as with fusarium infection situations. One approach to reduce the problem is to do some light tillage to encourage the grain to germinate. A burndown with glyphosate or a second cultivation 10 days later will destroy much of this grain. In a pure alfalfa stand, a grass herbicide could be used. In a pure stand of Roundup Ready HarvXtra alfalfa, glyphosate can be used.
Use Recommended Varieties
High performance, disease resistant, fast growing varieties should be used. Avoid the less modern, slower growing, less disease resistant varieties. Cheap seed is not a bargain and can be disappointing.
Alfalfa Following Alfalfa
Seeding alfalfa after alfalfa is high risk. Old stands of alfalfa release a toxin that reduces the germination and growth of new alfalfa seedlings. This is called “alfalfa autotoxicity”. Establishment problems can result if the existing stand was not plowed or sprayed at least 3 weeks before reseeding. These toxins are present for up to 6 months, and are sufficient to permanently reduce new stand yields. For maximum yields, one year of an alternate crop is required. The toxins are not present the first year in new seedings, so seeding failures can be reseeded without an autotoxicity effect. Carryover of disease is also a significant problem when seeding alfalfa after alfalfa, particularly on stands that died as a result of crown and root rots.
No-till summer seeding can be successful if proper attention is paid to seed placement (seeding equipment) and seedling competition from weeds (weed control). In dry conditions no-till can be a good option to preserve soil moisture. However, using no-till to reseed an existing alfalfa field in August is high risk. In addition to the autotoxicity risk, slugs and disease that may exist in the old sod can damage new seedlings.
Harvesting summer seeded alfalfa in the fall of the establishment year is not recommended. Summer seeded stands should sometimes be harvested a little later the following spring than established stands, to allow development of root reserves for strong regrowth.
Summer seeding of alfalfa can be an alternative to spring seeding. Good management and attention to some of the potential pitfalls is required to minimize the risk of establishment failure. Refer to “Successful Forage Establishment” https://www.qualityseeds.ca/post/successful-forage-establishment.