Grasses are broadly classified as summer annuals, winter annuals, or perennials. There are no biennial grasses. The intended use dictates which group is most suitable for a given situation.
Annual grasses are represented by the major grain crops (corn, sorghums, wheat, rye, barley, oats), and by many weedy types which infest fields and pastures. Broad categories include:
1. winter annuals: cool-season species which germinate in late summer or fall,
2. summer annuals
cool-season species seeded in the early spring, and
warm season species seeded in late spring or early summer.
Annuals complete their growth cycle in a single growing season and reproduce only by seed whereas perennial grases reproduce vegetatively as well as by seed. Seeds represent the major storage organ for excess photosynthate. With no storage organs, such as rhizomes, stolons, or tubers, there is no means for vegetative reproduction. Annuals usually grow back after mowing or grazing. Regrowth arises from buds found on the lower nodes of the stem. This type of regrowth is called aerial branching because the new shoots arise from adventitious buds on stems as opposed to basal buds in the crown zone. Aerial branching is an efficient regrowth mechanism. For example, annual ryegrass and sudangrass, which exhibit this growth habit, can be grazed several times during the summer. Many weedy grasses are noted for their ability to recover from defoliation. Their control usually involves use of selective herbicides.
Winter-hardy varieties of common cereal grains are planted in late-summer or fall, sufficiently early to allow seedlings to develop a crown and produce winter-hardy shoots (tillers). With resumption of growth in the spring, additional tillers are produced. With environmental conditions favoring floral induction, the shoot apex of each tiller produces a floral bud. The developing seedhead becomes a storehouse for sugars not needed to support further vegetative growth. As with annual grasses, winter annuals do not develop organs for storing food reserves; therefore, with advancing maturity the plant becomes senescent and dies.
Winter-annual cereal grains are often harvested for hay or silage when seedheads emerge from the boot. As seedhead development is disrupted, new tillers may arise from lower stem nodes as previously described with annual grasses. This recovery growth may represent an important source of forage.
There are several winter annual bromegrasses that are troublesome to many forage managers; hairy chess, downy brome, and cheat. Proper management can serve to reduce these unwanted species.
Summer annuals are species that are planted in the spring and complete their growth by the autumn. Summer annuals can be cool season or warm season. In northern latitudes, where cold temperatures threaten winter survival of fall-seeded cereals, growers select cultivars that are adapted to spring seeding (for example: spring wheat, spring oats, and spring barley). When seedheads ripen in early summer the plant becomes senescent and dies. However, if seed head development is disrupted by grazing or mechanical harvesting, further growth may follow due to aerial branching.
Forage type sorghums and millets (including the weedy types) represent warm-season annuals. Seed germination is favored by relatively warm soil temperatures, thus maximum vegetative growth occurs in late spring and early summer. Again, if seedhead development is disrupted, regrowth arises by virtue of aerial branching where new shoots arise from buds located in basal stem nodes. Sudangrass, related forage sorghums, and various millet cultivars provide mid-summer growth for managers who wish to calendarize their grazing systems.
Biennials are plants that take two entire seasons to reach the reproductive stage. The first year is a time for accumulating food reserves in storage organs. The second season produces reproductive flowers and seeds. This is in sharp contrast with winter annuals which germinate in the fall and die the follwoing season when seeds ripen.
There are no true biennial grasses. Nevertheless, in some climate zones, species like annual ryegrass may behave like a biennial, producing forage for two seasons when planted in the spring.
Although there are no biennial grasses, there are biennial forage crops. These include the Brassica family (turnips, rape, kale, etc.) and some legumes such as sweet clover (Melilotus spp.).
Horticultural root crops, such as beets, carrots, and parsnips, some vegetables like onions and cabbage, and some ornamental shrubs like hollyhock, are true biennials.
Perennials are plants that continue to grow indefinitely or that regrow each year. Most of the commonly used forage grasses function as perennials, reproducing vegetatively as well as by seed. With perennials, vegetative reproduction involves development of winter-hardy crown tisue which contains buds and tillers that resume growth with the onset of spring temperatures.
Forage grasses which perenniate for 3-5 years are typically referred to as short-lived perennials. Perennial ryegrass is an example of a short-lived perennial forage grass. However, any perennial that is mismanaged will be short lived.
Life Cycles of Common Forage Grasses
The following are examples of annual and perennial grasses:
Annuals: annual ryegrass, annual bluegrass, pearl millet, corn, and sorghum / sudangrass.
Perennials: orchardgrass, tall fescue, perennial ryegrass, kentucky bluegrass, smooth bromegrass, meadow foxtail, timothy, colonial bentgrass, bermudagrass, reed canarygrass, wheatgrasses, big bluestem, switchgrass, and indiangrass.
Annual species are by nature short-lived plants and must be planted each year. Most are planted in the spring. Winter annuals are planted in the fall, early if you want fall grazing or greenchop feed.
Winter annuals such as wheat, rye, winter oats, and winter barley are cereal gain crops, however, they can be used as cover crops or as nurse crops for new seedings of perennial grasses and legumes.
Cover crops vs companion or nurse crops: A cover crop is typically seeded in the fall to prevent erosion during the winter and to add organic matter to the soil. The cover is normally plowed or otherwise tilled into the soil in the spring prior to planting a crop such as corn, soybeans, vegetables and such. When tilled into the soil, cover crops may be called green manure crops-being used to improve soil fertility. Cover crops follow a crop. Companion or nurse crops are used concurrently. When used as a companion crop, the winter annual is seeded in early-fall together with a perennial grass and legume. The following spring, the companion crop is cut for hay and silage and the perennials species takes over.
Perennials have more uses. In crop rotations the sod crop may be plowed after only two or three years. With livestock as the major enterprise, the intent might be to maintain the sod for an indefinite period, to be reseeded when the desireable species disappear. Optimal management suggested by this project includes prompt, high regrowth rates after defoliation, and extended pasture life.