Perennial and annual grasses, legumes, and forbs all have different growth habits, and growth habits also vary even within the grasses group. Understanding the differences in how plants like to grow helps us design grazing systems that encourage plants to thrive, build healthy soils, and produce quality feed for livestock. Here, I’ll present an overview of plant types and growth habits.
Grasses include annuals, which live for just one season, and perennials, which are plants that live for more than two years. Let’s start with the perennials, and discuss the annuals later.
Perennial pasture grasses include both warm- and cool-season grasses. Cool-season perennials like to grow during cool, moist conditions, so they will be most productive in the cool times of spring and fall. These plants will produce most of their forage dry matter at the beginning of the grazing season. Some cool-season varieties, particularly in locations with hot dry summers, may go dormant for a period of time during midsummer. Warm-season grasses, by contrast, grow best and are more productive during hot, dry midsummer weather. Warm-season grasses can produce very high yields of forage for both grazing and harvested feed in the heat of summer.
Perennial grasses include species, such as Kentucky bluegrass, that are very well adapted to grazing. However, other perennial grasses such as smooth bromegrass or timothy are more susceptible to being grazed down too short. As discussed earlier, this is due to differences in how the plants grow. Learning about plant anatomy and physiology can help in understanding why some grasses do better than others in the pasture, and why some respond very differently to different grazing techniques.
When you look closely at the anatomy of a grass plant, you will notice that each plant is a collection of tillers–individual plant shoots–that grow from the base or crown. Tillers can develop from seeds, they can shoot out from the base of existing grass plants, or they can arise from stolons or rhizomes.
Each tiller consists of at least one leaf as well as the growing point. The growing point for each tiller is at or near the ground level. As the tiller begins to grow, additional leaves will emerge. In the spring after dormancy, new growth will be initiated from the tiller’s ground-level growing point. Later, as the tiller stem elongates, it will be easier to differentiate the nodes and internodes, and to see that each leaf attaches to the stem at a node. As the stem elongates, the internodes (stem areas between the nodes) become longer and some new growing points will be found higher up on the plant stem and leaf collars.
On each grass tiller, the older leaves are at the bottom; newer leaves are above them at the top. As a newly sprouted tiller grows, it will develop its own root system and can eventually sprout more tillers. Thus, each tiller has the ability to become another self-supporting grass plant if it is separated from the rest of the mother plant. As long as the new tillers remain connected to the rest of the grass plant, they can share stored energy reserves while they regrow after being grazed. This is one of the characteristics that helps grasses adapt to grazing.
The basic physiological stages of grass development include the vegetative stage, the elongation stage, and the reproductive stage. If the grass plant is growing from a new seed, there will also be an earlier stage: germination. The germination stage begins when temperature and moisture conditions in the soil make it possible for the seed to germinate. This is the stage in which the first shoot emerges from the seed.
During the early vegetative stage, the growing point remains compact near the soil line, where the crown of the grass plant is. This vegetative stage is when leaf growth and development occur. Later, the internodes on the stem will begin to elongate (this is also called jointing). This is the stage when the stems grow taller and become more visible in the pasture. This also elevates some of the growing points from near the soil level to higher up in the pasture canopy.
In addition to the growing points found in the crown where new tillers or stems form in these elongating grasses, there are also growing points at the collar or base of each leaf. The tips of the leaves are not where the growing points are found! When the leaf tip is removed, the tip doesn’t regrow. Instead, cells at the base of the leaf (at the collar or intercalary meristem) elongate to increase the length of the leaf blade.
Growing points may also be found in other locations in grasses, and may also be called buds. Buds at the crown of the plant produce new tillers. Buds may also be found on rhizomes and stolons. On some species, buds may also occur at nodes on the lower parts of the elongating grass stem.
After the growing point on a tiller changes to reproductive mode and starts to form the flower head, it will not produce more leaves. Instead, the stem will elongate to raise the seed head up. The flower will emerge and seeds begin to form. The final part of the reproductive phase occurs when the seeds ripen and eventually are fully developed.
When a grass plant is just beginning the early vegetative stage, it doesn’t have much leaf area, and the rate of growth will be slow. But as it starts to generate more green leaves, it can capture sunlight and grow faster. The fastest growth rate occurs in later vegetative stages. Once the stem has elongated and the plant shifts into the reproductive stage, the dry weight of the plant doesn’t increase; much of it is just redistributed between the different parts of the plant.
To judge when the plants in a pasture are ready to graze, it is helpful to be able to distinguish whether they are in the vegetative stage, making lots of highly digestible leaf, or if they are instead making a lot of less digestible stems as they get ready to flower and make seeds. This allows the farmer to make good decisions that care for the plants, which need enough time to grow, as well as for the cattle, which need high-quality digestible forage.
There are some additional categories of grasses that have different growing habits. These include sod-forming grasses, bunchgrasses, and grasses that elongate their stems even when they are not in the reproductive stage. Understanding these different growth habits makes it easier to observe what is happening in the pasture and make good management decisions.
Bunchgrasses include orchard grass, big bluestem, and tall fescue, while sod-forming grasses include Kentucky bluegrass, reed canary grass, and quack grass. When these species are present in a pasture together, each may be ready to graze at a very different height.
Some grasses, such as Kentucky bluegrass, keep their growing points at or below the ground level throughout the season. Others, such as timothy, reed canary grass, smooth bromegrass, and switchgrass, elongate their stems and elevate their growing points even when not yet in their reproductive stage. Because these plants produce more stem, they tend to produce less digestible forage. In addition, because they elevate some of their growing points, grazing is likely to remove growing points from these plants. Thus, they also require a longer period between defoliations so that they can regrow new leaves, photosynthesize, and replenish their stored energy reserves. These types of “jointing” or “elevating” grasses are thus less well adapted to grazing. They can still thrive in a well-managed pasture but require careful management so they are not grazed too short or too often.
Cool-season grasses with higher growing points (like timothy) will do best if animals don’t graze the bottom 3 to 4 inches. Some warm-season grasses need even more of the lower grass stem left behind after each grazing. By learning what types of plants are growing in the pasture, and how they each prefer to be grazed, the farmer can manage to improve those plants’ health and productivity.
Many of the grasses produce new tillers in the late summer and fall. These will be productive new grass plants and tillers in the following grazing season, so careful grazing of them in the fall will increase pasture productivity the next year. Leaving an adequate post-grazing plant residual, and giving the grasses enough recovery time to grow multiple leaves on these new tillers before the first killing frost, will allow them to build up a larger energy reserve with which to begin growth next season. Conversely, overgrazing those plants in the fall will reduce how productive they are in the next growing season. Each grass tiller has significantly more nodes for roots than for leaves. An individual tiller may have three to five leaves, but it may have 10 root nodes. Despite this, the amount of energy from photosynthesis channeled to the roots is relatively small compared with that devoted to the rest of the plant. Younger roots receive more energy than older ones. Less energy is directed to the roots during seed head development, after a summer drought, and following grazing. This is yet another reason that regrowth periods must be long after each grazing.
Cool-Season or Warm-Season?
As mentioned earlier, cool-season species are more productive in cooler weather with moist conditions. Some common cool-season perennial grasses suitable for grazing include orchard grass, Kentucky bluegrass, and perennial ryegrass. Warm-season grasses are more efficient at gathering carbon dioxide while using less water, which is why they can be more productive during hot, dry weather. Some warm-season perennial pasture grasses include switchgrass, big bluestem, and annual species such as sudan grass, corn, and millet.
The choice of which grasses to grow depends on climate. Not all grasses thrive in areas of the country that are too hot or dry, and some do not do well in cool northern climates. On some farms, having some fields in warm-season grass and other areas growing cool-season grass can increase pasture productivity over a longer growing season. Cool-season grasses can be grazed when they are more productive in the spring and fall, and warm-season grasses during midsummer
Warm-season grasses can produce highly palatable pasture when managed correctly, and they are at their most productive when cool-season grasses have gone dormant due to heat and drought. This makes them a good species to fill in the midsummer slump in pasture production. Some warm-season grasses are not winter-hardy, and will grow as a perennial only in southern regions. However, some warm-season plants such as switchgrass will persist in some northern regions, while others like millet or sorghum sudan grass will do well as annual summer crops.
The perennial warm-season grasses are often slow to establish; some species may take more than a year to become established well enough to graze. Allowing them to grow to 16 to 20 inches tall before grazing will help maintain plant health and vigor. It’s important to leave a tall post-grazing residual compared with most cool-season grasses. Warm-season grasses should be left with at least 6 to 8 inches of growth in the fall before they go dormant for winter.
Prescribed burning is often recommended to maintain stands of some warm-season grasses, but with good grazing management this should not be necessary. Burning is done to remove excess litter and woody weed species. However, grazing with a higher stocking density will also remove litter and weeds and will return the organic matter and carbon to the soil instead of putting more into the air.
Native warm-season grasses once covered the Canadian and American prairies where bison roamed in vast herds. When cattle grazing began on the prairies, though, poor grazing practices and tillage led to the replacement of native species with other grasses. The native warm-season species include switchgrass, big bluestem, and Indian grass.
Legumes are another important group of plants in pastures because they help build soil fertility and provide highly digestible, high-protein forage for livestock. Legumes have a symbiotic relationship with microorganisms called rhizobacteria that live in their roots. This relationship makes it possible to “fix” nitrogen from the air so that the plants can use it for growth. In exchange, the legumes provide carbohydrate energy to the rhizobacteria. The nitrogen collected in this mutually beneficial process also becomes available in the soil for other pasture plants such as grasses to use. Because of this, including legumes in a pasture mix can decrease or eliminate the need to apply nitrogen fertilizer. Legumes mixed with grasses also offer ideal forage quality for livestock. When planting legumes, it may be a good idea to include an inoculant of the right species of rhizobacteria with the seed.
As with grasses, regional differences in climate will determine which legumes will grow best on a specific farm.
Legumes grow differently from grasses in several ways. In the spring, legume stems begin to grow in length immediately; there is no shift from a vegetative to an elongation stage as in grasses. Legumes have many potential regrowth points and can produce flowers on either side branches or main stems.
Within the legumes, some species are perennial and some annual. Some are very tall upright plants with deep taproots, while others are low growing. For example, alfalfa has an upright growth habit, but white clover has a horizontal growth habit and bird’s-foot trefoil have intermediate growth habits.
Alfalfa plants are perennial and have a crown above the soil surface, so it may be damaged by trampling, which is why it may not persist under high-density grazing. Also, alfalfa will fare better when it is not grazed too short, because short grazing can allow livestock to actually consume the crowns, where many of the growing points of alfalfa plants are situated.
White clover, also a perennial, is one of the best grazing-adapted plants, due to its low-growing, spreading growth habit. However, it is also sensitive to shading and will disappear in overgrown pastures or pastures that are frequently left to grow up tall. White clover has relatively shallow roots, so it is also more likely to become less productive during midsummer’s hot, dry weather than deeper rooted alfalfa or red clover. For this reason, white clover can show significant seasonal variations in growth.
Following defoliation, white clover leaves will rapidly regrow from the horizontal stolons. Over time those leaves will age, and other plants such as grasses and broad-leafed forbs will also grow up into the sunlight. When new clover leaves form and emerge under the pasture plant canopy, they will be in less light due to shading. The clover plant “knows” this, however, so it responds by elongating this new bunch of leaf petioles to lift the new leaves up into the light. This adaptation is one of the reasons white clover does so well in pastures.
Clover has horizontal instead of vertical leaves as grasses have. This adds more density to the pasture canopy so it can capture as much sunlight as possible.
Clover exhibits phototrophic movement — movement that causes stems and leaves to orient themselves facing the sun. This is a growth advantage because the leaves then intercept more light throughout the day. This movement comes about when the plants elongate cells in the stems only on the side farthest from the light. This has the effect of bending or moving the stems and the leaves toward the sun.
White clover also responds to shade and sun on its stolons by either growing more branches or elongating. In densely shaded areas and clumps of grass, clover stolons (horizontal stems) tend to grow linearly. But when they reach a sunlit gap in the pasture, they will branch profusely to colonize the open area. This adaptation is one of the reasons that white clover spreads naturally in many pastures.
In addition to the perennial legumes discussed (white clover, red clover, alfalfa), there are also several annual legume species. Annual legumes such as peas and beans are generally grown as a grain crop, but can also be grown as a silage crop, as green manure, or for grazing. They are a high-protein feed and, when used as an annual pasture, are usually grown as part of a mixture with other plants. Growing annual legumes with upright annual grasses such as oats will provide support so the vining legumes stay upright. Mixing in grasses will also provide a broader range of forage nutrition. Strip grazing is one of the best ways to graze these annual mixtures, because it is easier to limit daily intake of the annual pasture and prevent trampling of ungrazed forage.
When grazing perennial and annual legumes, there can be a risk of bloat.
Whether they are intentionally planted or not, there will probably be some forbs in any pasture. The word forb is generally used to describe plants that aren’t grasses or legumes, though the official definition is “a flowering plant other than grass.”
Some forbs may be weeds, while others are potentially productive pasture species. Some of these are annuals, which must be replanted or reseed themselves each season, and others are perennials that may persist as part of the pasture mix for years. Beneficial forage pasture forbs include forage chicory, dandelion, plantain, and forage brassicas such as kale or turnip, which can play a useful role in a diverse pasture mix.
Many plants commonly considered weeds have interesting nutritional or medicinal uses. Dandelion, for example, is high in minerals and has some diuretic properties. Chicory is high in condensed tannins, which can help with management of internal parasites. In his 1955 book Fertility Pastures, Newman Turner wrote about “herbal leys.” These diverse pasture mixtures include several grass and legume species along with forbs including chicory, plantain, burnett (Sanguisorba minor), and other species.
Browse plants are often woody brush or tree species that are not grazing-adapted. They are commonly found in hedgerows, overgrown pastures, and forest understories. They include deciduous trees such as birch and poplar as well as shrubs such as blackberry, raspberry, spirea, and goldenrod.
Not all woody shrubs and trees will make good browse due to palatability or toxicity issues. However, for some small ruminant farms, managing to include some browse in the grazing rotation may be desirable. This may include managed grazing of hedgerows, woody plants, or brush. Many of these brushy or woody plant species, as well as some of the “regular” pasture species such as chicory and bird’s-foot trefoil, can serve as a source of condensed tannins, which can help with livestock parasite management. Additionally, the young leaves of these woody plants are highly digestible and nutritious.
If you make the decision to include browse in the pasture system, keep in mind that managing a pasture to maintain a significant amount of woody browse species is challenging, and may not always be practical or economic. If grazed too hard or too often, most woody browse species will die, and the pasture will convert to species such as grasses and clovers that are better adapted to grazing. To manage to maintain browse, graze the woody plants lightly. In addition, giving woody species a much longer regrowth and recovery period is often necessary. Some farms accomplish this by grazing these woody browse areas only lightly and only every other year. Another option is to maintain browse hedgerows, which can be grazed lightly on occasion. In brushy areas where the browse is being killed by grazing, planting some forbs such as chicory or plantain, which are better adapted to grazing than the woody browse plants, in the pastures can provide a diversity of plants as the brushy species die out under grazing pressure.
Annual Pasture Plants
Annual pasture species may also be a useful part of a grazing system. Annual plants can be particularly helpful in extending the grazing season into the fall or through the midsummer heat when perennials are growing slowly or have gone dormant. Annuals for grazing include both warm- and cool-season grasses, legumes, and some forb species. Some warm-season plants will grow as perennials in warmer climates, but in colder northern areas these may only grow as an annual crop.
Because annual pastures are replanted each year, they can be managed differently than perennial pastures. Perennial pastures, just like a hay field, must be managed carefully so that overgrazing does not damage the plants, which will reduce the next year’s yield. When grazing an annual crop such as millet or oats, there may be multiple grazings in one season, but the goal can be maximizing livestock feed intake instead of trying to balance feed intake with careful long-term perennial plant management. Highly productive annuals that can be grazed include millet, sorghum sudan grass, oats, annual ryegrass, and legumes such as peas and annual brassica crops.
In addition to these intentionally planted annuals, some species that reappear on their own in pastures each year are annuals as well. These plants reseed themselves naturally, and include both productive palatable species and some that are considered weeds.
Caution: There are risks associated with grazing certain annual crops. For example, prussic acid poisoning can result when grazing sorghum sudan grass. To prevent problems, avoid grazing sorghum sudan during or right after a frost, and be sure plant height is at least 18 to 24 inches before grazing. Millet, which can be grazed at 14 to 24 inches of height, does not cause prussic acid poisoning. These annual warm-season plants are commonly used in midsummer when perennial pasture growth slows.
Forage brassicas such as turnips, rapeseed, and kale are most commonly grazed in fall or late summer. They maintain quality well into freezing temperatures and can be used to extend the grazing season on some farms. Health problems can occur if brassica grazing is not done correctly. These can include bloat, atypical pneumonia, nitrate poisoning, and hypothyroidism. Following the appropriate management practices should prevent these health disorders. First, introduce animals to brassica pastures slowly (over three or four days); avoid sudden changes from regular pasture to lush brassica pastures. Second, brassica crops should not be the only pasture or forage in the ration. Supplement with dry hay, silage, or “regular” pasture while grazing brassicas.
For farms with the right soils and access to equipment, growing annual pasture crops can be a strategy to assure a regular supply of high-quality farm-grown feeds. For other farms, it is clearly best to keep the focus on producing perennial forages. Each farm will have to assess the feasibility of growing annual crops according to their own goals and unique farm situation. Annual tillage and seeding cost money and require significant labor. In contrast, well-managed perennial grass-legume pasture provides many benefits in building and maintaining soil fertility with lower annual expense and labor. With a good grazing management system, many farms may be able to use perennials only and avoid the additional labor and cost of growing annuals.
Reprinted with permission from The Art and Science of Grazing by Sarah Flack and published by Chelsea Green, 2016.