The vast majority of plants found in pastures in the Northeast region of the United States (U.S.) are cool-season grasses, legumes, and forbs. These plants begin growth in spring soon after snowmelt and are most active when soil temperatures are in the 40 to 60 degree F. range. Root growth begins at cooler temperatures than shoot growth. With the increasing soil temperatures of summer, growth slows. However, once fall returns and soil temperatures begin to cool, growth rates pick back up. This change in growth rate over the growing season produces a distinct seasonal pattern of production in most all cool-season grass and legume pastures such that about 50% of the seasonal growth occurs during the first 2 to 2.5 months while the remaining 50% is produced over the remaining 3.5 to 4 months.
It is important to understand the relationship between pasture growth rates and the seasonal pattern of forage production because even though growth rates and yields are highly variable during a single year, as well as from one year to the next, the general pattern of production is fairly predictable. And knowing the general pattern of how fast plants are growing and when establishes the basis for logically planning systems of grazing management that promote high forage yields, enhanced forage quality, increased harvest efficiency, and optimal animal performance.
It is also important to know how plants grow and respond to grazing. Because controlling – through management – the frequency, intensity, timing, and duration of the grazing events so that optimal plant growth can occur is vital to attaining optimal dry matter yields.
For example research conducted at Cornell University demonstrated the influence of grazing management on the productivity of pastures having the same soil type, fertility status, and plant species composition. Pasture treatments included a 16-paddock rotational system where cattle were moved to a new paddock every two days, a four-paddock rotational system where cattle were moved every seven to ten days, and a season-long continuously stocked system. Potential hay yields for the soils on the project were 7,500 to 8,000 lb/ ac/yr. However the only grazing system that allowed the pastures to produce as much forage as the site was potentially capable of producing was the 16-paddock rotational system. Conversely, the continuously stocked pasture produced less than half of the site’s potential forage yield.
Connected and Dependent Parts
A pasture plant can best be described as a living system comprised of two connected and dependent parts. The above-ground leaves and stems are solar collectors that, through photosynthesis, during the spring and summer convert light energy into carbohydrate energy for tissue growth. The below-ground roots and root hairs extract moisture and dissolved nutrients from the soil. Some 90% of plant growth is directly related to photosynthetic activity in the green leaves and stems, with the remaining 10% of growth related to the function of roots, root hairs, and stored carbohydrate reserves. While 10% does not seem like much, it can be argued this is the first 10% of growth, without which the other 90% never happens. Keep in mind, a plant without water is not much different than a fish out of water; they both simply bake in the sun.
Root volume and leaf tissue exist in a co-dependent relationship that is best described as what happens to one directly influences what happens to the other. Roots and root hairs provide the conduit by which moisture and dissolved nutrients enter the plant where they can be used in the synthesis of leaf tissue. The photosynthetic activity of the leaf tissue provides the carbohydrates used in the production of roots. It is generally the case that plants maintain a dynamic balance between root mass and leaf volume. In other words, when there is a large volume of one there is a large volume of the other. Conversely, when there is a low volume of one, there is also a low volume of the other.
This is because the leaves produce the carbohydrates for root growth. Without a large volume of leaves, there is no source of energy to keep a large root mass intact, thus it dies back to a volume that can be supported by the amount of green leaf present. Thus when a plant is defoliated to a low volume of leaf tissue, either through grazing or mechanical harvest, roots stop growing and slough off proportionately.
Over time, frequent close defoliation – as encountered in pastures that are continuously stocked – not only reduces the amount of leaf tissue produced, it also causes a decline in root mass. Subsequently, this reduces the plant’s ability to extract moisture and nutrients from the soil, which in turn reduces its capacity for growth even though adequate soil moisture and nutrients may be available.
This same phenomenon occurs even when pastures are grazed using rotational stocking methods. However, unlike pastures that are continuously stocked; with rotational stocking, the plants have the opportunity to recover – not only in leaf tissue but also in root mass – before they are defoliated again. Thus even though leaf tissue is removed and there is a commensurate reduction in root mass, when provided with adequate recovery periods, plant productivity can be maintained at high levels.
At one time, it was believed that plants stored an extensive reservoir of carbohydrate energy in their leaves and stems, rhizomes, stolons, and roots (depending on the kind of plant and species) where it could be mobilized to initiate new leaf growth after defoliation for many days in a row. Contemporary science suggests this is only partially correct. While legumes may store enough of this energy to “jump start” their regrowth after multiple defoliation events, most grasses only store enough for one or two. The function of the remaining stored carbohydrates is to keep the plant alive over winter, to provide nourishment during periods of environmental stress, and to initiate growth in the spring. In addition, most of the carbohydrates that plants mobilize for regrowth following defoliation are stored in the bottom few inches of the stems and leaves and in the top few inches of the root mass. Thus, when plants are severely defoliated (less than two inches of residual leaf length) not only has the photosynthetic capacity of the plant been severely reduced, so has the amount of stored carbohydrate energy the plant would use to initiate regrowth. Collectively, these two influences slow plant recovery and production.
Biological Growth Response
Pasture plants grow at different rates at different times during the season. They also grow at different rates during a single growth cycle. There is an early slow growth period, a mid-rapid growth stage, and a late mature phase characterized by a decline in growth rate. Each time a plant is defoliated, its rate of growth passes through these three phases, taking different amounts of time depending on the time of year.
In phase one, the plants are leafy and immature, high in quality, low in volume, and as a result of the lack of leaf area, growing slowly. In the second phase, growth rates are at their highest, the plants are leafy, growing toward maturity, and high in both quality and volume. In the last phase, the plants become stemmy and over-mature, grow slowly (if at all), yield is highest, but quality is at its lowest. During spring and early summer this phase is characterized by the appearance of reproductive tillers and seed heads. However, because some forages do not produce reproductive tillers later in the season, in summer and fall, there may not be reproductive tillers present, but the forage will consist of coarse low quality vegetation, with much of it in a state of decay.
Plant growth rates also change with the time of year. Spring and fall are generally times of accelerated growth rates, while summer is a time of reduced growth and lower forage yield. The result of which is, the recovery period between subsequent grazings is about twice as long in summer as it is in spring.