As noted in a previous post, the further development of collaborative monitoring tools for plant phenology will be critical to developing the concept of phenological grazing planning. Implementing a phenology monitoring program is the first step in phenological grazing planning, but it is not necessary to have many years of data to incorporate these concepts into your planning. Many land managers are often aware of the annual cycles of the principle species on the landscape; taking the time to think through and write down these key moments in species lifecycles can help in the planning process.
Mapping your landscape is also an important part of this process. Species composition is determined by variable landscape features, like soil type and elevation. Understanding the relative abundance of a species or a plant community will help you to understand what plants exist where, and as a result you will know which species life cycle patterns you must give the greatest attention on a per paddock basis.
Additionally, phenological grazing planning requires an understanding of how grasses and other plants grow. While individual plants do not move through space, they do move through time. As they do so, plants undergo changes in structure, palatability, and sensitivity to grazing. Presented below is a review of some of the key phases in a grass plant’s growth and development. This analysis is not comprehensive, but it will provide a strong basis for thinking about these concepts.
C3 or C4?
The differences between C3 and C4 plants are primarily metabolic, related to the different enzymes used in the photosynthetic apparatus of the plant. For our purposes, it is enough to say the C3 grasses tend to be cool season plants, and C4 grasses are generally warm season grasses. Some common C3 plants are wheatgrass, needlegrass, bromegrass, and bluegrass. Cool season grasses initiate growth in the early spring, when soil temperatures reach a minimum of 40 degrees F.
These cool-season grasses produce high-quality forage early in the growing season. However, they do not grow during the hot periods in midsummer, and often become semi-dormant. They may grow again in the fall as temperatures cool and late summer precipitation replenishes soil moisture. Thus, there may be two growing periods for these grasses: early spring and late summer or fall. 1
Common C4 grass species include blue grama, buffalograss, and bluestems. These grasses grow in a temperature range between 70 and 95 degrees F, although soil temperatures can be between 60 and 65 degrees F. C4 grasses are more effective at using soil moisture, especially under droughty conditions. In general, they are less palatable to grazers than cool season grasses. As Trlica notes:
…because C3 grasses often enter the reproductive period at about the time that C4 grasses begin growth, livestock normally seek out this new growth from warm-season species. New foliage is always more digestible than more mature foliage, whether it be from a C3 or C4 species.2
The Phases of Growth
There are several identifiable stages of grass growth: seedling germination/establishment, vegetative stage, the “boot” stage, the reproductive stage, and the dormant stage. Cereal producers have used more detailed tools for agronomic purposes. One such example is the Zadocks scale. For our purposes, the five stages listed above should be sufficient, and will be given consideration below.
Seedling Germination and Establishment
Seed germination occurs under proper environmental conditions, and is highly species specific. Some species will only germinate when soil moisture and soil temperatures have reached a critical threshold; other species require fire, insects, or passage through an animal’s digestive tract to germinate properly. Know your species, and take good notes of what you see in the field.
When planning your grazing, remember the following:
The most critical period in the life of a grass seedling is when the primary roots begin to die and the secondary roots may not have developed enough to properly feed the shoot. This period is particularly critical if soil moisture in the surface few inches of soil is limited and no subsoil moisture is present.3
Vegetative Growth
Plants enter the vegetative growth stage immediately after coming out of dormancy in the spring, or in the case of seedlings shortly after the development of secondary roots. Range scientist L.L. Manske has written concisely about the vegetative growth stage of grass plants, with specific reference to the effects of grazing. Below is an excerpt from his writings detailing key aspects of vegetative grass growth:
… the grass shoot’s production of three to three and a half new leaves during the growing season is important. When the shoot reaches the third-leaf stage, the apical meristem begins to produce flower buds rather than leaf buds, although formed leaf buds continue to grow and develop. Defoliation of leaf material before the shoot has reached this stage can disrupt the formation of leaf buds and leaves for the shoot, weaken the plant and diminish the plant’s ability to produce herbage.
Most native cool-season grasses reach the third-leaf stage around early June, and most native warm-season grasses follow in about two weeks. On strategies that begin grazing before the third-leaf stage, such as early spring grazing started in mid May, 45 to 60 percent of the potential herbage biomass will not be produced.
Defoliation of the shoot that has reached the third-leaf stage can stimulate the natural biological processes grass plants have developed in response to grazing. These processes include stimulation of vegetative reproduction, the growth of new tillers from the grazed shoot’s axillary buds. Properly timed grazing that removes only a small portion of the leaf activates beneficial processes that can result in a 30 to 45 percent increase in herbage production.
Implementing grazing management strategies that start after the third-leaf stage and coordinate rotation grazing periods with grass growth stages can activate beneficial plant processes that result in increased herbage production and in turn reduce pasture and forage costs…4
The “Boot” Stage
The “Boot” stage of grass growth is the transitional stage from vegetative to reproductive growth. This growth stage is initiated by changes in day length, and the transition is controlled by plant physiology and hormone releases. According to Manske, “The first external sign of flower stalk development is the swelling of the sheath that encloses the flower head.”5
Annual seed production is not always required to maintain range health, often because grasses can reproduce via tillering, or in the case of sod grasses via underground rhizomes. Since the forage quality and productivity of grass plants is much higher under vegetative growth, land managers might often prefer to graze plants after the third leaf stage and before the boot stage, thereby maintaining the grass community in a vegetative state. As Trlica notes:
… a reproductive tiller may remain vegetative if the growing point (terminal [or apical] meristem) is removed by grazing. Vegetative growth, therefore, is favored by some grazing, which reduces the number of seedheads produced and may stimulate the formation of new tillers.
Manske provides insights on how phenological data can be used to plan grazing in relation to the boot stage: “Most cool-season plants enter the reproductive stage before June 21, the longest day of the year, and most warm-season plants enter the reproductive stage after June 21.
Reproductive Stage
Sometimes seed production is necessary, especially when a stand is in need of newly established seedlings. Some land managers actually manage a stand to produce a seed crop, which is harvested and sold on the market.
Planned grazing can be used to steer the successional trajectory of a particular paddock or pasture. In this context, timed grazing and controlled utilization may allow the manager to inhibit seed production of some plant species. Subsequently, grazing animals can be removed from the area to allow seed production of desirable species. This process can also be applied in reverse, depending on the plant species and their flowering times. This approach requires good data on plant phenology, hence our previous emphasis on establishing monitoring protocols and collaborative databases. For more information on managing successional trajectories, please see our video on Holistic Weed Management.
The Dormant Stage
Plants begin to prepare for dormancy during the growing season. Carbohydrates accumulate in roots and crowns. These sequestered resources are used for respiration during winter dormancy and then again for the first flush of green growth once growing conditions resume.
Phenological grazing planning is an implicit principle embodied in the concept of planned grazing. It is our attempt here to offer tools and ideas for making this concept explicit in our thinking and planning processes. The idea is based on three principles: phenology monitoring, understanding the grass life cycle, and planned grazing. Hopefully, readers will find these concepts useful, and will apply them in their management process. HMI will continue to explore these concepts in greater detail. Ultimately, we aim to provide our community with useful tools for applying these concepts in practice.
1Trlica, M.J. Grass Growth and Response to Grazing. No. 6. 108. Range: Natural Resources Series. Colorado State University. Online at: http://www.ext.colostate.edu/pubs/natres/06108.html
2Ibid.
3Stichler, C. Grass Growth and Development. Texas Cooperative Extension. Available online: http://publications.tamu.edu/publications/Forages/scs-2002-22a.pdf
4Manske, L.L. Kraus, A.M. Jirik, T.C. Manipulating Grass Plant Growth Can Enhance Forage Production. North Dakota State University. Available online: http://www.chaps2000.com/bin/ccs2r2.pdf
5Ibid.