Using Phenology— Considering Grass Lifecycles in Your Grazing Planning by Frank Aragona
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ccording to Wikipedia, “phenology is the study of periodic plant and animal life cycle events and how these are influenced by seasonal and inter-annual variations in climate.” The discipline of phenology is often attributed to the great naturalist and author Aldo Leopold. In fact, phenology is an ancient science that human beings have studied and applied for millennia. Phenological phenomenon are expressed in the annual cycles of nature: a bear emerging from hibernation, a spring ephemeral blooming in the forest, or a grass seedling germinating and establishing itself with the assistance of summer rains. Phenology is proving a critical science in studying the effects and pace of climate change around the globe. Understanding phenology, moreover, is a critical part of land management, and grazing planning is no exception. To date, attempts at phenological grazing planning have been subtle or poorly documented. It is imperative that we improve our existing planning tools; this can be achieved by monitoring ecosystem phenology, and by incorporating phenological data into the grazing planning process. Monitoring is the first critical element of phenological grazing planning. Phenology is highly place specific—factors like elevation, latitude, seasonal variation, and species composition will affect annual ecosystem cycles. Many groups have felt the need to develop a landscape-scale, collaborative phenological monitoring protocol and database. This need has led to the USA National Phenology Network. They have developed data collection sheets, documentation on collecting field data, and a collaborative web-based interface to enter and retrieve data from across North America. Learning how to use and expand upon these kinds of tools will be critical for the continued development and refinement of planned grazing. In 2011, HMI will be looking to build a stronger relationship between our community and organizations like the USA National Phenology Network. We will encourage the use of these monitoring tools via outreach and education, and also explore ways to integrate these tools within the framework of holistic planned grazing. Implementing a phenology monitoring program is the first step in phenological grazing 6
IN PRACTICE
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 highquality 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. 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
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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.
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 Zadock’s 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.” This is especially important to remember in cases where new plant establishment is of concern.
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