Growth and Defoliation of Pasture Plants: how the biology of pasture plants relates to grazing levels and pasture productivity

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1 Growth and Defoliation of Pasture Plants: how the biology of pasture plants relates to grazing levels and pasture productivity David B. Hannaway Forage Program Director Crop & Soil Science Department Oregon State University

2 Biology and Application The biology part is learning about the various parts of grasses and legumes and how they are important in maintaining growth potential. The application part is helping land managers gain the most from their land while maintaining the health of the pasture ecosystem.

3 Specific Purpose Statement This afternoon, we will learn the basic concepts of grass growth and regrowth along with management implications for forage-livestock enterprises. The goal is to increase the understanding of regrowth mechanisms of grasses (and legumes) and thereby improve the management, profitability and sustainability of croplands, pasturelands, and rangelands.

4 Goal: Improved Defoliation Management Pastures have higher yields and better quality when they are grazed or cut at the optimal time. Understanding grass growth and regrowth will maximize pasture potential through improved management.

5 Importance and Multiple Uses Grasses are often considered of little importance due to their presence all around us. Nevertheless, they are of great importance as: Human feed (rice, wheat, corn) Livestock feed (pasture, hay, and silage) Erosion reducing plants Water filtering plants Air purifying plants Beautification Recreation

6 Biology of the Grass Plant A grass plant is a collection of tillers or shoots that arise from buds at the base of the plant called the crown. Each tiller is composed of a series of repeating units of leaf, stem node, stem internode, and a bud. Each leaf is attached to the stem at a node.

7 Biology of the Grass Plant (cont.) Early in the development of a grass plant, the distance between nodes (internodes) is very short and the stem remains compact at the base of the plant. At the top or apex of the stem is the growing point where new leaves and stems are originated. As long as this growing point remains intact it is capable of initiating new leaves.

8 Biology of the Grass Plant (cont.) Later in the development of the tiller, the growing point will undergo a change and cease initiating leaves and begin developing the inflorescence or reproductive structure of the plant. Once this transition occurs, some of the upper internodes will begin to elongate and raise the inflorescence to the top of the tiller.

9 Biology of the Grass Plant (cont.) There are three primary growth stages in grasses that you need to be able to recognize for grazing management: 1) vegetative 2) elongation 3) and reproductive.

10 Biology of the Grass Plant (cont.) Vegetative Stage In the vegetative phase, shoots consist predominantly of leaf blades. Leaf blade collars remain nested in the base of the shoot and there is no evidence of sheath elongation or culm development.

11 Biology of the Grass Plant (cont.) Elongation / Transition / Jointing Stage In response to critical temperature regimes, daylengths, and necessary leaf blade area for sensing these climatic variables, the apical meristem is gradually converted from a vegetative bud to a floral bud. This is called floral induction. This conversion phase is termed the transition phase.

12 Biology of the Grass Plant (cont.) Elongation / Transition / Jointing Stage (continued) During the transition phase, leaf sheaths begin to elongate, raising the meristematic collar zone to a grazable height. Culm internodes also commence elongation in an "un-telescoping" manner beginning with the lowermost internode thereby raising the meristematic zone (floral bud and leaf bases) to a vulnerable position.

13 Two key points: Grass Growth 1. Grasses grow by capturing light energy and fixing carbon (photosynthesis). 2. Growing points (meristems) must be protected during critical periods. As a manager, your decisions greatly impact both keys to optimal grass growth and regrowth.

14 Photosynthesis Photosynthesis is the basic reaction in green plants that converts solar energy to chemical energy. This reaction is directly or indirectly responsible for all life on earth. Photosynthesis provides the energy (as carbohydrates) for plant growth. Plants then provide the energy (as structural and nonstructural carbohydrates) for animal growth.

15 Photosynthesis (2) Different Systems: 1. Cool season plants grow best at C (C 3 system; RUBPCase) 2. Warm season plants grow best at C (C 4 system; PEPCase) Cool-season pastures are more productive in the cool, moist spring and fall months. Warm-season pastures are more productive in hot, dry summer months. Cool Season Warm Season

16 Photosynthesis (3) Management Decisions: 1. Plant type selection (C 3 vs C 4, species and cultivars) affects seasonal production profile. 2. Defoliation intensity (number of animals for how long) affects light capturing capacity (solar panels).

17 Meristems Second Fundamental Key: 1. Grasses have a variety of growing mechanisms (meristems). 2. These meristems permit multiple harvests within a season if protected during critical periods. 3. Grasses benefit from timely, moderate defoliation.

18 Meristems (2) Apical and Intercalary Meristems Main growing point consists of the apical and intercalary meristems: 1. Apical meristem allows for plant maturation (development of a seedhead or inflorescence) is low in the plant canopy during vegetative growth is elevated as the plant prepares for reproduction 2. Intercalary meristem allows for leaf blade formation and expansion located at the blade base (collar)

19 Other Meristems: 1. Buds allows plant to develop daughter plants (tillers) are found in various locations (leaf axil, crown) 2. Rhizomes and stolons allow for lateral tiller development Meristems (3) (a) shoot apical meristem, (b) leaf primordia, (c) axillary bud primoridum, (d) leaf, (e) stem;

20 Regrowth Depends On: 1. Growth phase Vegetative phase regrowth is rapid providing that adequate leaf area is maintained for photosynthesis. Transition phase is critical even moderate grazing may destroy the apical meristem if internode elongation has begun. 2. Plant structure Grass Shoot Regrowth Intercalary meristems may be elevated to a vulnerable height removal will cause leaf death (no regrowth meristem).

21 Grass Shoot Regrowth (2) Grasses Vary Widely, Two Types: 1. Regrowth culms with elevated apical meristems Same sensitivity as with initial culm development period Even moderate grazing may destroy the apical meristem if elevated within culm 2. Culmless sterile regrowth Meristems remain near the soil (not vulnerable to removal)

22 Grass Shoot Regrowth (3) Species with more defoliation tolerance/rapid recovery after cutting vegetative tillers: Orchardgrass Tall fescue Meadow brome Perennial & Italian ryegrasses Kentucky bluegrass Creeping & meadow foxtails Crested & Siberian wheatgrasses White clover Red clover

23 Species with less defoliation tolerance/slower recovery after cutting vegetative tillers: Smooth brome Timothy Reed canarygrass Intermediate & pubescent wheatgrasses Quackgrass Bluebunch wheatgrass Warm-season perennial grasses Alfalfa Grass Shoot Regrowth (4) Birdsfoot trefoil

24 Plant growth above ground is a mirror to what is occurring below ground A small root system can only support a small amount of plant growth Grass Root Growth

25 Leaf Removal Effect on Root Growth (Adapted from Grassroots, Bob Kingsberry) Leaf Volume Removed (%) Root Growth Stoppage (%) 10 to to to

26 Relative Below and Above Ground Perennial Grass Growth Rate Root Regeneration Occurs two times during the year. Fall and spring, but fall is the most critical time. Old roots are shed and new roots grow. above ground growth root activity Summer Fall Winter Spring Summer Primary root growth period Secondary root growth period Root shedding

27 Grass Root Growth New, white roots grow in the fall and spring

28 Management Implications (1) Key Points: 1. Managers must understand the specific growth and regrowth mechanisms of their grasses: Phasic development (vegetative, transition, reproductive) Type, location, and vulnerability of meristems 2. Grazing (defoliation) must Be controlled (severity and timing) to ensure continued growth and regrowth Apical meristem

29 Management Implications (2) Key Points (continued): 3. Managers must examine plants and make decisions based on plant (and animal) conditions rather than calendar date or historical traditions (or need): Locate growing points Determine available forage Adjust grazing pressure accordingly 4. Pastures of mixed species are more difficult to manage: Culmed and culmless regrowth

30 Summary Management Implications (3) The plant needs two things: (1) photosynthetic leaf area and (2) regrowth meristems. How much leaf area? (sigmoid growth curve) What are the end points? i.e. on offer and residual dry matter?

31 Management Implications (4)

32 Management Implications (5) Vegetative Stage Grazing Early spring growth is from vegetative tillers. This means that the apical meristem is in the crown or even below the soil surface and protected from removal. Thus, the primary concern is with photosynthetic leaf for regrowth.

33 Management Implications (6) Transition Stage Grazing All grasses have a transition phase during which time the vegetative shoot apex is transformed into a floral bud. During this phase the sheaths of flowering shoots elongate raising their respective leaf collars to the extent that the leaf could be severed by grazing or cutting below the meristematic zone. By this stage internode elongation may have raised the shoot apex to a vulnerable height. This meristem represents the currently active regrowth mechanism, if it is destroyed (decapitated), there will be no further production from the shoot. Recovery growth depends largely on an alternative under-ground regrowth mechanism (basal buds in the crown zone).

34 Management Implications (7) Reproductive Stage Grazing Grazing before seedheads emerge and plants flower is important to stimulate summer tiller density and persistence. Clipping a pasture as seedheads emerge can redirect plant energy from reproductive growth to other areas.

35 Summary Key Issues Maintain sufficient leaf area for photosynthesis. Maintain regrowth meristems (apical, intercalary, bud, rhizome, stolon). Manage to remain in Exponential Growth Phase by monitoring on offer and residual dry matter levels. Ensure adequate fall regrowth period for root growth and energy restoration. Sigmoidal growth curve of a typical forage stand indicates how yield, growth rates and rest periods change over the growing season. (Voisin 1988).

36 Web segment: Additional Information Comprehensive web segment regarding growth and regrowth

37 Publications: Additional Information (2) Barnhart, Stephen K How Pasture Plants Grow. Iowa State University Extension. PM Fransen, S. and T. Griggs Growth, Development, and Defoliation Responses of Pasture Plants. Chapter 5 of Pasture and Grazing Management in the Northwest. PNW Kallenbach, Robert L Dairy Grazing: Growth of Pasture Plants. University of Missouri Extension Service, M Moore Ken Watching Grass Grow: The Key to Successful Grazing. Iowa State University.

38 Acknowledgements Special grant from Western SARE USDA s Sustainable Agriculture Research & Education program: wsare.usu.edu Merle Teel (Professor Emeritus, University of Delaware and Purdue University) Kimberly Japhet (Education Specialist, Instructor C&SS Department)

39 Acknowledgements (2) Slide sets from: Steve Fransen, Washington State University Tom Griggs, West Virginia University Glenn Shewmaker, University of Idaho