Plant Growth and Development as the Basis of Forage Management
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Edward B. Rayburn, Ph.D
West Virginia University
Extension Forage Agronomist
December 1993

The growth and development of forage plants have a major affect on how plants respond to harvest management and on forage quality. By understanding these effects the forage manager can optimize the production and utilization of forages.

Growth is the process by which a plant increases in the number and size of leaves and stems. The result of plant growth is forage production and the amount harvested by animal or machine is forage yield. The growth of both plants and animals requires energy. Animals get their energy by digesting the plants they eat. Plants get their energy from the sun through photosynthesis. Photosynthesis is the process where the green pigment in the plant's leaf (chlorophyll) absorbs energy from sunlight and, using this energy, water, and carbon dioxide, produces oxygen and simple sugars. The plant then uses these sugars to make more complex sugars and starches for storage as energy reserves, to make cellulose and hemicellulose for cell walls or with nitrogen, to make proteins. How the plant uses its energy depends on the developmental stage of the plant and on environmental conditions.

When leaves are removed from a grass or clover plant, new leaves develop and grow from buds on the crown or stems of the plant. This growth requires energy which comes from reserve carbohydrates (sugars and starches) or from actively photosynthesizing leaves remaining on the plant. Most closely harvested plants have a predictable plant growth-energy reserve cycle (Figure 1). Manipulating this cycle is a useful tool in managing forages.

Let us look at orchardgrass as an example. When the leaves of an orchardgrass plant are harvested, new leaves will start to grow. If all the leaves are removed, the new growth will have to come from energy reserves stored in the leaf bases of the plant. Over a period of days the usable carbohydrate reserves will decrease. If more leaf area is left on the plant, the use of reserve carbohydrates will be less and regrowth will be faster since it can also be powered by photosynthesis.

As new growth increases the leaf area, more sunlight is intercepted and photosynthesis increases, providing more energy for additional leaf growth. At some point photosynthesis is great enough to produce more sugar than is needed for leaf growth. This results in an increase in the reserve carbohydrates in the plant. As the leaf area increases further, leaves start shading one another, and net growth slows as older leaves in deep shade do not get enough sunlight and begin to die.

Optimum forage yields are achieved by harvesting plants when they reach a height or growth stage corresponding to a high point in the energy reserve cycle and harvesting them at a height which removes most of the forage but leaves an adequate stubble. The optimum post-harvest forage height depends on the forage species or mixture used. Table 1 gives the recommended pre- and post-harvest plant heights for different forage species and mixes which should optimize forage yield and quality. The timing of harvests will be affected by plant species and growing conditions. In warm spring weather (mean temperature about 600oF) orchardgrass-clover stands take about 3 weeks to grow to the recommended 8 to 10-inch height after grazing. Later in the summer when growth is slower, it will take 5 to 6 weeks or more under drought conditons. Alfalfa is ready for harvest when the plant is in the late bud to early flower stage of development. This requires between 5 and 6 weeks of growth depending on the temperature and the time of year.

Harvesting most of the accumulated growth increases yield. If too much stubble is left, on the assumption that if a little leaf area is good more is better, yield decreases. This is because of failure to harvest part of the previous growth. Extra leaf area does not necessarily result in an increase in growth rate. The leaves at the base of the plant are the oldest and are less efficient in photosynthesis. By removing these leaves and their stems, new, more efficient leaves can grow from the crown. If we fail to harvest these old leaves, they will soon die and be lost from harvested yield.

Management of mixed grass-legume stands requires a harvest which is optimum for the mix but may not be optimum for one of the species if grown alone. The optimum management for orchardgrass when grown alone under nitrogen fetilization, is to allow it to grow to an 8 to 10-inch height, then harvest the stand to a 4-inch stubble. This management maintains high energy reserves in the plant and enough residual leaf area for rapid regrowth. However, when grasses are grown alone, nitrogen fertilizer must be applied to obtain reasonable yields. By growing legumes like clover and alfalfa with orchardgrass, we can let the legumes fix nitrogen from the air and save money. If a mixed orchardgrass-clover stand is managed using an orchardgrass-nitrogen harvest strategy, we will loose the legume. With the 4-inch harvest height the orchardgrass is able to regrow quickly since it has an adequate leaf area and high energy reserves. At a 4-inch stubble height, all of the functional legume leaves are lost, and the legumes have to start new leaf growth at ground level in the shade of the orchardgrass. Without sunlight on the new leaves, the legumes must use more reserve energy before the leaves reach full sunlight. With white clover the leaves may never make it into full sunlight and the plants may die.

To maintain clover or alfalfa in a mixed stand, we need to harvest to a shorter stubble height to set the orchardgrass back and give the legume a competitive advantage. When harvesting orchard-grass at a 2-inch height, we will take off most of the orchardgrass leaves, resulting in a slower regrowth. At a 2-inch harvest height no additional legume leaf is lost compared to cutting at 4-inchs and the new legume leaves are in direct sunlight. With the orchardgrass growth slowed, the legumes have a better chance to compete for sunlight and moisture. If the legumes are expected to respond to this harvest management, they must have adequate soil pH, phosphorus, and potassium.

Grass-legume stand yield an average of what pure grass stands yield if fertilized with 150 lb. per acre of nitrogen (325 lb. of urea). By using the clover, a producer can save $30 to $40 per acre in annual production costs. Another advantage in using legumes with grasses is that animals eat more forage if legumes are in the stand. Moderate levels of legumes (25 to 40 percent) will increase yearling steer daily gains by 0.25 to 0.30 pounds and dairy cow milk production by 6 to 10 pounds per day over nitrogen-fertilized grass. These advantages emphasizes the importantance of knowing how management affects the balance of legumes and grasses in mixed stands.

Root growth determines the ability of a plant to take up nutrients and water. Root growth is determined by the plant's actively photosynthesizing leaf area since the roots depend on energy captured by the leaves. When energy is in short supply, it is used by plant tissue nearest to the site of photosynthesis. Therefore, roots receive energy only when more energy is produced by photosynthesis than is being used by top growth. The energy reserve cycle depicted in Figure 1 is very consistent under favorable growing conditions. However, under drought conditions the lack of water may reduce top growth while photosynthesis remains active. This results in the accumulation of carbohydrates at relatively low canopy heights. Forage stands that have growth retarded by drought can then have a vigorous regrowth once good rain is received. Another result of drought stress is that even though yields are reduced, forage quality at a given stage of development is increased. The effects of cool weather in the spring or fall are similar. Cool temperatures reduce plant growth rates more than photosynthesis. This allows for an increase in reserve carbohydrates in the plant and higher forage quality. In the fall this also allows plants to develop root reserves and continue root growth, as long as there is adequate photosynthesizing leaf area, even though there is little increase in top growth.

Plant development is the process of a plant changing from one growth stage to another. This could be the development of tillers on a grass plant or flower buds on a legume plant. Plant development is the major factor affecting forage quality (Table 2). As plants change from vegetative to reproductive stages, forage quality decreases. When in the reproductive stage, both grasses and legumes produce

Figure 1. Changes in plant height and carbohydrate energy reserves in a forage plant during regrowth.

stemmy growth. As a plant matures, it increases in fiber and decreases in digestibility, crude protein, and intake by livestock. The rate and timing of reproductive development is determined by species, day length (photoperiod), and temperature.

Most cool-season grasses require a short-day/long- night photoperiod with low temperatures during late autumn and early winter to produce reproductive (seed head) tiller buds. This process is called vernalization. These buds over winter, then begin growth in the spring when the air temperature reaches 40oF. With the long-day/short-night photo- period of spring, these reproductive tillers elongate, causing the plant to produce seed heads. The development stages that a grass plant goes through during the spring are vegetative, jointing, boot, heading, blooming, and seed development (Figure 2).

The tillers increase in number and size until the grass enters the jointing phase. However, tiller growth and shading cause small, nonflowering tillers to die. Also as the seed heads develop, they produce plant hormones which retard the development of new tiller buds at the base of the plant. This "apical dominance" prevents or reduces forage regrowth. If aftermath forage is needed, the manager should harvest hay or graze and clip pastures soon after the seed heads emerge. This reduces tiller death due to shading and reduces the length of time that new tillers are suppressed by apical dominance. Also, if seedhead removal is put off too long, forage regrowth will occur in midsummer when low soil moisture and heat can reduce growth. Plants clipped early have the opportunity to take advantage of the moisture and cool temperatures available in late spring and early summer.

Most cool-season grasses (orchardgrass, bluegrass, and tall fescue), require vernalization to develop reproductive tillers. Therefore, once the seed heads are removed they usually produce only vegetative growth for the remainder of the year. Exceptions to this rule are found in some of the ryegrasses, which produce many seed heads in the regrowth, and timothy which produces seed heads when there are favorable growing conditions.

The plant development process of jointing and seed head emergence, along with the cool, moist growing conditions in the spring, produces the most vigorous forage growth. Growth rates during the jointing to early heading stage will average 60 to 90 pounds dry matter per acre per day under grazing management and exceed these levels under hay management. Growth rates are then greatly reduced as the seed heads flower (shed pollen) and set seed. Seed maturation terminates the life of the reproductive tiller. During the summer new tiller development is sparse. Under favorable summer growing conditions forage dry matter growth rates average 20 to 40 lb./a/day for bluegrass and orchardgrass and up to 50 lb./a/day for alfalfa. With the cool, short days of the fall tiller development increases and the growth and development cycle begins again.

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Figure 2. Stages in the development of a cool-season grass. A-growing point in a reproductive tiller, B & C-boot, D-early heading, E-late heading, F-late bloom, G-vegetative regrowth.

Another aspect of plant development is how forage species grow in the aftermath. Species such as orchardgrass, bluegrass, bentgrass, tall fescue, white clover, and Empire type birdsfoot trefoil have growing points which remain near the soil surface. The leaves grow up but the growing point stays low, out of the reach of grazing livestock and mowers. These species are tolerant of close or untimely harvest and can survive under continuous grazing. Other species, such as bromegrass, timothy, reed canarygrass, quackgrass, alfalfa, red clover, alsike clover, and Viking type birdsfoot trefoil, have growing points which start at the soil surface but as growth continues rise above the ground where livestock or mowing machines can cut them off. These species are sensitive to close or untimely grazing and are best suited to rotational grazing or hay management.

Legume development is similar to that of the cool-season grasses. The stages of development in legumes such as alfalfa, red clover, alsike clover, and birdsfoot trefoil are vegetative, bud, flowering, and seed maturation. These developmental stages are controlled by day length and temperature. Reproductive growth occurs in legumes during all regrowth cycles.

Fall management has a major effect on the productivity and survival of forage crops. Fall is a time for root growth, tiller development, and carbohydrate storage needed by the plant for over- wintering and for spring growth. Fall management affects vigor of the stand the following year. Good tiller development in the fall depends on adequate energy reserves, leaf area, soil fertility, and soil moisture. If the harvest recommendations in Table 1 are followed throughout the year, the plants will be in good condition for overwintering.

Practical application of these plant growth and development principles can be summarized as follows:

1. Obtain high forage yields under rotational grazing and hay management by allowing plants adequate rest intervals between harvests and by harvesting plants close to utilize a high percentage of the growth but leaving adequate stubble for vigorous new leaf growth. Keep forage growth rate high under continuous grazing by maintaining adequate leaf area for good ground cover and light interception resulting in high levels of photosynthesis.

2. Produce high quality hay by harvesting the first cut when the grasses are in the late boot to early head stage of development. Harvest the second cut when the legumes are in the late bud to early flower stage of development. For high quality pasture, maintain forage in a vegetative growth stage and allow adequate forage to permit high levels of forage intake. In all cases maintaining a reasonable percentage of legumes in the forage crop increases animal performance since legumes increase forage intake.

3. Obtian good summer regrowth by grazing or mowing seed heads early in the season to stimulate the development and growth of new tillers. Also manage the forage for maximum root development to enable the plants to reach additional reserve water.

4. Control the growth rate of forages by managing plant energy reserves and leaf area. To maximize growth, allow plants to maintain or come back to a point of high energy reserves before harvest. To minimize regrowth, harvest the plants to remove most of the leaf area at a time when the plants are at the low point in energy reserves. This point is when the plant is about half regrown (Figure 1). This management will reduce the growth rate to about half its potential and is useful for setting back plants when using cattle to walk clover seeds into a pasture.

5. Spring growth will be affected by the energy reserves accumulated in the plant the previous fall.

Table 1. Plant height at which to harvest the regrowth of different species or mixes and the stubble height to leave for optimal forage production and quality. First cut grass-legume hay should be harvested at the grass early head stage. First cut legume hay should be harvested in late bud to early bloom stage of development.
Plant Height
Rotational Grazing or Haying

Forage Species or Mix

Pre-harvest

Post-harvest
Continuous
Grazing
Approximate Rest
Interval


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Weeks

Grass Legume Mixes
Bluegrass-clover 4-6 1/2-1 2-3 3-6
Orchardgrass-clover 8-10 2-3 4-5 3-6
Tall fescue-clover (E+)1 5-6 1-2 2-4 3-4
Tall fescue-clover (E-)1 8-10 2-3 4-5 3-6
Alfalfa - grass early bloom 2-3 not recommended 5-6

Nitrogen Fertilized Grasses
Bluegrass 4-6 1-2 3-4 3-6
Orchardgrass 8-10 4-5 4-6 3-6
Tall fescue (E+)1 5-6 1-2 2-4 3-4
Tall fescue (E-)1 8-10 4-5 4-6 -6
Bromegrass2 pre or late jointing 2-3 not recommended 5-6
Timothy2 pre or late jointing 2-3 not recommended 5-6
Reed canarygrass2 pre or late jointing 2-3 not recommended 5-6

1E+ - endophyte infected E- - endophyte free
2These species should not be grazed close during the early jointing stage of development.

 

Table 2. The effect of a plant developmental stage on the nutritive quality (as percent dry matter), relative dry matter intake, and relative yield of legume and grass forages.


Developmental stage

TDN1

ADF2

NDF3

CP4

RDMI5

Rel Yield6


Alfalfa and Red Clover
Late vegetative 63 29 38 20 1.02 .70
Early bloom 60 31 40 19 1.00 .90
Mid bloom 58 35 46 18 .94 .95
Full bloom 55 37 49 16 .91 1.00
Late bloom 52 39 52 14 .88 1.00
Mature 50 44 58 13 .83 .95


Orchardgrass and Tall Fescue
Early vegetative 72 31 55 18 .92 .40
Boot 71 32 57 17 .91 .65
Early bloom 65 34 61 15 .86 .85
Mid bloom 57 41 68 11 .78 .95
Late bloom 54 45 72 8 .74 1.00

1 TDN - total digestive nutrients
2 ADF - acid detergent fiber
3 NDF - neutral detergent fiber
4 CP - crude protein
5 RDMI - relative dry matter intake
6 Rel Yield - relative yield


Programs and activities offered by the West Virginia University Extension Service are available to all persons without regard to race, color, sex, disability, religion, age or national origin. Issued in furtherance of Cooperative Extension Work, Acts of Congress of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture. Rachel B. Tompkins, Director, Cooperative Extension Service, West Virginia University.