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IV. Stages of Seedling Development
C. S. COOPER
Black (1956) reported emergence of subterranean clover seedlings from medium
and large seed 1 day earlier than seedlings from small seed when sown at a depth
of 3.2 cm. Jensen et al. (1972) reported that the emergence force of seedlings
from large-seeded alfalfa, red clover, alsike clover, and narrow leaf birdsfoot
trefoil was greater than that for small seeds. Williams (1956) found that differences in emergence force of seeds from several legume species was directly
related to seed size.
Depth of seeding is of major importance to emergence and establishment.
Stickler and Wassom (1963) obtained 53, 25, and 18% emergence of birdsfoot
trefoil from planting depths of 1.0, 2.5, and 3.8 cm, respectively. Moore (1943)
found best emergence of red, crimson, and alsike clover, white and yellow sweet
clover, alfalfa, and several Lespedeza spp. from a depth of 0.6 or 1.3 cm.
Seedlings from deeper plantings emerged more slowly and were less vigorous.
Erickson (1946) found that a 0.6-cm depth was most favorable for small alfalfa
seeds and that a 1.9-cm depth was best for large alfalfa seeds. He suggested a
1.3-cm depth as a desirable compromise. Peiffer et al. (1972) found that
‘Penngrift’ crown vetch had similar emergence from soil depths of 1.3, 1.9, and
2.5 cm but reduced emergence at 3.8 cm. Birdsfoot trefoil emergence decreased
at 2.5 cm and red clover and alfalfa at 3.8 cm.
For most forage legumes, seeding depth should be no greater than 1.3 cm.
Deeper planting may appear to be advantageous in order to place seeds in a
moist soil. However, deeper planting often results in weakened seedlings and
prolongs the period when seedlings are most susceptible to disease. Depth of
seeding may be important in terms of seedling competition. In mixtures, some
seeds will have an advantage over others at a given seed depth. Stapledon and
Wheeler (1948) concluded that optimum establishment of herbage seeds could
follow only from sowing the different fractions of a seed mixture at depths
suited to the individual seed size. Such a practice would be difficult with most
commercial seeding equipment.
During the heterotrophic stage both the amount and rate of germination may
be affected by osmotic concentration. Uhvits (1946) germinated alfalfa seeds in
substrates supplied with NaCl and mannitol at osmotic pressures ranging from 1
to 15 atmospheres. The rate and percentage of germination decreased as the
osmotic concentration increased. Germination was practically inhibited at 12 to
15 atmospheres of NaCl. Up to 9 atmospheres, however, alfalfa germination was
83% after 10 days of germination compared to 88% for tap water. Similar results
were obtained with a number of legumes germinated at a range of osmotic
concentrations by Young et al. (1970).
The legume seedling can assimilate and use externally supplied nutrients
at an early age. McWilliam e f al. (1970) reported an increase in weight of
legumes receiving nutrient solutions 5 days after imbibition. They reported
six- to tenfold increases in nitrogen content during the first 12 days of
LEGUME SEEDLING GROWTH
growth. Subterranean clover absorbed 32P as early as 4 days after imbibition
B. TRANSITIONAL STAGE
When the cotyledon emerges, the seedling derives energy from photosynthesis
as well as from stored reserves. The duration of the transitional stage of forage
legume seedling growth may be very short and is dependent upon the amount of
reserves left at emergence. The quantity of reserves remaining at emergence is
primanly affected by depth of planting (Black, 1955). Temperature affects rate
of reserve utilization, which in turn determines rate of growth (QuaUs and
Cooper, 1968). The role of the cotyledon as a storage organ ends with the
complete utilization of reserves. At t h s time, the major role of the cotyledon is
photosynthesis (Cooper and Fransen, 1974). McWilliam et al. (1970) detected
photosynthesis in subterranean clover on the third day after imbibition and on
the fourth day the compensation point between respired C 0 2 and accumulated
COz was reached. Black (1955) found that the percentage of cotyledonary
reserves remaining at emergence was dependent upon depth of seeding and
temperature. When seeded at a 1.3-cm depth at a temperature of 2I0C, 61%of
the reserves were present at emergence, but when seeded at a 5-cm depth and a
temperature of 28OC, only 33% were present at emergence. He states that
cotyledon reserves at emergence are not likely to limit seedling growth. In
sainfoin, however, cotyledonary reserves decreased at the same rate when seeds
were germinated in the dark or the light even though seedlings grown in light had
green cotyledons by the third day (Cooper and Fransen, 1974). In this species, it
appears that cotyledonary reserves are nearly completely utilized before photosynthesis begins. Stored energy in cotyledons of seedlings grown in darkness was
not sufficient to allow normal first leaf formation. In some crops, such as corn,
new leaf area formed must be able to supply energy at a rate equivalent to the
energy derived from endosperm if seedling growth is t o progress normally
(Cooper and MacDonald, 1970). Removal of part of the endosperm will decrease
During the transfer of cotyledonary reserves t o the seedling axis, total weight
of the seedling decreases due to weight losses from respiration until reserves are
utilized. Sainfoin lost 38% of its weight from imbibition until 9 days of age at a
temperature of 20°C (Cooper and Fransen, 1974). Greenhouse-germinated birdsfoot trefoil seedlings lost more weight when grown under low light intensity
than under high (Lin, 1963). The amount of dry matter lost varied from 25 to
45% among different varieties grown under 25% of greenhouse light intensity.
The efficiency of conversion of stored substrate to new growth, calculated by
dividing new growth by weight of dry matter lost from cotyledons of endo-
C. S. COOPER
sperm, varies among crops. Conversion efficiencies of 56% (Cooper and Fransen,
1974), 65% (Cooper and MacDonald, 1970), and 57% (Cooper, unpublished
data) have been reported for sainfoin, corn, and barley, respectively, when
grown in the dark.
Cotyledons have not been extensively studied in forage legumes. In other
species, epigeal cotyledons have been shown to vary in size and shape and in the
length of time in which they become functional (Love11 and Moore, 1971). Some
plants such as cucumber (Cucumis sativus L.) develop leaflike structures with
high photosynthetic rate. Others such as Phaseolus spp. have thick cotyledons
which senesce rapidly following utilization of reserves. Forage legume cotyledons are between these two extremes, but they differ in size and thickness and
probably in rate of senescence among species. In alfalfa, birdsfoot trefoil, and
sainfoin, each milligram of seed weight produced a cotyledonary area of 14.0,
10.1, and 3.5 mm, respectively (Lin, 1963). In studies with birdsfoot trefoil
(Lin, 1963), subterranean clover (Black, 1957), and sainfoin (Fransen and
Cooper, 1976), however, the ratio of cotyledonary area to seed size within a
species appears nearly constant.
Within a species, seed size is the major factor affecting the size of the
cotyledon. The initial growth of legumes in a newly seeded field is dependent
upon the amount of cotyledonary area present per unit of land surface. The
quantity of cotyledonary area in turn is related to the number of seedlings per
unit of land area and to the size of seed. The contribution of the cotyledon in
total seedling photosynthesis decreases as total leaf area increases (Cooper and
Differences in cotyledonary senescence which might occur among different
forage species have not been studied. Opik and Simon (1963, 1966) state that
cotyledon senescence begins as food reserves are utilized. At this time, cotyledons have a maximum water content, but dry weight and respiration rate have
declined rapidly. Chloroplasts begin breaking down, leaving the nucleus with an
irregular membrane. Cotyledons turn different hues of yellow owing to breakdown of chlorophyll, allowing the carotenoids to become visible. Finally, an
abscission layer forms between cotyledons and the remaining plant, and cotyledons are shed.
C. AUTOTROPHIC STAGE
Upon exhaustion of food reserves within the cotyledon, the legume seedling
becomes a true autotroph. Its ability to establish itself and to compete with
weeds and other crop species is dependent upon inherent seedling vigor and the
effects of environment.
Some species such as cicer milk vetch, crown vetch, birdsfoot trefoil, and
LEGUME SEEDLING GROWTH
sericia lespedeza, are slow to become established and often have thin unproductive stands because of competition from companion crops or weeds (Hensen and
Tayman, 1961). Numerous studies have been reported relative to legume seedling response to light. Cooper (1966, 1967) using growth analysis techniques,
studied growth responses of alfalfa and birdsfoot trefoil when grown under low
light intensity in growth chambers and when grown under various levels of
shading in the field. Under light intensities of 21 and 86 Klux in growth
chambers, or 8 t o 100% of full sunlight in the field, birdsfoot trefoil had a higher
relative growth rate than alfalfa. Thus, the poorer seedling vigor of birdsfoot
trefoil was not due to differences in the effect of decreased light intensity on
relative growth rate. He states that “although shading tolerance of birdsfoot
trefoil and alfalfa may be similar, birdsfoot trefoil is more susceptible to being
shaded, both in the seedling stage and later. Seedlings and mature plants of
birdsfoot trefoil begin growth later in the spring and recover more slowly after
clipping. These factors increase the likelihood of birdsfoot trefoil becoming
shaded by associated species.” Gist and Mott (1958) reported that growth
response of birdsfoot trefoil to low light was similar to alfalfa and red clover, but
seedling growth was always less. Cooper (1966) found that the ability of alfalfa
to outyield birdsfoot trefoil in the seedling stage is due entirely to initial seed
size or, in terms of Watson’s (1952) definition of growth dependency, to “initial
Shading or low light intensity affects distribution of dry matter into tops and
roots. Cooper (1966) reported that less dry matter is partitioned into seedling
roots with decreasing light intensity. As a result, shaded seedlings may become
more susceptible to drought because of restricted root development (Cooper,
1966; Shirley, 1945). Cooper and Ferguson (1964) reported that rooting depth
of birdsfoot trefoil was only 20 cm and alfalfa only 40 cm when the barley
companion crop with which they were grown was harvested. At the same time,
roots of both species had penetrated to a depth greater than 61 cm when grown
without a companion crop. McKee (1962) found that birdsfoot trefoil shaded
for 5 weeks required 6 to 11 weeks of growth in full daylight t o restore its
original top/root ratio. He also found that the leaf area per plant of “Pennscott”
red clover often increased with shading.
Pritchett and Nelson (1951) reported that one of the most striking effects of
reduced light intensity on alfalfa was the proportional decrease in nodulation.
They found that nodulation ceases at less than 2.6 Klux and felt that this may
contribute to loss of seedlings in the field. McKee (1962) reported that “Vernal”
alfalfa and “Empire” and “Viking” birdsfoot trefoil required at least 25% of
daylight to be functionally nodulated and 50% to be adequately nodulated. In
contrast, functional nodules of Pennscott red clover were observed at 12.5% of
Temperature has a marked effect on metabolic processes and consequently
C. S. COOPER
affects the growth of the legume seedling. Richards et al. (1952) state "Such
physiological phenomena as cytoplasmic streaming, bio-electrical potential, synthesis of organic materials, translocation and respiration are all influenced by
Temperature affects nutrient absorption. Potassium (K), nitrate, and bromide
ion uptake increases with temperature increases from 6" to 30°C (Hoagland and
Broyer, 1936). Temperature coefficients are higher for anion absorption than for
cation absorption (Wanner, 1948). In alfalfa, K increased in tops and decreased
in roots with increasing temperature, but Mg and Ca content of both tops and
roots decreased with increasing temperature. Potassium content of soybeans
increased with increasing temperatures of 12', 22", and 32"C, while divalent
cation content decreased (Wallace, 1957).
Each legume species has an optimum growth temperature (Stapledon and
Wheeler, 1948). When temperatures exceed this optimum, roothop ratios are
reduced, and when temperatures are less than optimum, root/top ratios are
increased (Sprague, 1943).
Smoliak et al. (1972) germinated and grew alfalfa, cicer milk vetch, and
sainfoin with soil temperatures controlled at 7", 13", and 27°C for a 28-day
period. Cicer milk vetch failed to emerge at 7°C and emerged and grew slowly at
18°C. Both alfalfa and sainfoin emerged and grew at 7°C. Alfalfa and cicer milk
vetch developed best at 27°C but sainfoin grew equally well at 18" or 27°C. Late
seeding at warmer temperatures seems to favor cicer milk vetch establishment,
and sainfoin can evidently be established early in the spring with cool temperatures.
McKell et al. (1962) studied growth response and phosphorus (P) utilization of
four native and four introduced legumes at 10.Oo, 15.5", and 21.1"C. Root and
top growth and P content increased with increasing temperature. Trifolium
subterraneum and T. incamaturn grew better at 15.6"C than did the other six
species. Four species, T. tridentatum, T. subterraneum, T. incamaturn, and
Medicago hispida responded to P at 15.6" and 21.1"C better than T. variegatum,
T. hirtum, T. microcephalum, and Lotus purshianus. Stimulation of root growth
with P fertilization during cool winter months was suggested as a major factor in
the subsequent production of these legumes. Many winter annuals of the genus
Wfolium germinate rapidly at temperatures of 20°C or below but have limited
germination above 30" to 35°C.
The temperature environment of seedling legumes is largely dependent upon
the climate of the area in which they are seeded. Temperature during the early
stages of growth may be controlled to some degree by time of seeding, although
extremes in temperature variation from year to year are likely to be the rule
rather than the exception. The presence or absence of competition will also
affect temperature of the microenvironment. Cooper and Ferguson (1964)
LEGUME SEEDLING GROWTH
found soil temperatures 1.6” to 4.4”C lower under a barley companion crop at
soil depths of 7.6 to 61 cm. The presence of an overstory species such as a
companion crop or weeds reduces the insolation intensity and influences the
wavelength of radiation reaching the soil surface. Geiger (1950) reported that
the insolation intensity of the soil surface under plants 1 meter high was only
one-fifth that at the surface of bare ground. Reflectivity was 8-20% for visible
light but rose to 45% for infrared (Geiger, 1950). Thus, both intensity and
quality of light reaching the soil surface differ under plant cover and may
influence soil and air temperatures near the soil surface. Geiger (1950) points
out that the intensity of incoming radiation upon a growing crop and bare soil is
the same during the day. Likewise, the intensity of outgoing radiation at night is
equal. The effect of the plant cover is on the distribution of heat gained or lost,
Dense stands of plants result in cooler air temperatures near the ground during
the day. At night, however, outgoing radiation is from the top surface of the
vegetation and the air is consistently warmer near the ground. In contrast, in the
absence of an overstory species, radiation is from the soil surface, resulting in a
cooler layer of air near the ground. As a result, legumes seeded with a companion crop have lower growth temperatures during the day but warmer growth
temperatures during the night than legumes seeded in pure stands.
Competition for the young legume seedling is generally from weeds or companion crops. Legume species differ in their abilities to compete, primarily as a
result of differences in growth rate. These differences may affect the vegetative
composition of those mixtures which contain more than one legume, since those
legumes which compete best may have a greater survival rate under the stress of
competition. Blaser er al. (1956) ranked aggressiveness of some common forage
legumes as follows:
Alfalfa (“Kansas common”)
Red clover (“Kenland” or “Northern Neck”)
White clover (Ladino)
Birdsfoot trefoil (“Granger” or “Italian”)
White clover (S-100, Virginia, and Lousiana)
Birdsfoot trefoil (“Empire”)
From that ranking, it is evident that the degree of aggressivenessvaries within a
species as well as among species.
C. S. COOPER
Improvement of Legume Seedling Vigor
Seedling vigor of legumes is proportional to seed size. However, cultivars of a
species with the same seed size may differ in vigor. Thus, in breeding for seedling
vigor, both seed size and inherent seedling vigor must be considered. Birdsfoot
trefoil has received much attention in breeding programs for seedling vigor,
mostly because of difficulties encountered in establishing this species. Draper
and Wilsie (1965) in three cycles of recurrent selection increased seed size of
“Viking” birdsfoot trefoil 30% per cycle and of “Empire” 6% per cycle.
Twamley (1974) increased seedling vigor of “Leo” birdsfoot trefoil 35 to 40% in
three cycles of selection. In his program, selection was made for large-seeded
plants and then for the most vigorous seedlings from seed of those plants.
VI. Seedbed Preparation
The ideal seedbed for forage legumes should be weed-free and firm enough
that a man walking across it does not leave a footprint deeper than 0.3 cm, and
should have enough loose surface soil to cover seed to a depth of 0.6 to 1.2 cm
deep (Cooper et al., 1973). Any tillage equipment that will provide this type of
seedbed will be satisfactory.
The moldboard plow turns under crop residue and makes the soil easier to
prepare for seeding. It prepares a loose seedbed 10-25 cm deep, which requires
additional treatment before it is ready for seeding. The major advantage of the
moldboard is that it destroys existing vegetation and buries seeds of grassy weeds
such as cheatgrass (Bromus tectorum L.) and foxtail (Hordeumjubacum L.) deep
enough to prevent their germination and reestablishment. Disadvantages are the
loose seedbed and a relatively high cost per hectare.
Disk plows are not as effective as the moldboard in eliminating existing
vegetation but are more effective under drier conditions in handling residue and
shrubby growth. The one-way disk plow effectively covers heavy residue and
controls weeds but leaves the soil loose. An offset disk is more effective than the
one-way disk in breaking down sod and large clods and in killing small weeds.
Following disking, harrowing will smooth the seedbed and firm the soil, although not to the degree needed for seeding. Spike harrows smooth the soil but
leave it subject to wind erosion. Spring tooth harrows leave the seedbed less
subject to blowing because they leave more clods on the surface.
Most seedbeds require rolling or cultipacking to acquire the degree of firmness
needed for the seeding operation. In the arid regions of the West, cultipacking is
essential for obtaining a good firm seedbed. In the more humid areas of the
Midwest and East, cultipacking during the spring is less necessary and may lead
LEGUME SEEDLING GROWTH
to excessive soil compaction (Tesar and Jackobs, 1972). Firming is best accomplished by rolling with a heavy roller or by cultipacking. If the needed equipment is not available for this, a weighted spike harrow or an irrigation float or
leveler will compact the soil before seeding.
VII. Seeding Forage Legumes
A. SEED TREATMENT
Before seeding, legume seed should be inoculated with symbiotic bacteria
(Rhizobium spp .).
The symbiotic bacteria are specific for many legumes such as birdsfoot trefoil
and sainfoin but in some cases bacteria will cross-inoculate with several species.
Inoculation is essential when a legume is seeded in an area for the first time, For
successful legume inoculation the following procedures should be adhered to:
1. Select the proper inoculant for the legume t o be grown.
2. Store the commercial culture in a cool, dark place until it will be used.
3. Plant seed within 48 hours after inoculation, or reinoculate.
4. Inoculate in all cases of doubt, and always inoculate on new land.
Small amounts of seed and inoculant may be mixed in a tub or bucket. Larger
amounts may be mixed in a small concrete mixer or by hand on a cement floor
or on the bottom of a truck bed. Addition of sticking agents such as milk or
diluted syrup will help inoculant adhere to seed, Most companies that sell
inoculant also sell sticking agents.
B. CALCULATION OF SEEDING RATES
Seeding rates are recommended to provide a given number of viable seeds per
linear meter of drill row. The percentage of viable seed in a seed lot is calculated
by multiplying germination percentage by purity percentage and dividing by
100. The value obtained is called pure live seed index (PLS). Thus if a seed lot
has a percentage germination and purity of 90 and 85, respectively, the PLS is
90 X SS/lOO = 76.5%. For legumes, the percentage of hard seed is added to the
germination percentage before multiplying it by the purity percentage.
The number of seeds planted per meter of row is dependent upon the number
of seeds per kilogram, the kilograms of seed planted per hectare (ha) and row
spacing. It may be computed as follows:
seed per meter of
= Number of PLS/kg X planting rate in Kg PLS/ha
row m in ha at width to be planted