Timothy J. Johnson
INTRODUCTION
In 1992, Iowa had 1.55 million acres of alfalfa and alfalfa mixed pastures worth $512 million. Iowa ranked seventh in the nation, ahead of Minnesota and Wisconsin, in production of hay baled with 6,615 tons (USDA, 1993). When put into monetary terms, the pastures represent an investment. Whether it is a long or short term investment can rely upon the management used by the producer.
It is not enough to choose the right varieties and planting rates for the best use of pastures. The pasture should endure for as long as possible. In 1989, Marten et al., defined the "persistence" of forage legumes as a function of location, animal nutritional needs, economics, production systems, ecological situation, pest problems, climate, soil and feeding alternatives.
While not covering the entire scope of a farming system this paper will deal with the regrowth following grazing of four legume pasture crops commonly grown in Iowa.
Grazing
The use of grazing as an alternative to feedlot animal rearing is being revisited by producers. A 1992 study from Canada showed a $102 advantage per animal in favor of grazing when comparing the two methods. But, doubts still persist on the ability of pastures to provide high quality forage and consistent yield within and between years (Parker et al., 1992).
A higher level of management is needed to make the best use of pastures. Pastures can be either overgrazed or undergrazed, and the botanical species of the pasture may be effected by the intensity of grazing. The choice of grazing scheme might be as important as the plants in the pasture. Lack of proper grazing management is responsible for the ruining of pastures more than any other single cause (Rhoweder et al., 1973).
Pressure Applied by Different Animal Species
The botanical makeup of a legume-grass pasture can be greatly influenced by the animals grazing it. Sheep tend to graze broadleaf herbage or legumes more than do cattle. An example of dietary preference between cattle and sheep showed a higher rate of decline in a subclover component when sheep grazed alone or in a 2:1 ratio with cattle in a subclover-grass pasture (Bedell, 1973). Cattle selected the grass and the sheep selected the subclover while it was plentiful. This is not a problem with sub clover alone; "...the selectivity of a species by grazing livestock is modified by plant maturity, sward height, density and leafiness" (Hoveland, 1989). Another study indicated that proportion of legume in the diet is a function of legume species as well as availability in the sward. Cattle consumed red clover in quantities equal to either alfalfa or birdsfoot trefoil though it had lower availability in the pasture (Forwood et al., 1989).
Animals also physically manipulate plants differently while feeding; "Cattle grasp at grass with their tongues and pull it back into their mouths... Horses grip the herbage with their lips and cut off more cleanly and closer to the ground than cattle...Sheep can cut off grass at soil level, taking away the part of the plant from which tillers emerge....Uncontrolled sheep grazing is often the cause of serious damage to pastures..." (Voisin, 1960). The forage plant may be damaged by the act of feeding.
The most important impacts animals have on the persistence of legumes can be summarized in these factors: treading, selective grazing, nutrient return, seed dispersal from feces, and the frequency/intensity of grazing (Forde et al., 1989).
ALFALFA (Medicago sativa L.)
Alfalfa forms a deep taproot system and woody crown like red clover and birdsfoot trefoil. The crown is formed by branching at the base of the plant, starting from buds in the axils of the cotyledons and first leaves. Low axillary buds on these stems produce more basal shoots, forming a branched crown of short perennial stems just above the ground which is main source of regrowth after defoliation. From 5-20 stems may grow from the crown during the growing year as the older stems mature and are harvested. The bulk of alfalfa leaves and growing points are borne on upright stems and it is vulnerable to excessive or badly timeddefoliation. Continuous grazing will reduce yield and vigor of the plant (Forde et al., 1989).
Erect growing species, like alfalfa with most of the leaf area high on the plant, are almost totally dependent on stored foods for recovery when almost all or most of the photosynthetic area is removed (Blaser et al., 1973).
Alfalfa will produce three distinct growths in the year if left uncut, all three initiating from the crown (Smith, 1962). Active extension growth of new shoots at the base of the plant also occurs after cutting or when the previous crop of shoots has reached a certain age of maturity, usually the production of flower buds (Forde et al., 1989).
Grazed alfalfa will start to regrow more quickly than mowed plants. Grazing at an early growth stage possibly hinders development of apical dominance and allows lateral branches to begin rapid growth (Allen et al., 1978).
Alfalfa genotypes with spreading crowns are probably more suitable than erect genotypes for prolonged grazing (Leach and Clements, 1984). Short-growing species are not totally defoliated with close cutting or grazing because the leaf area is near the ground. The plant is not as dependent on stored foods for the initiation of recovery growth. A part of the energy comes from intact leaves (Blaser et al., 1973).
Root Storage of Carbohydrates The principle storage organ of alfalfa is the root. Alfalfa is known to follow a cyclic pattern of root carbohydrate storage and use (Smith et al., 1989). Starches are stored to start growth in the spring and after each cutting (Blaser et al., 1973). Defoliation is followed by rapid regrowth and root reserves are slowly built up. Frequent defoliation results in depletion of root reserves and eventually death (Forde et al., 1989).
Alfalfa seldom remains an effective pasture component after about 3 years, but no single explanation for this is evident (Leach and Clements, 1984). Rests of only 3 weeks between grazing decreased alfalfa productivity compared with rests 2-3 times as long. Alfalfa also performed poorly in experiments where rest periods were short (Leach and Clements, 1984). Alfalfa should be harvested closely but not frequently (Hanson and Barnes, 1973).
WHITE CLOVER (Trifolium
repens L.)
White clover plants form rooted stolons that can persist after the original plant has died (Forde et al., 1989). The primary stolons arise from axillary buds of the primary stem and extend radially. The growth is apical and indeterminate. An axillary bud of a stolon may develop into either a flower, a branch stolon or it may remain dormant. Branch stolons are like primary stolons structurally and functionally (Hay et al., 1991).
Stolon branching increases a plants capacity to produce leaves and provides renewal of ground coverage near the primary axis. Flowering reduces stolon branching and leaf production (Gibson, 1985). Growth of a mature plant involves production of new nodes at stolon apices, death of nodes at the basal end of the primary stolon and birth and death of apices(branches) which form from the axillary buds at the nodes (Hay et al., 1991). The primary stem and root usually die before or during the second year and future growth depends on the stolons and their adventitious roots (Gibson, 1985).
Management greatly effects the longevity of clover stands. During the productive season, the pasture should be grazed to a height of about 5 cm at intervals of 15-30 days. Rapid regrowth of clover depends on the amount of functional leaf left after grazing. Therefore, forage should not be grazed shorter than 5 cm (Gibson, 1985). When stolon density (number of stolons intersecting a linear meter) was determined in November after a year of different grazing management, the average stolon density of rotationally stocked white clover was 10 times greater (10 stolons/m vs 1 stolons/m) than continuously stocked white clover (Brink and Pederson, 1993). Stolons were longer and heavier per m2 when white clover was rested for 35 days compared to 14 days following grazing by sheep (Sheldrick et al., 1993). Despite varying effects of grazing method on growth and morphology, stolon survival of white clover is shown to be greater under rotational grazing (Brink and Pederson, 1993).
The phenotypic plasticity of the white clover plant is well known. It has been shown that heavy grazing of indigenous populations resulted in small-leaved prostrate forms of white clover (Frame and Newbold, 1986). Paralleling reduction in leaf size is a reduction in stolon size, with the whole plant decreasing in size so as to maintain itself (Brock et al., 1988).
White clover utilizes current photosynthesis for regrowth assimilates. When intermittently defoliated (hay harvest or intensive rotational grazing) the plants may become completely defoliated. Then stolon reserves play an important role in leaf regeneration. Frequent defoliation shifts the assimilate dependence to current photosynthesis from the remaining leaves, with assimilates being largely utilized for new growth.
Severe grazing by sheep of stolons and therefore removal of terminal apices gives stolons less chance to assimilate material and spreading may be curtailed (Frame and Newbold, 1986). Root initiation and elongation vary with the season of the year as a direct response to weather conditions and photoperiod (Kendall and Stringer, 1985). Root growth during cool, moist periods is proportionately greater than top growth (Gibson, 1985).
RED CLOVER (Trifolium pratense
L.)
All red clovers can be grouped into three divisions: early flowering, late flowering and wild red. Most American red clovers are the early flowering type, known collectively as medium red clover (Taylor, 1973). Red clover forms a taproot and woody crown like alfalfa. Like alfalfa, it will produce three distinct growths if left uncut (Smith, 1962). The red clover primary shoot forms a rosette of leaves with the growing point at ground level. Growth of axillary buds from the cotyledons, primary leaves and early branches forms a crown at the top of a branched taproot system which can penetrate to 3 meters in favorable conditions. Adventitious roots from the crown region often form the major part of the root system in 3-4 year old plants (Forde et al., 1989).
Early-flowering strains produce relatively few stems at the first flowering, but a succession of flowering stems is produced throughout the season and rapidly regrown after cutting, leaving few dormant buds to continue growth in the following year. Late-flowering varieties produce a single heavy flush of flowering shoots and are slow to recover from cutting. This leaves many dormant buds to continue growth for next season (Forde et al.,1989).
Red clover depends on reserve assimilates for recovery growth, perhaps associated with suppressed new lateral bud development during growth. It appears that shoot initiation in red clover occurs in waves resulting when established shoots suppress the further initiation of shoots, as in alfalfa. If the plant is severely defoliated, growth zones of existing shoots are removed as well as all of the leaf area (Kendall and Stringer, 1985).
Total nonstructural carbohydrate (TNC) levels in the roots decline at the beginning of regrowth but increase again prior to flowering. Seasonal changes of TNC levels are typical of perennial legumes. The maximum concentration of starch in the roots occurs at full bloom. Root TNC levels also decline during seed settings. During regrowth, the increase of root yield parallels the accumulation of herbage yield. Defoliation causes a rapid reduction in root elongation which is followed by a gradual recovery (Bowely et al., 1984). As with alfalfa, late defoliation in cold regions may result in premature death because of depletion of root reserves (Forde et al.,1989).
Birdsfoot trefoil has a well developed taproot, not quite as long as alfalfa but with a more thick, fibrous root system in the upper soil (Forde et al., 1989). Mature plants have many well-branched stems arising from a single crown. Under favorable growing conditions the main stem can attain a length of 60-90 cm.
Stems are generally smaller in diameter and less rigid than those of alfalfa (Seaney, 1973). Prostrate forms produce stems from crown buds below ground level and grow for several inches as short rhizomes before emerging, but these do not generally root. The stems are indeterminate and bear flowers in stalked clusters towards the tips (Forde et al., 1989).
When left uncut birdsfoot trefoil will produce only one growth from the crowns, the growth being initiated in the early spring. The crowns are generally inactive during the growing season. After the primary stems have ceased to elongate, new growth is produced and continued from upper axillary buds on the old stems (Smith 1962). During the summer, birdsfoot trefoil regrows almost exclusively from axillary buds (Greub and Wedin, 1971). Regrowth arises from the stubble buds, and root reserves are maintained at a low level until autumn storage occurs. The remaining stubble must carry green leaves to support the regrowth of the upper axillary buds (Forde et al.,1989).
The root reserves of birdsfoot trefoil decrease during early spring and remain at low levels until restored in the fall months (Smith, 1962). Unlike alfalfa or red clover at bloom, a tall stubble needs to be left when birdsfoot trefoil is cut or grazed so that green leaves are available to furnish energy needed for regrowth since little energy is available from the roots (Seaney, 1973).
Frequent removal of forage by grazing or mowing may tend to inhibit rooting of prostrate stems by lowering carbohydrate reserves or by preventing development of woody stems (Waasom and Barnett, 1971).
These four legumes are examples of two different growth types. They are plants that form a deep taproot and woody crown (alfalfa, red clover and birdsfoot trefoil), and plants that form rooted stolons which can persist after the original plant has died (white clover).
Persistence of the original plant is most successful by the tap-rooted crown formers because they monopolize environmental resources by their large, long lived structures (Sheaffer, 1989). These species will tolerate considerable drought and cold because of the taproot depth and the protection of the partially buried crown. They also show good regrowth characteristics with top growth suppression of lateral buds (Forde et al., 1989). However, where growing points are elevated and exposed to defoliation, caution must be taken to not overgraze or cut too often (Smith, 1962; Chatterton et al., 1977; Forde et al., 1989). Plants of this type, particularly alfalfa, are spot bound and cannot compete well against more competitive species in a mixed pasture sward (Jung, 1982; Smith et al.,1992).
White clover is known for its "guerilla" regrowth. This is a typical description of clone formers whether they spread by rhizomes or stolons (Forde et al., 1989). In rotational swards, white clover may be more competitive with grass than the other legumes described in this paper since species of guerilla habit are aggressive competitors at low densities but are suppressed at high densities of associated grass species (Leffel et al., 1973; Hay et al., 1989).
As with other resources in agriculture, pasture legumes must be managed with the future in mind. Respondents to a survey from Indiana, Iowa, Nebraska and Wisconsin predicted that legume use in pastures would increase in the next 5 to 10 years. Two of the reasons given for the increase included greater emphasis on sustainable agriculture and an increasing awareness by producers of the benefits of legumes (Matches, 1989). But, the potential benefits of growing legumes will not be realized unless the plants are managed for persistence.
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