Roots function to anchor plants to the soil, as storage organs, and to absorb and conduct water and essential minerals from the soil. These processes vary depending on the water and minerals available in the plant and soil. The roots of grass plants are sent deeply into the soil before much emerges from the soil surface. This gives a foundation for growth. For example, 2-3 inches of root are developed before a shoot emerges. The root may reach 6 inches in depth within 2 weeks.
Two root systems exist for grasses: 1) the primary roots that develop from the embryo during seed germination, and 2) the adventitious roots that emerge from nodes of the crown and lateral stems.
The primary or seminal (seed) roots usually do not live beyond the first season following planting. Secondary or adventitious roots begin forming soon after the first leaf emerges. Adventitious roots may function throughout the life of the shoot, however, root longevity is governed in part by defoliation management as well as with seed production.
As with cool-season grasses, root growth of warm-season grasses resumes in the spring with the onset of climatic conditions favorable for the respective grasses. With wise management, warm-season species extend roots to somewhat greater depths in the soil profile than cool-season types due to greater tolerance to heat and drought.
Root growth occurs from expansion of newly developed cells resulting from division of meristematic cells located just behind the root cap. Maturation and differentiation of elongated cells result in the development of specialized tissues for transporting water and nutrients to other plant parts.
Water is absorbed by root hairs and diverted into the endodermal cells following an osmotic gradient into the conducting cells of the xylem. Nutrients absorption depends on energy from respiration. Grasses growing in severely compacted or waterlogged soils may have inadequate oxygen for respiration causing a restriction of transport within the root.
Grass species can be designated as perennial rooting or annual rooting, referring to how often a perennial plant's rooting system is replaced. The roots of Kentucky bluegrass remain for over a year (perennial rooting) while bentgrasses, bermudagrass, perennial ryegrass, and others generate new roots each year (annual rooting).
Reference: Oswalt, D.L., A.R. Bertrand, and M.R. Teel. Influence of Nitrogen Fertilization and Clipping on Grass Roots. Soil Sci. Soc. Amer. Proc. 23:No 3, 1959.
When a grass shoot with the potential to produce a flowering culm is defoliated during early-jointing, or later, in a manner which removes all of the above ground meristem, root growth is severely impeded. The duration of this effect varies not only with the effects of defoliating the flowering shoots, but also with the shoots not induced to flower. The "mother" root system of a shoot senesces in conjunction with seedhead development. Thus, aftermath shoots must develop a new root system. Timely grazing or clipping of pastures to prevent seedhead development triggers growth of aftermath shoots and subsequent root development prior to the onset of arid soil conditions. The above management practice reduces the severity of summer dormancy of smooth bromegrass.
The essence of good pasture management involves preservation of meristems until roots are well established. Meristems involved are the apical meristem (found in the shoot primordium) and the intercalary meristem (located in the collar in the base of the leaf blade). When a grass shoot enters the transition stage, where culm internode elongation raises the shoot growing point (primordium) to vulnerable height, a grass shoot might be defoliated in three different ways, each having a different impact on the roots.
1) Intensive defoliation destroys the ""elevated"" shoot primordium on a high percentage of flowering shoots. This effect is commonly referred to as "decapitation." With decapitation, the potential seed head and critical intercalary blade meristem have been destroyed. The rate of regrowth from new tillers arising from adventitious buds in crown tissue will vary with the degree of development of new shoot initials and the level of available food reserves. Roots will be deprived of energy from photosynthesis until the new shoots supply more energy than is being used for vegetative renewal.
2) Less intensive defoliation will safeguard the growing point of the shoots, however, due to sheath elongation, the more advanced leaves may be severed below the collar zone, i.e. through sheath tissue rather than through blade extension. Thus, as the culm continues to develop, only the flag leaf may remain. This effect is referenced as "denuding" since the culm is left essentially naked. Denuded shoots have minimal photosynthetic capacity. Stored energy reserves are also minimal as early vegetative growth demands essentially all of the carbohydrates produced via photsynthesis. Available energy sources will be used for culm and seed head development. Roots will be deprived of an energy source until new tillers produce excess sugars.
3) Defoliation at the commencement of internode elongation, commonly referred to in grazing management as the "early bite," imposes minimal risk to prompt recovery provided the shoot growing point is preserved and sufficient leaf blade tissue remains as part of the stubble. With the above precaution, recovery growth is largely independent of food reserves in crown tissue due to the prompt resumption of leaf blade development.
The meristematic growing point together with the intercalary meristem located in the collar zone of the developing blades, represent the currently active regrowth mechanisms. When preserved, these systems enusre competitive regrowth with minimal disruption of root development.
During the late fifties, an Australian livestock specialist summarized the results of a multi-year pasture study involving three cattle ranches. The weaning weight of the calves on one ranch consistently outweighed those from adjacent ranches by 70 to 100 pounds per head. The differences in weaning weights could not be explained by variability associated with breeds, dates of calving, soil types, forage species, etc. However, the rancher with the heavier calves practiced "topping" his pastures. This practice was the only variable identified that might explain the variation in weaning weights. Topping was explained as clipping the pasture with a rotary mower at early heading to prevent further flower stalk development. If flowering stems are not consumed before seed head emergence, they become woody and unpalatable. With timely clipping, while the grass is still palatable, livestock will readily consume the residues.
How does topping relate to the variation in weaning weight? Grass physiologists have shown that disruption of seed head development triggers production of new shoots (aftermath) from basal buds in crown tissue. Timely topping of pastures results in earlier aftermath production. The lush aftermath provided a higher quality ration than was available on the adjacent ranches.
In summary, the livestock gained a two-fold benefit from the above practice; they consumed flowering stems that would otherwise have been rejected, and they enjoyed a higher quality mid-summer ration comprised of lush aftermath shoots.