Swards Of Subterranean Clover Research Articles

The interrelationship of solar radiation and leaf area index in determining the rate of dry matter production of swards of subterranean clover (Trifolium subterraneum L.)

The relationship of crop growth rate, leaf area index, and solar radiation has been examined for swards of subterranean clover, of the variety Bacchus Marsh. Leaf area index was varied by growing swards at eight densities of sowing, and radiation varied by two levels of shading, in addition to full daylight; the whole experiment was done three times—in summer, early winter, and spring—at the Waite Agricultural Research Institute, Adelaide. Crop growth rate was measured as the difference between the dry weights of the swards on two harvest occasions, and was calculated from smoothed values obtained by fitting an appropriate equation to the yield : density relationship. Thus a set of nine curves were available, each showing the response of growth rate to varying leaf area index at a specific level of radiation. These nine curves formed a consistent series, the growth rate rising with leaf area index until a maximal value was reached at the "optimum leaf area index", thereafter falling. At low levels of radiation, growth rate fell to zero at high leaf area indices. Both the maximal growth rate and the optimum leaf area index increased with radiation, and an analysis of the data indicated that these growth indices were dependent on radiation, the effect of temperature being negligible. A generalized series of curves was calculated by extrapolation, so that crop growth rates can be derived for any combination of leaf area index and radiation. A comparison of maximal crop growth rates and percentage utilization of light energy of subterranean clover and other crops suggests that this clover is relatively inefficient in integrating its environment.

Border and orientation effects and their elimination in experimental swards of subterranean clover (Trifolium subterraneum L.)

Two experiments were carried out in swards of subterranean clover, of the variety Bacchus Marsh, to determine the width of the border required firstly at the edge of swards and secondly between harvest areas. Peripheral growth was restricted by (a) root panels, (b) leaf panels, and (c) root and leaf panels; a fourth treatment with no lateral restrictions was also used. The results showed that in a sward growing without restrictions, a border at the edge of only 2 in. was required before homogeneous dry weights were reached. This border effect could be eliminated by using leaf- or leaf- and root-restricting panels, but not by root panels alone. The data for borders between harvest areas were more difficult to interpret, but growing swards without panels while allowing for a 3 in. border, both at the periphery and between defoliated areas, is recommended as the simplest procedure. From a consideration of marginal growth under the different peripheral restriction regimes it was concluded that water and nutrients were non-limiting, but that sward growth was limited by the amount of light energy intercepted.

The influence of defoliation on the components of seed yield in swards of subterranean clover (Trifolium subterranean L.)

Six defoliation treatments were imposed on two strains of subterranean clover (Dwalganup and Yarloop) grown in dense swards. Seed yield (weight per unit area) was increased by 27% over controls by July, or July plus early September, defoliations. With progressively later defoliation, yields fell to the control level, then to 15% below it. The number of seeds per unit area showed an increase of almost 40% with early defoliation, whereas with later defoliation the numbers fell to the control level. The increase was due primarily to larger numbers of inflorescences per unit area and a greater proportion of buried burrs. Supplementary data from grazed swards confirmed the promotive effect of early defoliation on seed production. The promotive and detrimental effects of defoliation are discussed, and brief consideration is given to practical implications.

The production and life span of seed of subterranean clover (Trifolium subterraneum L.)

At an elevation of 2040 ft on the southern tablelands of Kew South Wales, seed production by swards of subterranean clover was apparently governed by mininnnn temperatures (frequency and intensity of frosts) during the flowering period. The aggregate deficit of the daily minimum temperature below 40°F in the 21 days from the commencement of flowering showed a high correlation with seed production per unit area in four successive seasons. Spring frosts account for the altitudinal limit of about 4000 ft to which subterranean clover grows in the alps of south-eastern Australia. Lateness of flowering has specific survival value at high altitudes in this region and in such areas as the high parts of the plateau of Spain. On the average 92 per cent. of the seed crop germinated in the year following its production, with 6.3 per cent. in the second year, and with falling values to 0.07 per cent. in the fifth year. Germination thereafter was nil or negligible. The early-flowering variety Dwalganup appeared to show a genetic difference in the persistence of more of its seed into the second and subsequent years. There was a regular seasonal pattern of seed germination, with the peak of germination occurring in the late summer and autumn. The "carry over" of seed into the second and subsequent years, and the germination peak in the autumn, have each considerable ecological significance.

Competition between plants of different initial seed sizes in swards of subterranean clover (Trifolium subterraneum L.) with particular reference to leaf area and the light microclimate

A study was made of the development and fate of individual plants growing from contrasting seed sizes in swards of subterranean clover (var. Bacchus Marsh). Three sets of swards were grown in boxes, from large, small, and mixed (large and small) seeds respectively, all sown at the same number of plants per unit area; the swards were harvested on five occasions. Leaf area in each 2 cm layer of the sward was determined separately and measurements of the light intensity reaching each layer were obtained at each harvest. Counts of the plants on each occasion revealed a steady drop of about 30 per cent. over the whole period; in the "mixed" swards, only plants from small seed sizes died, the number of large-seeded plants remaining unchanged. The data of leaf area and numbers in each layer showed that leaf area is concentrated in a relatively shallow band at the top of the sward, the area attained by individual leaves increasing with height in the sward and with time. Leaves are not retained in the canopy at heights below that at which light extinction occurs. An unexpected finding was that, as the sward developed, the leaf area index (the ratio of leaf area to ground covered) required for the absorption of all incident light energy increases, and it is suggested that this is due to changes in leaf morphology. The importance of the position at which leaves of individual plants are held in the canopy is stressed; although at the first sampling occasion, the small-seeded plants had 25 per cent. of the leaf area of the mixed sward, they were so located as to absorb only 10 per cent. of the incident light energy; at the last sampling occasion, the values were 10 and 2 per cent. respectively. It is concluded that the disappearance of the small-seeded plants from the mixed swards was due to shading from the large-seeded plants.

The growth of swards of subterranean clover with particular reference to leaf area

An experiment was conducted to study the growth of subterranean clover (Trifolium subterraneum L.) sown at different densities; the control swards were not defoliated while others were subjected to a single defoliation at various dates. During the final month the rate of dry matter production (tops only) increased to a maximum when the leaf area index (the ratio of the area of the leaves to the area of the ground surface — L.A.I.) was about 4-5, falling by about 30 per cent. as the L.A.I. increased to 8.7. The rate of leaf production was greatest at about L.A.I. 4-5, falling to zero at L.A.I. 8.7. Climatic conditions during the growing season influenced the relationship of L.A.I. to growth; as conditions became more favorable the values of the optimum LA.1. for growth and of the ceiling L.,4.1. progressively rose. Irrespective of the density, all swards tended towards a common ceiling L.A.I. and yield by the end of the season. The effect of defoliation depended on the L.A.I. at which defoliation occurred, on the value to which the L.A.I. was reduced, and on current climatic conditions. If swards near the ceiling L.A.I. were defoliated, total dry matter production was slightly increased and there was a great increase in leaf production. On the other hand, defoliation of swards from about the optimum L.A.I. to very low L.A.I. values led to a substantial reduction in both dry matter and leaf production. It is suggested that all these effects depend on the light relationships within the sward and their influence on the balance of photosynthesis and respiration. Pasture at the optimum L.A.I. will give greater production than swards of lower or higher L.A.I.; defoliation can give greatly increased leaf production, unless L.A.I. is reduced to very low values.