There has been little study on the growth and morphology of individual plants constituting the population of white clover in mixed swards under grazing. Such information is required if the mechanisms governing plant productivity and persistence are to be understood.Intact white clover plants were sampled from intensively sheep-grazed pastures under set stocking, rotational grazing, and a combination of both systems, by taking turves (250 x 250 mm), and washing out the plants, every month for a year. Characters measured for every stolon of each plant were: presence of a growing point; numbers of leaves, roots and axillary buds; stolon length. Total plant leaf and stolon dry weight were also recorded. Plants were classified according to degree of branching, and the contribution of each branching order to the population determined.There were strong seasonal variations in plant size (leaf and stolon dry weight, stolon length, and numbers of stolons and leaves per plant) which showed a significant decrease in spring with recovery over the following summer. This was paralleled by a rapid increase in the proportion of less branched plants (1st and 2nd branching order) in the population from 60 to 80% in spring, as higher-order plants broke up into smaller-and lower-ordered plants at this time. Numbers of roots per plant increased over winter to peak in early spring then declined in the following summer-autumn. While system of grazing management had no significant effect on branching structure of plants, it had a large effect on plant dry weight; rotationally grazed plants were 2-5 times larger than set stocked plants (0-182 cf. 0073 g respectively).Other general features of plant morphology were that each successive order of branch stolons was shorter and length before branching was less than that of their preceding parent stolon. The highest branching order observed was 6th order. There was no relationship between branching and numbers of roots; in branched plants only 55% of stolons were rooted regardless of plant order, but rooted stolons accounted for 85% of total stolon length and carried 62, 48 and 90% of the leaves, growing points and axillary buds per plant, respectively.Comparison with other studies suggests that the processes outlined in this report may be common to white clover growth under grazing over a wide range of favourable environments.
ThJ ~rowth and N-fixation capacities of pure stands of white clover (Trifolium repens L), suckling clover (T. dubium Sibth.), and Lotus pedunculatus Cav. were compared under high and low phosphate conditions in mown plots for 3 years.With high inputs of P, white clover yielded 10% more herbage DM and nitrogen than lotus, but with low P input lotus yielded 30% more than white clover. The main factor operating was the greater ability of lotus to take up P from the soil. Full development of an efficient N-fixing system took a year in lotus, much longer than for white clover. Suckling clover performed better than white clover at low P during its growing season.White clover and lotus accumulated twice as much soil N as suckling clover. Phosphate had no effect on soil N accumulation.The mean annual amount of N fixed (herbage + soil) at high P was 570, 590, and 265 kg/ha, and at low P was 400, 410, and 195 kg/ha for white clover, lotus, and suckling cI over respectively.
Understanding the grazing conditions under which plant populations are limited by seed availability (seed limitation) is important for devising management schemes that aim to manipulate the establishment of weed and forage species. Seeds of three weed species (Cirsium arvense, C. vulgare and Rumex obtusifolius) and five forage species (Lolium perenne, Lotus uliginosus syn. L. pedunculatus, Paspalum dilatatum, Plantago lanceolata and Trifolium repens) were broadcast sown into L. perenne-T. repens pastures in Manawatu, New Zealand and five sheep-grazing and two slug-grazing (with and without molluscicide) treatments were imposed in a split-plot design. Of the five sheep-grazing treatments, four compared continuous grazing with rotational grazing at intervals of 12, 24 and 36 d in spring, with all four grazed under a common rotation for the remainder of the year. The fifth treatment was continuous grazing all year. Seed sowing increased seedling emergence of C. vulgare, L. perenne, P. lanceolata, R. obtusifolius and T. repens under all sheep-and slug-grazing treatments, with differences in seedling densities persisting for at least 21 months. Seed sowing did not increase seedling densities of C. arvense, L. uliginosus or P. dilatatum. The effects of sheep-grazing management on seedling emergence and survival were uncoupled. For the five seed-limited species, seedling emergence was greater on pastures that were rotationally grazed during spring compared with those that were continuously grazed. However, seedling survival was lower in pastures grazed rotationally during summer, autumn and winter, so that after 21 months seedling numbers were greater on plots that were continuously grazed all year. Exclusion of slugs increased seedling recruitment of T. repens but had no impact on the other species. As weed and forage species responded in a similar way to sheep-grazing management (increased under continuous, decreased under rotational), it is unlikely that the goals of reducing weed invasions and enhancing forage species establishment could be carried out concurrently in established pastures with the same management.
Following germination, the ontogeny of white clover is characterized by two distinct morphological growth phases, a seminal taprooted stage followed by a clonal growth stage. Death of the seminal taproot and primary stolon initiates a process of fragmentation of the taprooted plant into a variable number of independent clonal fragments (plants) which comprise the initial population of the clonal growth stage. The objective of this study was to characterize the plant morphology of field-sown white clover populations from germination through to established clonal populations. Populations of eight white clover cultivars were assessed when sown with perennial ryegrass or tall fescue in pastures established under a common grazing regime for 16 months prior to imposition of continuous or rotational grazing treatments. One year from sowing, taprooted plants attained maximum size, with a mean plant branching order of 3n35, stolon DW of 460 mg and lateral spread of 250 mm, with some individuals having 6th order branching, 3n5 g stolon DW and 1m lateral spread. These taprooted plants were 4-5 times the size of plants in the subsequently formed clonal population. Nine months after sowing, the first individual taprooted plants fragmented into clonal plants. Twelve months from sowing, fragmentation processes were occurring at a linear rate, eliminating 6 % of the original taprooted population each month. This resulted in a 12-15 month transition period when the white clover population comprised both taprooted and clonal plants. During this transition period, the initial clonal fragments produced from taprooted plants were large, and this maintained a larger mean plant size in the clonal plant proportion of the transition population than measured in the later fully clonal population. This process was also considered to act to prevent the development of the expected differences between grazing managements, as it was not until the third year when all taprooted plants had disappeared that the clonal populations developed characteristics reflecting the expected influence of grazing management. Variation due to white clover cultivar and companion grasses was minor. The substantial differences in plant size and branching structure between taprooted and clonal populations has significant implications for the evaluation of breeding lines.
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