Studies on persistence strategies of perennial pasture legumes in lowland pastures

Abstract

Legume persistence is a major problem in most pastoral areas of the world. The objective of this thesis was to obtain a general conceptual understanding of the most relevant factors and processes affecting perennial legume persistence under grazing. Three perennial legume species, birdsfoot trefoil (Lotus corniculatus, L.), Caucasian clover (Trifolium ambiguum Bieb.) and white clover (Trifolium repens L.), with contrasting strategies for persistence were chosen to investigate persistence processes in the field.

The effect of four grazing intervals, 2, 4, 6 or 8 weeks, on stand productivity, plant morphology and plant population dynamics of an established birdsfoot trefoil pasture was assessed over the period November 1994-March 1997 at Lincoln, Canterbury, New Zealand. The effect of three closing dates, November, December or January, on seed production and quality was also evaluated. The November and December closing resulted in the highest seed yields (65.1 and 60.5 g/m² respectively). Seed quality components were similar for November and December closing treatments and generally superior to those from the January closing date. The overall seasonal pattern of seedling recruitment was not affected by grazing interval. Seedling recruitment was highest during the autumn and high seedling losses were observed during winter and summer. The 4-week grazing interval treatment provided a suitable environment for seedling recruitment and gave the highest average seedling density and survival over the experiment. The soil seed pool had a lower viability, higher germinability and a lower proportion of hard seed than the original seed input. The 2-week grazing interval treatment caused a severe reduction in productivity, a progressive decline in the growth potential of the remaining plants and promoted weed invasion. Rotational grazing with regrowth periods of longer than 4 weeks are recommended to promote long-term persistence in established birdsfoot trefoil pastures.

A second experiment was conducted at Winchmore, Canterbury, New Zealand, during the period February 1994-March 1995. Measurements on this trial enabled the description of the population dynamics and persistence strategies of Caucasian clover and white clover in mixtures including perennial ryegrass (Latium perenne L.) and cocksfoot (Dactylis glomerata L.), both in dryland and irrigated pastures. The experimental plots were rotationally grazed with long rest periods (30-70 days) after severe defoliation (20mm post-grazing stubble height).

Number and weight of growing points in each grass/legume combination was measured in fixed quadrats prior to grazing. In the last sampling date below-ground biomass in each fixed quadrat was evaluated. Irrigation had a major positive effect on growing points weight and dynamics. Standing biomass and number and weight of growing points was significantly higher in the irrigated treatments than in dryland at most sampling dates. The number of Caucasian clover growing points tended to increase in all the treatments during the experiment. The irrigated mixtures including Caucasian clover gave the highest total biomass over the experiment (1568 and 2203 g DM/m² in the cocksfoot and perennial ryegrass treatments respectively). In the irrigated treatments Caucasian clover caused a marked reduction in the number and weight of perennial ryegrass tillers and tended to dominate, whereas cocksfoot showed compatibility with Caucasian clover. A significant increase in the number of legume and grass growing points was observed in the irrigated Caucasian clover/cocksfoot treatment during the experiment. In the dryland treatments the total number of growing points of Caucasian clover remained below 500 growing points/m² but they tended to increase gradually over the experiment.

The stoloniferous growth form of white clover enabled coexistence with perennial ryegrass. Conversely, cocksfoot tended to exclude white clover in all the treatments. Below-ground biomass was significantly higher in the irrigated treatments than in dryland. Total below-ground biomass was highest in the irrigated Caucasian clover treatments (8491.7 and 4699.3 g/m³). The grasses had the highest underground biomass due to massive, fibrous root systems. The effects of the grazing system on the pattern of vegetative spread of the legumes and on the grass/legume coexistence mechanisms is discussed. The results highlight the potential of Caucasian clover as a component of mixed pastures on high fertility sites.