A S Laidlaw1, 2 J D Patterson1 and J A Withers2
1 Department of Applied Plant Science, Queen's University of Belfast and2 Applied Plant Science Division, Department of Agriculture for Northern Ireland, Newforge Lane, Belfast, Northern Ireland BT9 5PX
Introduction
Materials and methods
Results and discussion
References
Although it was originally considered that grass was aggressive towards white clover due to its shading effect on the leaves of clover, evidence suggests that this is not always the case as clover petioles can extend and place the laminae at the upper surface of the canopy (e.g. Woledge, 1988). However stolon branching may be influenced by canopy development due to change in light quality at the base of the canopy.
Two experiments were carried out to study the effect of different canopy structures on white clover development. The effect of constant and varying canopy heights on clover growth relative to that of grass was investigated in continuously grazed swards while a cutting experiment, simulating a silage regime, investigated the changes which took place in the contribution of clover to a developing canopy with and without application of N fertilizer, taking particular account of the relative contribution of leaves from branches on the main stolon axes as the canopy developed.
Grazing experiment. Steers were continuously stocked to produce target sward surface heights of 5, 7 and 9 cm (actual mean heights of 5.6, 7.4 and 9.4 cm achieved, respectively) from 14 April to 1 July (Phase 1) in a perennial ryegrass (cvs Frances, Morenne and Bastion)/timothy (cv. Motim)/white clover (cvs Aberystwyth S184 and Grasslands Huia) sward in its third year. From July to 30 September (Phase 2) swards were grazed to target heights of 7 or 9 cm (actual means of 7.7 and 8.9 cm achieved, respectively). Treatment plots were 0.5 ha in size, the treatments (target heights for Phase 1 and for Phase 2) being assigned randomly in a factorial design in two blocks i.e. 12 plots in total.
Tissue turnover rates were measured on 20 tillers and growing points per plot in a 1-week period commencing on 20 May while protected from grazing for the week. This was increased to 30 per plot for further 1-week periods of exclosure commencing 27 June, 28 July and 24 August. Population densities were measured at 2-monthly intervals and again in May in the following year.
Regrowth experiment. Twelve 5 × 2.5 m plots were laid down on a perennial ryegrass (cv. Condesa)/white clover (cv. Grasslands Huia) sward, which had been previously intermittently grazed then cut to 4 cm on 25 July. In each pair (block) of plots one received 100 kg N ha-1 as 'Nitro-chalk' (27.5% N). Approximately half of each plot was divided into 40 × 40 cm quadrat sites for future sampling.
The relative contribution of leaves on stolons to the canopy structure was assessed at different stages of development, namely 1 to 5 nodes (young branches), 6 to 12 nodes (advanced branches) and more than 12 nodes (main stolons). An inclined point quadrat (100 'hits' per plot) at 2-3 weekly intervals for 7 weeks was used in the half of the plot not subdivided into quadrat sites. This was related to the total LAI for each component. Only data from the first (31 July) and last (20 - 23 September) sampling times are presented.
While tiller population density was lowest in the tallest swards in each grazing phase (data not presented) stolon density was adversely affected only by the tall sward in autumn, the effect becoming more marked in the following spring possibly due to a slower turnover in stolon branch buds during winter in the shorter sward (Table 1).
Clover production per unit area was adversely affected by the low grazing height in the first phase, mainly due to the lower rate of production per stolon growing point (Table 1). Production per tiller was also lowest in the shortest sward in this phase but the higher tiller density compensated for it. In the second phase, clover production was not affected by sward height but tiller population density was lower in the taller sward. However, higher production per tiller more than compensated for the lower tiller density.
Table 1. Net daily growth (kg DM ha-1 d-1 during Phases 1 and 2 of grazing at different target sward heights and stolon population density (x103 m-2) in autumn sward height treatments in the following spring.
|
|
Sward height (cm) |
||||
|
5 |
7 |
9 |
SE signif |
||
|
Net growth |
|
|
|
|
|
|
Phase 1 |
Grass |
31.4 |
32.0 |
26.5 |
2.93 |
|
|
Clover |
25.0 |
44.8 |
44.3 |
4.43* |
|
Phase 2 |
Grass |
|
24.2 |
33.1 |
2.45* |
|
|
Clover |
|
38.5 |
46.8 |
8.20 |
|
Stolon density |
|
|
|
|
|
|
May (Year 2) |
|
1.79 |
1.40 |
0.062** |
|
Even within only one regrowth period there was evidence that N fertilizer reduced the contribution made by the leaves on young stolon branches to the clover canopy (Table 2). Indeed in both treatments the contribution made by these young branches was reduced from 22% of the total canopy at the beginning of the period to 12 and 2% in the 0N and 100N treatments, respectively, 7 weeks later. The likely cause of this reduction is a decrease in the rate of production of stolon branches and possibly death of some of the weaker branches as the canopy developed, similar to that found during regrowth in lucerne (Gosse et al., 1988).
Table 2. Contribution to clover LAI by leaves of stolons categorised according to stage of development, and LAI of total clover and grass.
|
|
Main axis |
Advanced branches |
Young branches |
Total clover |
Grass | |
|
31 July |
0.5 |
0.2 |
0.2 |
|
0.9 |
1.1 |
|
20 Sept. |
|
|
|
vx+1 |
|
|
|
0N |
2.3 |
0.6 |
0.4 |
1.17 |
3.3 |
2.2 |
|
100N |
1.7 |
0.5 |
0.05 |
1.03 |
2.3 |
4.6 |
|
SE |
0.18+ |
0.08 |
|
0.045+ |
0.30* |
0.41*** |
(+ p< 0.10)
By the end of experiment 2, the N-fertilized treatment had a mean LAI of 6.9 compared to 5.5 for the unfertilized treatment. A canopy of dense foliage reduces the red: far red ratio, inhibiting tiller bud expansion (Thompson, 1993) and low photosynthetically active radiation under such canopies will deny the young branches of clover the opportunity to develop as the smaller leaves on stolons in this category are less able to compete. Therefore low irradiance within the canopy would seem to be an issue in the adverse effect of N fertilizer on white clover development in mixed swards. The fact that the main stolons contributed less to clover LAI in the 100 N than the 0N treatment shows that the adverse effect of N is not restricted to the smaller branches. Other data from this experiment suggest that the high N treatment resulted in fewer older leaves remaining on these stolons.
Irrespecive of management, whether grazing or cutting, care should be taken to avoid excessive accumulation of the canopy otherwise the wellbeing of young branches which are the future main axes of the clover component may be jeopardised.
GOSSE, G., LEMAIRE, G., CHARTIER, M. AND BALFOURIER, F. (1988). Structure of a lucerne population (Medicago sativa L.) and dynamics of stem competition for light during regrowth. Journal of Applied Ecology, 25, 609-617.
THOMPSON, L. (1993). The influence of radiation environment around the node on morphogenesis and growth of white clover (Trifolium repens). Grass and Forage Science, 48, 271-278.
WOLEDGE, J. (1988).Competition between grass and clover in spring as affected by nitrogen fertilizer Annals of Applied Biology, 112, 175-186.
