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Physiological and Morphological Responses of Tall Fescue and Perennial Ryegrass to Leaf Defoliation

P.D. Kemp, H. Tavakoli and J. Hodgson

Institute of Natural Resources, Massey University, Palmerston North, New Zealand

ABSTRACT

Swards of tall fescue are not as persistent and productive as swards of perennial ryegrass when continuously stocked to maintain a surface height of 30 – 40 mm. The morphological and physiological responses of individual plants of tall fescue and perennial ryegrass to repeated defoliation to maintain none, one, two or three of the older fully expanded laminae on each tiller were examined over seven leaf appearance intervals. The responses to defoliation of the two species were generally similar but tall fescue growth was relatively more affected than perennial ryegrass by the most severe defoliation. The poorer regrowth of tall fescue resulted from its slower leaf appearance, lower specific leaf area, and greater allocation of biomass to roots and pseudostem. The longer leaf life span of tall fescue leaves resulted in a greater loss of photosynthate per leaf than for perennial ryegrass when all fully expanded laminae were defoliated.

KEY WORDS

Tall fescue, perennial ryegrass, defoliation, tillers, leaf appearance rate.

INTRODUCTION

Tall fescue is less productive and persistent than perennial ryegrass when they are continuously defoliated to a surface height of 30-40 mm (6). Under continuous stocking the proportion of young leaves in the sward increases as the intensity of grazing increases (4). The objective of this research was to use a controlled defoliation procedure to determine the relative importance of each individual leaf during steady state leaf defoliation to the physiological and morphological components of regrowth of tall fescue and perennial ryegrass. This information will assist with the definition of grazing strategies for tall fescue.

METHODS

Individual plants of tall fescue (Festuca arundinacea) cv. Grasslands Roa and perennial ryegrass (Lolium perenne) cv. Grasslands Nui were established from seed in pots under controlled environment conditions (21C (day)/15 C (night), 12 h photoperiod, 600 mol photons/m2/s (400-700 nm)). The oldest fully expanded lamina on all the tillers was removed regularly to maintain four (Control), three (Lax), two (Medium), or one (Hard) live lamina per tiller for seven leaf appearance intervals. Lamina defoliation treatments were applied to both species when there was a mean of eleven tillers per plant (38 and 26 days after establishment for tall fescue and perennial ryegrass, respectively). Tall fescue was defoliated less frequently than perennial ryegrass because it had a longer leaf appearance interval (8.3 vs 6.8 0.17 days/leaf/tiller). There was a factorial combination of species and defoliation treatments in a randomised complete block design with six replicates.

The measurements used are fully described in Tavakoli (5). All removed leaves were included in the cumulative leaf and shoot mass per plant over the seven leaf appearance intervals. Four plants of each species were used for the initial shoot mass in the calculation of relative growth rates (RGR). Tillers per plant were counted at each leaf defoliation sequence. Leaf appearance rate was measured on three marked tillers per plant. Individual leaf characteristics were measured on the youngest fully expanded leaves of five tillers per plant. Photosynthetic rate was measured with an infra-red gas analyser (Li-Cor 6200). Water soluble carbohydrate concentration was measured with a modification of the method of Trent and Christiansen (7).

RESULTS

Lax defoliation treatment effects were the same as for Control defoliation so they are not presented.

Hard defoliation, all fully expanded laminae per tiller removed, decreased the shoot and root mass per plant of tall fescue and perennial ryegrass more than did Medium defoliation (Table 1), and increased both the ratio of shoot mass to root mass, and the ratio of leaf mass to shoot mass (Table 1). The decreased shoot, root and leaf mass of tall fescue under

Table 1. Effect of three leaf defoliation treatments (Control, Medium and Hard = 4, 1 and 0 fully expanded laminae per tiller remaining) on the growth and morphology of vegetative plants of tall fescue and perennial ryegrass over seven leaf appearance intervals.

Hard defoliation was relatively greater than that for perennial ryegrass (Table 1). For example, the shoot masses of tall fescue and perennial ryegrass under hard defoliation were 33 and 43 %, respectively, of their

shoot masses under the Control defoliation. In contrast, the Medium defoliation shoot masses were 76 and 78 % of the shoot masses under the Control treatment for tall fescue and perennial ryegrass, respectively.

The shoot RGR was relatively lower for Hard defoliated tall fescue than for perennial ryegrass when compared with the Control (Table 1). The RGR of perennial ryegrass was greater than that of tall fescue under all defoliation treatments. The leaf area per plant was similar for tall fescue and perennial ryegrass (3084 vs. 2743 345 cm2/plant) and was unaffected by defoliation. The mass and area of individual tall fescue leaves were greater than for perennial ryegrass leaves, but tall fescue leaves under Hard defoliation decreased more relative to Control defoliation than did perennial ryegrass leaves (Table 1). Leaf masses of the youngest fully expanded leaves after seven leaf appearance intervals of Hard defoliated tall fescue and perennial ryegrass were 29 % and 49 %, respectively, of the Control plants (Table 1). Tall fescue leaves were wider than perennial ryegrass leaves, with a lower specific leaf area (SLA) and slower leaf elongation rate (Table 1). Hard defoliation decreased leaf width and mass, and increased SLA, but did not affect leaf elongation rate (Table 1). Photosynthetic rate per unit leaf area of the two youngest leaves was similar for tall fescue and perennial ryegrass (12.1 vs. 13.3 0.44 mol CO2/m2/s), but photosynthetic rate per unit leaf mass was lower for tall fescue than perennial ryegrass, and was increased by Hard defoliation (Table 2). Leaf life spans for tall fescue and perennial ryegrass were 41.8 and 31.0 0.47 days, respectively, with a similar photosynthetic rate per unit area for the leaves over the first 26 days, but thereafter a significantly greater rate for tall fescue leaves (5).

There were fewer but heavier tillers per plant for tall fescue than perennial ryegrass (Table 1). Number of tillers per plant was decreased by Hard defoliation but not by Medium defoliation (Table 1). There was no species by defoliation treatment interaction (Table 1). Leaf appearance rate, site filling and consequently tillering rate were lower for tall fescue than perennial ryegrass (Table 1). Site filling was decreased by Hard defoliation (Table 1). The water soluble carbohydrate (WSC) concentrations in the tiller stem bases were similar for tall fescue and perennial ryegrass (Table 2). Hard defoliation greatly decreased WSC concentration, to 28 % of that in the Control stem bases (Table 2).

Table 2. Effect of three leaf defoliation treatments (Control, Medium and Hard = 0, 4, 1 and 0 fully expanded laminae per tiller remaining) on the physiology of vegetative plants of tall fescue and perennial ryegrass over six leaf appearance intervals. L1: youngest leaf, L2: second youngest leaf. Photosynthetic rate of L2 in Hard defoliation measured immediately to defoliation.

DISCUSSION

Plant mass and RGR were more sensitive in tall fescue than perennial ryegrass to the Hard defoliation treatment relative to the Control and Medium defoliation treatments. Both species required the retention of at least the youngest fully expanded leaf (Medium defoliation) to produce similar growth to that when all four leaves were retained. Davies (2) showed that removal of perennial ryegrass leaves had little effect on the growth of the plant until all fully expanded leaves were removed. The greater sensitivity of tall fescue to Hard defoliation was due to its inherent morphological and physiological differences from perennial ryegrass. There was little evidence of a species by defoliation interaction for the morphological and physiological components of growth measured. However, the leaf mass of the youngest fully expanded leaf (immediately prior to defoliation) after seven leaf appearance intervals was relatively more affected by Hard defoliation when compared with the Control and Medium treatments on tall fescue than perennial ryegrass.

Although the leaf area per plant and photosynthetic rate per unit leaf area were similar for tall fescue and perennial ryegrass, tall fescue had lower SLA, and allocated relatively more of its biomass to roots and psuedostem than perennial ryegrass. Also, the longer lived, heavier leaves of tall fescue appeared and elongated more slowly than did the leaves of perennial ryegrass, which would have resulted in a greater loss of potential photosynthate for tall fescue when leaves were defoliated. Tall fescue leaves have a similar photosynthetic rate per unit leaf area to perennial ryegrass leaves for their first 26 days, but are capable of photosynthesis for 12 days more than perennial ryegrass leaves(5). The slower leaf appearance interval, lower site filling and resultant fewer but heavier tillers per plant of tall fescue when compared with perennial ryegrass resulted in tall fescue plants producing fewer tillers over the seven leaf appearances. However, the effect of Hard defoliation on tiller mass was greater than that on tiller number. Similarly, Tavakoli et al. (6) found that the lower herbage mass produced by a continuously stocked tall fescue sward maintained at a surface height of 30-40 mm, compared with one maintained at a surface height of 50-60 mm, was primarily due to the lower mass per tiller than to a decrease in tiller density.

The responses of individual plants of tall fescue and perennial ryegrass to the defoliation of leaves were similar to those in other studies, but did not include some effects observed when grass swards are cut or grazed to a specified height. The pseudostem sheath height of tall fescue is known to affect the minimum sward height at which it can be maintained (1). At a stubble height of 30 mm tall fescue sheath length lacks the plasticity to shorten sufficiently to enable residual leaf area for regrowth to be retained. Similarly, Matches (3) showed that tall fescue was more productive at a sward height of 25 mm if ten per cent of tillers were left intact after cutting than when all tillers were cut. The lamina defoliation used in the experiment reported here was independent of sheath length, and resulted in tall fescue being defoliated less frequently than perennial ryegrass.

CONCLUSION

It is concluded that the factors that resulted in the regrowth of tall fescue being less tolerant of defoliation of all fully expanded leaves than perennial ryegrass were longer leaf life span, slower leaf appearance rate, lower SLA, and greater allocation of biomass to non-leaf plant organs. The accumulated effects of these factors decreased the tiller mass and number of tall fescue plants, and thereby decreased shoot mass and shoot RGR. This was due to a shortage of photosynthate resulting from leaves being defoliated before they had provided the plant with a full return on the carbon invested in them. The practical implications are that for tall fescue to be productive and persistent under continuous stocking it needs to be maintained at a sward height that allows tillers to maintain their leaves for approximately 40 days after full expansion.

REFERENCES

1. Chapman, D.F. and Lemaire, G. 1993. Morphogenetic and structural determinants of plant regrowth after defoliation. In: Grasslands for our World. (Ed. M.J. Baker) (SIR Publishing: Wellington, New Zealand). Pp. 55-64.

2. Davies, A. 1974. Leaf tissue remaining after cutting and regrowth in perennial ryegrass. Journal of Agricultural Science, Cambridge. 82: 165-172.

3. Matches, A.G. 1966. Influence of intact tillers and height of stubble on growth responses of tall fescue (Festuca arundinacea Schreb.). Crop Science. 6: 484-487.

4. Parsons, A.J.; Johnson and I.R.; Williams, J.H.H. 1988. Leaf age structure and canopy photosynthesis in a rotationally and continuously grazed swards. Grass & Forage Science. 43: 1-14.

5. Tavakoli, H. 1993. Physiological and morphological responses of tall fescue and perennial ryegrass to defoliation. Unpublished PhD thesis, Massey University, Palmerston North, New Zealand.

6. Tavakoli, H., Hodgson, J. and Kemp, P.D. 1993. Response to defoliation of tall fescue. Proceedings of the XVII International Grassland Congress pp. 155-156.

7. Trent, J.D. and Christiansen, S. 1986. Determination of total nonstructural carbohydrates in forage tissue by p-Hydroxybenzoic acid hydrazide flow-injection analysis. Journal of Agriculture and Food Chemistry. 34: 1033-1037.

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