Discussion

The productivity and sustained production of grasses are conferred by the successive production of relatively short-lived tillers (Langer, 1 963; Jewiss, 1 972). In the current experiment tillering was affected by defoliation intensity, particularly when plants were cut frequently to 30 mm stubble height either in H1 or H2 treatment (Table 4.5, Figure 4.2). Mitchell and Coles ( 1 955) reported that cutting perenn ial ryegrass to 20 mm repeatedly checked the tillering rate. Tillering of tall fescue was reduced 1 00% relative to uncut plants when plants were defoliated repeatedly to a 25 mm stubble height (Matches, 1 966).

Decreasing leaf elongation rate with increasing severity of defoliation (Figure 4.4) is consistent with the results of Ryle (1 964), Davies (1 977), Bircham and Hodgson (1 983), Grant et al. (1 983) and Butler and Hodgson {1 993). Herbage mass harvested per tiller for both species decreased over time and with increasing severity of defoliation (Table 4.4) which corresponds with results obtained by Mitchell and Coles (1 955) under close and repeated defoliation on perennial ryegrass. Reduction in tiller weight with decreasing defoliation height (Table 4.7) is in agreement with the findings of Bircham and Hodgson (1 983), Xia {1 993) and Brock & Hay (1 993) in ryegrass, and Zarrough et al. {1 983a) in tall fescue. Although tall fescue had heavier tillers they were more sensitve to cutting than perenn ial ryegrass. Study on the causes of reduction in leaf growth and tiller weight is followed in experiments under controlled environment.

The number of tillers per plant for perennial ryegrass plants was greater than for tall fescue plants (Figure 4. 1 ). A slightly higher tillering rate in perenn ial ryegrass resulted in an accumulating advantage in tiller number With time, thus at plant harvest perennial ryegrass plants had 1 .6 times more tillers than tall fescue. On the other hand, tall fescue had about 1 .6 times heavier tillers than perennial ryegrass, indicating that tall fescue invests more assimilate in individual tiller development, and less i n tiller initiation, than does

perennial ryegrass. lt is generally accepted that perennial ryegrass is a more vigorous plant than tall fescue in terms of tillering activity (e.g. Ryle, 1 964; Rhodes, 1 968; Brock, 1 983), but the physiological or morphological mechanism responsible for the different behaviour of tall fescue and perennial ryegrass are not fully understood and need further investigation.

A reduction in herbage yield in response to increasing severity of defoliation can reflect a lower tiller n umber (Davies, 1 971 ), a lower tiller weight (Zarrough et al. , 1 983a; Bartholomew & Chestnutt, 1 978), or both (Korte et al.,

1 985). In the present experiment, reduction in herbage harvested per plant was due to both reduction in tiller number and tiller weight. Also, lower tiller numbers per plant and lower leaf elongation rates (65% of perennial ryegrass) in tall fescue than perennial ryegrass were the causes of lower levels of herbage harvested per plant in tall fescue relative to perenn ial ryegrass. However, tiller weight of tall fescue was higher than perenn ial ryegrass.

Reduction in plant root growth as the severity of defoliation is increased has often been reported (Davidson & Milthorpe, 1 966b; Schuster, 1 964; Wilson, 1 988). Apparently reduction in supply of photosynthate to the roots because of reduced photosynthesis, and also greater allocation to shoot meristematic and leaf growth regions after defoliation (Richards, 1 993), results in a reduction in root growth under more intense defoliation. Root mass of both species decreased as intensity of defoliation increased {Table 4.9).

Removal of top growth in tall fescue is almost always associated with a drop in total plant weight (Booysen & Nelson, 1 975) and the recovery of plants from the effects of defoliation takes time and depends on its severity (Booysen & Nelson, 1 975). If plants are defoliated again before recovery is complete, then recovery will be even more prolonged and plant · death may result. The recovery of perennial ryegrass and tall fescue plants cut to 30 mm every 5 days from different initial stages of development (H1 and H2) declined progressively and resulted in death of 75% of tall fescue plants and 50% of

perennial ryegrass plant by the end of experiment. However, plants cut to 60 mm stubble or higher were .able to recover sufficiently to survive the same interval between cuts (see Plate 4. 1 ), in accord with the results obtained by Matches (1 966) on tall fescue. Cutting to leave 60 mm of stubble height on a prostrate ryegrass did not affect the amount of regrowth (Wilson & Robson, 1 970), but closer cutting progressively reduced regrowth compared with uncut plants.

lt was thought that plants under H 1 and H2 might behave differently under defoliation due to smaller and shorter tillers in H2, so leaving a greater proportion of leaf after defoliation relative to treatment H 1 . In fact treatm ents H 1 and H2 produced similar results. Apparently less development of the root system and low initial stubble weight under H2 resulted in failure of the plants to recover after repeated defoliation.

In general, results obtained in this experiment with 5 day cutting intervals together with the findings of Matches (1 966) on tall fescue with 1 0 day cutting intervals with similar cutting heights, confirmed the responses of plants to defoliation under grazing conditions (Chapter 3). Under the hard cutting treatment with 30 mm cutting height at 5 day intervals in the present experiment, and under 25 mm cutting height with 1 0 day intervals in the study of Matches (1 966), plants failed to recover over short defoliation intervals. However, under grazing conditions (Chapter 3) with a tiller grazing interval of about 9 days, plants tolerated hard grazing better. For example, tillering rates remained constant under hard grazing from November until April (Table 3.5). The fact that close grazing is less detrimental than uniformly close cutting may be because grazing typically leaves some ungrazed tillers on a plant while removing others (Vallentine, 1 991 ), thus allowing for the transfer of assim ilate from undefoliated tillers to the defoliated tillers (Ryle & Powell, 1 975; Ong, 1 978). Greater growth rates are observed in tall fescue plants following defoliation as the percentage of undefoliated tillers within the plant increased (Matches, 1 966).