The efTect of level of nitrogen application upon the dynamics of herbage growth in a continuously grazed sward of tall fescue was investigated during two successive years. In order to obtain a large range of sward structural conditions, the experiments were carried out with two contrasting cultivars: cv. Clarine and cv. Barcel, and, in Year 2, with two different sward heights or leaf area indices (LAls). During each of five experimental periods (2-3 weeks), swards received either optimum (N2) or deficient (Nl) N applications, were maintained at their target LAI, and leaf growth was measured on labelled tillers. With continuously defoliated tillers, N»-shortage had only a small effect on the leaf elongation rate compared with tillers protected by cages. The leaf production per tiller was only slightly reduced by N shortage, and it was mainly by the means of a reduction in tiller density that the N deficiency resulted in reduced herbage growth per hectare. These results indicate that, in continuously grazed swards, in contrast with results previously found in intermittently defoliated swards, leaf elongation is not the only important component of difference in herbage growth and that the promotion of tillering rate is an additional pathway for N response in such management regimes.

The effect of a reduced level of nitrogen fertilizer application upon the herbage consumption of a continuously grazed sward of tall fescue has been investigated during two successive years. Two contrasting cultivars were used and two levels of leaf area index were maintained on the swards in order to obtain a wide range of sward structure. The herbage growth, herbage senescence and herbage consumption were measured directly on labelled tillers. It was possible to estimate the different components of the defoliation process (frequency and intensity) at the level of individual tillers of each leaf category. The results show that the effect of reduction in N fertilizer on herbage consumption rate is proportionally more important than that on herbage growth rate. In consequence, the efficiency of herbage utilization in continuously grazed conditions is reduced when nitrogen nutrition becomes limiting for herbage growth. The data suggest that this phenomenon is not directly attributable to nitrogen deficiency but is an indirect effect owing to the reduction of the stocking density at the low N level, which leads to a lower frequency of defoliation of individual tillers.

processes (Touraine et al., 1994), a high N requirement during late winter and early spring has been observed For some species, mathematical models have been developed to for annual ryegrass (Salette et al., 1984). However, since describe tissue N dilution during crop growth and to estimate the the rate of soil N mineralization varies throughout the plant N status applying the N nutrition index (NNI), the ratio betweenyear, mainly in response to soil temperature and water the actual tissue N concentration and the tissue N concentration needed to obtain the maximum instantaneous crop growth rate (criti-

Two glasshouse experiments were carried out to evaluate the morphological and physiological responses to water deficit of Mediterranean and temperate tall fescue (Festuca arundinacea Schreb.) cultivars. In Expt 1, 3 cultivars were studied: 2 temperate cultivars, Grasslands Advance (GA) and El Palenque (EP); and a Mediterranean cultivar, Maris Kasba (MK). Water deficit was induced in containers of 4 plants of a single cultivar by withholding water. In Expt 2, plants of EP and MK were grown together in the same container and received water daily with gradation in intensity of water deficit achieved by varying the daily water ration per container.All cultivars in each experiment exhibited commonly reported responses to water deficit, characterised by diminished evaporative surface area and increased root : shoot ratio. The response of MK was primarily morphological and MK plants had smaller plant size, higher root : shoot ratio, and a lower growth rate compared with temperate cultivars. By contrast, response of temperate cultivars was primarily physiological; stomatal conductance of temperate cultivars was lower and these cultivars had a greater tendency for leaf lamina osmotic adjustment than MK under the most stressful water deficit conditions studied. The morphological adaptations of MK were very effective in delaying the onset of water deficit in Expt 1 when plants were grown with others of the same cultivar, but rendered MK plants uncompetitive and more susceptible to water deficit in Expt 2 where plants of MK where interspersed with plants of a temperate cultivar.Additional keywords: plant water relations, roots. S . G . A s s u e r o , C . M a t t h e w , P . K e mp , D . J . B a r k e r , A . M a z z a n t i A R 0 1 0 2 3 Wa t e r d e f i c i t e f f e c t s o n c o n t r a s t i n g t a l l f e s c u e s S . G . A s s u e r o e t a l .

This study describes the effect of herbage mass of species on defoliation patterns in a multispecies temperate± subhumid natural grassland of the Flooding Pampas at low, medium and high sward herbage mass [761 8á7, 989 137 and 1591 44á0 kg green dry matter (DM) ha ±1 respectively] maintained by continuous grazing of cattle at variable stocking rates. Four native and naturalized warm-season species were studied: the grasses Cynodon dactylon, Leersia hexandra and Paspalum dilatatum and the legume Lotus tenuis. Sward state of the four species was described in terms of sward surface height, herbage biomass and the number of individual grass tillers and legume stems per unit area. The defoliation pattern was monitored twice-weekly during an experimental period of 3 weeks on permanently marked individual plants. Proportions of individual plants defoliated and the ranking of species selected differed among sward treatments and species. The results showed that the density of the species was a major factor determining the pattern of species defoliation and suggest that, in continuously grazed mixtures, the pattern of defoliation might be more dependent on the abundance of a species in the plant community than on species preferences. The results show the importance of considering this structural characteristic of swards in understanding the process of species selection by large herbivores in multispecies vegetation.