Defoliation Patterns Research Articles

Spatiotemporal patterns of large-scale defoliation caused by the spruce budworm in Ontario since 1941

Spatiotemporal patterns of large-scale defoliation caused by the spruce budworm in Ontario since 1941

Forecasting the susceptibility and vulnerability of balsam fir stands to insect defoliation with Landsat Thematic Mapper data

The potential of remote sensing to aid in the forecast of insect defoliation and the associated impact on forest stands was tested by examining the relationships between i) preoutbreak forest structure and growth characteristics, ii) multispectral values recorded by the Landsat Thematic Mapper (TM) sensor, and iii) subsequent patterns of defoliation and volume loss in the balsam fir ( Abies balsamea L. Mill) forests of Western Newfoundland. Forest structure and growth characteristics were measured in the field in 1992–1993 and reconstructed to represent conditions 3 years prior to an infestation of black-headed budworm, Acleris variana (Fern.), which began in 1988. Preoutbreak Landsat TM data were acquired on 8 August 1985. Spectral values were converted to approximate reflectances using PCI atmospheric correction routines and related to the field measurements. Logistic regression techniques were used to develop models for predicting forest susceptibility (probability of attack) and vulnerability (response to attack) and to assess the accuracy of the susceptibility and vulnerability forecasts. In general, susceptible stands were younger and had lower basal area and tree density, but higher leaf area index than stands that were not attacked. Vulnerable stands were older, had lower vigor and growth efficiency indices (stemwood growth per unit of leaf area), and also displayed the highest near infrared spectral values. The best predictions combined selected spectral measurements with provincial forest inventory data. These models produced classifications with accuracies of 81%, 67%, and 78% for predicting susceptibility and pre- and postoutbreak vulnerability, respectively.

Causes and consequences of sectoriality in the clonal herb Glechoma hederacea

The causes of sectoriality and consequences for clone behaviour are examined using data from the stoloniferous herb Glechoma hederacea. The proximal causes of physiological integration patterns are investigated using anatomical studies, acid fuchsin dye to reveal patterns of xylem continuity between ramets, and 14C as a label to reveal quantitative photoassimilate translocation patterns in the phloem. Dye movement in the xylem was acropetal and sectorial, and the sectoriality was determined by phyllotaxy. Patterns of 14C-labelled photoassimilate allocation were qualitatively similar to those of xylem based resources, although there was some basipetal movement of photoassimilate. The patterns of physiological integration and independence between ramets are shown to be governed by rules which depend on vascular continuity and discontinuity between ramets. Physiological support to stolon apices results in acquisition of relative branch autonomy (branches become semi-autonomous integrated physiological units, IPUs). This paper evaluates whether observed physiological integration patterns may be modified by altering normal source-sink relationships or by modifying environmental conditions. An experiment using different defoliation intensities, and different defoliation patterns at the same overall intensity, demonstrated that the precise positions of leaves removed from a clone had unique consequences for its subsequent development. Individual ramets of a given clone may be located in microhabitats of differing quality. An experiment in which competition was either present or absent throughout the space occupied by the clone, or patchy in distribution, showed that G. hederacea did not respond to competition at the whole clone level. Instead, connected stolons (IPUs) responded independently to local competition. Sectoriality may promote the restriction of lethal, localised environmental factors within the affected IPU. A study investigating the uptake and translocation of zinc by clones revealed that quantified patterns of zinc distribution resembled patterns of 14C movement in the phloem, and that there was no significant transport of zinc from one stolon to another. Although sectorial patterns of resource movement in G. hederacea can be modified in the short term, in the long-term, physiological integration may not allow this species to integrate the effects of environmental heterogeneity. A mobile clonal species with a high growth rate and relatively short-lived ramets, such as G. hederacea, is likely to benefit from a semi-autonomous response to patch quality at the level of the stolon, since the alternative of widespread intra-clonal support may increase the residence time of the clone in unfavourable pathches.

Recent cooling and recession of Norway spruce (Picea abies (L.) Karst.) in the forest—alpine tundra ecotone of the Swedish Scandes

Abstract. Repeated crown condition surveys, 1974–94, of subalpine clonal groups of Norway spruce (Picea abies (L.) Karst.) were carried out in the Swedish Scandes. Complementary analyses concerned radial and vertical growth, sexual regeneration and range limit responses of other plant species. Significant defoliation of spruce progressed linearly over the period of study, reaching cumulative values of about 85%. It is inferred that defoliation was preconditioned by decreasing radial growth since the thermal climax in 1937 and was proximately initiated by the extremely cold winter of 1965/66 and paralleled by consistently declining radial growth and staggering vertical increase. It appears that severe and prolonged ground freezing invoked winter desiccation (xylem cavitation), extensive needle loss and reduced radial growth. Hypothetically, from circumstantial evidence, these processes are interrelated in a positive feedback system, implying increasing sensitivity to climatic stress and decreasing ability to take advantage of positive climatic anomalies. Thus, the total demise of the supranival stems is cautiously predicted, by linear regression of the 20‐yr defoliation pattern, to be less than a decade ahead. The recession of P. abies, clearly relevant in a landscape perspective, conforms with analogous responses of Pinus sylvestris L. and Betula pubescens Ehrh. ssp. tortuosa (Ledeb.) Nyman and a significant altitudinal range‐limit retraction of certain silvine field‐layer species. The structural development examined in this study concurs with long‐term climate cooling and cold events and strongly contrasts with simulations of the performance of this system in response to a putative enhanced‘greenhouse’effect.

Sectorial root growth in cuttings of Coleus rehneltianus in response to localized aerial defoliation

Asymmetries in root growth in response to localized aerial defoliation were examined in Coleus rehneltianus (Lamiaceae). We confirmed that assimilate transport was sectorial by examining the distribution of 14C-labeled carbohydrates following a 24-h chase period. Integrated physiological units (IPUs), or sectors, extended from the leaves into the roots, and this was reflected in the differential growth of roots following artificial defoliation of part of the leaf canopy. When defoliation was localized within leaves or leaf halves within sectors, roots grew asymmetrically, with decreased root growth in defoliated sectors. Three root populations were identified by their location and growth responses: stem side, stem corner, and bottom side roots, and asymmetric growth was observed in all three populations. Only the growth of stem corner roots, which made up 35–90% of dry mass of the total root population, was influenced by the pattern of aerial defoliation. In contrast, asymmetries in the growth of the other two root populations appeared to reflect the distribution of leaf biomass prior to defoliation.

Vertical stratification of feeding by Japanese beetles within linden tree canopies: selective foraging or height per se?

Adult Japanese beetles, Popillia japonica Newman (Coleoptera: Scarabaeidae), tend to aggregate and feed most heavily in the upper canopy of host plants, defoliating them from the top down. We examined characteristics of linden (Tilia cordata L.) foliage from different canopy zones and tested two hypotheses that might account for vertical stratification of feeding by this vagile, polyphagous folivore. In the field, P. japonica caused 4-12 times more damage to upper canopy leaves in full sunlight than to lower canopy leaves in sun or shade. However, this within-tree pattern apparently cannot be explained by differences in nutritional parameters (protein, water, and sugar content) or "defensive" properties (toughness, tannins) of leaves. Furthermore, beetles did not discriminate between foliage from different canopy zones in laboratory choice tests, nor were fecundity or longevity higher for beetles fed upper canopy, sun-exposed leaves. Clonal grape plants suspended from vertical posts in the field at 3.65, 1.83 or 0 m above ground showed a top-down defoliation pattern identical to that seen in linden trees. This suggests that the height of foliage per se strongly affects initial orientation and attack by P. japonica. Recent related studies have shown that both sexes of P. japonica are strongly attracted to host volatiles induced by feeding of conspecific beetles. We suggest that Japanese beetles begin to feed in the upper canopy for reasons unrelated to host nutritional variation (e.g., behavioral thermoregulation, visual orientation to the host silhouette), and that top-down defoliation follows as additional beetles are attracted to feeding-induced volatiles acting as aggregation kairomones.

The role of a stand dynamics model in the spruce budworm decision support system

Stand dynamics models that predict effects of pests on forest development are key components of pest and forest management planning decision support systems (DSS). Such stand models are required to quantify the marginal effects and costs–benefits of alternative strategies, and thereby to assist in evaluating management options and defining tactics and strategies. The spruce budworm (Choristoneurafumiferana (Clem.)) DSS allows users to project forest inventories under alternative budworm and protection (insecticide use) scenarios, to plan protection programs based on marginal timber supply benefits, to incorporate budworm effects on stand or forest development into management plans, and to evaluate silvicultural and forest management alternatives to limit damage. The STAMAN stand model is an integral part of the spruce budworm DSS, used to calculate stand impact matrices used by two DSS components. Simulations, using STAMAN, for 13 classes of spruce–fir stands under various levels of constant, cumulative defoliation for a 10-year period, resulted in predicted volume reductions of 1–6% with 20% defoliation, 27–42% with 60% defoliation, and 82–99% with 90% defoliation. Another set of simulations using a 30-year cyclical defoliation pattern (including 5 years of light defoliation, 8 years of moderate–severe defoliation, and 17 years of very light or nil defoliation) resulted in volume reductions similar to published data for actual spruce budworm outbreaks.

Spatial distribution patterns of pine sawflies (Hymenoptera: Diprionidae) in Arizona, US and Sichuan, PR of China

In a comparison study of sawfly distribution patterns in the western US and southwestern China, we found that defoliation and oviposition were limited to certain forest stand characteristics. Both species ( Neodiprion autumnalis Smith in Arizona, US and N. xiangyunicus (Xiao and Huang) in Sichuan, P.R. China) were restricted to pure stands of their hosts at specific elevations. Defoliation of ponderosa pine ( Pinus ponderosa Dougl. ex Law.) by N. autumnalis was limited to a narrow elevational band (2410–2440 m) within the range of its host at the study site (approximately 2350–2650 m). Defoliation of and oviposition on yunnan pine ( P. yunnanensis Franch.) by N. xiangyunicus was confined to a specific but wider elevational band (1850–2050 m) within the host range of the area studied (1650–2050 m). The density of ponderosa pine trees (measured as basal area (m 2 ha −1)) also appears to play a role in the distribution of N. autumnalis. Defoliation by this species was limited to stand basal areas of less than 4.6 m 2 ha −1. Stand density does not appear to have a significant influence on N. xiangyunicus defoliation patterns. However, a significant interaction between stand density and vertical crown position on oviposition by N. xiangyunicus was detected. Proportionately more egg clusters were found in the top of crowns at low stand densities, while egg clusters were located in the middle and bottom of crowns equally at all stand densities. Significant differences in defoliation intensity were not detected between crown level for either species. In addition to discerning these distribution patterns, we established that foliage characteristics varied across stand density and elevation gradients in both Arizona and Sichuan. Needles in low density compared with high density stands in Arizona were longer, wider, tougher, and had less water. Pine needles in Sichuan followed this same general pattern when comparing the foliage from low elevations with foliage from high elevations. We propose that longer needles of open-grown trees in Arizona allow ovipositing females to lay more eggs per needle which in turn benefits larval performance through increased feeding and defense against natural enemies. The 20% more tough needles of yunnan pine at low elevations in Sichuan may confine oviposition and larval feeding to higher elevations.

Production and rain use efficiency in short‐grass steppe: grazing history, defoliation and water resource

Abstract. Grassland in the semiarid shortgrass steppe, subjected to 50 years of heavy, light, and no grazing intensity, was clipped to simulate the natural pattern and intensities of defoliation by cattle or not clipped. A level of water resource treatment was superimposed upon the grazing and clipping treatments. Half of the plots were supplemented with additional water to simulate a wet year and half were not supplemented in a year of average precipitation. All three treatments interactively determined above‐ground production. Water treatment had the largest overall effect on above‐ground production. Current‐year defoliation had no direct significant effect on production, but mediated differences between both longterm grazing and watering treatments. Long‐term ungrazed compared to grazed grassland was capable of responding to high amounts of precipitation, but was also most affected by low amounts of precipitation and, therefore, displayed greater variability in above‐ground production and rain use efficiency. Only in the year of average precipitation, defoliation increased rain use efficiency in long‐term lightly, but not heavily, grazed treatment. This suggests a water conservation mechanism of defoliation that is reduced with heavy grazing.

Pattern of Defoliation and Its Effect on Photosynthetic Capacity in Oenothera Biennis

I Five different patterns of tissue removal were used to mimic natural herbivore damage in Oenothera biennis. The effects of defoliation by: (1) half edge (tissue removed from one side of the leaf, parallel to and exclusive of the midrib); (2) double edge (tissue removed from leaf perimeter); (3) perforation (roughly circular portions removed, leaf edges and midrib intact); (4) tip (apex of leaf removed); and (5) entire leaf defoliation on photosynthesis were examined. 2 Plants were subjected to removal of tissue from either a single leaf or throughout the plant. Photosynthesis, stomatal conductance, nitrogen content and water potential of the remaining portion of the youngest fully emerged leaf at the time of defoliation was examined over a one to two week period. 3 Tip defoliation at both the single leaf and whole plant level enhanced photosynthesis relative to undefoliated controls. Half edge and entire leaf defoliation at the whole plant level also enhanced photosynthesis relative to controls, but not to the same extent as tip defoliation. Double edge and perforation defoliation at the single leaf level depressed photosynthesis relative to controls and tip defoliation. 4 Differences among defoliation treatments can best be explained by intraleaf variation in photosynthetic capacity and nitrogen content and by the extent of wounding to residual tissue. Results from this study illustrate that the way in which tissue is removed can have a dramatic effect upon photosynthetic capacity of the remaining tissue.

Patterns of balsam fir foliar production and growth in relation to defoliation by spruce budworm

Changes in foliar production caused by spruce budworm (Choristoneurafumiferana (Clem.)) defoliation and patterns of volume increment of surviving trees during and after the outbreak were determined in 20 mature balsam fir (Abiesbalsamea (L.) Mill.) stands on Cape Breton Island, Nova Scotia. Following the cessation of defoliation, the number of shoots increased 4-fold and mean shoot length doubled, resulting in a 12-fold increase in needle biomass. Average specific volume increment declined from 0.17–0.25 to 0.02–0.04 cm3•cm−2•year−1 after 4 years of severe defoliation; maximum increment reduction was 74–92%. Periodic radial increment and volume increment reduction (percent of mean increment in the previous decade) were both significantly related to cumulative (summed current annual) defoliation; a logistic regression equation explained 77% of the variation in volume loss. Differences in growth recovery among trees were determined by the temporal patterns of defoliation, with a 1-year lag before the beginning of volume increment recovery. Following 5 to 8 years of recovery, surviving trees had regained 48–82% of their predefoliation increment; however, absolute volume losses during the outbreak averaged 12–33 dm3/tree, or 32–48%.

Forage quality in relation to long-term grazing history, current-year defoliation, and water resource.

Forage nitrogen concentrations, nitrogen yields, and in vitro digestibilities were assessed in shortgrass steppe that had been ungrazed, lightly, or heavily grazed for 50 years. Caged plots were defoliated in amounts based upon removals observed in naturallygrazed reference plots or not defoliated. This was done in a year of average precipitation and with a supplemental water treatment to simulate a wet year. In general, current-year defoliation had positive effects, and longterm grazing and supplemental water had negative effects, on forage nitrogen concentrations and digestibilities. However, defoliation interacted with long-term grazing in determning forage nitrogen concentrations, and with grazing and with watering in determining digestibilities. Nitrogen concentration and digestibility increased with defoliation in lightly, but not in heavily, grazed treatments. The dilution effect of supplemental water an digestibilities through increased plant growth was offset by defoliation. The negative effects of long-term grazing on forage quality were small, equally or more than compensated for by defoliation in a year of average precipitation, but more pronounced in the simulated wet year. Nitrogen yields and digestible forage production were usually increased by defoliation, but this depended upon grazing and watering treatments. Increased nitrogen and digestible forage yields and concentrations in response to defoliation were greater than the biomass response in lightly grazed grassland. For both nitrogen and digestibility, yields were greater in grazed than ungrazed treatments in the year of average precipitation, but less in the simulated wet year. Optimizing quantity and year-to-year stability of nitrogen and digestible forage yield may best be achieved with light grazing rather than no or heavy grazing. Clipping was conducted in a manner closely resembling the natural pattern and intensity of defoliation by the cattle, and confirms the potential for a positive feedback of increased forage quality with defoliation observed in pot experiments. Long-term heavy grazing can diminish this response. Quantily (aboveground primary production, ANPP), quantity of quality (digestible and N yields), and quality (concentrations) do not necessarily respond similarly in interactions between current-year defoliation, long-term grazing history, and level of water resource.

Why Does a Branch Suffer More after Branch-Wide Than after Tree-Wide Defoliation?

We investigated the way in which the pattern and extent of defoliation (a single branch or all the branches of the tree) and fertilization the growth and foliage carbon/nitrogen ratios of Scots pine (Pinus sylvestris) branches of different ages and thus different relative growth rates. Our objective was to contrast two hypotheses, operating at different levels, explaining consequences of defoliation : the carbon/nutrient (C/N) balance hypothesis, which builds on defence-related changes in foliage carbon vs nutrients, and the source/sink hypothesis which emphasizes defoliation-caused disturbances in the source/sink balance, and the consequent quantitative changes in growth (...)

Defoliation patterns of three grass species in sourveld grazed by cattle and sheep

Abstract A point‐based technique was used to determine the effects of grazing, due to stocking rate and cattle‐to‐sheep ratio, on the defoliation pattern of three grass species. At each of 100 systematically located points per grazing treatment the nearest plant of each of three target species was recorded as being in one of five defoliation categories which ranged from ungrazed to severely, but uniformly, grazed. Each treatment was sampled at the end of each of three consecutive grazing seasons. The data for each species were subjected to log‐linear and canonical correspondence analyses. In these analyses the proportional composition of defoliation categories was the dependent variable and stocking rate, cattle‐to‐sheep ratio and year were the independent variables. The results indicated that (1) stocking rate had an overriding effect on defoliation pattern; (2) ratio had a lesser but significant influence on the defoliation pattern of all three species; (3) similar defoliation patterns may be achieved a...

Little Bluestem Tiller Defoliation Patterns under Continuous and Rotational Grazing

Defoliation patterns of little bluestem [Schizachyrium scoparium (Michx.) Nash] on tallgrass prairie were compared using continuous and rotational grazing systems on six 24-ha pastures for each system over a range of stocking rates (0.28 to 0.49 AU ha-1) in 1991 and 1992. We tested the generalization that rotational grazing provides greater managerial control over the frequency, intensity, and uniformity of tiller defoliation compared to continuous grazing. Rotational system pastures were subdivided into 8 paddocks with 4 grazing cycles (3-7 day graze periods) per grazing season. Tillers were sampled biweekly in continuous system pastures and at the beginning, midpoint, and end of each grazing period in rotational system pastures. Multiple regression prediction equations were developed for grazed height, number of defoliation events in a grazing season, percent of tillers defoliated per sampling period (of continuous system) and grazing cycle (for rotation system), and number of defoliation events within a grazing cycle (rotational system). Grazed height decreased as stocking rate increased, but was not influenced by grazing system. The number of cumulative defoliation events per tiller increased with increasing stocking rate over the grazing season. Under similar stocking rates, a higher percentage of tillers were defoliated during the grazing season in the continuous than in the rotational grazing system. Within both grazing systems, percentage of tillers defoliated increased with increasing stocking rates. The percentage of tillers defoliated biweekly in continuous system pastures was similar over the grazing season; the percentage of tillers defoliated per cycle increased as grazing periods lengthened in rotational system pastures. A large number of tillers were defoliated during the second half of each grazing period. Less than 10% of tillers were regrazed within a grazing cycle, even at the highest stocking rate and longest grazing period. Rotational grazing provided greater managerial control over the frequency and uniformity of tiller defoliation; intensity of tiller defoliation was similar between the 2 grazing systems. We hypothesize higher range condition will be maintained over the long-term in rotational system pastures as little bluestem will remain more competitive and productive resulting from fewer defoliation events throughout the grazing season.

Tiller Defoliation Patterns under Frontal, Continuous, and Rotation Grazing

An investigation was conducted to characterize the intensity and frequency of tiller defoliation in 'Plains' Old World bluestem (Bothriochloa ischaemum (L.) Keng) under frontal, continuous, and 2-paddock rotation grazing systems. Frontal grazing allows cattle a continuous opportunity to graze fresh forage via a livestock-pushed sliding fence that allocates and controls grazing within a pasture. Nearly 100% of frontal grazing tillers were defoliated at least once during a 3-hour period as the frontal fence was advanced over the transect ares. The initial defoliation intensity of tillers under frontal grazing was also significantly higher and remaining tiller height less than that of tillers under rotation or continuous grazing (P < 0.05). Tillers under frontal grazing were defoliated at a faster rate compared to rotation or continuous grazing, but cattle had access to them for only 6 to 8 days of the entire grazing season. Season-long defoliation frequency was estimated to be 2.4, 4.6, and 4.7 times for frontal, continuous, and rotation grazing, respectively. Tillers that originated from the perimeter of a tussock were initially taller than those arising from the center (P < 0.05); however, frequency and intensity of defoliation was similar for both tiller locations. Significant relationships were also described between defoliation frequency and stocking rate and between defoliation frequency and herbage allowance. Defoliation frequency increased linearly as stocking rate increased; and conversely, defoliation frequency decreased quadratically as herbage allowance increased. Data from this study suggest that the pattern of tiller defoliation under frontal grazing enhanced forage production which allowed the maintenance of higher stocking rates.

Modelling Patterns of Defoliation by Grazing Animals in Rangelands

This study describes techniques and models for identifying the distribution of grazing by domestic animals in large (> 100 km 2 ) rangeland paddocks in arid central Australia. It also examines the relationship between grazing distribution and the characteristics of the vegetation cover. Models of grazing distribution and resultant defoliation through time use the inverse Gaussian distribution function. Additional model components describe the effects of natural decline in cover over time and the effect of past grazing on the spatial distribution of palatable species. Models are calibrated using remotely sensed vegetation cover data derived from Landsat MSS and closely reflect observed animal distributions (...)

Tobosa Tiller Defoliation Patterns under Rotational and Continuous Stocking

Continuous low animal density grazing of tobosa [Hilaria mutica (Buckl.) Benth.] in the northern Chihuahuan desert results in nonuniform forage utilization. Stocking smaller tobosa rangeland paddocks with high numbers of cattle for short periods of time may facilitate more uniform forage utilization. Two grazing periods in each of 2 consecutive years were monitored to investigate the frequency with which tobosa tillers were defoliated and the intensity of defoliation (change in height) in relation to grazing pressure under high-density seasonal rotational and low-density seasonal continuous grazing. Approximately 40% of tiller height, including leaves, was removed at each defoliation in the rotational treatment, while intensity of defoliation per grazing event remained consistent. In the continuous treatment, amount of tiller removed varied widely and was not consistent among the 4 periods. Percentage of tillers defoliated in the rotational treatment was always greater than 75%, and always less than 30% in the continuous treatment. The probability that a tiller would be grazed at least once in the rotational treatment was more than twice as great as in the continuous treatment. However, within the rotational treatment, the probability of multiple grazing events (greater than or equal to 2) on an individual tiller was less than the probability of a tiller being grazed just once. In general, high-density rotation grazing promoted more uniform. forage utilization of tobosa than low-density continuous grazing.

Relationships Between Patterns of Defoliation by the Fruittree Leafroller (Lepidoptera: Tortricidae) and Foliage Morphology of Baldcypress in Forested Wetlands of Louisiana

Baldcypress, Taxodium distichum (L.) Richard, has been threatened recently by a new insect herbivore, the fruittree leafroller, Archips argyrospila (Walker) (Lepidoptera: Tortricidae). Baldcypress foliage varies from long-leaved, open-faced branchlets with leaves arranged in two-ranked fashion to short-leaved, appressed, imbricate branchlets. Field surveys revealed that the tallest trees contained 72 – 85% appressed foliage, whereas intermediate and small-sized trees contained 55–92% open type foliage. Over a three-year period, open morphology trees repeatedly exhibited significantly higher levels of defoliation (percentage of crown loss) by the fruittree leafroller, compared with appressed morphology trees. There was no ovipositional preference (number of egg masses) associated with morphology type. Differences in defoliation levels, therefore, were attributed to morphologically related influences on foliage suitability for larval stages.

Grazing preference for pasture species by sheep is affected by endophyte and nitrogen fertility

The grazing response of ewe hoggets offered tall fescue, endophyte-infected (+E) or endophyte-free (-E) perennial ryegrass all sown with white clover into a fertile silt loam was examined in April 1990. In a second experiment 2 grazings were observed where 5 grasses had been overdrilled into old lucerne growing on stony infertile land. Pregnaht ewes (August) and ewes with young lambs (October) grazed 3 replicates of tall fescue, +E perennial ryegrass, cocksfoot, phalaris and prairie grass. After the August grazing nitrogen fertiliser was applied at 0 and 300 kg N/ha to 5 x 2 m subplots in each 30 x 9 m grass plot. April observations showed that on day one 76% of grazing time was on tall fescue. During the first 4 days of grazing grass leaf height decreased 60 mm in tall fescue, 43 mm in +E and 42 mm in -E ryegrass while in the last 5 days decreases were 5, 3 and 25 mm. Over the 9 days' grazing, pseudostem height declined 5.2, 1.5 and 5.0 mm, green grass cover from 62 to 30,65 to 44, and 68 to 33%, and herbage removal was 940, 1100 and 1300 kg DM/ ha from tall fescue, +E and -E ryegrasses. During the first 3 days of the August grazing, grass leaf heights of cocksfoot and +E ryegrass declined at a much slower rate than in the other 3 species. The October grazing showed a similar defoliation pattern on plots without N. Addition of 300 kg N/ ha resulted in very rapid defoliation of all 5 species during the first day of the 7-day grazing period. April grazing showed that initially sheep preferred tall fescue. Even though grazing time was similar for +E and -E ryegrasses the hoggets consumed less +E than -E ryegrass, possibly owing to reluctance to penetrate the +E pseudostem horizon. The August and October grazings demonstrated the ability of sheep to discriminate between grass species and strong rejection of cocksfoot and +E ryegrass. However, N at a rate similar to a urine patch produced leaf in all species which was equally attractive to sheep. The reduced intake of +E ryegrass measured in the April grazing may explain some of the lower animal performance of sheep on +E ryegrass pastures. Equally, grazing preference shown by rate of canopy height decline in any pasture is probably highly correlated with pasture intake and animal productivity. Keywords: Acremonium lolii, Bromus willdenowii, Dactylis glomerata, Festuca arundinacea, Lolium perenne, nitrogen-fertilised pasture, Phalaris aquatica, sheep grazing preference