High Grazing Intensity Research Articles

Effects of White‐Tailed Deer on Populations of an Understory Forb in Fragmented Deciduous Forests

The effects of grazing by white‐tailed deer (Odocoileus virginianus) on populations of Trillium spp. were examined in remnant, old‐growth patches of the highly fragmented Big Woods forest ecosystem in southeastern Minnesota. We conducted three separate studies involving an exclosure experiment, transplant experiments, and comparisons of Trillium populations among study sites. The highest grazing intensity was observed where deer occurred at high overwinter concentrations (~25–35/km2); significantly lower grazing intensities occurred at low overwinter density (~5–10/km2). Deer focused their grazing on large, reproductive plants; at sites with high deer density, Trillium population structure was skewed toward small plants, and deer consistently caused over 50% reduction in reproduction during the growing season. Protection of individual plants from deer for two growing seasons resulted in dramatically increased flowering rates and significantly greater leaf area compared to control plants. No significant impact of current‐year herbivory on reproduction in the following year was detected. Nevertheless, flowering rates at one site with high overwinter deer densities for at least the past 5 years suggest that the cumulative effects of grazing over several years can reduce reproduction in subsequent years. Transplant experiments with Trillium grandiflorum also showed that deer had significant effects on growth and reproduction where deer occur at high density. Our results suggest that changes in landscape structure and local deer abundance have altered plant‐deer relationships such that grazing can lead to the local extirpation of sensitive forbs such as Trillium spp. As a result, active, long‐term management of deer at low densities appears necessary for the conservation and restoration of fragmented forest communities in eastern North America. Efectos del Venado Cola Blanca en Poblaciones de Trillium spp. en Bosques Decidous Fragmentados Los efectos de ramoneo por venado cola blanca (Odocoileus virginianus) en poblaciones de Trillium spp. fueron examinadas en parches remanentes de bosque maduro del ecosistema altamente fragmentado del bosque Big Woods en el sureste de Minnesota. Realizamos tres estudios separados que involucraron un experimento en un encierro, experimentos de transplante y comparaciones de poblaciones de Trillium entre sitios de estudio. La intensidad del ramoneo más alta se observó donde los venados se concentraron para pasar el invierno (~25–35/km2), mientras que intesidades menores de ramoneo ocurrieron a bajas densidades durante el invierno (~5–10/km2). Los venados enfocaron su consumo en plantas reproductivas grandes; en sitios con alta densidad de venados la estructura poblacional de Trillium estuvo sesgada a plantas pequeñas y los venados consistentemente ocasionaron una reducción mayor al 50% en la reproducción durante la temporada de crecimiento. La protección de plantas individuales de la depredación de los venados por dos temporadas de crecimiento resultó en un dramático incremento en la tasa de florecimiento y una cobertura de hojas significativamente mayor comparando con plantas control. No se detectó un impacto significativo de la herbivoría del año en curso en la reproducción del año siguiente. Sin embargo, las tasas de florecimiento en un sitio con alta densidad de venados durante el invierno por al menos cinco años consecutivos sugiere que el efecto acumulativo del ramoneo a lo largo de varios años puede reducir la reproducción en años subsecuentes. Experimentos de transplantes con Trillium grandiflorum mostraron también que los venados tienen un efecto significativo en el crecimiento y reproducción en sitios donde los venados ocurren el altas densidades. Nuestros resultados sugieren que los cambios en la estructura del paisaje y la abundancia local de venados han alterado las relaciones planta‐venados a un punto en el que el ramoneo podría conducir a la extirpación local de especies sensitivas como lo son Trillium spp. Como resultado, aparenta ser necesario el manejo activo a largo plazo de venados a bajas densidades para la conservación y restauración de comunidades de bosques fragmentados en el este de Norteamérica.

Simulated effect on dairy cow and herd production of different grazing intensities

The effect on cow and herd level of different grazing intensities was evaluated using results from short term grazing experiments combined with different dairy herd management strategies from twelve commercial herds using a dynamic stochastic model (SIMHERD) for the simulation of dairy herd production and a deterministic model (SIMCOW) for the simulation of cow production. High grazing intensity had a negative effect on milk production and live weight gain on cow level in all twelve herds, but the energy efficiency and the distribution between milk production and live weight gain varies between herds. The same general effects were simulated on herd level, but there was no direct connection between the results on cow level and on herd level on the production in the grazing period of different intensity. The annual results of different grazing intensities showed a positive carry-over effect on milk production of decreased grazing intensity but a negative effect on live weight gain.

The impact of grazing by macropods on coastal foredune vegetation in southeast Queensland

Abstract The extent of grazing by two macropodids, the agile wallaby (Macropus agilis) and the swamp wallaby (Wallabia bicolor) on coastal foredunes on South Stradbroke Island in southeast Queensland was investigated to determine potential impacts on the principal sand colonizing species, sand spinifex grass (Spinifex sericeus). Grazing on spinifex grass on the foredunes of South Stradbroke island can be attributed principally to agile wallabies. Foraging activity by wallabies was higher in areas of high spinifex abundance, however, grazing intensity and impact on spinifex was only important on foredunes with low spinifex abundance. Spinifex consumption by wallabies was also related to a number of factors, especially composition and structure of vegetation in adjacent habitats. Spinifex consumption increased when the abundance of ground cover components (grasses, sedges, forbs) in adjacent habitats was low and structural complexity was high. Grazing on foredunes by wallabies significantly affects the species composition of the foredune community by excluding the establishment of a number of perennial foredune plant species. This may have implications for community succession in coastal ecosystems.

Stocking Rate Risk for Pasture-Fed Steers under Weather Uncertainty

A biophysical model, GRAZE, is used to simulate beef forage performance for stocker steers pastured on common bermudagrass. Eight alternative stocking rates, ranging from low to high grazing intensity, are simulated over 14 simulated over 14 states of nature using historical weather data. The impact of weather variability on animal weight gain and economic performance is assessed and empirical cumulative distributions of net returns are developed. The risk efficient stocking rate strategies are identified for alternative decision-maker risk using generalized stochastic dominance. Under improved pasture conditions in Arkansas, results show that (a) expected weight gain per head is largely independent of grazing intensity until a critical stocking rate (6 hd/ha) is attained; (b) the highest expected net return per hectare is achieved under a lower stocking rate rate (10 hd/ha) than one which results in highest expected weight gain per hectare (12 hd/ha); and, (c) an increase in the stocking rate is accompanied by greater production (weather) risk which is reflected in increased variance of weight gain and net returns as well as a higher frequency and magnitude of economic losses.

Effects of season and intensity of sheep grazing on tree regeneration in a British upland woodland

This paper reports some effects of experimental alteration of season (summer, winter) and intensity of sheep grazing on tree regeneration, growth and browsing damage in an upland broadleaved woodland in Cumbria, UK. During the first 3 years of grazing treatments, seedling recruitment was primarily Betula pubescens, Sorbus aucuparia and Fraxinus excelsior. After 7 years of grazing treatments, seedlings and saplings of many species were numerous, with fewer seedlings in winter-grazed plots but a higher proportion of these were browsed. More seedlings had reached sapling stage in winter-grazed plots. Sapling numbers increased with decreasing grazing intensity, with lower proportions browsed at low grazing intensities. Within the first 3 years grazing treatments, recruitment of Betula pubescens seedlings was greater in grazed than ungrazed plots, and greater in winter-grazed than in summer-grazed plots. Browsing damage to these seedlings increased with grazing intensity in winter, and height increments were greatest in ungrazed plots. After 7 years, B. pubescens seedlings and saplings showed no significant treatment effects. After 3 years, recruitment of Sorbus aucuparia seedlings was greater in grazed than ungrazed plots, and greater in summer-grazed than winter-grazed plots. Fewer seedlings were browsed in low grazing intensity plots, and seedling height increments were greatest in ungrazed plots. After 7 years, more S. aucuparia seedlings were browsed in winter than summer, but saplings showed no treatment effects. After 3 years, recruitment of F. excelsior was greatest in low grazing intensity plots and smallest in medium grazing intensity plots. More seedlings were browsed at higher grazing intensities, but in summer-grazed plots height increments tended to be greatest in the most heavily grazed plots. After 7 years, F. excelsior seedlings were more numerous in summer-grazed plots but were not differentially browsed; sapling numbers were not affected by grazing treatments but browsing damage was very high in all except summer low grazing plots. The implications of the results are discussed in relation to woodland grazing management issues.

Grazing behavior of horses on pasture: Predisposition to strongylid infection?

Summary This study was designed to determine how the behaviors of horses on a fenced-in pasture in the northeastern United States may contribute to infection by Strongylus vulgaris . The infective stages of strongylid parasites of horses develop in the feces, and they must be ingested for infection to occur. Domestic horses are thought to be coprophobic, and to avoid grazing in areas contaminated with their feces. 1,2 In this study, nine Standardbred colts were observed on two pastures three times a week during two observation periods (January, 1995 and February through mid-March, 1995). Nine adult Standardbred mares and geldings were also observed on the same pastures during one observation period from April through mid-May, 1995. On each pasture, the roughs (areas of group defecation associated with long grass), lawns (short-cropped grass) and bare areas (where hay was fed in racks), were mapped. Grazing occurred equally on roughs, lawns, and bare patches of the pasture in two of the three observation periods (January and April/May). In the February/mid-March observation period, most of the grazing activity occurred on the bare areas, where hay was provided, but there was still grazing in the roughs. Defecation and urination activities occurred at similar frequencies in all areas of the pastures during all observation periods. These data suggest that horses kept at high grazing intensities will graze near feces in the roughs, and will defecate in the “grazing” areas. It was also determined that the infective larvae of S. vulgaris are attracted to horse feces, but show no responses to grass, in laboratory chemical migration assays. These data suggest that the parasites may not migrate from the feces to the grass as part of their transmission strategy as previously reported. Thus, parasite infection may be occurring when horses graze in the roughs or near feces in the lawns and bare areas, and pasture management strategies for controlling these parasites may be most effective if focused on reducing the infective larvae present on the pasture.

Effect of grazing frequency and intensity onLolium perenneL. pastures under subtropical conditions: Herbage quality

Abstract The cellulase dry matter disappearance (CDMD) and herbage nitrogen (N) of Lolium perenne (perennial ryegrass) were evaluated for pastures grown under subtropical conditions for two years, under five combinations of grazing frequency and intensity, each applied in a rotational grazing system. These quality factors were also evaluated for a simulated continuous grazing (CG) system. Low frequency, low intensity grazing produced lower CDMD and herbage N levels than higher grazing frequencies and intensities. These differences were, however, generally small. Overall, levels of herbage digestibility (estimated by CDMD) and N fell within similar ranges to those observed for perennial ryegrass in temperate environments. Seasonal trends in quality were characterized by high levels during late autumn to early summer (May to October) and low levels during mid‐summer to early autumn (January to April). These data would tend to support high frequency grazing as part of the management programme for perennial r...

Effects of season and intensity of sheep grazing on a British upland woodland: Browsing damage to planted saplings

Summary The effects of experimental alteration of season (summer/winter) and intensity of sheep grazing on browsing damage to planted saplings in an upland broadleaved woodland in Cumbria are reported. For saplings of Fraxinus excelsior, Ilex aquifolium and Quercus petraea planted in 1986, browsing of current year's shoots was greatest in summer but browsing of previous years' shoots was greatest in winter. For saplings planted in 1988, season of grazing only significantly affected Quercus petraea, with more damage in winter than summer. Intensity of grazing affected saplings planted in 1988 of all three species, with increased damage at higher grazing intensities. It is suggested that, for sheep, the availability of non-woody forage may be an important determinant of the degree of browsing damage to saplings, which has implications for the choice of herbivore for woodland grazing management.

Predicting Demographic Change in Response to Herbivory: A Model of the Effects of Grazing and Annual Variation on the Population Dynamics of Anthyllis Vulneraria

1 Documenting the impact of herbivores on perennial plants is a particularly acute problem due to the difficulties of quantifying their effects on lifetime reproductive success and survival. 2 Our purpose here is to extend our previous short-term empirical results that two contrasting sheep grazing regimes have a significant effect on seed output but no effect on the establishment and survival of seedlings in a natural population of Anthyllis vulneraria by combining them with stochastic long-term stage-transition models in which the life history stages were classified according to age and developmental state. Our objectives were to predict how: (a) variation in fecundity, rate of germination and seed survival, and (b) the frequency of years with either high or low seed production, may regulate population growth in relation to sheep grazing either as 'one-off' instantaneous effects of high-intensity grazing or via effects due to repeated low intensity grazing. 3 The results of the simulations predict that periods of decline in population growth can be followed by an increase in the rate of population growth with the pattern depending on the sequence of poor and favourable years since these determine annual variation in seed output. The occurrence of consecutive poor years caused a shift in age structure towards a population biased towards older aged plants. As a result, seed output was greatly improved by a subsequent favourable year due to the high reproductive capacity of older plants. 4 The limit that variation in seed output may impose on population growth, which is more severe under repeated free grazing than under a single episode of controlled high intensity grazing, will thus depend on how the population is regulated by the succession of poor and favourable years for seed production. This indicates that the demographic characteristics of natural plant populations may be markedly influenced by an interaction between environmental conditions and herbivory.

Biomass of Grazed, Burned, and Undisturbed Paramo Grasslands, Colombia. I. Aboveground Vegetation

Estimations of the amount of aboveground vegetation mass were made for four piramo grassland sites with different grazing management and burning histories in the Central Cordillera of Colombia. The total mass of live plus dead grassland vegetation showed a decrease from 2820 ? 190 g m-2 at the undisturbed to 868 + 73 g m-2 at the intensively grazed and burned sites. Stem rosette mass was highest at both the undisturbed and the heavily grazed sites (666 ? 168 and 1029 + 245 g m-2, respectively), but considerably lower at the burned sites (397 ? 94 and 285 ? 78 g m-2). Eighty percent of the total undisturbed vegetation mass consisted of standing dead material and litter. The decrease of dead material mass along the disturbance gradient may have large implications for ecosystem stability. The proportion of live material increased along the grazing and burning gradient, resulting in a similar live material mass at all sites. Under high grazing intensities and in the absence of burning, the vegetation can transform into ground-covering mats, attaining a moderately high biomass. Where burning took place, this transformation does not occur, and both grassland and stem rosette biomass were reduced, leaving many patches of bare ground.

Integrated pastoral management strategies for Hieracium control

There are four main options for controlling Hieracium: agricultural development, herbicides, biological control and grazing management. Grazing management appears one of the most feasible current strategies for limiting Hierucium spread on low-input land. Exclosure studies in Canterbury and.Otago show grazing can reduce Hierucium flower density 40-fold, limiting expansion by seed. Autumn recruitment from seed varied from 46 to 95 seedlings /m* , suggesting this may be an important dispersal mechanism in wet summers. Low-intensity spring-summer grazing significantly reduced plant number and ground cover of upright Hieracium species, but not prostrate species. Conversely, high-intensity grazing may assist establishment. In a 16-year grazing trial in Otago, Hierucium cover, an4 that of similar flatweeds, was greatest at high stocking rates. Farming with Hierczcium in the future will require development of grazing strategies integrating the requirements for the optimal management of different Hieracium species. Keywords: Hieracium, hawkweed, high country, grazing management

Simulated Effects of Grazing on Soil Nitrogen and Mineralization in Contrasting Serengeti Grasslands

The roles of edaphic characteristics and grazing in regulating nitrogen cycling and belowground processes were simulated for the shortgrass and tallgrass regions of the Serengeti National Park, Tanzania, by coupling a model of grassland productivity with a model of belowground decomposition processes. Grasslands of each region were simulated under ungrazed and observed grazing intensities to establish reference points for sensitivity analyses using a wide range of grazing intensities and different microbial biomasses. The consequences of patchy carbon and nitrogen deposition were also simulated. Results indicate that nitrogen cycling processes are affected differentially by grazing in the shortgrass and tallgrass regions. Net nitrogen mineralized by the microbial biomass in the shortgrass simulations declined from the ungrazed condition at low grazing intensities, and proceeded to increase from moderate to high grazing intensities. Plant nitrogen uptake was significantly correlated with net mineralization, and soil mineral nitrogen levels were greatest at moderate grazing intensities. In tallgrass simulations, soil mineral nitrogen increased with grazing intensity; net nitrogen immobilization occurred over much of the grazing intensity range because of low decomposition substrate quality. Tallgrass plant nitrogen uptake responded to urine nitrogen input to the soil and was uncorrelated with net mineralization. The potential for nitrogen competition between grasses and microbes was greater in the tallgrass than in the shortgrass because of lower total system nitrogen and lower grazing intensity in the tallgrass. The ecosystem—wide soil nitrogen gradient of the Serengeti governs most properties of nitrogen cycling, while within—region edaphic conditions and grazing create significant variation in local ecosystem processes.

Water-Extractable Organic Matter from Plant Litter and Soil of Rough Fescue Grassland

Little is known about the chemical composition of throughfall, or the water that falls through, and drips from, the grass canopy of Rough Fescue Grassland during the grazing season. Water-extractable C, N, organic acids, and monosoccharides from litter and from soil in the upper 2 cm of the Ah horizon collected at monthly intervals in 1988 were at Stavely, Alberta. Rough fescue (Festuca campestris Rydb.) were stocked at tither light (1.2 AUM/ha) or very heavy (4.8 AUM/ha) fixed rates for 39 years or were ungrazed in exclosures located within each field for an equal period of time. At the high grazing intensity, the soil and litter N was less water-extractable. The C/N ratios of the water-extractable organic matter from litter and soil averaged 11.2 and 2.3, respectively. Soil monosaccharides were essentially not water-extractable. The quality of the litter as reflected by the water-extractable constituents often differed over the season between fields. Observations at regular time intervals are essential. The effect of the quality of leachates of litter on soil was not predictable. The 3 major long-chain fatty acids identified, palmitic, stearic, and arachidic acids, from soil in grasslands that are in good condition because of the low grazing pressure, could well contribute to the resistance of those grasslands to the encroachment of invading species.

Spread of Introduced Lehmann Lovegrass Along a Grazing Intensity Gradient

1. Changes in density of Lehmann lovegrass and native grasses, and the proportion of lovegrass present along a livestock grazing intensity gradient were measured on six occasions in permanent plots during 1972-90, on the Santa Rita Experimental Range, Arizona, USA. The gradient included grazing exclosures and plots radiating away from a cattle watering point. Lehmann lovegrass spread from sowings done during 1945-59 4km from the water point and an average 1 8 km from the exclosures. 2. Lovegrass density increased with time but was not affected by different grazing intensities. Native grass density decreased, and lovegrass relative abundance increased with time and as grazing intensity increased. Lovegrass density and relative abundance did not differ between adjacent ungrazed and grazed areas. 3. Livestock grazing was not necessary for Lehmann lovegrass to spread, but the lovegrass relative abundance was greater at higher grazing intensities.

Supplementation Programs for High-Intensity Grazing Systems and High-Quality Forages-A Review

Newer grazing systems such as Intensive Early Stocking and Short Duration Grazing for native ranges and improved pastures typically involve heavier stocking densities than extensive systems. High-quality forages such as small grains are almost always stocked heavily. Because intensively grazed cattle are more likely to be marginally over-stocked and consume slightly less daily forage than cattle grazed extensively, the effect of a supplement on forage intake and digestibility must be considered very carefully. Effects of supplementation on gain per ha must be considered along with gain per animal. Feeding supplements to cattle on high-quality pastures and intensively grazed patterns is most likely to be efficient when forage is limited and supplementation permits a greater carrying capacity. The best supplement ingredients will be those high in digestible fiber and low in starch because forage utilization is not adversely affected. The supplements are safe and gain per ha as well as gain per head may be efficiently enhanced.

Influence of Nutrients and Grazing on the Response of Stream Periphyton Communities to a Scour Disturbance

We examined the effect of nutrient input and grazing intensity on resistance and resilience of periphyton communities in laboratory streams to a scour disturbance. Previous theoretical studies using stream models indicated that resilience of nutrient-limited streams, as defined by the return time to the initial state, is strongly related to the residence time of nutrients in the system and to the presence of herbivores. To experimentally test this prediction we established two levels of nutrient input by altering the flow regime (once-through flow and 98% recirculated flow) and two levels of grazing intensity by the presence or absence of the snail Elimia clavaeformis, resulting in a 2 × 2 factorial design with two replicate streams per treatment. After the streams reached an approximately stable level of periphyton biomass, a scour disturbance was imposed on all streams. Measures of the resistance and resilience of the periphyton to this disturbance were made. The resistance of most structural characteristics of the periphyton was significantly higher in streams with snails than in streams without snails. Nutrient input had little effect on resistance of all periphyton characteristics. Resilience was measured by two parameters: rate of increase of a particular characteristic over time following the disturbance (r), and time to return to the pre-disturbance state (T_s). The experimental results on resilience were complex. In general, high grazing intensity resulted in low values of r for most periphyton characteristics, but also shorter T_s for some characteristics. The shorter T_s values were attributed to lower pre-disturbance biomass and metabolism in the streams with snails compensating for lower r for these characteristics following the disturbance. Contrary to model predictions, low nutrient input generally did not result in low resilience for most characteristics in the streams without snails, probably because increased nutrient recycling compensated for low inputs. However, for the streams with snails, resilience of most periphyton characteristics was lower in streams with low nutrient input compared with streams with higher nutrient input. This contrast in resilience suggests a positive feedback between biomass accrual and nutrient recycling that helps maintain high productivity despite low nutrient inputs during recovery of stream periphyton communities from disturbance.

DEFOLIATION EFFECTS ON BIOMASS YIELD COMPONENTS OF WINTER WHEAT

Short duration, high intensity grazing is often used to maximize production of grazed forage crops. Relatively little information is currently available for this practice on dual purpose cereal crops such as wheat. Components of soft white winter wheat biomass yield were studied during the 1979 and 1980 crop years near Corvallis, Oregon. Treatments were a factorial arrangement of two defoliation times (mid-April and mid-May) and three defoliation intensities (undefoliated and clipped to either an 8-cm or 4-cm stubble). All defoliation regimes reduced grain yields compared to undefoliated controls. However, biomass yields of 8-cm vs. 4-cm treatments were similar within defoliation dates. Grain yields primarily reflected the most recent defoliation event, suggesting that plant phenological stage and/or the length of the regrowth period following defoliation was more important than the intensity of the defoliation event in determining plant yield.Key words: Short duration grazing, grazing wheat

Grazing Impact by Small Rodents in a Steep Cyclicity Gradient

Small rodents show a gradient of increasing cyclicity in population density northwards in Scandinavia. This has been interpreted as regulation by generalist predators in the south, with increasing snow cover giving way to food limitation in the north. Another hypothesis is that the amount of agricultural land, instead of snow conditions, decides the regulatory ability of generalist predators. The importance of food, snow and agricultural land, in contrast to social regulation, was examined by measuring vole population variations, grazing impact and food selectivity in five Swedish localities characterized by a steep increase in cyclicity. Peak population densities and effects on the habitat increased strongly northwards, with the most pronounced crash also in the northernmost locality. At the same time, peak grazing impact was at least one order of magnitude higher in the northernmost locality than in the southern area. At that high grazing intensity a much broader range of plant species, including lignoses, were attacked by the voles. The increase in densities and grazing was strongly related to snow cover but not clearly to the amount of agricultural land. There were indications that the crash was amplified by disease outbreaks. Taken together, these observations demonstrate increasing population fluctuations and a deterioration of the food situation with increasing snow cover, which cannot be explained by any intrinsic mechanism of population regulation.

The Impact of Controlled Grazing by Clethrionomys rufocanus on Experimental Guilds of Boreal Forest Floor Herbs

Thirty plots representing herb-rich middle boreal forest floor vegetation were established in a greenhouse in January 1984. During the spring, the vegetation of the plots grew under simulated summer conditions and was exposed to different intensities of grazing by the vole Clethrionomys rufocanus. Survival of marked shoots was followed and the plots were harvested 10 May. Moderate grazing intensity had practically no impact upon the vegetation. High grazing intensities resulted in decimation of populations of small, broad-leaved herbs with soft leaf texture. Under very high grazing intensity, all herbs with umbrella shape (leaves attached apically to erect shoots) suffered heavily, regardless of toxicity. The results have following implications: (1) Serious food depletion by C. rufocanus during the summer in luxuriant habitats requires population densities which exceed observed maxima about ten times. (2) Toxic substances are of limited value as defense against voles, whereas shoot morphology is crucially important. (3) Differences between the grazing tolerance of umbrella and rosette herbs are great enough to have consequences for the structure of relatively unproductive plant communities where moderate vole densities suffice to create high grazing pressure. In particular, vole grazing may prevent temperate spring herbs, adapted to a cool and short growing season, from invading northern forest and tundra.

Effects of grazing management on herbage production and botanical composition of ‘Grasslands Nui’ ryegrass—paspalum — white clover pasture 1. Effect of intensity of grazing by cattle in different seasons

Abstract The effects of high and medium grazing intensities at different times of the year (except from mid October to end of November) on composition and on net herbage increments (NHI) of a perennial ryegrass — paspalum — white clover pasture was measured at a site near Hamilton. Store cattle were used and the experiment continued for 3.5 years. No treatment, even high grazing intensity from mid March to mid October, significantly affected annual NHI. Possible reasons for this are proposed. High grazing intensity in early summer increased the percentage of paspalum (Paspalum dilatatum Poir.) at the expense of ‘Grasslands Nui’ ryegrass (Lolium perenne L.). The effect was marked where hard grazing was continued throughout the summer. In early spring, hard grazing raised the proportion of paspalum in the sward provided temperatures were high enough for this species to take advantage of the weakened competition from ryegrass. An increase in the proportion of paspalum depressed NHI in early winter without in...