Frequency and severity of defoliation of grass and clover by sheep at different stocking rates

The frequency and severity of defoliation of individual grass tillers and clover plant units was studied in Lolium perenne-Trifolium repens swards grazed by sheep at stocking rates ranging from 25 to 55 sheep ha^-1 and either receiving no N fertilizer or 200 kg N ha^-1. On average, sheep at the highest stocking rate defoliated individual tillers once every 4·2 d compared with once every 9·2 d at the lowest stocking rate with the removal of 58% and 47% of the leaf length of each tiller leaf at these stocking rates. Clover plant units were defoliated once every 4·2 d at the highest stocking rate and once every 7·2 d at the lowest stocking rate with the removal of 51% of its leaves and 12% of its stolon at the high stocking rate and 42% and 4% respectively at the low stocking rate. Differences in frequency and severity of defoliation between N fertilizer treatments were smaller than between stocking rates. Grass tillers and clover plant units were both defoliated less frequently and less severely in swards fertilized with N, though the difference in defoliation frequency between fertilizer treatments decreased as stocking rate increased. Defoliation frequency was related to the length of grass leaf per tiller or number of clover leaves per plant unit, and to the number of these tillers and the herbage on offer.     

Frequency of defoliation of native and naturalized species of the Flooding Pampas (Argentina)

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  ±  8·7, 989  ±  137 and 1591  ±  44·0 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.     

Response of herbage regrowth and water-soluble carbohydrate concentration of ryegrass species to defoliation practices when grown in a Mediterranean environment

Three experiments were conducted to determine the association between leaf number per tiller at defoliation, water‐soluble carbohydrate (WSC) concentration and herbage mass of juvenile ryegrass plants when grown in a Mediterranean environment. Seedlings of ryegrass were grown in nursery pots arranged side‐by‐side and located outside in the open‐air to simulate a mini‐sward in Experiments 1 and 2, and a mixture of annual ryegrass and subterranean clover (Trifolium subterraneum L.) was grown in a small plot field study in Experiment 3. Swards were defoliated mechanically with the onset of defoliation commencing within 28 d of germination. Frequency of defoliation ranged from one to nine leaves per tiller, whilst defoliation height ranged from 30 mm of pseudostem height that removed all leaf laminae in Experiment 1, to 50 mm of pseudostem height with some leaf laminae remaining post‐defoliation in Experiments 2 and 3. A positive relationship between herbage mass of ryegrass, WSC concentration and leaf number per tiller at defoliation was demonstrated in all experiments. In Experiment 1, the herbage mass of leaf, pseudostem and roots of tillers defoliated at one leaf per tiller was reduced to 0·10, 0·09 and 0·06 of those tillers defoliated less frequently at six leaves per tiller. However, the reduction in herbage mass from frequent defoliation was less severe in Experiment 2 and coincided with a 0·20 reduction in WSC concentration of pseudostem compared with 0·80 measured during Experiment 1. In Experiment 3, the highest harvested herbage mass of ryegrass occurred when defoliation was nine leaves per tiller. Although the harvested herbage from this sward contained senescent herbage, the in vitro dry‐matter digestibility of the harvested herbage did not differ significantly compared with the remaining treatments that had been defoliated more frequently. Leaf numbers of newly germinated ryegrass tillers in a Mediterranean environment were positively associated with WSC concentration of pseudostem and herbage mass. A minimum period of two to three leaf appearances was required to restore WSC concentrations to levels measured prior to defoliation thereby avoiding a significant reduction in herbage mass. However, maximum herbage mass of a mixed sward containing ryegrass and subterranean clover was achieved when defoliation was delayed to nine leaves per tiller.     

THE FREQUENCY OF DEFOLIATION OF INDIVIDUAL TILLERS IN A SET-STOCKED SWARD

A method designed to study the defoliation of individual tillers in a set-stocked sward is described. This was used on S23 ryegrass swards carrying 19 (medium-stocked) and 30 (heavily-stocked) sheep/ac.
The tillers in the heavily-stocked sward were defoliated, on average, every 7–8 days, and those in the medium-stocked sward every 11–14 days. The total green leaf length (GLL) of tillers grazed by the sheep was generally greater than that of tillers not grazed. On average 27 and 40% of the GLL of tillers in the medium- and heavily-stocked swards respectively was removed at each grazing. Older leaves on the grazed tillers were defoliated much less frequently than were younger leaves.
It is suggested that a rational analysis of the relationship between the grazing animal and the grazed sward cannot be made without more work of this nature.     

Effect of nitrogen fertilization on herbage production of tall fescue swards continuously grazed by sheep. 2. Consumption and efficiency of herbage utilization

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.     

Sward height, structure and leaf extension rate of Lolium perenne pastures when grazed by overwintering barnacle geese

Abstract The grazing of agricultural pastures during winter and spring by geese is considered an important agricultural problem in parts of the U.K. This study describes the sward structure, leaf extension and senescence rates of Lolium perenne‐dominated pastures that are frequently grazed by barnacle geese (Branta leucopsis) during winter in South‐west Scotland, as well as the conducting of a field experiment that simulated grazing to investigate the effects of defoliation. Gross leaf extension and senescence rates were strongly related to temperature, daylength and average tiller size, resulting in positive values of net leaf extension rate per tiller for most of the winter. Total tiller length declined from November to January but increased from January to April. Sward height, however, declined consistently from October to April, suggesting that swards were becoming trampled by repeated visits by flocks of geese over this time. The structure of individual tillers was found to vary slightly over the winter, with tillers becoming more dominated by younger leaves towards the end of the winter. Experimental defoliation of tillers suggested that absolute leaf extension rates did not respond in an under‐ or over‐compensatory manner, even when tillers were nearly completely defoliated. The results suggested that sward structure and leaf extension rates are not unduly affected by repeated grazing by overwintering geese and that short‐term depletion and trampling are the main impacts.     

Defoliation and productivity of a phalaris-subterranean clover sward, and the influence of grazing experience on sheep intake

Swards of Phalaris aquatica-Trifolium subterraneum were subjected to four defoliation treatments—zero, low (11 sheep ha⁻¹) and high (22 sheep ha⁻¹) stocking rates, and weekly cutting. At high stocking rate the annual grass Hordeum leporinum dominated while clover was dominant at low and zero stocking rates. Weekly cutting suppressed species other than clover and so failed to simulate grazing.
There were similarities in net herbage production between zero and lightly grazed swards and between heavily grazed and repeatedly cut swards. Net herbage production decreased in the order undisturbed sward < lightly grazed sward < heavily grazed sward < repeatedly cut sward.
When sheep grazed swards where herbage mass was low their daily consumption of herbage, and therefore liveweight change, depended on their recent grazing experience. Sheep accustomed to swards where herbage mass was low ate more because they grazed for much longer each day than unaccustomed sheep, although they selected a diet of similar digestibility.     

The effect of nitrogen fertilization upon the herbage production of tall fescue swards continuously grazed with sheep. 1. Herbage growth dynamics

The effect 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. Clarìne and cv. Barcel, and, in Year 2, with two different sward heights or leaf area indices (LAIs). During each of five experimental periods (2-3 weeks), swards received either optimum (N2) or deficient (N1) 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.     

Interaction between water-soluble carbohydrate reserves and defoliation severity on the regrowth of perennial ryegrass (Lolium perenne L.)-dominant swards

A field-study was undertaken in Hamilton, New Zealand to determine if there was an interaction between water-soluble carbohydrate (WSC) reserve content and defoliation severity on the regrowth of perennial ryegrass-dominant swards during winter. Perennial ryegrass plants with either low or high WSC content were obtained by varying the defoliation frequency. At the third defoliation at the one-leaf stage and at the first defoliation at the three-leaf stage (harvest H₁), swards were mown with a rotary lawnmower to residual stubble heights of 20, 40 or 60 mm. All swards were then allowed to regrow to the three-leaf stage before again defoliating to their treatment residual stubble heights (H₂). Frequently defoliated plants contained proportionately between 0·37 and 0·48 less WSC in the stubble after defoliation, depending on the severity of defoliation. There was no interaction between WSC content and defoliation severity for herbage regrowth between harvests H₁ and H₂. Herbage regrowth was lower from swards containing low WSC plants compared with high WSC plants (2279 vs. 2007 kg DM ha⁻¹). Furthermore, swards defoliated to 20 or 40 mm had greater herbage regrowth compared with those defoliated to 60 mm (2266, 2249 and 1914 kg DM ha⁻¹ for swards defoliated to residual stubble heights of 20, 40 and 60 mm, respectively). Regrowth of perennial ryegrass was positively correlated with post-defoliation stubble WSC content within defoliation severity treatment, implying that WSC contributed to the defoliation frequency-derived difference in herbage yield. However, the effect of defoliation severity on herbage regrowth was not associated with post-defoliation stubble WSC content.     

Growth, colonization and productivity of two white clover cultivars strip-seeded into an upland Festuca-Agrostis sward

Small plots of a Festuca-Agrostis upland sward on a peaty gley podsol were strip-seeded during late June 1986 with white clover cvs Aberystwyth S184 or Menna at 4 kg ha⁻¹ and defoliated early (20 August) or late (3 September) and then frequently or infrequently (every 2 weeks or 4 weeks) until the end of September. All plots were defoliated in early November, at 3-weekly intervals during the growing season in 1987 and then grazed rotationally during 1988.
Satisfactory seedling establishment, representing 46% emergence, was achieved 5 weeks after sowing. The differential defoliation regimes had no persistent significant effects on clover development. S184 soon produced more leaves per seedling than Menna and a smaller proportion of its leaf number and weight were removed at each defoliation. Following large losses of leaves over the 1986–87 winter, SI84 had significantly more leaves per stolon than Menna; subsequently it also colonized the sward at a quicker rate. During 1987 amounts of herbage harvested (6.1 t ha⁻¹) were similar with the two clover cultivars, with S184 contributing 47% and Menna 44% of this respectively. SI84 made a larger contribution to yield during May and June but Menna was more productive during September and October. During 1988 clover populations were maintained with rotational grazing without additional fertilizer inputs.
The results show that, despite initial soil and climatic contraints, both small and medium-leaved clovers can be strip-seeded into upland swards with large subsequent benefits to yield and herbage quality. However, they also indicate the need for further experiments to determine the influence of sward morphology and defoliation regime on stolon branching rates and accumulation of growing points which, in turn, govern sward colonization.     

Effects of spring defoliation and fertilizer nitrogen on the growth of white clover in ryegrass/clover swards

An examination was made of the effects of different spring treatments on the growth of white clover in a ryegrass/white clover sward. Plots were either cut once (in February, March or April) or twice (in February and April) or left uncut. Nitrogen was applied to half of the plots in each instance. The clover was sampled at intervals of approximately 3 weeks from February to June to determine numbers of leaves and growing points and weights of plant parts. Rates of leaf appearance were also observed and estimates were made of total herbage mass from ground-level cuts.
Percentages of white clover in the herbage were higher in unfertilized than in fertilized plots and in defoliated than in undefoliated plots. The percentage increases that followed defoliation were usually maintained into later regrowth, showing that clover content was not automatically reduced as herbage mass increased. Increases in growing points were recorded after the beginning of April in defoliated unfertilized plots but not in undefoliated fertilized plots or in plots fertilized and defoliated twice during the spring period, in which numbers fell substantially.
Inverse relationships were found between rates of leaf appearance, or the number of green leaves retained per stolon, and herbage mass, whereas heights of clover and grass leaves and the percentage of dry matter allocated to petiole rather than leaf in the clover increased with increasing herbage mass.
We suggest that the observed differences between spring treatments in clover percentage result primarily from their differential effects on the formation and death of tillers and growing points in the early stages of regrowth.     

Chloris gayana Kunth under different defoliation regimes. Morphogenesis,sward structure and leaf area index

Subtropical pastures are an important alternative to increase forage yields to fulfil cattle nutritional requirements. Despite the increasing expansion of these pastures in the semiarid subtropical region of Argentina, there is very little information about their responses to grazing management. The aim of this study was to evaluate the effect of different defoliation regimes on morphogenesis, sward structure and leaf area index of one of the most expanded forage species in this region, Chloris gayana Kunth. A combination of two defoliation frequencies (300 and 500 GDD) and two defoliation intensities (1 and 3 green stubble leaves) was compared by a controlled experiment that comprised 1,500 GDD. Defoliation frequency significantly affected leaf elongation rate (LER) and leaf area index (LAI). Under the high defoliation frequency, LER and LAI resulted almost half than under low defoliation frequency (0.34 ± 0.08 vs. 0.67 ± 0.08 mm·tiller^?1·GDD^?1; 8.31 ± 2.27 m²/m² vs. 13.27 ± 1.59 m²/m², at 300 or 500 GDD respectively), regardless of the intensity. Defoliation frequency or intensity did not affect leaf appearance rate, leaf lifespan, leaf size, number of green leaves per tiller nor tiller density at the end of the experiment. We conclude that to maintain high LER and LAI in Chloris gayana Kunth cv. Épica INTA‐Pemán pastures, defoliation frequency could be of 500 GDD. Since leaf lifespan was 415 ± 110 GDD, under this defoliation frequency, a maximum accumulation of green leaf tissues with very little dead tissues may be achieved.     

Animal production evaluation of herbage varieties. 1. Comparison of Aurora with Frances, Talbot and Melle perennial ryegrasses when grown alone and with clover

Comparative herbage characteristics and sheep production (ewes and lambs until weaning and lambs thereafter) from swards of Aurora (very early flowering), Frances (early flowering), Talbot (intermediate flowering) and Melle (late flowering) varieties of perennial ryegrass ( Lolium perenne ) were studied under a continuous variable stocking management based on sward surface height guidelines. The varieties were assessed as grass-only (215 kg N ha⁻¹ fertilizer) and grass/clover (83 kg N ha⁻¹ fertilizer) pastures.
Over 2 years (1985-86) total annual lamb production per hectare from grass-only swards of Aurora was 19% more than that from Frances despite similar herbage productivity. Differences between the varieties in lamb output were more pronounced as grass/clover swards, with Aurora producing 29 and 18% more lamb than Melle and Talbot respectively and with Frances also giving 16% more than Melle. Overall lamb production from grass/clover swards was 10% more than that from the grass-only pastures, mainly due to 69% advantage in individual lamb growth rates after weaning. Herbage organic matter digestibility, during the post-weaning period, was higher on grass/clover than on grass-only swards, and similarly with Aurora versus the other varieties. Under a frequent cut simulated grazing regime the relative herbage productivity of the four varieties differed, with Aurora 13% less productive than Frances.
The results are discussed in relation to the limitations of assessing performance under cutting, and the significance of the interaction in animal productivity between the two sward types.     

The influence of sward condition on rates of herbage growth and senescence in mixed swards under continuous stocking management

A study was conducted to determine the influence of herbage mass under continuous stocking management with sheep on rates of herbage growth (G), senescence (S) and net production of green herbage (NP) in a mixed-species sward of Lolium perenne. Poa annua and Trifolium repens. Plots were maintained in as nearly constant herbage mass as possible at 500, 700, 1000 and 1700 kg organic matter (OM) ha⁻¹ by continuous but variable stocking with sheep for the period May to July inclusive. Estimates of G, S and NP were obtained for each species from repeated measurements over two 2-week periods on individually identified grass tillers or clover stolons. Estimates of species population densities enabled estimates of G. S and NP per unit area to be made.
Rates of G were higher and of S were lower for Lolium tillers and Trifolium stolons than for Poa tillers. The laminae of Poa tillers occupied inferior positions in the canopy of the swards maintained at higher herbage masses and were less accessible to the grazing animals, so that Poa tissue was less well utilized.
The total population densities of tillers and stolons declined at herbage masses both above and below 700 kg OM ha⁻¹. The rate of G for the three species combined increased with herbage     

Sward growth under cutting and continuous stocking managements: sward canopy structure, tiller density and leaf turnover

The change in structure of continuously grazed versus infrequently cut swards of perennial ryegrass ( Lolium perenne L), cv. S23, was investigated during their first full harvest year. Measurements were made from early May until late September. The intensity of stocking by sheep in the grazed sward was adjusted in an attempt to maintain a high level of radiation interception and the cut sward was harvested at approximately monthly intervals.
The herbage mass, lamina area index and radiation interception of the cut sward varied in a cyclic pattern between harvests but in the grazed sward these parameters showed considerably less variation, although they all increased early in the season and then declined later. The proportion of dead material above ground increased throughout the season in both sward types but was more marked in the grazed sward.
There were major differences between the grazed and cut swards in the number of tillers per unit ground area; the difference became more marked throughout the season and by September the tiller densities in the grazed and cut swards were 3·20⁴ m^-2 and 6·20³ m^-2 respectively. Divergence in tiller density was associated with differences in specific stem weight and leaf area per tiller.
Rates of appearance and death of leaves on tillers in the grazed sward were determined. During May, leaf appearance exceeded leaf death but this was reversed in June. During the rest of the season as a new leaf appeared on a tiller so the oldest leaf died.     

Efficiencies of ryegrass and white clover herbage utilization in mixtures continuously grazed by sheep

A method for measuring the growth, senescence and defoliation fluxes in a mixture consisting of alternate rows of perennial ryegrass (Lolium perenne) and white clover (Trifolium repens) was used to calculate the actual and potential efficiencies of grass and clover utilization under continuous sheep grazing. White clover contributed relatively more to the growth than to the herbage mass of the mixture and its relative growth rate was usually significantly greater than that of its companion grass. The primary reason for the greater potential efficiency of herbage use of the white clover component was, however, its lower rate of senescence. Greater potential efficiency was not reflected in a greater actual efficiency because the contribution of white clover to the total herbage removed was always smaller than its contribution to the growth flux of the mixture. Despite the commonly assumed sheep preference for white clover, the legume was usually the least defoliated species, presumably because of the lower surface height of white clover compared with grass, which resulted in a lower vertical availability of white clover, thereby restricting its defoliation rate. The positive net herbage accumulation observed with white clover at a constant sward surface height occurred partly through stolon and bud development and partly through the growth of undefoliated leaves from axillary growing points and contributed to a net herbage accumulation per unit dry weight in white clover greater than that in ryegrass. This resulted in an increase in the white clover content of the sward, which was observed independently by destructive measurements.     

Effect of defoliation management, based on leaf stage, on perennial ryegrass (Lolium perenne L.), prairie grass (Bromus willdenowii Kunth.) and cocksfoot (Dactylis glomerata L.) under dryland conditions. 1. Regrowth, tillering and water-soluble carbohydrate concentration

A field study was undertaken between April 2003 and May 2004 in southern Tasmania, Australia to quantify and compare changes in herbage productivity and water‐soluble carbohydrate (WSC) concentration of perennial ryegrass (Lolium perenne L.), prairie grass (Bromus willdenowii Kunth.) and cocksfoot (Dactylis glomerata L.) under a defoliation regime based on leaf regrowth stage. Defoliation interval was based on the time taken for two, three or four leaves per tiller to fully expand. Dry‐matter (DM) production and botanical composition were measured at every defoliation event; plant density, DM production per tiller, tiller numbers per plant and WSC concentration were measured bimonthly; and tiller initiation and death rates were monitored every 3 weeks. Species and defoliation interval had a significant effect (P < 0·05) on seasonal DM production. Prairie grass produced significantly more (P < 0·001) DM than cocksfoot and ryegrass (5·7 vs. 4·1 and 4·3 t DM ha^?1 respectively). Plants defoliated at the two‐leaf stage of regrowth produced significantly less DM than plants defoliated at the three‐ and four‐leaf stages, irrespective of species. Defoliation interval had no effect on plant persistence of any species during the first year of establishment, as measured by plant density and tiller number. However, more frequent defoliation was detrimental to the productivity of all species, most likely because of decreased WSC reserves. Results from this study confirmed that to maximize rates of regrowth, the recommended defoliation interval for prairie grass and cocksfoot is the four‐leaf stage, and for perennial ryegrass between the two and three‐leaf stages.     

The effects of defoliating grass in winter or spring on herbage yields and ensilage characteristics

Under Irish conditions, the digestibility in May of grass managed for silage production is sometimes lower than expected. In each of two successive years, replicate field plots were established to examine the effects of three defoliation heights (uncut or cut to a stubble height of 10 or 5 cm) applied in winter and/or spring on herbage yields harvested in May and again in July, and on chemical composition and conservation characteristics associated with first‐cut silage. Swards that were not defoliated in December or March had a dry‐matter (DM) yield and in vitro DM digestibility (DMD) in mid‐May of 6597 kg ha^?1 and 736 g kg^?1, respectively, in Year 1, and corresponding values of 7338 kg ha^?1 and 771 g kg^?1 in Year 2. Defoliating swards to 5 cm in December reduced (P < 0·001) May DM yields compared to swards that were not defoliated in both December and March, while herbage DMD in May increased (P < 0·001) when defoliated in December or March. There were no clear effects of defoliation height or its timing on herbage ensilability or resultant conservation efficiency characteristics. The effects of defoliation on July yield were the reverse of those observed for May, while the total yield of the December and March defoliations plus the two silage harvests increased as defoliation height was lowered in Year 2 only. It is concluded that defoliation in winter and/or spring can increase herbage digestibility but will likely reduce DM yields in May.     

Defoliation pattern, foraging behaviour and diet selection by lactating dairy cows in response to sward height and herbage allowance of a ryegrass-dominated pasture

The effects of sward surface height (SSH) and daily herbage allowance (HA) on the defoliation pattern and grazing mechanics of early lactation dairy cows grazing on irrigated perennial ryegrass–white clover pasture were studied. The hypothesis tested was that SSH and HA affect intake and diet selection through their effects on the pattern of defoliation which is influenced by the resistance to prehension bites. Factorial combinations of two initial SSH (14 and 28 cm) and two daily HA (35 and 70 kg DM cow^?1 d^?1) were examined in a replicated experiment. The peak longitudinal tensile force required to break the sward portion encompassed in a 100 cm² area [bite fracture force (BFF₁₀₀)] was measured as an index of the resistance to prehension. The volume of herbage defoliated and herbage intake increased with SSH (P < 0·05) and HA (P < 0·01). Corresponding to an increase in HA from 35 to 70 kg DM cow^?1 d^?1, there was a proportional increase in the total defoliation area (TDA) and intake by 0·24 and 0·55 in the short sward compared with 0·16 and 0·32 in the tall sward respectively. The results of this experiment suggest that a consistent spatial pattern of reduction of the canopy exists during defoliation by cows and that the volume of sward canopy defoliated is the major variable affecting herbage intake. The BFF increased down the sward profile at a rate that was higher (P < 0·05) for the taller sward than for the shorter sward. It is proposed that a relatively lower resistance to prehension in the short sward compared with the tall sward explains the greater proportionate increase in TDA and intake corresponding to an increase in HA. The rate at which BFF₁₀₀ increases down the sward profile is suggested as a sward physical variable that can influence the defoliation process. The estimated time and energy costs of prehension bites are discussed in the context that larger bites are handled more efficiently than smaller bites.     

THE RELATIONSHIP BETWEEN COMPETITION AND DEFOLIATION IN PASTURE

Field trials were carried out on a downland permanent pasture at the Grassland Research Institute, Hurley, between 1953 and 1955, to investigate the factors involved in competition in pasture undergoing periodic defoliation. Response to competition was estimated by comparing the yield of species units (either individual plants or close groups of shoots) growing normally in the pasture with the yield of matched units from which the surrounding herbage had been removed. The species studied were cocksfoot, red fescue, creeping bent, plantain, ox-eye daisy and bulbous buttercup. In addition to the estimation of normal competition for environmental resources in pastures of differing height, two special effects upon species due to the presence of surrounding herbage were investigated following unusual results from competition tests in swards under periodic defoliation. First, the influence of surrounding herbage in modifying the growth habit of a species, causing it to grow in a more erect manner, and consequently to suffer more on defoliation at a specific height than in the absence of competition. Secondly, a decreased yield after defoliation, not associated with changes in the growth habit of the species, which was ascribed to an increase in top/root ratio brought about by shading during the preceding period of competitive growth. The effect of selective defoliation of a species in relation to the herbage surrounding it was also studied. Differences in the degree of defoliation of a species and its surrounding herbage had a marked effect upon the degree of competition exerted by the latter upon the former, increasing it when the species was preferentially defoliated and decreasing it when the surrounding herbage was preferentially defoliated. The factors involved in pasture competition under periodic defoliation are listed and discussed.