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.     

Inheritance of the concentration of water-soluble carbohydrates and its relationship with the concentrations of fibre and crude protein in herbage of cocksfoot (Dactylis glomerata L.)

The concentration of water‐soluble carbohydrates (WSC) of cocksfoot is lower than that of other temperate grasses. Increasing the WSC concentration in cocksfoot is important in increasing its digestibility and preference by ruminants. The genetic variation in mono‐ and disaccharides, fructans and total WSC concentrations and their inter‐relationships with crude protein (CP) and fibre concentrations of cocksfoot (Dactylis glomerata L.) at the vegetative growth stage in half‐sib cross populations were assessed in two experiments conducted under spaced planting and sward conditions. There was a wide range in the means for concentrations of fructan, WSC and neutral‐detergent fibre (NDF) in parents and progeny whereas there was a narrow range of the means for concentrations of mono‐ and disaccharides, CP and acid‐detergent fibre (ADF). Mean concentrations of mono‐ and disaccharides showed the smallest range. Mean squares of entries in parents and progenies were significant for mono‐ and disaccharides, fructan and total WSC concentrations in all populations. The entry × year interactions for fructan and total WSC concentrations were significant in the parents and the half‐sib progeny. However, the entry × year interactions for mono‐ and disaccharide concentrations in progeny were not significant. Concentration of WSC was under genetic control as mean squares of the concentration values were significant and variance components for all traits were significantly larger than zero. There were negative genetic correlations between WSC and ADF and NDF concentrations. Narrow‐sense heritabilities (h_n) estimated from variance components of progeny for total WSC concentrations were 0·59 and 0·53 in sward and spaced planting conditions respectively. The h_n of fructan and WSC concentrations were similar in both sward and spaced planting conditions, whereas that of mono‐ and disaccharide concentrations varied from 0·20 to 0·69. It was found that the genetic variation of the total WSC concentration in cocksfoot depended mainly on genetic variation in fructan concentration. These results suggested that the forage quality of cocksfoot at the vegetative growth stage was influenced by an additive gene effect and could be improved genetically by recurrent selection.     

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.     

A model describing the utilization of herbage by cattle in a rotational grazing system

A model that describes the utilization of herbage by cattle in a rotational grazing system is presented. The model considers swards as being structured into two phases: a long phase associated with old dung patches, and a short phase. Both phases are treated as consisting of a series of horizontal strata corresponding in depth to a bite depth. The sward is divided into feeding stations consisting of either the long or the short phase. In each, only the surface stratum is available for grazing at each time step. At any time step, the individuals of a herd of cattle, distributed at random, encounter the entire range of strata. The rate of intake of each member of the herd depends on the intake properties of the stratum that it has encountered. The number of cattle that encounter each stratum type is variable so that the mean intake per member of the herd is the weighted mean. The core feature of the model is the simulation of the change over time in the frequency distribution of exposed stratum types and the distribution of grazing across this range of strata. The members of the herd are assumed to select a feeding station based on preference for leafiness of the encountered stata and the phase. The decision to graze or not is based on the comparison between the current vs. the previous feeding station. Model parameter values were based on published data. The proportion of leaf and bulk density of a phase or the strata were determined from an analysis of a sample of sward profiles. Using bite dimension, bite weight, biting rate, search time, feeding station area and selective behaviour, it was possible to simulate sward depletion that is very similar to the observed data from grazed paddocks in experiments in south‐east Ireland. The model of herbage utilization adequately described the changes in intake and sward structure during grazing and it was concluded that it was suitable for use as part of a simulation of a grazing system.     

The use of herbage nitrogen status to optimize herbage composition and intake and to minimize nitrogen excretion: an assessment of grazing management flexibility for dairy cows

A framework for managing rotationally grazed pastures for dairy cattle which enables the cows’ energy and protein requirements to be met while simultaneously limiting the amount of N excreted in order to reduce N losses is described. The first objective is achieved by ensuring that lamina mass and the N concentration of herbage do not limit herbage intake or feeding value. The second objective is achieved by limiting N fertilizer supply or increasing the interval between defoliations to reduce the N concentration of herbage. Lower and upper thresholds for the N concentration of herbage and lamina mass were estimated from published data. The method is illustrated using two vegetative regrowths (beginning and end of spring) in a cutting experiment with two fertilizer treatments, 0 or 120 kg N ha^?1 (?N and +N), and early or late cutting. Decreasing N supply led to a reduction in grazing management flexibility, i.e. the defoliation interval ranges which were compatible with the required sward characteristics (minimum lamina mass and N concentration of lamina) for herbage intake and to meet the protein requirements of dairy cows. Aiming for the upper threshold N concentration of herbage increased the minimum interval between defoliations only for the +N treatments. Nevertheless, grazing management flexibility remained the highest for the +N treatments.     

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.     

Effect of different grazing management systems on the herbage mass and pasture height of a Nardus stricta grassland in western Scotland, United Kingdom

The aims of this study were to examine the effect of three grazing treatments (year‐round stocking rates of 0·8 ewes ha^?1, 0·5 ewes ha^?1 and 0·5 ewes ha^?1 plus grazing cattle in summer), imposed for 4 years, on the herbage mass and surface height of a Nardus stricta‐dominated grassland in western Scotland and to obtain estimates of annual productivity of this grassland. Nardus stricta‐dominated grassland comprised proportionately 0·20 of the grazing area. Stocking rate of sheep had no significant effect on the herbage mass of the grassland in the first 2 years of the experiment, although mean summer pasture heights were significantly higher under the lower stocking rate of sheep. The pasture on the treatment with cattle grazing in summer had a significantly lower herbage mass and lower surface height than the two sheep‐only grazing treatments. Year‐to‐year variation in the herbage mass and surface height of herbage in summer was greater than the effect of treatments. Despite changes in surface height, the structural diversity of the grasslands was not increased by the treatments. The annual production of vascular plant material ranged from 417 g DM m^?2 in 1994 to 628 g DM m^?2 in 1996.     

Effects of grazing and precipitation on herbage production,herbage nutritive value and performance of sheep in continental steppe

The present study highlights the effects of sheep grazing and precipitation on herbage and animal performance in a grazed steppe of Inner Mongolia. Experimental data were collected during grazing periods of four consecutive years (2005–2008), and effects were analysed across a gradient of seven grazing intensities. Variation in annual precipitation, reflected by the effect of ‘year’, was the major factor affecting herbage; i.e., the production and nutritive value of herbage increased with increasing precipitation. Herbage parameters were also affected by grazing intensity, as herbage production (HP) and herbage nutritive yields decreased, while herbage nutritive values increased with increasing grazing intensity. The grazing‐induced decrease in herbage nutritive yields suggests that decreases in HP offset the positive effect of grazing on the nutritive value. Liveweight gain (LWG) was predominantly affected by grazing intensity, as LWG per sheep and per ha decreased and increased, respectively, with increasing grazing intensity. However, responses varied among years: LWG per sheep was maximized by light grazing in the drought year and by moderate grazing the wet year. Our results showed that herbage shortage at high grazing intensities reduces LWG per sheep and thus diminishes responses in LWG per ha. Nevertheless, the highest grazing intensity provides highest animal production per ha in the short term; however, this is not sustainable in the mid‐ and long term because decreasing HP induces degradation processes. Based on our results, a reduction in grazing intensity that still provides 78% of the maximum LWG per ha meets the requirements of a sustainable grazing management.     

Manipulation of herbage production by altering the pattern of applying nitrogen fertilizer

The redistribution of herbage production during the growing season to synchronize herbage supply with feed demand by livestock by altering the application pattern of a range of nitrogen (N) fertilizer rates was studied. Application rates of N were 50, 150 and 250 kg N ha^?1 per annum and patterns were with 0·60 of N fertilizer applied before June (treatment RN) and with only 0·20 of N fertilizer before June (treatment IN). Treatments were imposed in a cutting (simulated grazing) experiment (Experiment 1), which was conducted for 2 years and a grazing experiment (Experiment 2) which was conducted for 3 years. In both experiments, herbage production was reduced in April and May and increased in the June–October period on treatment IN relative to RN, but annual herbage production was not significantly affected except in the third year of Experiment 2, when treatment RN had significantly (P < 0·05) higher herbage production than treatment IN. Crude protein (CP) concentration of herbage was lower in April and May on treatment IN than treatment RN. However, CP concentration of herbage was rarely below 150 g kg^?1 DM and so it is unlikely that livestock productivity would be compromised. On treatment IN, concentrations of CP in herbage were higher in the late summer than on treatment RN, which may increase livestock productivity during July and August when livestock productivity is often lower. Altering the strategy of application of N fertilizer did not affect in vitro dry matter digestibility of herbage.     

Defoliation patterns and their implications for the management of vegetative tropical pastures to control intake and diet quality by cattle

This study assessed the use of pasture attributes to control daily intake and diet quality during progressive defoliation on pastures of Axonopus catarinensis. Three consecutive 12‐day grazing treatments of progressive defoliation were conducted with Brahman cross‐steers. Daily forage intake and defoliation dynamics were assessed using a pasture‐based method. The treatments differed in initial sward height (33, 44 and 61 cm) and herbage mass (1030, 1740 and 2240 kg ha^?1). The post‐grazing residual sward height, at which forage intake decreased, appeared to increase with the initial sward height (12·3, 14·6 and 15·5 cm). Steers grazed up to four distinctive grazing strata in all treatments. The depth and herbage mass content of the top grazing stratum were at least five times higher than the lower grazing strata in all treatments. This explains why forage intake decreased when the top grazing stratum was removed in approximately 93% of the pasture area in all treatments, equivalent to approximately 7% of the pasture area remaining ungrazed. We conclude that the residual ungrazed area of the pasture, rather than residual sward height, can be used to develop grazing management strategies to control forage intake and diet quality in a wide range of pasture conditions.     

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.     

Seasonal changes in herbage mass and nutritive value of a range of grazed legume swards under Mediterranean and cool temperate conditions

Seasonal changes in herbage mass and herbage quality of legume‐based swards under grazing by sheep or cattle were investigated at four locations in climatically different zones of Europe: Sardinia (Italy), southern France, northern Germany and south‐west England (UK). At each location standard treatments were applied to legumes typical of species widely used in each locality: Medicago polymorpha in Italy, Medicago sativa in France, and Trifolium repens in Germany and in UK. At each site comparisons were made of two other legumes: Trifolium subterraneum and Hedysarum coronarium in Italy, Onobrychis sativa and Trifolium incarnatum in France, Trifolium pratense and Lotus corniculatus in Germany, and Trifolium ambiguum and L. corniculatus in UK. Legumes were sown in mixture with locally appropriate companion grasses, and measurements were made over two or three grazing periods. In Italy M. polymorpha swards gave the greatest herbage mass in grazing period 1 but H. coronarium was more persistent. At the French site all legumes established poorly with no significant herbage mass differences between treatments. At both the UK and German sites L. corniculatus maintained a high proportion of legume in the sward; T. repens showed poor persistence under continuous sheep grazing in UK but persisted under cattle grazing in Germany, while T. ambiguum was slow to establish in the UK, and T. pratense proved to be of comparable herbage mass to the standard T. repens‐based sward in the last year of the experiment. The concentration of crude protein and in vitro digestibility of organic matter in the dry matter of herbage showed greater within‐season variation than between treatments at each site. It is concluded that, in addition to currently used species, legume‐based swards containing H. coronarium, O. sativa and L. corniculatus all have potential to contribute to forage production for low‐input grazing and their use merits further consideration in systems of livestock production in Europe.     

Evaluation of the effects of ewe prolificacy potential and stocking rate on herbage production,utilization, quality and sward morphology in a temperate grazing system

This study investigated the effect of ewe prolificacy potential (PP; predicted number of lambs born ewe^?1 year^?1), stocking rate (SR; ewes/ha) and their interaction on herbage dry matter (DM) production, utilization, quality and sward morphology within a temperate grass‐based lamb production system. The study had a 2 × 3 factorial design, consisting of two ewe PP as dictated by sire breed (180 medium prolificacy potential (MP—Suffolk crossbred) and 180 high prolificacy potential ewes (HP—Belclare crossbred)) and three SR: low (LSR; 10 ewes/ha), medium (MSR; 12 ewes/ha) and high (HSR: 14 ewes/ha). Each treatment was managed in a rotational grazing system, with LSR, MSR and HSR treatments grazing to target post‐grazing sward heights (PGSH) of 4.55, 4.15 and 3.75 cm respectively. Herbage DM production (above target PGSH) and utilization were highest at the HSR, intermediate at the MSR and lowest at the LSR (p < .001). Ewe PP had no effect on herbage DM production, utilization, quality or sward morphology (p > .05). The proportion of leaf in the sward (above target PGSH) was 4% greater in MSR and HSR compared with LSR (p < .05). In conclusion, findings demonstrate the potential to support increased ewe PP through the selection of ewe genotypes of a genetically higher PP and lower mature live weight and increased SR within a temperate grass‐based lamb production system.     

Effects of post‐grazing herbage height and concentrate feeding on milk production and major milk fatty acids of dairy cows in mid‐lactation

Milk fatty acids (FA) were compared in mid‐lactation dairy cows in four feeding systems combining grazing management and supplementation. The four treatments were factorial combinations of compressed herbage grazed to 3·7 or 4·6 cm post‐grazing height, with or without concentrate feeding (3·6 kg cow^?1 d^?1). Milk yield and composition were measured for four groups of eight Friesian × Jersey dairy cows over 3 weeks in mid‐lactation for cows that had grazed treatments for 64 d from early spring. Milk yield was higher in cows fed concentrate plus herbage (23·9 kg d^?1 cow^?1) than cows fed herbage only (20·3 kg d^?1 cow^?1). Milk fat percentage was higher in cows fed herbage only (5·5%) than that fed herbage plus concentrate (5·1%). Milk protein percentage was higher in cows fed herbage plus concentrate (4·0%) than that fed herbage only (3·7%). The concentrations of conjugated linoleic acids c9, t11, C18:0, C18:1 t11 and C18:2 t9, c12 FA were lower where concentrate was fed. The concentrations of C18:1 t10, C18:1 t5, t8 and C18:2 c9, c12 FA were higher where concentrate was fed. The concentrations of C18:1 c6, C18:1 c9, C18:1 t9 and C18:3 c6,9,15 were unaffected by concentrate feeding. Post‐grazing herbage height had no significant effect on milk yield or concentration of milk FA. Provided dairy cows are harvesting leafy material of similar nutrient and FA concentration, post‐grazing herbage height does not appear to alter milk FA and the supply of high energy concentrates is more influential on milk FA profiles.     

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.     

Intake characteristics of perennial ryegrass varieties when grazed by yearling beef cattle under rotational grazing management

Four intermediate‐heading perennial ryegrass (Lolium perenne L.) varieties, which in previous studies had been associated with high‐ or low‐intake characteristics when swards containing them had been continuously stocked with sheep, were sown as monocultures. They were rotationally grazed, using 1‐d paddocks, with core groups of four yearling Simmental × Holstein beef heifers in 2002 and 2003 and ingestive and ruminative behaviour, and sward factors, were measured. There were two diploid (Belramo and Glen) and one tetraploid (Rosalin) perennial ryegrass varieties and one tetraploid hybrid (Lolium × boucheanum Kunth) (AberExcel) variety. Intake rate (IR) was significantly higher in August 2003 for heifers grazing Glen than those grazing Belramo [27·5 vs. 20·6 g dry matter (DM) min^?1; P = 0·019], but there were no significant differences between varieties in two other measurement periods. This is in contrast to previous results with sheep when IR were significantly higher for Glen than Belramo and for AberExcel than Rosalin. Total jaw movement rates during grazing were significantly higher for heifers on the tetraploid swards than those on the diploid swards (87·7 vs. 83·6 jaw movements min^?1; P = 0·023) in September 2002. Ruminating time was significantly lower for heifers on the tetraploid swards than those on the diploid swards (453 vs. 519 min 24 h^?1; P = 0·012) in July 2002. Digestibility of grass snips was significantly higher on the tetraploid than the diploid swards [697 vs. 680 g digestible organic matter (DOM) kg^?1 DM; P = 0·042] in September 2003 and, within diploids, was significantly higher for Glen than Belramo (696 vs. 663 g DOM kg^?1 DM; P = 0·014). There were significant differences in sheath tube and leaf lengths and in the population density of tillers between and within ploidies, which might have been expected to have influenced intake characteristics, but this was not generally found under rotational grazing with cattle. In order to separate the effects of defoliation interval from those of grazing style of the different ruminant species, it is suggested that grass variety evaluations using continuously stocked cattle swards are required.     

Performance of cultivated perennial grass mixtures under different grazing intensities in the alpine region of the Qinghai-Tibetan Plateau

In 1998, seeds of four cultivated native perennial grasses, Bromus inermis (B), Clinelymus nutans (C), Elymus nutans (E) and Agropyron cristatum (A), were sown as mixtures with different seed rates in three combinations (B + C, B + E + A and B + E + C + A) in a field study in the north‐eastern part of the Qinghai‐Tibetan Plateau, China. A grazing experiment was conducted in 2000 to investigate the performance of these grass mixtures at leaf, plant and sward scales under different grazing intensities (GI). Four GIs, expressed as the proportion of herbage consumed by Tibetan lambs in relation to the available biomass (IP), were applied in the summer: no grazing, and 0·30, 0·50 and 0·70 of IP. Tiller numbers of the grasses increased and leaf photosynthetic rate, sward leaf area index (LAI) and herbage mass declined with increase in GI. No effect of GI on specific leaf area was observed. Interactions between GI and grass mixtures on the dynamics of species composition, swards’ LAI and herbage mass were found. Large fluctuations in species composition with different GIs showed the poor species compatibility and sward persistence of these grass mixtures under high GI by sheep.     

Effect of altering the grazing interval on growth and utilization of grass herbage and performance of dairy cows under rotational grazing

The effects of short grazing intervals in the early part of the grazing season on the growth and utilization of grass herbage, and the performance of grazing dairy cows, in a rotational grazing system were examined. Seventy-six cows were allocated to two grazing treatments: a normal rotation treatment (20-d rotations for the first 60 d) and a short rotation treatment (12-, 8-, 8-, 8-, 12- and 12-day rotations). Thereafter, both treatments had the same grazing interval and over the season as a whole both treatments received the same amount of nitrogen fertilizer and were stocked at the same rate. The short rotation treatment significantly reduced pre- and post-grazing sward heights and pre-grazing herbage mass in May and June. Total herbage production was significantly lower on the short than the normal rotation treatment as a result of a significant reduction in the growth rate of herbage in May and June. The short rotation treatment had a significantly lower milk output per cow. Grazing shorter swards, as a result of shorter rotations, significantly reduced herbage intake, reflecting reductions in intake per bite, grazing time and total bites per day. Treatment had no significant effect on herbage quality or pre- and post-grazing sward height in August and September, despite the increased grazing severity in May and June with the short rotations. The severity of rotation length in this instance had a detrimental impact on animal performance, whereas a more modest reduction in grazing interval may control herbage production, without reducing animal performance.     

Effects of autumn and spring defoliation management on the dry‐matter yield and herbage quality of perennial ryegrass swards throughout the year

A small‐plot experiment was conducted in south‐west Ireland to investigate (i) the effects of pre‐closing regrowth interval and closing date on dry‐matter (DM) yield and sward structural and composition characteristics, during the autumn–winter and spring opening periods, and (ii) subsequent carryover effects. The study used a randomized block design with a factorial arrangement of treatments (4 closing dates × 2 opening dates) with a split plot (two pre‐closing regrowth intervals). The long pre‐closing (LPC) interval began on 9 August, and the short pre‐closing interval (SPC) started on 15 September. The autumn closing dates were as follows: 1 October (CD1), 15 October (CD2), 1 November (CD3) and 14 November (CD4). Plots were defoliated again on 1 February (EOD) or 1 March (LOD). On the LPC treatment, herbage yield increased from CD1 (2463 kg DM ha^?1) to CD3 (3185 kg DM ha^?1). On the SPC treatment, herbage yield was similar for CD3 and CD4, indicating a ceiling in herbage accumulation. For each 1‐d delay in closing date between CD1 and CD4, the opening herbage yield was reduced by 10 kg DM ha^?1. Herbage quality decreased as the closing date was delayed; DMD and CP decreased by 0·06 and 12 g kg DM^?1, respectively, between CD1 and CD4. The EOD resulted in increased leaf and decreased dead proportions over the LOD treatments. A balance between autumn CD and spring OD needs to be achieved to ensure a sufficient supply of high‐quality grass in spring.     

Ingestion and excretion of nitrogen and phosphorus by beef cattle under contrasting grazing intensities

There is increased global interest in the environmental impacts of farming, including the need to prevent the contamination of soil, water and air with excessive amounts of nitrogen (N) and phosphorus (P) in grazed systems. Reduction in grazing intensity has frequently been recommended to meet biodiversity and production goals in sustainable grazing systems. The objective of this experiment was to measure N and P ingestion and excretion by beef cattle grazing semi‐natural pastures at two grazing intensities (Moderate or Lenient). The cattle grazing at Moderate grazing intensity had significantly more defaecations each day than those grazing at Lenient intensity (9·5 vs. 7·5) and tended to have more urinations (7·0 vs. 5·8). For the Moderate and Lenient treatments, respectively, 113 vs. 76 g N d^?1 was excreted compared with 136 vs. 94 g N d^?1 ingested; 12 vs. 8 g P d^?1 was excreted compared with 13 vs. 10 g P d^?1 ingested and urine N comprised 0·51 and 0·52 of the total N excreted each day. In improved, intensively managed grassland systems, urine N comprises a much higher proportion (approximately 0·70–0·85) of the daily total N excreted. The lower level found here is likely to impact on potential volatilization, denitrification and leaching losses, and these aspects should be examined further to see the extent to which semi‐improved grasslands containing increased plant diversity compared with improved grasslands can deliver higher resource protection, as well as enhanced grassland faunal diversity and abundance.