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- Materials and methods
The objective of this study, which was part of a larger grazing-systems experiment, was to investigate the cumulative impact of three levels of grazing intensity on sward production, utilization and structural characteristics. Pastures were grazed by rotational stocking with Holstein–Friesian dairy cows from 10 February to 18 November 2009. Target post-grazing heights were 4·5 to 5 cm (high; H), 4 to 4·5 cm (intermediate; I) and 3·5 to 4 cm (low; L). Detailed sward measurement were undertaken on 0·08 of each farmlet area. There were no significant treatment differences in herbage accumulated or in herbage harvested [mean 11·3 and 11·2 t dry matter (DM) ha−1 respectively]. Above the 3·5 cm horizon, H, I and L swards had 0·56, 0·62 and 0·67 of DM as leaf and 0·30, 0·23 and 0·21 of DM as stem respectively. As grazing severity increased, tiller density of grass species other than perennial ryegrass (PRG) decreased (from 3,350 to 2,780 and to 1771 tillers m−2 for H, I and L paddocks respectively) and the rejected area decreased (from 0·27 to 0·20 and to 0·10 for H, I and L paddocks respectively). These results indicate the importance of grazing management practice on sward structure and quality and endorse the concept of increased grazing severity as a strategy to maintain high-quality grass throughout the grazing season. The findings are presented in the context of the need for intensive dairy production systems to provide greater quantities of high-quality pasture over an extended grazing season, in response to policy changes with the abolition of EU milk quotas.
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- Materials and methods
Worldwide demand for dairy products is expected to increase as a result of projected population growth and increases in per capita disposable income (Delgado, 2005). Grass-based dairy production systems, which convert cheaply produced grazed grass into milk, are highly profitable and provide pastoral dairy farmers with an advantage over other production systems within an increasingly competitive global milk production environment (Dillon et al., 2008). Grazed grass remains the cheapest source of feed (Shalloo et al., 2004) and is a key profit driver for all dairy systems (MacDonald et al., 2011). Dillon et al. (2008) suggested that, in countries in which grass makes up a high proportion of the diet of dairy cows, a 0·10 increase in the proportion of grazed grass included in the cow's diet reduces total production costs per litre of milk by €0·025. As pastoral farming systems develop and expand, increased quantities of herbage are required to feed larger herds, sometimes within the same land base, and therefore grazing management practices must focus on increasing herbage production and utilization within such systems. Additionally, within a European Union (EU) context, the abolition of milk quotas in 2015 will allow increased milk production. This may be achieved partly through increased milk production per cow, but also through higher stocking rates and increased production per hectare, which will result in increased on-farm feed demand. During the EU milk-quota era, grass-based production systems have focused on high productivity per animal, but post–milk quotas, per-hectare grassland productivity will increasingly limit grass-based milk production (Shalloo et al., 2004). Consequently, grazing management practices will need to be adopted to realize increased grass production and quality and sustain higher overall stocking rates even at the expense of individual animal performance (McCarthy et al., 2011). Improvements in grass production, utilization and quality arising from changes in grazing pressure could also positively impact on the environmental sustainability of grazing if increased quantities of milk production can be achieved with a reduced requirement for chemical fertilizers or the import of feed supplements to the grazing system (Soussana et al., 2004; Dillon et al., 2008). Maintaining a high proportion of grazed grass in the diet of dairy cows reduces CH4 emissions per cow per day, per kg milk solids produced and per kg DM intake compared with a total mixed ration (TMR) diet (O'Neill et al., 2011).
Efficient exploitation of grass through grazing will require the development of grazing systems designed to achieve high per-cow intakes while producing a greater quantity of high-quality pasture over a long grazing season (Peyraud et al., 2001). Achieving the correct balance between herbage quality and quantity is critical in grass-based production systems, and changes in defoliation severity impact on sward growth, morphology and structure. Michell and Fulkerson (1987) found that high relative grazing severity produced higher growth rates of grass leaf but at the expense of total growth, due to a reduction in stem production. In contrast, other studies have reported reduced pasture growth, albeit in association with lax grazing management, mainly due to increased plant senescence and decay (Hunt and Brougham, 1967; Tainton, 1974). More recently, Lee et al. (2008) and MacDonald et al. (2008) reported that herbage production was reduced at lower stocking rates. In addition to the effects on herbage accumulation, the severity of grazing may also affect sward components. Both Stakelum and Dillon (1990) and O'Donovan and Delaby (2005) observed significantly more green leaf and less dead material in severely grazed swards, resulting in improved sward nutritive value in comparison with swards subjected to less-severe grazing management.
Many classic studies have reported the impact of grazing severity, typically investigating the effects of large differences in post-grazing sward height (often in excess of 3 cm) on traditional mixed species pastures (Brougham, 1956; Binnie and Harrington, 1972; Grant et al., 1981). While these studies have provided the basis for our understanding of grazing management impacts, the differential in defoliation severity was usually outside the practical range implemented on efficiently managed grazing dairy farms (O'Donovan et al., 2011). More recently, grazing management studies investigating the effects of defoliation severity on herbage characteristics have been limited to small-scale plot and glasshouse studies (Fulkerson, 1994; Fulkerson and Slack, 1995) or, if performed under grazing, were limited in time (Baker and Leaver, 1986; Hoogendoorn and Holmes, 1992; Pulido and Leaver, 2003). Additionally, much of the emphasis within these studies has focussed on animal performance (Greenhalgh et al., 1966) or overall system efficiency (MacDonald et al., 2001) rather than specifically on the agronomic effects.
The objective of the present experiment was to examine the cumulative effects of consistently applying three post-grazing sward heights on sward dry matter (DM) production, structure, morphology and chemical composition, under an intensive grazing regime, across an entire grazing season. The hypothesis of this study was that low post-grazing sward heights would increase the annual productivity and utilization of pastures based on modern sown varieties of perennial ryegrass (PRG; Lolium perenne L.).
Materials and methods
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- Materials and methods
The experiment was conducted at Curtins Research Farm, Teagasc, Animal & Grassland Research and Innovation Centre, Moorepark, Fermoy, Co. Cork, Ireland (52o7′N, 8o16′W). It focused on sward characteristics and was a component of a larger study investigating the effect of stocking rate (SR) and calving date on dairy cow performance (McCarthy et al., 2012). A permanent grassland site with 2- to 3-year-old PRG swards was used to measure the effects of three post-grazing sward heights on sward production and characteristics. The cultivars sown were cv. Tyrella (diploid) and cv. Bealey (tetraploid). No clover was present in the sward. The three blocks used in the study were sown between 2006 and 2009 in the autumn period as diploid–tetraploid mixes, reflecting the grass swards commonly sown in Ireland (Gilliland et al., 2011). Seeding rate was 28 kg ha−1. The soil type was a free-draining, acid brown earth with a sandy-loam to loam texture. The experiment was undertaken from 10 February to 18 November 2009. Rainfall and air and soil temperatures (at 100 mm soil depth; Table 1) were recorded daily at a meteorological station <1 km from the experimental site. The findings of this study are reported as three seasons: spring (10 February–15 April), mid-season (16 April–31 July) and autumn (1 August–18 November). The grazing and forage terminology used in this study follows Allen et al. (2011).
Table 1. Mean air and soil temperature (oC) and total rainfall (mm) in 2009 and the 10-year average (1999–2008) at Moorepark
| ||Rainfall (mm)||Mean soil temperature at 100 mm (oC)||Mean air temperature (oC)|
|2009||10-year average||2009||10-year average||2009||10-year average|
Experiment and treatments
A grazing area of 4·1 ha (0·08 of the 48·1 ha farm) was used to compare the effects of three post-grazing sward heights on pasture production and sward characteristics. Holstein–Friesian dairy cows were used in this experiment, as described previously by Coleman et al. (2009). Further information on animal performance during the larger study was reported by McCarthy (2011) and McCarthy et al. (2012). The three post-grazing sward heights treatments were high (H), intermediate (I) and low (L), with target post-grazing compressed sward heights of 4·5 to 5 cm, 4 to 4·5 cm and 3·5 to 4 cm respectively. Three grazing blocks of similar size were selected within the farm (1·36 ± 0·092 ha). Within each block, there were three paddocks, one assigned to each treatment (0·40–0·52 ha, depending on post-grazing sward height treatment). Applications of artificial fertilizer were kept at a constant rate for each treatment, at 250 kg nitrogen (N) ha−1 year−1 (30 kg N ha−1 ± 0·9 kg following each grazing period). Soil nutrient status was similar and adequate [pH 6·5, and Soil index 3 for phosphorus and potassium (Teagasc, 2008)] for each block during the study, and recommended maintenance macronutrient applications were made during the grazing season (Teagasc, 2008).
Paddocks within each treatment were rotationally stocked for the 10-month grazing season (February to November), and restricted access time to pasture was practised during periods of inclement weather to avoid pasture damage (Kennedy et al., 2009). The decision rules applied to grazed blocks in the experiment were consistent with the larger experiment and are outlined in McCarthy et al. (2012). The residency time within each paddock ranged from 1·5 to 2·5 days per paddock, and this was dictated by the time required to reach the desired post-grazing sward height. Treatments were managed independently and thus the rotation length varied between them. The experimental paddocks were grazed exclusively, and no cutting for silage or mechanical topping took place in these paddocks during the study period. The number of grazing rotations was 12, 11·7 and 11 for H, I and L treatment paddocks respectively. Although there is a difference of one rotation between H and L, there is no indication of any difference between these two treatments in each of the seasonal periods.
Pre- and post-grazing herbage mass, herbage accumulated and herbage harvested
Pre-grazing herbage mass (HM) above 3·5 cm was determined by cutting two strips per paddock (1·2 m wide × 10 m long) with a Agria motor mower (Etesia UK Ltd., Warwick, UK) and taking the average of the two harvests, similar to Kennedy et al. (2009) and McEvoy et al. (2010). The herbage harvested from each cut strip was collected, weighed and sampled, and a subsample (100 g) was dried overnight at 90°C in a forced-draught oven to determine DM content. Pre-grazing HM below 3·5 cm was measured within each cut strip using a 0·5 × 0·2 m quadrat and scissors, and all collected material was washed to remove any soil contamination and then dried overnight at 90°C in a forced-draught oven to determine DM content. Total HM was then calculated by adding pre-grazing HM above and below 3·5 cm. Post-grazing HM above 3·5 cm was determined after grazing by cutting a 20-m strip per paddock, following a similar procedure to pre-grazing HM.
Herbage accumulated between grazings was calculated as the pre-grazing HM, minus the post-grazing HM of the previous rotation. Herbage harvested (kg DM ha−1 utilized; above 3·5 cm) in any given rotation was calculated as the pre-grazing HM less the post-grazing HM within the same rotation. Total herbage accumulated and harvested during each period and over the entire grazing season was calculated as the sum of all herbage accumulated and harvested. The proportion of available herbage utilized (harvesting efficiency) was calculated on an individual-paddock and grazing-event basis as the proportion of the pre-grazing HM that was harvested. The proportion of available herbage utilized reflects the grazing severity of an individual grazing event (and approaches 1·00 when paddocks are grazed to 3·5 cm).
Pre- and post-grazing sward heights were measured with the rising plate meter (Jenquip, Feilding, New Zealand), taking 35 measurements across the diagonals of each paddock. Sward bulk density (kg DM cm−1 ha−1) was calculated using the following equation:
Sward structure, rejected areas, sward morphology and tiller density
Extended tiller height (ETH) and extended sheath height (ESH) were measured on 200 random PRG tillers immediately pre- and post-grazing across the diagonals of each paddock using a graduated ruler. The ETH was measured from ground level to the highest point of the tiller. The ESH was measured from ground level to the point of the highest ligulae (longest leafed sheath). Free leaf lamina (FLL) was then calculated by subtracting the ESH from the ETH, as described by Gilliland et al. (2002).
The proportion of rejected area, or those areas which were poorly grazed due to the presence of faecal and urine deposits, was visually assessed using a 1 × 1 m quadrat placed randomly at 10 points along the diagonal of each paddock, when ETH and ESH were measured. The number of tillers that were selected from rejected (taller grass) or non-rejected areas (grazed to the expected grazing height) when measuring ETH and ESH were consistent with the proportion of rejected area previously measured with the quadrat.
The morphological composition of the herbage was determined before each grazing period. Twenty grab samples of herbage (c. 80 g in total) were cut at random to ground level with scissors in each paddock. The vertical structure of the cut sample of the sward was preserved by securing it with an elastic band and placing it in a bag. A 40-g subsample was selected from the sample while fresh and separated into two portions: above and below 3·5 cm. Each portion was subsequently separated into leaf, stem (including true stem, pseudo-stem and flower head, if present) and dead material. Each component was dried overnight in an oven at 90°C to determine morphological composition on a DM basis. The DM weight of each component was divided into the total DM weight of the sample to estimate the proportion of each component in the sample. Leaf, stem and dead yields were estimated using the proportion of each component from the herbage accumulated. Leaf, stem and dead component availabilities were calculated as proportions of the pre-grazing HM.
To estimate tiller density, 10 turves per paddock (10 × 10 cm) were removed from each treatment paddock on the 5 February, 17 June and 2 November. The tillers were separated into PRG or other grass species (Poa pratensis, Poa trivialis and Agrostis stolonifera) and counted, and then multiplied by 100 to estimate number of tillers m−2 (Jewiss, 1993).
Herbage, representative of that selected by cows grazing each treatment, was sampled weekly from each paddock with Gardena (Accu 60; Gardena International GmbH, Ulm, Germany) hand shears, taking cognizance of the previous defoliation height recorded from each treatment. A subsample was dried at 40°C for 48 h and subsequently milled through a 1-mm screen prior to chemical analysis, which consisted of ash, neutral detergent fibre (NDF; Ankom Technology, Macedon, NY, USA), organic matter digestibility (OMD; Fibered Systems, Foss, Ballymount, Dublin, Ireland) and crude protein analyses (CP; Lecco FP-428, Lecco Australia Pty Ltd., Castle Hill, NSW, Australia).
All sward measurements were analysed using SAS (Statistical Institute, 2006). Analysis of variance was performed with a mixed model that included the fixed effects of block, treatment and rotation, and their interaction, and the random effect of paddock. Alternative covariance structures were investigated based on biological plausibility including compound symmetry, unstructured and autoregressive options both with homogenous or heterogeneous variances. Using the Akaike information criterion, a compound symmetry error structure was determined as the most appropriate residual covariance structure. The significance of treatment effects were tested against the mean squares of the interaction treatment by paddock and treatment by rotation. Rotation length was analysed using the GLM procedure with a linear model that included the effects of treatment, block and rotation. The proportion of available herbage utilized was analysed after root square transformation with PROC MIXED by seasons. The means and 0·95 confidence intervals (CI) are presented after back-transformation.
The effect of post-grazing sward height is considered in each season separately as well as over the entire grazing year. Block had a significant effect on sward structure and productivity; however, as there were no interactions between grazing intensity treatment and block, only the main effects of grazing intensity treatment are reported.
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- Materials and methods
Similar to many previous grazing experiments (Baker and Leaver, 1986; Hennessy et al., 2008; Lee et al., 2008; McEvoy et al., 2009), this study was undertaken during a single grazing season, and so the potential longer-term effects of the imposed grazing regimes are not considered. Additionally, although a significant block effect was evident from the analysis, there were no significant treatment by block interactions observed, and so only the main effect of post-grazing sward height is reported in this study.
While the differential in post-grazing sward height between treatments in this experiment is relatively small, this differential represents the likely range of practical grazing severity within grass-based systems at stocking rates of 2·5 to 3·3 LU ha−1 and where up to 90% of the diet comes from grass (McCarthy, 2011). In addition, the consistent imposition of these post-grazing sward height differentials had a significant effect on individual animal performance, resulting in milk production levels of 5684, 5061 and 4955 kg of milk per cow for the H, I and L treatments respectively (McCarthy, 2011).
Although the swards were sown as a diploid–tetraploid variety mixture, it would be expected that the tetraploid would be the dominant component of the mixture (Gilliland et al., 2011).
Pre- and post-grazing herbage mass, herbage accumulated and herbage harvested
Air and soil temperatures in January and February were below the 10-year average, resulting in low pasture growth rates, feed shortages and, consequently, similar low post-grazing residual sward heights for all treatments during the spring period. As a result, all treatments exhibited similar sward structure and morphology during spring, whereas the differences in post-grazing sward heights between treatments became significant during the mid-season and resulted in differences in many sward measurements between treatments as the grazing season progressed.
Mean pre-grazing HM in mid-season was low (1108 kg DM ha−1 >3·5 cm) for all treatments and below the levels recommended for optimum plant growth by McEvoy et al. (2009), Curran et al. (2010) and Tuñon et al. (2011; 1500–1700 kg DM ha−1). Consequently, daily sward growth rates during mid-season were low (60·6 kg DM ha−1) for all treatments in the current experiment as compared with values reported by McEvoy et al. (2009) and Curran et al. (2010; 68 to 77 kg DM ha−1 day−1). Similar to previous experiments (Bircham and Hodgson, 1983; Stakelum and Dillon, 1990; O'Donovan and Delaby, 2005), the increased grazing severity of the I and L treatments increased herbage utilization, sward leaf proportion and PRG tiller density.
Total pasture DM yield and leaf accumulation above 3·5 cm were unaffected by treatment in the current study. This contrasts with Carton et al. (1989) and Stakelum and Dillon (2007) who found higher leaf yields with increased grazing pressure, but those results were observed on swards with significantly greater pre-grazing HM than those reported here. The increased grazing severity of I and L swards in this study (4·2 and 3·6 cm, respectively, compared with 4·9 cm for H) may have deleteriously impacted on net herbage accumulation in comparison with maintaining a ‘plant optimal’ post-grazing sward height, which Fulkerson et al. (1994) and Lee et al. (2008) concluded is approximately 5 cm. Bircham and Hodgson (1983) and Stakelum and Dillon (2007) suggest that pasture production above 3·5 cm is insensitive to grazing management over a broad range of grazing conditions. Similarly, Fariña et al. (2011) suggested that, while resulting in increased herbage utilization and quality, grazing treatment has little effect on net herbage accumulation above 3·5 cm.
Previous studies have provided conflicting evidence of the impact of defoliation severity on grass production. Both Hunt and Brougham (1967) and Tainton (1974) reported reduced pasture growth with lax grazing, mainly due to increased plant senescence and decay, while Lee et al. (2008) and MacDonald et al. (2008) reported reduced herbage production through repeated lax defoliation. In contrast, Michell and Fulkerson (1987) found that high relative grazing severity reduced total growth and has been associated with a substantial reduction in the rate of photosynthesis (Brougham, 1956; Parsons et al., 1988) due to the removal of all the leaf material. Chapman et al. (2011) reported that where PRG swards are continuously severely defoliated, the sward adapts to maximize leaf area after grazing by reducing sheath length and maintaining additional leaf tissue below the severe grazing height. However, this effect was not observed in the current study as leaf proportion below the 3·5 cm horizon was not significantly affected by treatment.
Sward structure, sward morphology and tiller density
Treatment affected ESH >3·5 cm in mid-season and autumn. Persistent low post-grazing sward height reduces sheath height as the animals graze tighter into the sward than under high post-grazing sward height grazing conditions. This also resulted in increased FLL >3·5 cm in mid-season, but not in autumn. In the L treatment, cows grazed to 3·7 cm in mid-season and 3·5 cm in autumn, thereby removing all the sheath and leaf above this. The increased leaf content >3·5 cm observed with increasing grazing severity is a function of the shorter sheath height, and hence the likely increased leaf extension post-grazing (Grant et al., 1981), although leaves would be shorter (Casey et al., 1999).
The proportion of leaf, stem and dead material observed compares favourably with previous studies at Moorepark (Stakelum and Dillon, 2007; Curran et al., 2010). Increased grazing severity in I or L treatments produced swards with higher contents of green leaf and digestible nutrients, but lower contents of grass stem and senescent material compared with the H treatment. The similar pre- and post-grazing FLL length and reduced pre-grazing ETH and ESH lengths of L and I swards during summer and autumn indicate that the reduction in stem yield is mainly responsible for the increase in leaf proportion associated with increased grazing severity during mid-season and autumn. In addition, while the L treatment achieved a consistently lower post-grazing FLL length during the entire grazing season (4·2 cm), it also achieved a similar pre-grazing FLL length compared with H and I treatments (15·5 cm; Table 4). This can be associated with high photosynthetic activity of young leaves (Gay and Thomas, 1995), which make up a greater proportion of the leaf population of a severely grazed sward compared with a less-severely grazed sward. High post-grazing sward heights are associated with a higher proportion of older leaves that have decreased photosynthetic capacity (Woledge, 1978).
The increase in stem and dead material content of swards that are not severely grazed has been widely reported (Hoogendoorn and Holmes, 1992) and explained by the greater age of the plant tissue (Korte et al., 1984). Reduced penetration of light to the lower strata of leniently grazed swards (Curll, 1982) causes tiller death (Ong, 1978) and hence increased dead proportions. As grazing severity increased (I and L swards) herbage harvested increased and little residual herbage tissue remained after grazing. In contrast, the H treatment allowed animals to selectively remove leaf in preference to stem and dead material. Frame and Hunt (1971) observed that 0·15 to 0·40 of the leaf that remains ungrazed within the sward eventually senesces, causing a decrease in leaf proportion. Consistent with Baker and Leaver (1986), the L treatment resulted in a smaller proportion of rejected area (0·10) compared with H (0·27) due to restricted herbage availability and hence less selective grazing.
Even though the composition of the base of the sward (below 3·5 cm) was quantified in this study, no significant sward composition differences were found and the main differences in leaf, stem and dead material content occurred in the grazing horizon. Similar to this study, both Mayne et al. (1987) and Michell and Fulkerson (1987) showed that HM increased at the sward base in the latter part of the season, due to stem and dead material accumulation.
Perennial ryegrass swards can use their tillering ability to adapt to defoliation regimes. Tiller size and density may have an inverse relationship (Langer, 1963) meaning that as HM increases, individual tillers are fewer but larger, and LAI and herbage accumulation remain constant (Matthew et al., 1995). Thus, any effect of management practice on the sward can be masked by size/density compensation (Yoda et al., 1963). The H treatment showed a slower increase in tiller numbers between the first two measurement dates (5 February and 17 June) in contrast to the other treatments (11 vs. 25 tillers day−1). There could have been an increase in death of vegetative tillers in the H treatment due to a failure of large tillers to supply assimilate to smaller tillers (Ong, 1978), due to the flowering axis inhibiting the growth of tiller buds (Laidlaw and Berrie, 1974) or due to shading at the base of the sward (Davies and Simons, 1979). Net tiller density change between the second and the third measurement dates showed no differences between treatments, similar to Wade (1979) and L'Huillier (1987). Differences in tiller density caused by management are generally due to changes in the amount of light penetrating to tiller bases, which influence tiller initiation and appearance (Mitchell, 1953; Langer, 1963). Peak tiller density seems to occur at 2–3 cm post-grazing sward height. Above this, tiller density changes following the self-thinning rule (Yoda et al., 1963) until it reaches an upper limit of 16 cm (Davies, 1988). This can help explain why grazing management did not affect herbage accumulation in the present study.
The significant reduction in non-PRG tiller density in the L sward as the grazing season progressed was also observed by both Korte et al. (1984) and Lee et al. (2008) and is due to the reduced opportunity for selective grazing afforded to the S treatment animals.
Increased OMD and reduced NDF as grazing severity increased was observed in mid-season and autumn due to increased sward leaf content and reduced stem content in the L swards compared to the H and I swards and is consistent with the differences in leaf, stem and dead proportions of the various swards, and similar to previous results (L'Huillier, 1987; Michell and Fulkerson, 1987; Hoogendoorn and Holmes, 1992; Stakelum and Dillon, 2007). Hoogendoorn and Holmes (1992) and Hurley (2007) showed that severe grazing in the early to mid-summer period reduces the ratio of reproductive to vegetative tillers, thereby avoiding large accumulations of stem in the sward. Likewise, Stakelum and Dillon (1990) and O'Donovan and Delaby (2005) also found greater OMD values with swards that were grazed to low relative post-grazing heights and hence had greater proportions of leaf. The results of this study indicate that grazing to a post-grazing sward height of 3·5–4 cm (similar to L and I swards) over the entire season can be an effective management strategy to reduce the proportion of stem and dead material in PRG dominant swards, and arrest the decline in sward digestibility and green leaf content, typically observed during the grazing season.
Productivity of perennial ryegrass swards as influenced by grazing intensity
The results of this study indicate that, within the range of post-grazing sward heights investigated, grazing management practice has an important impact on the feed-production capability of modern PRG swards for intensive grazing dairy production systems. While resulting in similar herbage accumulation above 3·5 cm, the results of this study indicate that increasing grazing severity to a consistent post-grazing residual height of 3·5–4 cm over the entire season can be an effective management strategy to increase the productivity of grass-based systems, resulting in increased herbage utilization, swards with higher concentrations of green leaf and digestible nutrients and less rejected herbage. The H, I and L treatments carried stocking rates of 2·5, 2·9 and 3·3 LU−1 ha−1, respectively, producing milk yields of 13 362, 14 115 and 15 469 kg−1 ha−1 (McCarthy, 2011) largely from grazed grass. While large differences in sward structure emerged during the grazing season, the results also suggest that future grazing research should consider the longer-term and potentially cumulative effects of grazing management practice over multiple grazing seasons to fully quantify the effects of grazing management practice on sward structure and productivity.