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Keywords:

  • post-grazing sward height;
  • tiller density;
  • rejected area;
  • leaf yield;
  • Lolium perenne ;
  • Ireland

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgments
  9. References

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.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgments
  9. References

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

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgments
  9. References

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)
200910-year average200910-year average200910-year average
January106·897·64·55·94·15·6
February3970·85·16·04·95·8
March88·281·07·17·17·37·0
April59·362·58·98·810·29·6
May38·373·21111·61313·0
June52·569·314·613·917·116·0
July142·761·814·815·617·617·3
August23·182·614·915·717·316·8
September102·194·712·913·714·914·7
October82·6115·611·810·512·911·1
November97·7101·47·87·98·48·1
December36·5101·33·26·346·2

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).

Grazing management

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.

Measurements

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:

  • display math
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).

Sward quality

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).

Statistical analysis

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.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgments
  9. References

Weather

Average daily air temperature (oC), soil temperature (oC) and rainfall for 2009 are shown in Table 1. Compared with the period from 1999 to 2008 (annual mean 1011 mm), total rainfall during the study was 0·28 higher (1293 mm). Mean daily temperature (9·7°C) was 0·5°C lower than the 10-year average (10·2°C), and mean daily soil temperature at 100 mm was similar in 2009 to the 10-year average (11°C).

The effect of grazing intensity on sward productivity

Grazing intensity had no significant effect on total herbage accumulation over the duration of the experiment.

Spring (10 February–15 April)

Each treatment area was grazed twice during the spring period. The average regrowth interval between grazings was 46 ± 3·5 days, and average herbage growth rate was 12·7 kg DM ha−1 day−1 during this period.

Pre- and post-grazing herbage mass, post-grazing sward height, herbage accumulated and herbage harvested

Average HM above and below 3·5 cm, pre-grazing height and sward density were similar for all treatments (881 and 1137 kg DM ha−1, and 6·7 cm and 261 kg DM cm−1 respectively). Post-grazing sward height, herbage accumulated and herbage harvested were similar for each treatment during spring (3·6 cm, and 1076 and 1701 kg DM ha−1 respectively). The effect of treatment on herbage utilization (= 0·13) was 0·87 (CI, 0·76–0·99) for H, 0·93 (CI, 0·81–1·05) for I and 1·08 (CI, 0·96–1·22) for L treatments.

Sward structure, sward morphology, tiller density and sward quality

Above and below the 3·5 cm horizon, the proportions of leaf (0·72 and 0·05 respectively), stem (0·14 and 0·56 respectively) and dead material (0·15 and 0·38 respectively) were not affected by treatment. There was also no effect of treatment on densities of PRG and non-PRG tillers (Figure 1; 4259 and 953 tillers m−2 respectively). Quality of grazed herbage was unaffected by treatment during spring (OMD = 785·4 g kg−1, NDF = 439·7g kg−1 and CP = 229 g kg−1; Table 2).

Table 2. Effect of post-grazing sward height on sward chemical composition (CP, crude protein; OMD, organic matter digestibility; NDF, neutral detergent fibre) in swards grazed to high (4·5–5 cm), intermediate (4–4·5 cm) and low (3·5–4 cm) post-grazing sward heights in spring, mid-season and autumn
 Post-grazing sward heights.e.Level of significance
HighIntermediateLow
  1. Values with different letters are significantly different at < 0·05; s.e., standard error of the mean.

  2. NS, not significant; *< 0·05; **< 0·01; ***< 0·001; †< 0·10.

Spring
 CP (g kg−1)228·5230·8228·33·62NS
 OMD (g kg−1)786·0783·3786·815·95NS
 NDF (g kg−1)437·2453·8428·08·47NS
 Ash (g kg−1)143·0136·7148·75·13NS
Mid-season
 CP (g kg−1)192·9191·6193·33·57NS
 OMD (g kg−1)760·6a770·7ab782·9b7·34 *
 NDF (g kg−1)464·3a469·6a448·8b4·53 *
 Ash (g kg−1)129·8123·7127·13·61NS
Autumn
 CP (g kg−1)207·1189·1208·95·56NS
 OMD (g kg−1)721·2a706·7a751·4b11·2 **
 NDF (g kg−1)511·0a505·3a485·7b7·44 *
 Ash (g kg−1)109·2110·7121·95·90NS
Entire grazing season
 CP (g kg−1)198·6198·2203·42·72NS
 OMD (g kg−1)752·6a750·8a769·5b5·95
 NDF (g kg−1)472·1a484·5b462·7a3·00 ***
 Ash (g kg−1)125·4ab121·9a129·1b2·16
image

Figure 1. Effect of post-grazing sward height [high (4·5–5 cm), intermediate (4–4·5 cm) and low (3·5–4 cm)] on (a) perennial ryegrass tiller density (high ▲, intermediate ● and low ♦) and (b) non-perennial ryegrass tiller density (high Δ, intermediate ○ and low ♢) during the grazing season. ***< 0·001.

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Mid-season (16 April–31 July)

There were 6·1 ± 0·2 grazing rotations completed by cows grazing each treatment during the mid-season period. The interval between grazings (16·6 days) and herbage growth rate (60·6 kg DM ha−1 day−1) were unaffected by treatment.

Pre- and post-grazing herbage mass, herbage accumulated, sward density and herbage harvested

There was no significant difference in pre-grazing HM >3·5 cm (1109 kg DM ha−1), leaf yield >3·5 cm (4118 kg DM ha−1), herbage accumulated (5704 kg DM ha−1) or sward density (248 kg DM cm−1) between treatments during the mid-season period (Table 3). Below the 3·5 cm grazing height, HM was similar for all treatments (2344 kg DM ha−1). Post-grazing HM was highest for H, intermediate for I and lowest for L paddocks (< 0·01). The effect of treatment on herbage harvested during mid-season approached significance (= 0·08), with reduced herbage harvested on the H treatment (4761 kg DM ha−1) compared with both I and L treatments (5864 kg DM ha−1). The L treatment achieved the highest (< 0·001) level of available herbage utilization, while I was intermediate and H lowest. The proportion of grazing area rejected by the cows was lowest in L paddocks (0·08), highest in H paddocks (0·21) and intermediate in I paddocks (0·16; < 0·001; Figure 2).

Table 3. Effect of post-grazing sward height [high (4·5–5 cm), intermediate (4–4·5 cm) and low (3·5–4 cm)] on pre- and post-grazing herbage mass, sward structure and sward morphology during mid-season (16 April–31 July 2009)
 Post-grazing sward heights.e.Level of significance
HighIntermediateLow
  1. Values with different letters are significantly different at < 0·05; s.e., standard error of the mean.

  2. Numbers between brackets are 0·95 confidence intervals after back-transformation.

  3. NS, not significant; *< 0·05; **< 0·01; ***< 0·001; †< 0·10.

Pre-grazing
 Herbage mass >3·5 cm (kg DM ha−1)113811871001112·1NS
 Height (cm)7·78·17·40·31NS
 Sward density (kg DM cm−1)24725024811·4NS
 Herbage accumulated (kg DM ha−1)532059595834398·2NS
 Extended tiller height (cm)23·723·622·40·68NS
 Extended sheath height (cm)12·1a10·4ab9·5b0·67 *
 Free leaf lamina (cm)11·7a13·2b12·8b0·450·12
 Leaf proportion >3·5 (cm)0·56a0·60b0·64c0·02 **
 Stem proportion >3·5 (cm)0·33a0·27b0·24b0·01 *
 Dead proportion >3·5 (cm)0·110·130·120·01NS
 Leaf yield >3·5 cm (kg DM ha−1)402444413888201·7NS
 Stem yield >3·5 cm (kg DM ha−1)2475a1880ab1413b224·4
 Dead yield >3·5 cm (kg DM ha−1)933a848a683b45·7
Post-grazing
 Herbage mass >3·5 cm (kg DM ha−1)401a263b103c36·7 **
 Height (cm)5·1a4·5b3·7c0·12 ***
 Extended tiller height (cm)14·8a14·0a10·8b0·53 **
 Extended sheath height (cm)10·0a9·1a6·7b0·47 **
 Free leaf lamina (cm)4·8a5·1a4·1b0·300·11
 Herbage harvested (kg DM ha−1)4761a5969b5758b286·4
 Proportion of herbage utilized0·65 (0·61-0·69)a0·81 (0·77-0·86)b0·95 (0·90-1·01)c  ***
image

Figure 2. Effect of post-grazing sward height [high (4·5–5 cm), intermediate (4–4·5 cm) and low (3·5–4 cm)] on the proportion of rejected area at each grazing rotation (Rotation 5 = mid/late June, Rotation 11 = late October/early November; high ▲, intermediate ● and low ♦). ***< 0·001.

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Sward structure, sward height, sward morphology and tiller density

There was no significant difference in pre-grazing sward height between treatments (7·7 cm; Table 3). Leaf proportion >3·5 cm differed significantly (P < 0·01) between treatments and was highest for the L treatment (0·64), lowest for the H treatment (0·56) and intermediate for the I treatment. Stem proportion >3·5 cm was similar for the I and L treatments (0·26; < 0·05), which was less than that of the H treatment (0·33). Pre-grazing stem yield tended to be higher on H paddocks (1062 kg DM ha−1) compared with the L paddocks (1413 kg DM ha−1) while I was similar to both treatments. Yield of dead herbage tended to be lower on L paddocks, compared with the other treatments, by 208 kg DM ha−1 (< 0·1). Below the 3·5-cm horizon, leaf (0·05), stem (0·53) and dead proportions (0·42) were similar for the three treatments. The H treatment paddocks had a lower PRG tiller density (6144 tillers m−2; < 0·05) than the other two treatments (7461 tillers m−2) during mid-season, while there was no difference in non-PRG tiller density (2951 tillers m−2; Figure 1). Pre-grazing extended sheath height was significantly (< 0·05) higher for H paddocks (12·1 cm) compared with I and L paddocks (10·4 and 9·5 cm respectively). Post-grazing sward height was highest for H, intermediate for I and lowest for L treatments. Post-grazing ETH and ESH were lowest in L paddocks (10·8 and 6·7 cm respectively) while the I and H treatments were similar (14·4 and 9·6 cm respectively). Post-grazing FLL tended to be less on L swards (4·1 cm) compared to I and H swards (5·0 cm).

Sward quality

The chemical composition of the swards in mid-season is shown in Table 2. Neither CP content nor ash was influenced by treatment (192·6 and 126·9 g kg−1 respectively). The OMD of the L swards was higher (< 0·05; +22·3 g kg−1) than the H swards (760·6 g kg−1) while the I treatment was intermediate (770·7 g kg−1). The NDF contents of the H and I treatments (467·0 g kg−1) were similar and higher (< 0·05) than that of the L treatment (448·8 g kg−1) during mid-season.

Autumn (1 August–18 November)

There were 3·3 grazing rotations completed during the autumn period. The average rotation length was 27·0 days, and daily herbage growth rate was 46·8 kg DM ha−1 day−1.

Pre- and post-grazing herbage mass, herbage accumulated and herbage harvested

There was no effect of treatment on pre-grazing HM (1395 kg DM ha−1), sward density (239 kg DM cm−1) or herbage accumulated (4538 kg DM ha−1) during autumn (Table 4). Post-grazing HM differed significantly between treatments (< 0·01) and was highest for H (395 kg DM ha−1), intermediate for I (115 kg DM ha−1) and lowest for L paddocks (8 kg DM ha−1). The most herbage was harvested from the H treatment (4516 kg DM ha−1; < 0·05) during autumn compared with both I and L (3870 and 3771 kg DM ha−1 respectively). The L treatment achieved the highest (< 0·05) level of available herbage utilization (0·92) compared with H (0·69), while I (0·83) was intermediate. The proportions of grazing area rejected during autumn were 0·33, 0·24 and 0·13 for H, I and L treatment paddocks respectively (< 0·001; Figure 2). Below 3·5 cm, HM was not different between treatments (2226 kg DM ha−1).

Table 4. Effect of grazing post-grazing sward height [high (4·5–5 cm), intermediate (4–4·5 cm) and low (3·5–4 cm)] on pre- and post-grazing herbage mass, sward structure and sward morphology during autumn (1 August to 18 November 2009)
 Post-grazing sward heights.e.Level of significance
HighIntermediateLow
  1. Values with different letters are significantly different at < 0·05; s.e., standard error of the mean.

  2. Numbers between brackets are 0·95 confidence intervals after back-transformation.

  3. NS, not significant; *< 0·05; **< 0·01; ***< 0·001; †< 0·10.

Pre-grazing
 Herbage mass >3·5 cm (kg DM ha−1)146613711349142·2NS
 Height (cm)9·49·19·00·35NS
 Sward density (kg DM cm−1)23324723812·4NS
 Herbage accumulated (kg DM ha−1)459744404576303·2NS
 Extended tiller height (cm)26·926·225·20·99NS
 Extended sheath height (cm)12·4a7·2ab4·8b1·25 **
 Free leaf lamina (cm)15·817·719·61·71NS
 Leaf proportion >3·5 (cm)0·55a0·60ab0·73b0·04 *
 Stem proportion >3·5 (cm)0·28a0·23ab0·17b0·03 *
 Dead proportion >3·5 (cm)0·18a0·18a0·10b0·02
 Leaf yield >3·5 cm (kg DM ha−1)311828623033196·4NS
 Stem yield >3·5 cm (kg DM ha−1)1702a970b652b195·6 *
 Dead yield >3·5 cm (kg DM ha−1)1045a722b364b136·6
Post-grazing
 Herbage mass >3·5 cm (kg DM ha−1)395a115b8c40·6 **
 Height (cm)5·2a3·9b3·5c0·13 ***
 Extended tiller height (cm)19·6a14·6b9·6c0·68 ***
 Extended sheath height (cm)14·6a8·3b5·1c0·81 ***
 Free leaf lamina (cm)5·55·64·20·72NS
 Herbage harvested (kg DM ha−1)4516a3870b3771b121·3 *
 Proportion of herbage utilized0·69 (0·59-0·79)a0·83 (0·73-0·95)ab0·92 (0·81-1·04)b  *
Table 5. Effect of post-grazing sward height [high (4·5–5 cm), intermediate (4–4·5 cm) and low (3·5–4 cm)] on pre- and post-grazing herbage mass, sward structure and sward morphology over the entire season (10 February–18 November 2009)
 Post-grazing sward heights.e.Level of significance
HighIntermediateLow
  1. Values with different letters are significantly different at < 0·05; s.e., standard error of the mean.

  2. Numbers between brackets are 0·95 confidence intervals after back-transformation.

  3. NS, not significant; *< 0·05; **< 0·01; ***< 0·001; †< 0·10.

Pre-grazing
 Herbage mass >3·5 cm (kg DM ha−1)12541268112763·4NS
 Height (cm)8·28·68·00·65NS
 Sward density (kg DM cm−1)24623623112·3NS
 Herbage accumulated (kg DM ha−1)11 05511 60911 291514·1NS
 Extended tiller height (cm)24·8a24·0ab23·4b0·46
 Extended sheath height (cm)11·2a8·2b7·3b0·53 **
 Free leaf lamina (cm)13·615·115·50·84NS
 Leaf proportion >3·5 (cm)0·56a0·62b0·67b0·021 *
 Stem proportion >3·5 (cm)0·30a0·23b0·21b0·016 *
 Dead proportion >3·5 (cm)0·140·150·120·010NS
 Leaf yield >3·5 cm (kg DM ha−1)856986298019285·1NS
 Stem yield >3·5 cm (kg DM ha−1)4439a3072ab2275b391·4
 Dead yield >3·5 cm (kg DM ha−1)2230a1808b1297c152·4 *
Post-grazing
 Herbage mass >3·5 cm (kg DM ha−1)340a163b26c27·2 ***
 Height (cm)4·9a4·2b3·6c0·07 ***
 Extended tiller height (cm)15·7a13·8a10·6b0·49 ***
 Extended sheath height (cm)10·7a8·5b6·3c0·43 ***
 Free leaf lamina (cm)5·1a5·2a4·2b0·22 *
 Herbage harvested (kg DM ha−1)11 04311 50611 200554·9NS
 Proportion of herbage utilized0·71 (0·65-0·76)a0·83 (0·78-0·90)b0·97 (0·91-1·03)c  **
Sward structure, sward height, sward morphology and tiller density

Pre-grazing sward height was similar across treatments (9·2 cm; Table 4) while post-grazing sward height decreased with increasing levels of grazing severity. Pre-grazing ETH and FLL length did not differ significantly between treatments; however, ESH was highest (< 0·05) for H paddocks (12·4 cm) and lowest for L paddocks (4·8 cm), and the I paddocks were similar to both (7·2 cm). Although treatment had no effect on post-grazing FLL length (5·1 cm), H paddocks had the greatest post-grazing ETH (19·6 cm; < 0·001) and ESH (14·6 cm; < 0·01), while L were shortest (9·6 and 5·1 cm respectively; < 0·05) and I were intermediate (14·6 and 8·3 cm respectively). There was also no significant effect of treatment on leaf yield (3004 kg DM ha−1); however, both stem (< 0·05) and dead (< 0·1) material yields were higher for the H treatment (1702 and 1045 kg DM ha−1 respectively) compared with I (970 and 722 kg DM ha−1 respectively) and L treatments (652 and 364 kg DM ha−1 respectively).

Sward structure and morphology were affected by treatment during autumn. Leaf proportion >3·5 cm in the pre-grazing sward was higher (< 0·05) for L (0·73) than for H (0·55) while that for I was intermediate (0·60). Conversely, stem (< 0·05) and dead (< 0·1) proportions >3·5 cm in the pre-grazing sward were lowest for L (0·17 and 0·10) and highest for H (0·28 and 0·18), while that for I was intermediate (0·23 and 0·18 respectively). The proportion of leaf, stem and dead material >3·5 cm in the post-grazing sward was unaffected by grazing treatment. During autumn, L paddocks had the highest PRG tiller density (4969 tillers m−2; < 0·001) compared with H and I treatments (3381 and 3347 tillers m−2 respectively). Non-PRG tiller density was highest in H paddocks (6242 tillers m−2; < 0·001), lowest in L (1581 tillers m−2; < 0·001) and intermediate for I (4172 tillers m−2; Figure 1).

Sward quality

Neither CP content nor ash was influenced by treatment (201·7 and 113·9 g kg−1 respectively; Table 1). The L treatment achieved a higher OMD (751·4 g kg−1; P < 0·01) and lower NDF (485·7 g kg−1; P < 0·05) content compared with the I treatment (706·7 and 505·3 g kg−1 respectively) and the H treatment (721·2 and 511·0 g kg−1).

Entire grazing season (10 February–18 November)

There were 11·4 grazing rotations during the study, with an average rotation length of 24·9 days.

Pre- and post-grazing herbage mass, herbage accumulated and herbage harvested

Post-grazing sward height had no significant effect on pre-grazing HM, sward density, total herbage accumulated or herbage harvested during the grazing season (Table 1). While there was no significant effect of treatment on leaf yield (8405 kg), the H treatment produced more stem (4439 kg DM ha−1; < 0·05) and dead (2230 kg DM ha−1; < 0·05) material compared with both the I (3072 and 1808 kg DM ha−1) and L (2275 and 1297 kg DM ha−1) treatments. Average herbage utilization was 0·71, 0·83 and 0·97 for H, I and L respectively (< 0·01). The proportion of area rejected was 0·27, 0·17 and 0·10 for H, I and L respectively (< 0·001; Figure 2). Herbage mass below 3·5 cm did not differ between treatments (2099 kg DM ha−1).

Sward structure, sward morphology and tiller density

Pre-grazing sward height did not differ between treatments (8·3 cm). The H treatment paddocks had the greatest pre-grazing ETH (< 0·01; Table 4) and ESH (< 0·001). The L treatment paddocks were grazed 0·6 cm shorter than I and 1·3 cm shorter than H paddocks (< 0·001). Post-grazing FLL was shortest in L paddocks (1 cm shorter, < 0·001). Above the 3·5-cm horizon, L paddocks had the greatest proportion of leaf and the lowest proportion of stem (0·67 and 0·21 respectively; < 0·001). Conversely, H paddocks had the lowest proportion of leaf and the greatest proportion of stem (0·56 and 0·30; < 0·001). Below the 3·5 cm horizon, treatment did not affect the proportions of leaf, stem or dead components (0·04, 0·51 and 0·46 respectively). The L treatment paddocks tended to have a more dense PRG population than the other treatments (4746, 4832 and 5602 PRG tillers m−2 for H, I and L paddocks respectively; < 0·1) and contained the least number of non-PRG tillers (1771 m−2; < 0·001). Total tiller density was not different between treatments (7694 tillers m−2; Figure 1).

Sward quality

Treatment had a significant effect on sward nutritive value during the study (Table 1). While there was no significant treatment effect on CP content (200·1 g kg−1), the L treatment achieved a higher OMD content (769·5 g kg−1; = 0·06) compared with both the H and I treatments (752·3 and 750·8 g kg−1 respectively). In contrast, the NDF content of the I treatment was significantly greater (484·5 g kg−1; < 0·001) compared with the H and L treatments (472·1 and 462·7 g kg−1 respectively). Over the entire grazing season, the ash content of L swards was highest (129·1 g kg−1; = 0·06) compared with I swards (121·9 g kg−1) which was lowest, while H was intermediate (125·4 g kg−1).

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgments
  9. References

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.

Sward quality

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.

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgments
  9. References

Milk quota abolition in Europe has revived interest in increasing SR and hence increased grazing severity to harvest more grass on dairy farms. From an agronomic perspective, an increase in grazing severity through low post-grazing sward height will result in similar net herbage and leaf accumulation above 3·5 cm, and increased sward utilization and quality. These findings endorse the concept of increased grazing severity as a method of retaining high-quality grass throughout the grazing season while reducing the build up of senescent material and proliferation of non-PRG grasses. This study demonstrated that, under the post quota production environment grazing management practice on Irish dairy farms can have a major impact on characteristics and nutritive value of the sward.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgments
  9. References

We wish to acknowledge the financial support of the Walsh Fellowship programme and Irish Dairy Levy Trust. The authors thank M. Feeney, C. Fleming and N. Galvin (Animal and Grassland Research and Innovation Centre, Teagasc, Moorepark, Fermoy, Co. Cork, Ireland) for their technical assistance. We thank the staff at Curtins Farm, Moorepark for their care and management of the experimental farmlets. Special thanks to Brian McCarthy, Jeanne Guegan and Camille Terrasse for their very valuable help.

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  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgments
  9. References
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