Transitioning to low chemical nitrogen perennial ryegrass white clover pastures on wetland grazing dairy systems

Introduction

Sustainable and resilient agri-food systems are vital to safeguard the environment and ensure food security (FAO 2023). Within the European Union (EU) there is increased awareness of the environmental pressures from agriculture on greenhouse gas emissions (GHG), freshwater quality decline and biodiversity loss. Recent policies addressing these challenges include the Farm to Fork Strategy, the EU Biodiversity Strategy for 2030, the Common Agricultural Policy review (CAP) 2023–2027 (DECC 2021) and the European Climate Law. As an EU member state, Ireland has developed a national Climate Action Plan (Government of Ireland 2021) to reduce agricultural GHG emissions by 25% by 2030 (DAFM 2020), including a 25% reduction in national chemical nitrogen (N) fertiliser use. The challenge of maintaining farm productivity and profitability while reducing chemical N use within pasture-based systems will require the successful incorporation of legumes such as white clover (WC; Trifolium repens) within grazed pastures (Lüscher et al. 2014).

The majority of the research undertaken to date on WC incorporation in Ireland has been conducted on freely draining loam soils and advantageous climatic conditions (McDonagh et al. 2017; Egan et al. 2018; Guy et al. 2018), with minimal evaluation within poorly drained organic soils. In addition, few studies have documented the impacts of the transition from old PR swards to newly established and oversown WC swards. The objective of this study was to investigate the transition from PR-old swards to newly established PR swards receiving high rates of chemical N, and PRWC swards supported by reduced rates of chemical N on a wetland soil type. A preliminary report of this work has been published by Walsh et al. (2024).

Materials and methods

This study was completed at Teagasc, Ballyhaise College, Co. Cavan, Ireland (54°051′N, 07°031′W) for 3 years (2021–2023, inclusive). The experimental site was situated on a variety of soil types including alluvia, brown earth, brown podzolic and gleys which lay over a Silurian sandstone bedrock. Dramatically sloped (8–9°) drumlins and alluvial flatlands dominate the topography of the site. All procedures undertaken involving cows during the experiment were approved by the Teagasc Animal Ethics Committee (TAEC0323-371).

Results

Average monthly rainfall and mean daily air temperature (mm and °C, respectively) for the study period were similar to the 10 year average (2011–2020) values for this site (90 mm and 10.3°C average from 2021 to 2023 inclusive, compared to 87 mm and 9.5°C for 10 year average). The 10 cm soil temperature for the 3 year study period was comparable to the long-term average values, excluding July–November 2021 (113%), July 2022 (103%), February 2023 (142%) and August–October 2023 (111%), where above average soil temperatures were reported.

During the study, 11 grazing rotations were achieved during both 2021 and 2022, with 10 rotations during 2023. The total number of days during which cows grazed varied each year and between sward treatments (259 days for both PR and PRWC in 2021, 257 and 256 days for PR and PRWC in 2022, and 243 and 235 days for PR and PRWC in 2023, respectively).

The establishment of PR-new and PRWC-new swards had a significant impact on total pasture production, which reduced (P < 0.001) during the year of establishment from 14,182 kg DM/ha for PR-old to 8925 and 8561 kg DM/ha for PR-new and PRWC-new, respectively (Table 2). Where permanent pasture was oversown with clover (PRWC-over), total pasture production was also reduced during the year of establishment (11,330 kg DM/ha), which may be attributed to lower initial WC contents and delayed oversowing of pasture until early summer (particularly in 2022) due to inclement weather conditions. However, in each subsequent year after establishment, total pasture production increased for PR-new, PRWC-new and PRWC-over (P < 0.001) compared to PR-old (14,891, 15,642, and 15,218 kg DM/ha in PR-new, PRWC-new and PRWC-over, respectively). Significant deficits in conserved silage for PR and PRWC (66% and 65%, respectively) were recorded overall during the 3 year study. As a consequence of the reduction in total pasture DM production, grazing days (No./ha) were reduced in the initial year of newly established PR-new and PRWC-new swards (373 and 399 days, respectively) compared to both PR-old and PRWC-over (609 and 580 days, respectively).

The establishment of PRWC-new, and to a lesser extent PRWC-over, resulted in a significant reduction (P < 0.001) in mean chemical N fertiliser application, from 229 kg and 230 kg for PR-old and PR-new, to 119 kg for PRWC-over and 90 kg for PRWC-new. Mean annual sward clover contents were 210, 250 and 190 g/kg DM during 2021 (from early September onwards), 2022 and 2023, respectively, within the PRWC farm systems (Fig. 1). Establishment method had a significant effect on sward clover content, with a mean clover content of 296 g/kg in PRWC-new compared to 136 g/kg within PRWC-over during the study. Although similar mean monthly sward WC contents were achieved from March to mid-May for both establishment methods, considerable differences were observed from late May until the end of the grazing season (Fig. 2). Reseeded swards experienced a rapid increase in WC content from late May (120 g/kg) to July (290 g/kg), whereas an increase of only 30 g/kg was evident in PRWC-over (130–160 g/kg, respectively) during the same period. In addition, stolon mass measurements were conducted on three occasions during 2023, which confirmed the substantial WC content within the PRWC treatment area (1.45 t, 1.25 t and 1.71 t stolon DM/ha for February, June and October 2023, respectively).

Fig. 1.

The evolution of sward white clover contents during the grazing season (based on full reseed and oversowing during the study period) where , and represents clover contents in 2021, 2022, and 2023, respectively.

Fig. 2.

Mean monthly sward white clover content for reseeded and oversown perennial ryegrass white clover swards during the grazing season throughout the 3 year transition (2021–2023).

Both sward change and sward age had a significant impact on sward chemical composition (Table 3). Ash content was greater (P < 0.001) for both newly established swards (PR-new and PRWC-new; 103.6 g/kg DM) compared to both PR-old and PRWC-over swards (94.9 and 96.4 g/kg DM, respectively). Similarly, sward CP content was increased for newly established swards from the year of establishment (201.8 g/kg DM) compared to PR-old (195.0 g/kg DM), while oversown swards initially had the lowest (P < 0.001) CP content (172.0 g/kg DM) but increased to similar levels to the newly established swards during Year 2 and 3 (202.3 and 195.4 g/kg DM, respectively). Sward NDF content was also affected by sward change, with both PR-new and PRWC-over swards reaching higher values over the 3 years of the transition (416.4 and 413.6 g/kg DM respectively) compared to both PR-old and PRWC-new swards (407.2 and 401.2 g/kg DM, respectively). In contrast, both ADF and OMD content were unaffected by sward change or establishment age.

The effect of sward system on animal performance is outlined in Table 4. The current study demonstrated a tendency for greater (P = 0.08) milk yield per cow in the PRWC system compared to PR (5197 and 5092 kg, respectively) with modest increases also observed for fat plus protein yield (472.5 and 461.0 kg/cow, respectively). No significant differences in milk composition (fat, protein and lactose), BW and BCS were observed between sward systems during the current study.

Discussion

This study investigated the transition from old PR swards to newly established PR swards with similar high levels of chemical N application, or to PRWC-new and PRWC-over swards with reduced chemical N application on a wetland grazing dairy system in the border midland and western (BMW) region of Ireland. The results of the study are of particular relevance to the establishment of WC within livestock grazing systems on more marginal land types such as those in the BMW region of Ireland. While the rate of new pasture establishment within this study is beyond that recommended (10% per year; Creighton et al. 2011) and impractical for commercial farms, the accelerated transition undertaken herein is nonetheless indicative of the likely longer term impacts for commercial grazing farms within a more prolonged timeframe. Moreover, the novelty of reporting on the immediate transitional phase impacts from old to rejuvenated pastures with/without WC has not, to our knowledge, previously been reported within the published literature.

The successful establishment of WC with grazing swards is critical to the future productivity of pastures in Ireland, given that the annual rate of fertiliser N input permitted under the EU Nitrates Directive Statutory Instruments (SI) No. 101 (ISB 2009) has substantially reduced from 275 kg/ha in 2009 to 212 kg/ha in 2024, while the costs of fertiliser N have also increased significantly (Teagasc 2024). The level of WC establishment (by both content (250 and 190 g/kg DM during 2022 and 2023, respectively) and stolon mass (1.71 t DM/ha in 2023)) on a wetland soil type compares favourably with previous studies both in Ireland (Humphreys et al. 2009; McDonagh et al. 2017) and elsewhere (Lüscher et al. 2014) and is indicative of the resilience of WC across a range of soil types and climatic conditions. The grazing management practices under which this study was undertaken were also conducive to successful WC establishment as frequent grazing rotations (11 for 2021 and 2022, and 10 for 2023), appropriate post-grazing residual heights (40 mm) and adequate soil fertility have been widely acknowledged as critical requirements (Hoogendoorn et al. 1992; Chapman et al. 2017; Guy et al. 2018).

Notwithstanding the high level of WC content achieved during establishment, there was, nonetheless, a high level of variation in WC content between swards within the study. The results indicate that establishment methods had a significant impact on WC contents (296 g/kg and 136 g/kg for reseeded and oversown swards, respectively), as observed previously by Hume and Chapman (1993), which further emphasises the importance of complete sward renewal to substantially increase WC contents on commercial farms. It also emphasises the important impact of prevailing weather conditions and the appropriate timing of oversowing (Clavin et al. 2017) as WC content was reduced in particular during 2022, when oversowing occurred in July due to waterlogged soils in early summer, resulting in poor WC establishment. Although a reduction in mean WC content was observed during 2023 (190 g/kg DM) compared to 2022 (250 g/kg DM), nonetheless, stolon mass measurements during 2023 indicate that the presence of a significant stolon architecture will support high levels of WC content in each subsequent grazing season, similar to previous studies (Schwinning and Parsons 1996; Guy et al. 2018). Subsequent to this transition, the proportion of reseeding will be retained at the recommended annual rate (10%; Creighton et al. 2011) to sustain both pasture production and clover content in each subsequent year.

The fertiliser strategy for WC swards within the current study was similar to that reported previously by Frame (1987), Humphreys and Lawless (2006) and Søegaard (2009) and was designed to increase PR production while simultaneously enhancing clover contents (Humphreys et al. 2009; Enriquez-Hidalgo et al. 2016). Although there were substantial differences in fertiliser N inputs between the newly established PR and PRWC swards, biological N fixation (BNF) within the newly established PRWC swards provided sufficient N to support similar levels of pasture production compared to fertilised PR swards similar to previous studies (Egan et al. 2018; Jørgensen et al. 2023).

Although WC is naturally more digestible than PR (Ulyatt 1970; Thompson 1984) and is therefore expected to increase OMD (Leach et al. 2000; Ribeiro Filho et al. 2005), both Humphreys et al. (2009) and Grace et al. (2018) observed no significant differences in sward OMD or NDF content between PR and PRWC swards. The significant reduction (P < 0.001) in DM production within the year of sward establishment in this study (−6.7 t DM/ha) was due to inclement weather conditions and poor soil trafficability, which extended the fallow period before new sward establishment. While the intended closed period is generally anticipated to be 60 days (Teagasc 2014), an additional extension of the closed period occurred each year before an adequate seedbed could be established for the new swards (+35, +28 and +42 days for 2021, 2022, and 2023, respectively). When the significant reduction in pasture production during new sward establishment is combined with the high proportion of reseeding undertaken each year during the study, substantial shortages in winter feed production were recorded during the 3 year study period. Consequently, farmers attempting to increase WC content on commercial farms must make adequate provision for increased winter feed reserves during the period of new sward establishment.

The impact of WC inclusion within PR swards on animal performance has received significant attention but with contradictory findings reported (Humphreys et al. 2009; Glassey et al. 2013; Scully et al. 2021). Ribeiro Filho et al. (2005) reported no effect of WC inclusion on milk yield as the WC content of the sward was insufficient (270 g/kg). Dineen et al. (2018) observed a significant linear increase in daily milk yield with increasing WC content within grazed swards, suggesting that higher WC contents (> 320 g/kg) are required to observe significant benefits in individual animal performances. Although mean sward WC contents of 250 and 190 g/kg were present in the current study during 2022 and 2023, respectively, only a very modest increase in animal performance was observed during this study. On that basis, these results indicate that, while substantial saving in chemical N application can be realised immediately, only very modest impacts on animal performance can be anticipated during the transition from PR to PRWC swards.

Although the initial transition to WC swards was successful in establishing significant stolon mass and WC contents and reducing N fertiliser requirements, a long-term evaluation of the persistency and productivity of WC on such soils is merited to build confidence among the wider commercial industry to incorporate WC on farms. Moreover, given the significant reduction in chemical N fertiliser applications arising from the establishment of WC swards, an economic cost benefit analysis and environmental impact assessment of such systems could encourage a more accelerated adoption of WC on commercial grazing farms into the future.

Conclusion

The results of this study demonstrate the successful establishment of WC, with reduced chemical N on a wetland soil type in Ireland. This study observed similar levels of pasture production for WC reseeded swards, with lower rates of chemical N when compared to PR reseeds under higher N fertiliser application rates. Although a high level of clover establishment was achieved in terms of clover content, only modest increases in milk production were observed during the transition period. The results highlight the potential of WC swards under reduced chemical N application to maintain pasture and animal performance when compared to PR only swards receiving high chemical N rates.