The effect of nitrogen and phosphorus fertiliser application rate and strategy on herbage production and nitrogen response in spring

Abstract

Maximising herbage yield while reducing nitrogen (N) fertiliser input, particularly in spring, is essential to ensure environmental and economic sustainability on grassland farms. A plot experiment was conducted over 2 yr, comparing three different spring N application rates of 30 (30N), 60 (60N) and 90 (90N) kg N/ha using three different spring application strategies: 0:100 (S1), 50:50 (S2) or a 33:66 (S3) split across February and March, respectively. Half of the plots also received phosphorus (P) fertiliser with the first application of N at a rate of 13 kg P/ha. Nitrogen fertiliser application for the remainder of the year (April–September) was the same for all plots (23 kg N/ha/application). Both spring and cumulative herbage yields were significantly affected (P < 0.05) by N application rate; 90N had the greatest spring and cumulative herbage yield compared to 30N and 60N (10,925, 9,834 and 10,499 kg DM/ha, respectively); however, N response reduced as N application rate increased. Nitrogen application strategy had a significant effect (P < 0.05) on spring herbage yield, with S1 significantly lower than S2 and S3. Applying 13 kg P/ha in spring increased herbage yield at defoliations 2 (23 April) and 3 (15 May) (+133 and 56 kg DM/ha, respectively), relative to no application of P fertiliser, as well as increasing cumulative herbage yield (+241 kg DM/ha). The results of the current study indicate that N should be applied in early February and the strategic application of N and P during spring can increase spring and cumulative herbage yield.

Main article text

Introduction

Nitrogen (N) and phosphorus (P) fertilisation are essential to achieve sufficient levels of herbage production (Wall & Plunkett, 2020); however, surplus nutrients have negative environmental implications such as soil acidification, increased greenhouse gas (GHG) emissions and N leaching, causing reductions in water quality and aquatic biodiversity (Sainju et al., 2019). It is estimated that N and P fertilisers are responsible for 78% of global marine and freshwater eutrophication (Jwaideh et al., 2022). Within the European Union (EU), the Nitrates Directive (91/676/EEC) required farmers to reduce chemical N fertiliser use from 250 to 225 kg N/ha from 2023 (European Union, 2022), with further reductions required in future years. In Ireland, N fertiliser use peaked at 408,000 tonnes in 2018; however, the national government policy has set a target of 350,000 tonnes per year by 2025 and 325,000 tonnes by 2030 (Department of Agriculture, Food and the Marine, 2020). Current N fertiliser recommendations in Ireland state that at high stocking rates (>2.5 cows/ha) 90 kg N/ha should be applied in spring (Wall & Plunkett, 2020); however, this may need to be reduced to comply with national N fertiliser reduction targets. Given the reliance on grazed pasture in the diet on Irish grassland farms (O’Brien et al., 2018), it is essential that any reduction in N fertiliser application is achieved with minimal impact on herbage production as this would result in a reduction in stocking rate or increased import of other feedstuffs to fill feed deficits arising from reduced pasture production, resulting in increased N input (O’Donovan et al., 2021).

An important objective of pasture-based milk production systems is to increase the proportion of grazed grass in the diet of lactating dairy cows, and grass utilisation, particularly in early lactation, to reduce production costs (Kennedy et al., 2009). Due to the growth pattern of perennial ryegrass (Lolium perenne L.; PRG), there is limited grass growth during winter and early spring (Hennessy et al., 2008). As a result, grazing management practices (Kennedy et al., 2007) and chemical fertiliser application (Rumpel et al., 2015) need to be strategically managed to better meet spring herd feed demand from grazed grass (Hennessy et al., 2020). Earlier application of N fertiliser increased spring herbage production (O’Donovan et al., 2004; Murphy et al., 2013; McNamara et al., 2022). When N application was delayed from early January until late February at varying N rates (30, 60 and 90 kg N/ha), spring herbage production was reduced due to insufficient N supply (O’Donovan et al., 2004).

Nitrogen surplus is defined as the difference between total farm N inputs and N outputs (kg N/ha; Buckley & Donnellan, 2018). Applying excess N fertiliser can result in N surplus to plant requirements and therefore reduced N use efficiency (NUE) of the applied fertiliser which can be affected by a range of factors such as N rate (Vellinga et al., 2010), climatic conditions (de Boer et al., 2016), soil type (Vogeler et al., 2015), level of soil organic N available in spring, sward quality and the addition of N from grazing animals (Deenen & Middelkoop, 1992). Vogeler et al. (2015) reported that an increase in N fertiliser application also leads to higher levels of indirect nitrate (NO₃ ⁻) leaching due to higher concentrations of N in swards. Previous studies indicate that N losses can be considerably reduced by more precise application, whereby N is applied to match plant demand and during times when the risk of environmental loss is reduced (Cuttle & Scholefield, 1995; Cameron et al., 2013).

The simultaneous application of N and P has been shown to further increase overall herbage production as N application rate increases (Duru & Ducrocq, 1996), particularly in periods of reduced herbage production in spring (Saunders et al., 1987; Sheil et al., 2016). Phosphorus is an essential nutrient for both plants and animals (Alexander et al., 2008), increasing herbage DM production in soils with both inadequate (Herlihy et al., 2004; Schulte & Herlihy 2007; Burkitt et al., 2010) and even adequate (Wall & Plunkett, 2020) soil P levels, as P is not as readily available to plants during periods of low soil temperatures, particularly in spring (Saunders et al., 1987; Sheil et al., 2016). Current P fertiliser advice is based on maintaining or increasing soil P levels at >5.1 mg/L to ensure soils are index 3 or 4 (Alexander et al., 2008), while replacing P removed from soil by grazing animals and/or silage harvest (Wall & Plunkett, 2020).

In light of reducing N fertiliser allowances and the urgent need to reduce N losses to the environment, the objectives of the current study were to investigate the impact of N and P fertiliser application rate and N application strategy on spring and cumulative herbage production, herbage production response to N application and recovery of N in herbage.

Materials and methods

Experimental site

A plot experiment was carried out at the Teagasc, Animal and Grassland Research and Innovation Centre, Moorepark, Fermoy, Co. Cork, Ireland (52° 7′ 3″ N, 8° 16′ 42″ W), from February to November 2020 (year 1) and February to November 2021 (year 2). The soil type was a free-draining acid brown earth of sandy loam to loam texture. Soils had a pH of 6.4, P index of 4 (8.5 mg/L) and potassium (K) index of 3 (125 mg/L) (Alexander et al., 2008). The experimental site was south facing and approximately 40 m above sea level. The experimental area was sown in May 2019 with a PRG seed mix (cv. Astonenergy and Abergain) at 30 kg seed/ha. The experiment was carried out on predominantly PRG (90%) swards with the remaining 10% consisting of annual weed grass (Poa annua L.) and broadleaved grassland weeds. In the establishment year the plots received 37.5 and 75 kg/ha of P and K at sowing, respectively, and a total of 88 kg N/ha applied in four equal applications between sowing and 14 September 2019. Plots were defoliated as per Teagasc recommendations (Teagasc, 2017), with the final defoliation on 10 November 2019 in the establishment year. The meteorological data from the experimental site were collected throughout the trial. Average daily air temperature (°C), monthly rainfall (mm) and soil temperature (°C; to a depth of 10 cm) were recorded and are summarised in Table 1.

Table 1:

Average daily air temperature (°C), monthly rainfall (mm) and mean soil temperature to a depth of 10 cm (°C) between January and December for year 1 (2020), year 2 (2021) and the 10-year average (2010–2019) at the experimental site

	Jan	Feb	Mar	April	May	June	July	Aug	Sept	Oct	Nov	Dec
Mean daily air temperature (°C)
Year 1	6.1	6.6	6.6	9.8	11.9	13.9	15.3	16.2	13.7	9.7	8.4	5.6
Year 2	4.3	6.3	7.5	7.4	9.8	14.4	17.2	15.6	14.9	11.8	8.5	7.7
10-year average	5.5	5.6	6.5	8.7	11.4	14.0	15.8	14.8	13.2	10.6	7.2	6.2
Monthly rainfall (mm)
Year 1	90	153	48	65	37	73	73	145	43	102	119	153
Year 2	60	190	53	23	131	27	63	58	102	125	33	135
10-year average	110	85	76	75	63	79	69	75	72	100	111	126
Mean soil temperature to a depth of 10 cm (°C)
Year 1	5.7	6.1	6.7	11.0	14.7	16.3	17.0	17.9	15.2	10.3	8.7	5.5
Year 2	3.6	6.2	7.8	9.7	12.5	17.0	20.0	17.6	16.3	12.5	8.6	7.3
10-year average	5.2	5.5	7.0	10.2	13.5	16.7	18.6	17.2	14.9	11.5	7.6	5.9

Experimental design

The experimental design was a 3 × 3 × 2 factorial design comprising of four replicates. Three spring N application rates (30 kg N/ha [30N], 60 kg N/ha [60N] and 90 kg N/ha [90N]), three N application strategies in spring (split between 3 February and 19 March [year 1] and 3 February and 16 March [Year 2]); 0:100 (S1), 50:50 (S2) and 33:66 (S3) and two P application rates; (0 kg P/ha and 13 kg P/ha) (Table 2). All of the plots used in the experiment were 1.5 m × 6 m. Nitrogen fertiliser was applied in the form of protected urea (46% N) and P was applied to half of the plots on the first N application date (3 February) in the form of TOP-PHOS (complex super protected phosphate; 23% P, Groupe Roullier, Saint-Malo, Brittany, France) which is a water-soluble P fertiliser. All plots received the same N fertiliser application rate (23 kg N/ha per rotation) in the form of protected urea (46% N) after defoliation 2 (23 April) through to defoliation 7 (3 September). A total of 168 kg N/ha, 198 kg N/ha and 228 kg N/ha was applied to the 30N, 60N and 90N treatments, respectively, across the year. The 90N S3 treatment was included as the control as this was the recommendation for spring N application when the experiment was carried out (Wall & Plunkett, 2020). There was an additional 0N plot included within each replicate which was used to calculate N recovery and N response for the first and second N application dates in spring, which was not included in the herbage statistical analysis. These plots received 0N for application dates 1 and 2; however, they received the same N as the remainder of the plots after defoliation 2. The first defoliation was on 19 March in year 1 and 16 March in year 2 and the second defoliation was on 23 April in both years. Plots were defoliated when a target pre-grazing herbage mass of 1,300–1,500 kg DM/ha was achieved on the 90N treatment, using visual assessment (O’Donovan et al., 2002), for a total of eight defoliations each year, to investigate any carry-over effects. The average pre-grazing herbage mass across the full production year (≈1,300 kg DM/ha) was within the range of Teagasc recommendations (Teagasc, 2016) for mid-season grazing management. Defoliations 3, 4, 5, 6, 7 and 8 were carried out on 15 May, 10 June, 1 July, 24 July, 21 August and 4 November in year 1 and 24 May, 15 June, 12 July, 9 August, 3 September and 27 September in year 2, respectively. The plots were not defoliated between the last defoliation in year 1 (4 November 2020) and the first defoliation in year 2 (19 March 2021) to represent paddocks being closed over winter on Irish dairy farms.

Table 2:

Spring nitrogen (N) application rate and application strategy for applications 1 (3 February) and 2 (19 March in year 1 and 16 March in year 2)

Total spring N application rate (kg N/ha)	30						60						90
Nitrogen application strategy 1	S1		S2		S3		S1		S2		S3		S1		S2		S3 2
Nitrogen rate application 1 (kg N/ha)	0		15		10		0		30		20		0		45		30
Nitrogen rate application 2 (kg N/ha)	30		15		20		60		30		40		90		45		60
Phosphorus (P) application (kg P/ha) 3	0	13	0	13	0	13	0	13	0	13	0	13	0	13	0	13	0	13
Total annual N application rate (kg N/ha) 4	168						198						228

¹Nitrogen application strategy: quantity of N applied on first (3 February) and second (19 March in year 1 and 16 March in year 2) application in spring (S1 = 0N spread in application 1 and 100% in application 2, S2 = 50% in application 1 and 50% in application 2 and S3 = 33% in application 1 and 66% in application 2).

²Control treatment.

³Phosphorus fertiliser was applied to half of the plots with the first application of N.

⁴Total amount of N applied throughout the year.

Sward measurements

Herbage yield

The herbage yield was measured by cutting the plots to a post-cutting height of 4 cm (rising plate meter, Jenquip, New Zealand) using an Etesia mower (Etesia UK Ltd., Warwick, UK). The herbage from each plot was collected and the weight was recorded. A sample was taken from the mown herbage for each plot and a sub-sample of 100 g was taken from this to determine the DM content of each plot by drying the grass in a forced convection oven for 48 h at 60°C (Parsons Lane, Hope Valley, UK). Once dried, the sample was milled through a 1-mm sieve using a Cyclotec 1093 Sample Mill (Foss, Hillerød, Denmark) and stored for analysis. Cumulative herbage yield was calculated as the total annual herbage yield across all defoliations in each year. The response to the application of P fertiliser was calculated as the increase in cumulative herbage yield for plots that received P fertiliser (13 kg P/ha) compared to plots with the same N rate that did not receive P (0 kg P/ha).

Herbage quality

The milled herbage samples were analysed for crude protein (CP) content using near-infrared spectrometry (NIRS, Model 6500, Foss-NIR System, Hillerød, Denmark), which was calibrated by a trained laboratory technician weekly, using the equations derived by Burns et al. (2010), similar to Claffey et al. (2019) and Murray et al. (2023).

Estimation of herbage N yield, N response and N recovery

The herbage production response to the N fertiliser applied was calculated as the kilogram of DM produced per kilogram of N applied (kg DM/kg N applied) (O’Donovan et al., 2004; McNamara et al., 2022):

$$\frac{\begin{array}{l}(\text{Herbage}\hspace{0.17em}\text{yield}\hspace{0.17em}[\text{kg}\hspace{0.17em}\text{DM/ha};\hspace{0.17em}30\text{N},\hspace{0.17em}60\text{N}\hspace{0.17em}\text{or}\hspace{0.17em}90\text{N}]\\ \hspace{1em}\hspace{1em}–\hspace{0.17em}\text{herbage}\hspace{0.17em}\text{yield}\hspace{0.17em}[\text{kg}\hspace{0.17em}\text{DM/ha};\hspace{0.17em}\mathrm{0N}])\end{array}}{\text{Applied}\hspace{0.17em}\text{N}\hspace{0.17em}\text{rate}\hspace{0.17em}(30,\hspace{0.17em}60\hspace{0.17em}\text{or}\hspace{0.17em}90\hspace{0.17em}\text{kg}\hspace{0.17em}\text{N/ha})}$$

Herbage N yield (kg N/ha) was calculated as follows (O’Donovan et al., 2004):

$$\frac{\text{Herbage}\hspace{0.17em}\text{yield}\hspace{0.17em}(\text{kg}\hspace{0.17em}\text{DM/ha})\hspace{0.17em}\times \hspace{0.17em}\text{herbage}\hspace{0.17em}\text{CP}\hspace{0.17em}\text{content}\hspace{0.17em}(\text{g/kg})\hspace{0.17em}}{6.25\hspace{0.17em}\times \hspace{0.17em}1000}$$

The rate of N recovered in the herbage was calculated as the proportion of the applied N recovered in the herbage (kg N/ha) (O’Donovan et al., 2004).

Statistical analysis

Statistical analysis for this experiment was carried out using SAS 9.4 (SAS Institute Inc., Cary, NC, USA, 2002). The effect of N rate, N application strategy and P application on herbage yield, N response and N recovery was determined using the PROC MIXED procedure in SAS with N rate, N application strategy and P application and associated interactions used in the model. Plot was the experimental unit, with year as the random factor and defoliation number was the repeated measure. Defoliations 1 and 2 were analysed separately and cumulatively for herbage yield, N response and N recovery. Defoliations 3 and 4 and cumulative annual yield were analysed to investigate carry-over effects from N application rate, N application strategy and P application on herbage yield. All non-significant interactions were removed from the model.

Results

Meteorological data

Average daily air temperature (°C), monthly rainfall (mm) and mean soil temperature (°C; to a depth of 10 cm) for year 1 (2020), year 2 (2021) and the 10-year average (2010–2019) at the experimental site are presented in Table 1. The rainfall in spring (January, February and March) was similar for year 1 (291 mm) and year 2 (303 mm). Rainfall was greater during February in both years of the study compared to the 10-year average (+68 and 105 mm, respectively). Soil temperature was greater in March of year 2 compared to the 10-year average (+0.8°C).

Herbage production

The effect of N application rate and strategy on herbage yield is presented in Table 3. Year had a significant effect (P < 0.05) on herbage yield for defoliations 1–8. Cumulative herbage yield was greater in year 2 (10,854 kg DM/ha) compared to year 1 (9,985 kg DM/ha). Nitrogen application strategy had a significant effect (P < 0.05) on herbage yield in defoliation 1; S1 had a lower herbage yield (982 kg DM/ha average) compared to S2 and S3 (1,195 and 1,164 kg DM/ha, respectively). Nitrogen rate had a significant effect (P < 0.001) on herbage yield in defoliation 2 with 90N having the greatest yield, followed by 60N, while 30N yielded the least (Table 3). The application of P had a significant effect on herbage yield in defoliation 2 (+132 kg DM/ha; Table 4). Nitrogen rate, N application strategy and the application of P also all had a significant effect (P < 0.05) on the cumulative spring (defoliations 1 and 2) herbage yield (Tables 3 and 4).

Table 3:

The effect of nitrogen (N) application rate and strategy on herbage yield in defoliation 1 (19 March and 16 March in year 1 and 2, respectively), defoliation 2 (23 April in years 1 and 2), defoliation 3 (15 May and 24 May in years 1 and 2, respectively), defoliation 4 (10 June and 15 June in years 1 and 2, respectively) and cumulative DM yield (defoliations 1–8)

Spring nitrogen rate (kg N/ha)	0 1	30			60			90			P-values
Nitrogen application strategy 2		S1	S2	S3	S1	S2	S3	S1	S2	S3	SE	N rate	N strategy	P 3
Defoliation 1 (kg DM/ha)	944	949a	1,153bc	1,107ab	984ab	1,222c	1,175bc	1,014ab	1,209c	1,210c	67.4		0.003
Defoliation 2 (kg DM/ha)	774	1,279a	1,228a	1,208a	1,651b	1,569b	1,661b	1,811c	1,902cd	1,934d	41.3	<0.001		<0.001
Cumulative spring yield (defoliations 1 and 2) (kg DM/ha)	1,718	2,228a	2,381a	2,315a	2,634b	2,790b	2,835b	2,826b	3,111c	3,143c	87.0	0.001	0.008	0.03
Defoliation 3 (kg DM/ha)	1,169	1,379a	1,340a	1,392a	1,749c	1,566b	1,569b	1,891d	1,681bc	1,731c	40.1	<0.001	0.001	0.04
Defoliation 4 (kg DM/ha)	1,481	1,479b	1,339a	1,349a	1,521b	1,362a	1,415ab	1,537b	1,378ab	1,468b	35.9		<0.001
Cumulative yield (defoliations 1–8) (kg DM/ha)	9,288	9,909ab	9,774a	9,820a	10,681de	10,353bc	10,464cd	10,916ef	10,773def	11,085f	178.4	<0.001		0.03

Means within a row with different superscripts (a, b, c, d, e, f) differ significantly (P < 0.05).

¹0: Zero nitrogen (0N) plots, which were used to calculate nitrogen response and nitrogen recovery for defoliation 1 and defoliation 2 and are not included in the statistical analysis. The 0N plots received 0N during spring and 23 kg N/ha/application from defoliation 2 onwards (total N = 115 kg N/ha/yr).

²Nitrogen application strategy: amount of N applied on first (3 February) and second (19 March in year 1 and 16 March in year 2) application in spring (S1 = 0N spread in application 1 and 100% in application 2, S2 = 50% in application 1 and 50% in application 2 and S3 = 33% in application 1 and 66% in application 2).

³P = phosphorus.

Table 4:

The effect of nitrogen (N) application rate and phosphorus (P) application (with first N application; 3 February) on herbage yield for defoliation 1 (19 March and 16 March in years 1 and 2, respectively), defoliation 2 (23 April in years 1 and 2), defoliation 3 (15 May and 24 May in years 1 and 2, respectively), defoliation 4 (10 June and 15 June in years 1 and 2, respectively) and cumulative DM yield (defoliations 1–8)

Spring nitrogen rate (kg N/ha)	30		60		90		P-values
Phosphorus (kg P/ha)	0	13	0	13	0	13	SE	N rate	P 1	N rate × P
Defoliation 1 (kg DM/ha)	1,085	1,055	1,158	1,095	1,108	1,180	53.3
Defoliation 2 (kg DM/ha)	1,199a	1,277a	1,580b	1,674b	1,769c	1,995d	32.6	<0.001	<0.001	0.03
Cumulative spring (defoliations 1 and 2) (kg DM/ha)	2,284a	2,332a	2,738b	2,769b	2,878b	3,176c	69.5	<0.001	0.03
Defoliation 3 (kg DM/ha)	1,346a	1,395a	1,617b	1,639bc	1,719c	1,816d	32.9	<0.001	0.04
Defoliation 4 (kg DM/ha)	1,398	1,380	1,424	1,441	1,438	1,485	29.6
Cumulative yield (defoliations 1–8) (kg D/ha)	9,818a	9,909a	10,485b	10,425b	10,594b	11,285c	104.5	<0.01	0.006	0.002

Means within a row with different superscripts (a, b, c, d) differ significantly (P < 0.05).

¹P = phosphorus.

In defoliation 3, N rate, N application strategy and P application had a significant effect (P < 0.05) on herbage yield, where 90N had the greatest herbage yield compared to 60N and 30N. Herbage yield at defoliations 3 and 4 was greatest for S1. In defoliation 3, plots that received P had a greater herbage yield (+56 kg DM/ha; Table 4). There was also a significant interaction (P < 0.05) between N rate and P application for cumulative herbage yield. As N application rate increased, the application of P had a greater effect on cumulative herbage yield (+91, −59 and +691 kg DM/ha for 30N, 60N and 90N, respectively). Nitrogen rate and P application also had a significant effect (P < 0.05) on cumulative annual herbage yield. The greatest annual herbage yield was achieved with 90N, followed by 60N, and 30N had the lowest cumulative herbage yield (10,940, 10,455 and 9,863 kg DM/ha, respectively). The application of P also increased cumulative herbage yield (+241 kg DM/ha; Table 4).

Herbage crude protein content

Year had a significant effect (P < 0.001) on herbage CP content (g/kg DM) for defoliations 1–8. Herbage CP content was greater in year 2 (196 g/kg DM) compared to year 1 (168 g/kg DM). Nitrogen rate had an effect on CP content in defoliations 1, 2 and 3 and also cumulatively. The 90N treatment had the greatest CP content for defoliations 1–8 (186 g/kg DM), 30N had the lowest (178 g/kg DM) and 60N was intermediate (182 g/kg DM; Table 5). Nitrogen application strategy had a significant effect (P < 0.001) on CP content (g/kg DM) in defoliations 1, 2 and 3 and cumulatively (Table 5).

Table 5:

The effect of nitrogen (N) rate and N application strategy on crude protein (CP) content in defoliation 1 (19 March in year 1 and 16 March in year 2), defoliation 2 (23 April in years 1 and 2), defoliation 3 (15 May and 24 May in years 1 and 2, respectively), defoliation 4 (10 June and 15 May in years 1 and 2, respectively) and cumulative CP content (defoliations 1–8)

Spring nitrogen rate (kg N/ha)	30			60			90			P-values
Nitrogen application strategy 1	S1	S2	S3	S1	S2	S3	S1	S2	S3	SE	N rate	N strategy
CP content defoliations 1 and 2 (g/kg DM)	164a	167a	164a	175b	182cd	178bc	187de	200f	189e	1.81	<0.001	<0.001
CP content defoliations 3 and 4 (g/kg DM)	150ab	151ab	148a	154bc	152ab	153bc	158c	155bc	152ab	1.91	0.004
CP content defoliations 1–8 (g/kg DM)	179ab	179ab	176a	182bc	182bc	181bc	185c	189d	185c	1.30	<0.001	0.03

Means within a row with different superscripts (a, b, c, d, e, f) differ significantly (P < 0.05).

¹Nitrogen application strategy: amount of N applied on first (3 February) and second (19 March in year 1 and 16 March in year 2) applications in spring (S1 = 0N spread in application 1 and 100% in application 2, S2 = 50% in application 1 and 50% in application 2 and S3 = 33% in application 1 and 66% in application 2).

Nitrogen response and recovery

Nitrogen rate and N application strategy had a significant effect (P < 0.05) on N response in defoliation 2 (Table 6). Nitrogen response was greatest for 30N compared to 60N and 90N (+1.8 and 4.5 kg DM/kg N applied, respectively). Nitrogen application S2 had the greatest N response followed by S3 and S1 (27.3, 21.1 and 14.3 kg DM/kg N, respectively). Nitrogen rate also had a significant effect (P < 0.05) on N response for cumulative spring yield. Nitrogen response decreased 21 and 15.7 kg DM/kg N as N rate increased from 30N and 90N, respectively. There was a significant effect (P < 0.05) of N application rate on N recovery for defoliation 1 (Table 6). As N application rate increased, so too did N recovery (from 7.1 kg N uptake for 30N to 14.5 kg N uptake for 90N). Nitrogen application rate, N application strategy and P application also had a significant effect (P < 0.05) on N recovery for defoliation 2. Nitrogen recovery was greatest for 90N, lowest for 30N and intermediate for 60N. Nitrogen application S1 had a significantly greater (P < 0.001) N recovery than S2 and S3 (+5.3 and +4 kg N uptake, respectively). Nitrogen recovery was greater for plots that received P fertiliser (+3.8 kg N uptake) compared to the plots that did not receive P.

Table 6:

The effect of nitrogen (N) rate and N application strategy on N response and N recovery in defoliation 1 (19 March in year 1 and 16 March in year 2) and defoliation 2 (23 April in years 1 and 2)

Spring nitrogen rate (kg N/ha)	30			60			90			P-values
Nitrogen application strategy 1	S1 2	S2	S3	S1	S2	S3	S1	S2	S3	SE	N rate	N strategy	P 3
Nitrogen response (kg DM/kg N applied)
Defoliation 1		14.0	16.4		9.3	11.5		5.9	8.9	3.15
Defoliation 2	16.9bc	30.3f	21.7d	14.6ab	26.5e	22.2d	11.5a	25.1de	19.3cd	1.27	0.005	<0.001	0.001
Cumulative spring (defoliations 1 and 2)		22.1a	19.9a		17.8c	18.6c		15.5b	15.8b	1.71	0.013
Nitrogen recovery (kg N uptake)
Defoliation 1		8.4abc	5.7a		13.5bd	9.3ab		16.1d	12.9bcd	2.16	0.006
Defoliation 2	13.5a	10.5a	10.5a	29.8d	22.2b	26.5c	42.5f	37.1e	36.9e	1.42	<0.001	<0.001	0.002
Cumulative spring (defoliations 1 and 2)		19.0a	16.2a		35.6b	35.8b		53.3c	49.8c	2.89	<0.001

Means within a row with different superscripts (a, b, c, d, e, f) differ significantly (P < 0.05).

¹Nitrogen application strategy: amount of N applied on first (3 February) and second (19 March in year 1 and 16 March year 2) applications in spring (S1 = 0N spread in application 1 and 100% in application 2, S2 = 50% in application 1 and 50% in application 2 and S3 = 33% in application 1 and 66% in application 2).

²Nitrogen application strategy 1 has no N response or N recovery for defoliation 1 or cumulative spring as no N was applied prior to defoliation 1.

³P = phosphorus.

Discussion

Environmental regulations are resulting in reductions in chemical fertiliser use to reduce N surplus on farms. It is essential that reductions in herbage production associated with lower N rates are minimised in order to avoid feed deficits (Hennessy et al., 2020) which could result in increased importation of N in the form of feed stuffs. A balance has to exist between optimising grass growth, while minimising N surpluses with its associated loses via N leaching, denitrification and volatilisation (Cameron et al., 2013). The rainfall experienced during February in both years of the current study was considerably greater than the 10-year average which may have impacted spring herbage production and N response. The results of the current study demonstrate that the application of 60 kg N/ha during spring achieves the ideal balance of pasture production in addition to a high N response (kg DM/kg N applied) when compared to both 30 kg N/ha and 90 kg N/ha.

Similar to previous studies (O’Donovan et al., 2004; Murphy et al., 2013; McNamara et al., 2022), spring herbage yield increased linearly with increased N application. High spring N application in February may be greater than plant N requirements as N response was lowest for the 90N treatment at defoliation 1. This reduction in N response as N application rate increased may be caused by low grass growth rates and plant N uptake in early spring compared to early summer when peak grass growth and plant N uptake occur (Murphy et al., 2013). Christie et al. (2018) carried out an experiment in Southeast Australia investigating the seasonal response to N fertiliser in a pasture-based dairy system and reported that an optimal spring N application rate of between 36 ± 3 kg N/ha/month and 62 ± 4 kg N/ha/month was required to achieve 90% of maximum pasture production. In the current study, reducing spring N application from 90 to 30 kg N/ha led to a large decrease in spring herbage yield (−719 kg DM/ha) and may create a spring feed deficit; however, reducing spring N from 90 to 60 kg N/ha leads to a more manageable decrease in spring herbage production (−274 kg DM/ha; −10%) while maintaining high levels of N recovery and N response. Higher spring N applications can also increase cumulative annual herbage yield as the largest difference in cumulative yield in the current study occurred between the 30N (9,863 kg DM/ha) and 90N (10,940 kg DM/ha) treatments. On the other hand, these higher N rates have negative environmental implications such as reductions in N recovery, reduced herbage yield in response to N application and larger amounts of N losses to the environment (Vellinga et al., 2004). The cumulative annual herbage yield reported in the current study was lower than expected; however, it was similar to herbage yields reported by Creighton et al. (2012) on simulated grazing plots receiving 350 kg chemical N/ha at the same experimental location.

McNamara et al. (2022) reported that N application strategy had minimal effects on herbage yield as all application dates had the potential to increase herbage yield but was dependant on year, site and rate of N fertiliser. This contrasts with the current study, where S1 yielded least for defoliation 1 due to insufficient N supply in early spring, similar to previous studies (Stevens et al., 1989; O’Donovan et al., 2004; Hennessy et al., 2008). Plants have a large demand for N in spring and summer as biomass is rapidly increasing (Wang & Schjoerring, 2012); however, N supply from the soil is low and early spring N application is necessary to achieve optimum yields. Herbage yield was greater in defoliations 3 and 4 for S1 as all spring N was applied in application 2. Similar to Le Clerc (1976), withholding N from the first N application in spring and applying it later resulted in increased residual growth subsequently during the growing season; however, there was no increase in cumulative herbage yield. High N application in March with no N application in February does not result in increased levels of herbage production as N response was lower for S1 compared to S2 and S3. McNamara et al. (2022) reported that split applications of spring N increased herbage production compared to single applications of 60 or 90 kg N/ha. Previous research has shown that, although there are a number of factors to be considered for the optimum application date of N in spring, achieving soil temperatures above 5–6°C for an extended period of 3–4 wk is critical (Wilman et al., 1980). A range of dates for first spring N application may be utilised from early to mid-February depending on spring herbage demand and weather conditions (soil temperature and rainfall) to capitalise on increased herbage production and reduced levels of N leaching (O’Donovan et al., 2004; Ledgard et al., 2011). In the current study, S3 (33:66) increased both early spring DM yield response and N recovery during defoliation 1 while providing similar DM yield from a reduced February N application compared to S2. On that basis, S3 represents the best combination of DM production and N recovery in early spring when higher levels of rainfall increase the risk of N loss from fertiliser application.

In the current study N response decreased as N rate increased, similar to Humphreys et al. (2008) and Shepherd et al. (2015). The 30N treatment had the greatest N response compared to the 90N (+5.2 kg DM/kg N applied); however, 30N had the lowest herbage production. In order to meet spring herd demand and meet government targets to reduce emissions, the 60N treatment can be implemented on farms with early turnout and high spring grass demands, whereas the 30N treatment may be suitable in cases where cows are turned out later or spring herd demand is lower. Nitrogen rate had a significant impact on N recovery, which increases as N rate increased due to more N being available for uptake as N rates increase. Vellinga et al. (2004) reported that apparent N recovery was between 30 and 70% in the first cut after N is applied, similar to the current study as 44 and 60% of the N was recovered in defoliations 1 and 2, respectively. Murphy et al. (2013) reported N recovery to be 2 kg N/ha/wk when N was applied on 4 February similar to the current study at 1.8 kg N/ha N recovered for N applied on 3 February. Low soil temperatures and high rainfall in early spring hinder grass growth which reduces N uptake and leaves more N available to be lost (O’Donovan et al., 2004).

O’Donovan et al. (2004) reported N response on the first defoliation (18 March) for N applied on 3 February to be 12 kg DM/kg N applied which is comparable to the current study with an average N response of 11 kg DM/kg N applied on 3 February across all N rates. In contrast, N response was greater at defoliation 2 in the current study at 20.8 kg DM/kg N applied compared to O’Donovan et al. (2004) at 17.6 kg DM/kg N applied. The higher N response for defoliation 2 in the current study may be due to a later cutting date (23 April) compared to O’Donovan et al. (2004) (8 April). Additionally, protected urea was used in the current experiment, which could also contribute to the greater N response due to a reduction in ammonia losses (Forrestal et al., 2017). Smith et al. (2018) reported that strategic approaches to N fertilisation such as seasonally modified N application, whereby N is only applied when plants are actively growing and at a rate suited to plant growth at application, can improve NUE with minimal decreases in herbage production compared to a flat rate of N applied after each grazing (40 or 50 kg N/ha/application depending on location). The majority of herbage yield response to N application occurs within the first 4 wk after application (Murphy et al., 2013) and as such, increased rainfall during early spring reduces N recovery (Humphreys, 2007) as N is lost through leaching. In the current study, the S1 strategy had a significantly lower N response at defoliation 2 compared to S2 and S3 (−13 and 6.8 kg DM/kg N applied, respectively), which may be caused by high N application rates on the second application date, resulting in N applications in excess of plant requirements (Murphy et al., 2013). Nitrogen application S1 also reduced herbage yield at defoliation 1 and cumulative spring herbage yield which further indicates that it is not a suitable application strategy for the Irish pasture-based system, which requires high spring grass availability on farms to meet herd demand. Nitrogen application S2 and S3 had similar spring N response and N recovery; however, utilising S3 applies N at a lower rate in February which may help reduce N loss during the higher risk period as reported in the current study with 121 mm more rainfall in February compared to March.

Similar to the current study, P application has previously been reported to increase herbage yield (Schils & Snijders, 2004; Touhami et al., 2023), particularly as N rate increases (Duru & Ducrocq, 1996). Similar to Sheil et al. (2016), the herbage production response with the application of P fertiliser is greater during initial rather than later defoliations, with herbage yield greater for defoliations 2, 3 and 5 (+133, +56 and +55 kg DM/ha, respectively) with P application. This increase in spring herbage yield with P application may be as a result of the effect of lower temperatures which reduce the availability of soil P, causing plants to be more reliant on P fertiliser in spring (Grant et al., 2001). During the winter and spring period, when temperatures are low, P levels in the soil accumulate; however, soil P is not released until soil temperature increases (11–18°C) due to increased microbial and enzymatic activity and mineralisation (Perrott et al., 1990). Further research investigating various P application rates in spring is warranted to determine appropriate strategies to increase spring herbage production while reducing N fertiliser use.

Strategic N fertiliser application should be considered, particularly in early spring due to increased risk of environmental losses (Decau et al., 2004), whereby weather forecasts and growth rates, such as those predicted using the MoSt Grass Growth model (Ruelle et al., 2018), are used as decision-making tools for N application. Low herbage production increases the accumulation of N in the soil and, if followed by heavy rainfall, leads to higher rates of NO₃ ⁻ leaching (Di & Cameron, 2002; Cameron et al., 2013; Vogeler et al., 2015). Recent Irish studies (Leahy & Kiely, 2011; Domonkos et al., 2020) indicate that rainfall patterns during spring are becoming increasingly unpredictable with an increased frequency of high rainfall events with significant implications for nutrient loss during early spring (Murphy et al., 2024). In the current study, February rainfall was 102% of the long-term average values during the 2-yr study period and is indicative of the increased risks for farmers applying nutrients during early spring. On that basis, the development of additional precision management decision supports is required based on studies, such as reported here, to support improved decision-making to minimise pasture production loss while reducing nutrient losses to the environment.

Conclusion

The results of the current study indicate that reducing early spring N application, changing the N application strategy and applying P fertiliser during spring can improve N response with modest reductions in herbage yield. The results indicate that where such practices are combined with precision fertiliser application methods and using predicted rainfall and soil temperature information, further improvements in spring fertiliser N recovery can be achieved. Withholding spring N application in February significantly reduced spring herbage production and N response which may lead to feed deficits on Irish grassland farms as demand increases rapidly.

Acknowledgements

The authors wish to thank M. Liddane, A. McGrath and P. O’Connor for their technical assistance and all the staff of Moorepark research farm for their assistance during the experiment. This experiment was funded by Irish Dairy Levy Funding administered by Dairy Research Ireland and the Teagasc Walsh Scholarship Programme.

Conflicts of interest

All authors declare that there are no conflicts of interest.

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