Evaluation of calcium ammonium nitrate and urea-based fertilisers applied to grassland in Ireland

Transcription

1 An ASABE Meeting Presenttion Pper Number: Evlution of clcium mmonium nitrte nd ure-bsed fertilisers pplied to grsslnd in Irelnd D.L. Antille * Tegsc Johnstown Cstle, Environmentl Reserch Centre, Wexford, Co. Wexford, Irelnd. * Corresponding uthor (Present ddress): University of Southern Queenslnd, Ntionl Centre for Engineering in Agriculture (NCEA), Building Z2, West Street, 4350 Toowoomb, QLD, Austrli. Emil: d.l.ntille@gmil.com N.J. Hoekstr Tegsc Johnstown Cstle, Environmentl Reserch Centre, Wexford, Co. Wexford, Irelnd. M. Ernfors Swedish University of Agriculturl Sciences, Deprtment of Agrosystems, P.O. Box 104, Alnrp, Sweden. K. Richrds S.T.J. Llor Tegsc Johnstown Cstle, Environmentl Reserch Centre, Wexford, Co. Wexford, Irelnd. Written for presenttion t the 2013 ASABE Annul Interntionl Meeting Sponsored by ASABE Knss City, Missouri July 21 24, 2013 Abstrct. This study investigted the influence of N source nd rte, nd timing of N ppliction on dry mtter yield (DMY), N responses, N uptke nd N use efficiency (NUE) in grss crop. The experiment used three fertiliser tretments: CAN, ure nd nbtpt-coted ure (nbtpt-u), three N rtes (0 control, 25, 50 nd 75 kg h -1 ), nd 18 fertiliser ppliction timings. The gronomic performnce of ure ws lower thn CAN in erly spring. This included reltively lower N responses, lower reltive DMY (90%) nd N uptke (85%) which trnslted in lower NUE (0.45 vs kg kg -1 ). For N pplictions lter in the spring both ure nd nbtpt-u showed reltive DMY nd NUE which were within ±5% compred with CAN (100%). nbtpt enhnced the overll performnce of ure which ws shown with incresed temperture towrds the summer or incresed N rtes. In the summer, the efficiency of ure ws lower thn CAN or nbtpt-u in ll mesured prmeters. The vribility of ure nd nbtpt-u s N-sources for grss ws comprble to CAN but DMY with ure ws 95% tht of CAN (100%) t 8 out of the 19 ppliction timings. Incresing the ppliction rte of ure-n to offset its reltively lower efficiency my not be recommended since DMY of ure-n reltive to CAN decresed with incresed N fertilistion levels. However, with the use of nbtpt this my be possible, but fertiliser choice needs to be bsed on the reltive costs per unit N. Keywords. Ure, clcium mmonium nitrte, nbtpt, fertiliser use efficiency, temperte grsslnd. The uthors re solely responsible for the content of this technicl presenttion. The technicl presenttion does not necessrily reflect the officil position of the Americn Society of Agriculturl nd Biologicl Engineers (ASABE), nd its printing nd distribution does not constitute n endorsement of views which my be expressed. Technicl presenttions re not subject to the forml peer review process by ASABE editoril committees; therefore, they re not to be presented s refereed publictions. Cittion of this work should stte tht it is from n ASABE meeting pper. EXAMPLE: Author's Lst Nme, Initils Title of Presenttion. ASABE Pper No St. Joseph, Mich.: ASABE. For informtion bout securing permission to reprint or reproduce technicl presenttion, plese contct ASABE t rutter@sbe.org or (2950 Niles Rod, St. Joseph, MI USA).

2 Introduction Ure is the min source of N fertiliser used in griculture worldwide. Its consumption is set to increse from currently 50% of the totl N fertiliser usge to bout 60% to 70% by 2020 (Dmpney et l., 2003). In Irelnd, ure nd clcium mmonium nitrte (CAN) represent 17% nd 40%, respectively, of the totl N fertiliser pplied on mnged grsslnd nd their use s stright N sources hs incresed in recent yers (Llor et l., 2010). The study of the gronomic efficiency of ure reltive to CAN or mmonium nitrte (AN) pplied s top dressing in temperte grsslnd hs received considerble ttention. Scholefield (2003) indicted tht the mjority of the dt produced were obtined from field-scle trils conducted in the UK (e.g. Devine nd Holmes, 1963; Chney nd Pulson, 1988) nd Irelnd (e.g. Kene et l., 1974; Murphy, 1978, 1983) before the middle of 1980 s with limited informtion produced therefter (e.g. Lloyd, 1992; Chmbers nd Dmpney, 2009). The reltively lower price of ure-n compred to AN-N recorded before 1985 (Nix, ) brought bout the need for further reserch into the subject but the dt vilble re limited (Scholefield, 2003). The informtion vilble ws lrgely compiled by Tomlinson (1970) nd lter by Wtson et l. (1990). Subsequently, the report of the NT26 Project (DEFRA, 2003) presented nd discussed existing knowledge on the effects of the use of lterntive N- contining fertiliser mterils to AN; in prticulr, ure, by focusing on spects relted to crop performnce, fertiliser ppliction, environmentl impcts nd possible mitigtion options. Tomlinson (1970) highlighted tht the inherent risk of vribility in the effectiveness of ure rises in response to number of intercting fctors such s N rte nd method of ppliction, soil type, nd temperture nd rinfll ptterns. The initil ttempts to increse its efficiency s fertiliser hd been prtilly successful while the use of cotings hd resulted in chnges to the pttern of N vilbility (Tomlinson, 1970). The criticl spects concerning the use of ure on temperte grsslnds re its efficiency reltive to AN, the fctors ffecting it, nd the opportunities for incresing fertiliser use efficiency (Wtson et l., 1990; 2009). The efficiency of ure hs been shown to be similr (e.g. Kene et l., 1974; Murphy, 1983; Stevens et l., 1989) or higher (e.g. Herlihy nd Sheehn, 1977) compred with CAN when used in spring grss production. By contrst, other work (Chney nd Pulson, 1988; Swift et l., 1988) showed lower reltive efficiencies. Scholefield (2003), bsed on Devine nd Holmes (1963), indicted tht the use of ure on grss crops cn result in DMY reductions in the rnge of 10% to 15% compred with AN depending on the conditions in which the fertiliser ws pplied. Lloyd (1992) showed tht the effectiveness of ure compred with CAN incresed with the mount of rinfll tht occurred within three dys of fertiliser ppliction. Wtson et l. (1990) concluded tht the effectiveness of ure is similr to CAN or AN for pplictions in the spring but it cn be lower in the summer (reltive yields 95%). Their study, however, indicted tht there ws not sufficient evidence to suggest tht ure is significntly more vrible N source thn CAN for spring grss production but mximum DMY is expected to be lower. - Ure hs suggested lower susceptibility for NO 3 leching nd denitrifiction thn CAN (Jordn, 1989) but reltively higher potentil for NH 3 voltilistion which is the min reson for its inefficiency (Freney et l., 1983). These N losses represent n economic loss to frmers nd hve environmentl implictions for wter qulity nd GHG emissions (Wtson et l., 2009). The interction between temperture nd rinfll ws shown to be one of the min fctors influencing the reltive effectiveness of the two N fertiliser sources (Bussink nd Oenem, 1996; Scholefield, 2003). The risk of NH 3 emissions from pplied ure is greter thn tht of nitrtebsed fertilisers but this vries depending on the environmentl nd soil conditions following fertiliser ppliction (McGrry et l., 1987; Bhogl et l., 2003). 2

3 In Irelnd, the current limit for mmoni (NH 3 ) emissions under the EU Ntionl Emissions Ceilings Directive is 116 kt NH 3 per yer nd estimtes (EPA, 2012) suggested tht nturl emissions re 106 kt. This implies tht current NH 3 emission trgets do not pper to be restriction to dopting higher levels of usge of ure-bsed fertilisers in the short term but meeting this obligtion my be cumbersome if significnt switch in fertiliser prctices from CAN to ure ws implemented t ntionl level. Ammoni emissions from ure pplied to grss my be reduced with the use of urese inhibitors which offers dvntges compred with other lterntives (Bhogl et l., 2003). N-(n-butyl) thiophosphoric trimide (nbtpt) cts s urese inhibitor following conversion to its oxygen nlogue (Creson et l., 1990; Mnunz et l., 1999). The use of nbtpt llows for the N sved in voltilistion to be tken up by the grss crop incresing N uptke nd DMY compred with ure lone (Wtson nd Miller, 1996; Wtson et l., 2008). Wtson et l. (1994) showed tht nbtpt-treted ure t 0.05% incresed DMY by 9% nd reduced NH 3 emissions from 13% to 2.2% on verge compred with untreted-ure. The dt presented in this pper provide dtset which is intended to ddress some of the questions formulted by Wtson et l. (1990) concerning the use of ure on temperte grsslnd nd, in prticulr, under Irish conditions. Therefore, the objectives of this work were to: (1) determine the fertilising efficiency of ure nd nbtpt-coted ure (nbtpt-u) compred with CAN for rnge of fertiliser ppliction timings, nd (2) develop fertiliser-specific model tht cn be used to simulte DMY bsed on meteorologicl dt for short-term temperture nd rinfll before nd fter fertiliser ppliction. Mterils nd Methods Site description The study ws conducted t Tegsc Johnstown Cstle, Irelnd (52 29 N, 6 5 W) on modertely well drined (Schulte et l., 2005) cly lom soil (Grdiner nd Ryn, 1964). The site ws permnent grsslnd swrd (>5 yers old) dominted by perennil ryegrss (Lolium perenne L). Soil P nd K contents, nd ph (Morgn, 1941; Byrne, 1979; MAFF, 1986) for the bulked m lyer determined prior to the experiment were: 4.8 mg L -1 of P (soil P Index 2), 120 mg L -1 of K (soil K Index 3) nd soil ph of 5.3. Rinfll nd temperture records re summrised in Figure 1. Experimentl design The experiment ws conducted in 1 m 3 m plots subjected to the following tretments. Three types of fertiliser were used: ure (46% N), ure coted with N-(n-butyl) thiophosphoric trimide (nbtpt-u) t rte of 0.48 g kg -1, nd clcium mmonium nitrte (CAN, 27% N). These were hnd-pplied t N rtes equivlent to 0 (control), 25, 50 nd 75 kg h -1 over 19 ppliction timings (T1 to T19 except for nbtpt-u, T3-T19) on weekly bsis (26 Februry to 22 April 2010) or fortnightly bsis (6 My to 9 September 2010) (Tble 1). The experiment ws lid out in rndomised block design with four replicted blocks. Ech block consisted of 19 strips; one for ech fertiliser ppliction timing. The N ppliction rte nd fertiliser type tretments (three fertiliser types by three N ppliction rtes plus n unfertilised control plot) were rndomised within ech strip resulting in totl of 760 plots. Tble 1: Timing of fertiliser pplictions nd corresponding dtes of hrvest (Yer: 2010). 3

4 Timing Dte of Cut 1 Cut 2 Timing Dte of Cut 1 Cut 2 (T) ppliction (T) ppliction 1 26 Feb 31 Mr 29 Apr My 17 Jun 15 Jul 2 4 Mr 1 Apr 29 Apr 12 3 Jun 1 Jul 28 Jul 3 11 Mr 8 Apr 6 My Jun 15 Jul 11 Aug 4 18 Mr 15 Apr 12 My 14 1 Jul 28 Jul 24 Aug 5 26 Mr 22 Apr 20 My Jul 11 Aug 9 Sept 6 1 Apr 29 Apr 26 My Jul 24 Aug 21 Sept 7 8 Apr 6 My 3 Jun Aug 9 Sept 4 Oct 8 15 Apr 12 My 10 Jun Aug Apr 20 My 16 Jun 19 9 Sept 4 Oct 4 Nov 10 6 My 2 Jun 1 Jul Site mintennce A blnket ppliction of P (30 kg h -1 ) ws pplied to ll plots in lte Mrch 2010 to ensure these nutrients would not limit crop growth. During the experiment, ll plots tht were not scheduled for N ppliction in the subsequent four weeks received mintennce ppliction of N using CAN t rte of 15 kg h -1 of N every five weeks. Mintennce N ws pplied on 10 Februry (T7-T15), 16 April (T11-T15), nd 22 My (T14-T19). Plots tht were not under tretment were cut every one to three weeks depending on crop growth, with the hrvested herbge being mulched nd returned to the plot. Mesurements nd nlyses Grss yield ws determined t four nd eight weeks fter fertiliser ppliction (cuts 1 nd 2 respectively). The entire plots were cut to height of 40 mm using push lwnmower to replicte the stndrd field prctice before ppliction of the fertiliser t the corresponding timing. Totl fresh weight ws determined nd subsmple ws tken for determintion of dry mtter content nd totl N content (MAFF, 1986). N use efficiency (NUE, kg kg -1 ) of pplied N fertiliser ws estimted using the difference method (Eqution [1]) (Cssmn et l., 1998; Johnston nd Poulton, 2009). N N [1] F F 0 NUE N Rte Where: N F=0 nd N Rte re the N uptke of the control (zero-fertiliser) nd the N ppliction rte respectively. Sttisticl nlyses Sttisticl nlyses were undertken with GenStt (14 th Edition) nd involved nlysis of vrince (ANOVA) nd the lest significnt differences (LSD) to compre the mens with probbility level of 5%. Grss responses to the ppliction of N were investigted by mens of simple (liner) regression nlyses. 4

5 Non-liner (qudrtic) responses were lso tested nd results re discussed. Since nbtpt-u ws pplied from T3 onwrds, ANOVA ws conducted seprtely for the fertiliser pplictions corresponding to T1 nd T2, from those performed from T3 nd fter. Hence, comprisons between tretments for the first two fertiliser pplictions were only mde between ure nd CAN; subsequently, ll three fertiliser types were included in the nlyses. There were no dt collected for the ppliction of fertiliser corresponding to T18 t cut 2 due to wether difficulties restricting hrvesting opertions. Therefore, this ws excluded from ll nlyses nd the corresponding results re not reported. Repeted mesurement nlyses of vrince were conducted to compre levels of DMY nd NUE obtined t cuts 1 nd 2 respectively. A covrince nlysis (ANCOVA) using 5% probbility level ws conducted to investigte the effects of short-term temperture nd rinfll on N responses (β) for the three N sources. From this, model ws developed which simultes β for cut 1 nd 2 using the following vribles:. totl rinfll nd men temperture for the 4 weeks period between N ppliction nd cut 1, nd between cut 1 nd cut 2; nd b. totl rinfll nd men temperture within five dys prior to, nd within three dys fter N ppliction. These prmeters were included becuse of reported evidence (Bouwmeester et l., 1985; Herlihy nd O Keeffe, 1987; Lloyd, 1992; Bussink nd Oenem, 1996) which showed tht the effectiveness of N fertilisers cn be significntly influenced by temperture nd rinfll before nd fter brodcst ppliction. The model ws vlidted with experimentl dt from seprte tril which ws conducted in 2011 in neighbouring field in the sme frm but on slightly hevier, poorly drined, cly lom soil (Grdiner nd Ryn, 1964). The experiment used the sme fertiliser types but only four N ppliction timings (8 April, 1 My, 12 June nd 11 July) nd three rtes of N fertilistion (0 control, 25 nd 50 kg h -1 of N). DMY ws determined t four nd eight weeks fter fertiliser ppliction s described erlier. A simple (liner) regression nlysis ws conducted to determine the reltionship between observed nd predicted dt. Results nd Discussion Rinfll nd temperture Meteorologicl records for Johnstown Cstle re shown in Figure 1. Rinfll ws mesured t ground level using tipping bucket rin guge. Air temperture ws mesured t 1.52 m height with dry pltinum resistnce thermometer. The men totl nnul rinfll for the period ws 1059 mm compred with 913 mm for 2010 nd 840 mm for 2011 (Met Eirenn, 2012). Rinfll distribution for 2010 nd 2011 differed mrkedly from the 30 yers records. Totl rinfll for the period Jnury-June, nd August 2010 were below their corresponding long-term records wheres July nd September 2010 were well bove verge. The men tempertures in 2010 were reltively lower compred with the sme records. 5

6 Rinfll (mm) Rinfll Rinfll 2010 Rinfll 2011 Men Temp Men Temp 2010 Men Temp Men temperture ( o C) Figure 1: Rinfll nd temperture records for Johnstown Cstle, Wexford, Irelnd (Met Eirenn, 2012). Dry mtter yield (DMY) 0 Jn Feb Mr Apr My Jun Jul Aug Sep Oct Nov Dec At cut one, there ws significnt effect of the timing of fertiliser ppliction (P-vlues <0.05), the N ppliction rte (P<0.001); except when considering T1 nd T2 (P=0.26), nd the fertiliser type (P-vlues <0.05) on DMY levels. On verge, cross the entire experiment, DMY incresed with the N ppliction rte from pproximtely 1250 kg h -1 (control) to 2235 kg h -1 t 75 kg h -1 of N. The overll effect of the fertiliser type observed t cut one ws minly due to differences in DMY between ure nd CAN (Figure 2). For T1 nd T2, the ppliction of ure resulted, on verge, in lower DMY reltive to CAN. The vlues of DMY recorded t these timings were low (490 nd 600 kg h -1 for ure nd CAN respectively) but differences were significnt for n LSD vlue (5% level) of 92. For T3 to T19, the overll differences between these two fertilisers were significnt for n LSD (5% level) vlue of 62 but reltively smll (<85 kg h -1 ). The differences in DMY recorded between nbtpt-u nd CAN (T3-T19) were not significnt. At cut one (T1-T19), DMY with ure nd nbtpt-u reltive to CAN (100%) were, on verge, 95% nd 98% respectively. For nbtpt-u, this vlue my be slightly overestimted since it is not computing the reltive efficiencies for T1 nd T2. For ure, the reltive efficiencies t these two timings were low ( 82%). 0 6

7 DMY (kg h -1 ) Cut nbtpt-u Ure CAN Control T1-T2 T3-T9 b T10-T16 b c T17-T19 b b Ure CAN Control nbtpt-u Ure CAN Control nbtpt-u Ure CAN Control nbtpt-u Ure CAN Control Figure 2: DMY recorded t cut one s ffected by fertiliser type nd timing of fertiliser ppliction. The error brs show the LSD vlue (5% level). For T1-T2: P=0.005, n=8 (control) nd n=24 (tretments). For T3-T9: P>0.05, n=28 (control) nd n=84 (tretments). For T10-T16: P=0.03, n=28 (control) nd n=84 (tretments). For T17-T19: P>0.05, n=8 (control) nd n=24 (tretments). At cut two, there ws significnt effect of the timing of fertiliser ppliction (P<0.001); except when considering T1 nd T2 (P>0.05), the N ppliction rte (P-vlues <0.001), nd the fertiliser type (P=0.01); except when considering T3-T19 (P>0.05). On verge, cross the entire experiment, DMY incresed with the N ppliction rte from pproximtely 1100 kg h -1 (control) to 2000 kg h -1 t 75 kg h -1 of N. The overll effect of the fertiliser type on DMY t cut two is shown in Figure 3. For T1 nd T2, the differences recorded between ure nd CAN t cut two were greter compred with those encountered t cut one (c.150 vs. 110 kg DM h -1 respectively). This suggested tht losses of pplied N-fertiliser occurred to lrger extent in ure- compred with CAN-treted crop, nd tht there ws greter efficiency in the uptke of residul fertiliser N fter the first hrvest with the use of CAN. For T3 to T19, the overll differences between fertiliser types were mrginl (<25 kg DM h -1 ). At cut two (T1-T19), DMY with ure nd nbtpt-u reltive to CAN (100%) were, on verge, 98% nd 100% respectively. For ure, the reltive efficiency t T1 nd T2 ws 92% which indictes greter recovery of N compred with tht obtined t cut one t the sme timings but still lower thn CAN. At both cuts, the interctions timing fertiliser type (Figures 4 nd 5), nd fertiliser type N rte were not significnt, nd the sme effect ws observed when the fertiliser ppliction timing ws fctored in (P-vlues >0.05). 7

8 nbtpt-u T1-T2 Ure CAN Control DMY (kg h -1 ) Cut b c T3-T9 b T10-T16 b T17-T19 b Ure CAN Control nbtpt-u Ure CAN Control nbtpt-u Ure CAN Control nbtpt-u Ure CAN Control Figure 3: DMY recorded t cut two s ffected by the fertiliser type nd timing of fertiliser ppliction. The error brs show the LSD vlue (5% level). For T1-T2: P=0.01, n=8 (control) nd n=24 (tretments). For T3-T9 nd T10-T16: P>0.05, n=28 (control) nd n=84 (tretments). For T17-T19: P>0.05, n=8 (control) nd n=24 (tretments) nbtpt CAN Ure Control DMY (kg DM h -1 ) Cut T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 T13 T14 T15 T16 T17 T19 Fertiliser ppliction timing Figure 4: DMY recorded t cut one for ech of the fertiliser ppliction timings showing the control vs. the tretments (men vlues cross the three N ppliction rtes). The error brs show the LSD vlue (5% level); P-vlues >0.05; n=4 (control), n=12 (tretment). 8

9 nbtpt CAN Ure Control DMY (kg DM h -1 ) Cut T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 T13 T14 T15 T16 T17 T19 Fertiliser ppliction timing Figure 5: DMY recorded t cut two for ech of the fertiliser ppliction timings showing the control vs. the tretments (men vlues cross the three N ppliction rtes). The error brs show the LSD vlue (5% level); P-vlues >0.05; n=4 (control), n=12 (tretment). It cn be seen from Figures 4 nd 5 tht the fertiliser pplictions conducted between T1 nd T5 resulted in reltively higher (P-vlues <0.001) DMY levels t cut two (rnge of 1590 to 2050 kg h -1 ) compred with cut one (rnge of 475 to 1375 kg h -1 ) which ws observed for the three fertiliser types. However, over the rnge of fertiliser ppliction timings (T1-T19), men DMY ws higher (P<0.001) t cut one (1940 kg h -1 ) thn t cut two (1230 kg h -1 ). Using ±5% rnge with respect to CAN (100%), it my be possible to generlise tht the fertiliser choice is not significntly ffected when the corresponding gronomic performnce flls within tht rnge. Wtson et l. (1990) used similr pproch to clustering dt from wide rnge of studies so tht ure ws considered to be less effective N source thn CAN (100%) when its reltive efficiency (URY) ws below 95% nd vice-vers when it ws bove 105%. Bsed on cumultive DMY, the use of CAN my be preferred to ure for pplictions conducted t T3 or erlier (before middle of Mrch), nd lso between T11 nd T13 (lte My to middle of June). For pplictions conducted between T4 nd T10 (middle of Mrch to erly My), nd fter T14 (beginning of July), either CAN or ure my be recommended. Similrly, CAN my be preferred to nbtpt-u between T12 nd T13 (beginning to middle of June). Outside these dtes, both fertilisers my be recommended. In generl, nbtpt-u showed mrginlly better gronomic performnce thn ure but differences in reltive DMY (cumultive) between the two mterils were within ±3% for ll timings, except t T3 when reltive DMY of nbtpt-u (nry) ws 5% higher thn URY. Tempertures for Februry nd Mrch 2010 were low (men T=5.7 C) which restricted growth, ffected responses nd consequently DMY in the erly prt of the spring. These conditions re likely to hve ffected urese ctivity; hence, reducing ure-n vilbility. The effectiveness of ure nd nbtpt-u reltive to CAN showed some vribility which responded to the combined effect of the N ppliction rte nd the timing of fertiliser ppliction. Since the coefficients of vrition (c.v., %) for the dt corresponding to cumultive DMY were of similr mgnitude for ll N sources (rnge of 13.4% to 13.7%), the vribility in the effectiveness of ure nd nbtpt-u ws similr to tht of CAN when compred to the controls. 9

10 Nitrogen uptke There ws n effect of the timing of fertiliser ppliction (P-vlues <0.05) which ws observed in both cuts (except for T1 nd T2 t cut two, P>0.05). The overll effect of the fertiliser type ws significnt (P-vlues <0.05 except t cut two for pplictions between T3 nd T19). At cut one, for pplictions conducted t T1 nd T2, CAN showed N uptkes which were 14% to 25% higher thn ure. From T3 to T19, differences between fertiliser tretments were due to the use of CAN or nbtpt-u which incresed N uptke by bout 7% on verge compred with ure. There ws no effect of the interction fertiliser type N ppliction rte, nd the sme ws observed when the timing of fertiliser ppliction ws fctored in (P-vlues >0.05). In Figure 6, cumultive N uptkes (cuts one + two) showed significnt differences (P<0.001) with respect to the timing of fertiliser ppliction; except for T1 nd T2 (P=0.2). Reltively high N uptkes (rnge of 90 to 125 kg N h -1 ) were observed when N ws pplied t 50 nd 75 kg h -1 t T7, nd between T11 nd T13; especilly, in the grss treted with CAN nd nbtpt-u. The cumultive N uptkes were influenced (P-vlues <0.05) by the fertiliser type but the effect ws not observed when the timing of ppliction ws fctored in (P-vlues >0.05). The effect of the fertiliser type ws due to the use of CAN or nbtpt-u which incresed N uptke by bout 5% on verge compred with ure between T3 nd T19. For T1 nd T2, men N uptke in CANfertilised grss ws bout 12 kg h -1 of N higher thn tht of ure. There ws no effect fertiliser type N rte on cumultive N uptke nd the sme holds true when fctoring in timing of fertiliser ppliction (P-vlues >0.05). Totl N uptke (kg N h -1 ) nbtpt Ure CAN Control T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 T13 T14 T15 T16 T17 T19 Fertiliser ppliction timing Figure 6: Cumultive N uptke for ech of the fertiliser ppliction timings showing the control vs. tretment (men vlues cross the three N ppliction rtes). The error brs show the LSD vlue (5% level); P>0.05, n=4 (control), n=12 (tretment). The reltive vlues of cumultive N uptke of ure nd nbtpt-u compred with CAN (100%) were found to be 95% or lower in 11 out of 18, nd in 2 out of 16 timings respectively (Figure 7). By contrst, these were 5% tht of CAN in only two occsions (T5 nd T6) for both fertiliser mterils. The overll men N uptke of ure reltive to CAN ws 95% nd it rnged from 85% (T1 nd T2) to 119% (t T5) wheres for nbtpt-u the men mtched tht of CAN (100%) but it rnged from 92% (T13) to 123% (T5). 10

11 As for DMY, the vribility observed in reltive N uptkes resulted from the combined effects of the N ppliction rte, fertiliser type nd timing of fertiliser ppliction. The ltter closely relted to the effects of rinfll nd temperture on DMY (P<0.05) nd responses pre- nd postfertiliser ppliction s discussed lter. The c.v. (%) for the dt corresponding to cumultive N uptke were of similr order of mgnitude for ll three fertiliser mterils (rnge of 15.5% to 17.2%) when compred to the unfertilised controls; hence, the vribility in the effectiveness of N uptke from ure nd nbtpt-u ws comprble to CAN. 150 Reltive N uptke (% of CAN) nbtpt-u CAN (100%) Ure 50 T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 T13 T14 T15 T16 T17 T19 Fertiliser ppliction timing Figure 7: Percentge (cumultive) N uptke of ure nd nbtpt-u reltive to CAN for ech of the fertiliser ppliction timings. The error br shows ±5% rnge with respect to CAN (100%) denoted by the dotted line. Men vlues cross the three N ppliction rtes (n=12). Nitrogen responses The responses to the ppliction of fertiliser N were liner for the rnge of N ppliction rtes investigted. Bsed on erlier studies (Reid, 1970, 1978; Sprrow, 1979; Morrison et l., 1980), non-liner responses were first fitted to the dt which showed cceptble fits to the qudrtic function. However, the estimtes of prmeters for the squre term were not significnt (p-vlues >0.05) in most circumstnces. Therefore, responses were better explined by liner functions which showed significnce to the liner term. This ws expected given the rnge of N ppliction rtes investigted but non-liner functions my be possible with more complete dtset (Sprrow, 1979; Morrison et l., 1980). Liner regression nlyses for ech fertiliser type explined, individully, no more of the vrition thn it did common slope (P<0.001; R 2 =96%; s.e.=115) since there ws no systemtic fertiliser N rte effect. However, the vlue encountered for ure pplied t 75 kg h -1 of N decresed its yield to N response curve slightly which resulted in mrginlly lower response in the rnge of 50 to 75 kg h -1 of N, s predicted by the liner model, compred with the other two fertiliser mterils. Although the interction fertiliser type N rte ws not significnt (P=0.10), DMY obtined with CAN or nbtpt-u compred to ure t 75 kg h -1 of N ppered to be significntly higher for the clculted LSD vlue. Overll, responses were greter in the first (Figure 8 top) compred with the second cut (Figure 8 bottom) except for pplictions conducted between T1 nd T4 which were higher t cut two. Over the growing seson, the responses recorded in the first cut, followed well the pttern of temperture. 11

12 The reltively higher responses encountered in the second cut for pplictions conducted between T1 nd T4 were due to residul fertiliser N which resulted from the combined effect of low tempertures nd reduced N uptke erlier in the spring, nd the chrcteristic pttern of the sesonl rte of growth of grss (Anslow nd Green, 1967). The opposite effect ws observed for pplictions conducted fter T5; especilly, between T8 nd T13 when responses were higher thn bout 15 kg DM kg -1 N. 30 β coefficient (kg DM kg -1 N) Cut LSD 5% level (T1-T2) LSD 5% level (T3-T19) Cut 1 Ure CAN nbtpt Fertiliser ppliction timing 30 β coefficient (kg DM kg -1 N) Cut LSD 5% level (T1-T2) LSD 5% level (T3-T19) Cut 2 Ure CAN nbtpt Fertiliser ppliction timing Figure 8: Responses of grss (β) to the ppliction of fertiliser N recorded t cut one (top) nd two (bottom) for ech of the fertiliser ppliction timings. The error brs show the LSD vlues (5% level). 12

13 Nitrogen use efficiency (NUE) On verge, cross ll timings (T1-T19), NUE ws greter (P<0.001) t cut one (0.57 kg kg -1 ) compred with the recoveries obtined t cut two (0.17 kg kg -1 ). NUE ws influenced (P-vlues <0.05) by the timing of fertiliser ppliction, except t T1 nd T2 (P-vlues >0.05) nd it showed significnt decrese with the N ppliction rte (P-vlues <0.05). Fertiliser pplictions conducted in erly spring resulted in reltively lower NUE (<0.6 kg N kg -1 N) compred with those conducted lter in the seson, except t T16 nd T19 (Figure 9). There were no significnt differences in NUE s result of the fertiliser type for pplictions conducted between T3 nd T19 (P-vlues >0.05) but CAN showed reltively higher NUE thn ure nd nbtpt-u; except t T5 which ppered to be significntly lower for the clculted LSD vlue. For T1 nd T2, the use of CAN resulted in higher NUE (P-vlues <0.05) thn ure (0.7 vs kg N kg -1 N respectively). Bsed on cumultive N uptkes, the clculted NUE for ure nd nbtpt-u reltive to CAN (100%) were 91% nd 97% respectively between T3 nd T19 wheres for T1 nd T2 this ws 65% (ure only). The interction timing fertiliser type ws not significnt nd the sme ws observed when the N ppliction rte ws fctored in (P-vlues >0.05). N uptke showed significnt, positive, correltion with the response of the grss to the ppliction of N (β) which ws observed for ll fertiliser types (P<0.001; R 2 =41%; s.e.=6.9). A similr reltionship ws obtined between β nd NUE (P<0.001; R 2 =46%; s.e.=0.12) which indicted tht the higher the response from the N fertiliser pplied, the higher the N uptke nd the N recovery. This hs gronomic nd s well s environmentl implictions since the N pplied with the fertiliser is likely to be removed from the soil nd incorported into the crop biomss t fster rte; hence, reducing the risk of N losses from the fertiliser pplied nbtpt Ure CAN N use efficiency (kg kg -1 ) T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 T13 T14 T15 T16 T17 T19 Figure 9: N use efficiency of ure, nbtpt-u nd CAN clculted using the difference method nd bsed on cumultive vlues of N uptke. The error brs show the LSD vlue (5% level); P>0.05, n=12. Model predictions of dry mtter yield Fertiliser ppliction timing The generl form of the model is shown in Eqution [2] nd the vlues of specific prmeters re given in Tble 2. 13

14 2 2 R3 6 T 5 FijrT5 Fik 3 DMY F r [2] i Fi 1T5 2T3 3R5 4R3 5 rt Where: DMY is dry mtter yield (kg h -1 ); : constnt; F i nd δ F re fertiliser-specific coefficients; r is the N ppliction rte (kg h -1 ); δ n depends on men temperture ( C) nd totl rinfll (mm) recorded within 5 dys prior to, nd within 3 dys fter fertiliser ppliction which re denoted s T 5 nd R 5, nd T 3 nd R 3 respectively. Tble 2: Vlues of coefficients corresponding to Eqution [4]. SE is stndrd error of observtions; DF is degrees of freedom. Prmeter Fertiliser Estimte SE DF t-vlue P > t F i nbtpt-u F i CAN F i Ure δ F nbtpt-u δ F CAN δ F Ure δ <0.001 δ δ δ δ δ <0.001 δ Fij nbtpt-u δ Fij CAN <0.001 δ Fij Ure δ Fik nbtpt-u δ Fik CAN <0.001 δ Fik Ure The model indicted tht, ll other fctors being constnt, n increse in R 5 nd/or R 3 in the rnge of 0 to 40 mm will result in reduced DMY but differences in predicted responses between fertiliser types will be smll (<5%). Cumultive rinflls in excess of 40 mm do not yield stisfctory results since possible losses of N (e.g. by mens of leching or gseous evolution) cnnot be ccounted for with use of this model. An increse in T 3 up to mximum of 20 C is likely to result in incresed DMY but to greter extent (bout 10% to 15%) with CAN compred with nbtpt nd ure. Predictions mde for higher tempertures pper to be unrelistic nd to ignore the effect tht wrmer wether 14

15 hs on incresed voltilistion of mmoni with the use of ure. This upper limit suggested for temperture is, however, resonble if compred with historicl wether dt over the min growing seson (Figure 1). The interction N rte temperture showed significnt effect (P-vlues <0.05); therefore, the response for given N input cn be significntly modified by temperture pre- or post-n ppliction with ll fertiliser mterils. This interction ws not observed for rinfll (P>0.05) but the effect of rinfll on DMY before nd fter fertiliser ppliction ws significnt (P-vlues <0.05). The overll effect of R 3 is tht it will tend to decrese DMY becuse of the lrge negtive coefficient ssocited with its liner term (δ 4 ); this, within the suggested rnge, will offset the effect of its squre term which crries smller coefficient (δ 5 ). Predicted vlues of DMY for N ppliction rtes in excess of 75 kg h -1 (upper limit in this study) should be treted with cution becuse of the effect of the interction with temperture indicted erlier. Since δ Fik is positive, n increse in T 3 will hve beneficil effect on DMY which my not hold true for ure bove the suggested upper limits of temperture nd N ppliction rte. It ws shown tht ure-n pplied t 75 kg h -1 resulted in reltively lower responses compred with the other two fertiliser mterils (Figure 8). This ws ttributed to incresed voltilistion of mmoni t higher fertilistion levels; effect tht is not computed in this model. However, this my be offset by T 5 since the reltively lrge coefficient ssocited with its squre term is negtive but it is still n rtefct of the model. Predicted DMY from the model ws vlidted with observed experimentl dt collected in 2011 (Figure 10). Individul regressions for ech N source did not explin significntly more of the vrition (71%>R 2 >84%; P-vlues <0.001) thn it did common slope which showed resonbly good greement between observed nd predicted dt (R 2 =78%; P<0.001). However, predictions of DMY for individul N sources my still be mde to llow for comprisons of reltive efficiencies between fertilisers given the N rte, nd the short-term rinfll nd temperture prior or post-fertiliser ppliction. Figure 10: Predicted vs. observed DMY bsed on Eqution [2] nd experimentl dt for 2011 with common slope for the three N sources used in this study. The two curves on both sides of the fitted line represent its 95% confidence intervl. 15

16 Discussion Efficiency of pplied fertiliser N s ffected by rinfll nd temperture The bsence of significnt R 3 nd R 5 rinfll (<2 mm) between T1 nd T3 ppered to hve enhnced the efficiency of CAN (reduced N leching) compred with ure which ws reflected in their reltive yields (URY: 89%-93%). Fertiliser pplictions conducted erly in the seson resulted in reltively lower responses t cut one compred with cut two which ws ttributed to the low tempertures recorded in Februry (T MEAN =4.3 C) nd Mrch (T MEAN =6.1 C). Since ryegrss requires minimum temperture of 5 C to initite growth (Lwrence et l., 1973) nd urese ctivity is reduced t low tempertures (Moyo et l., 1989), these conditions restricted growth, N vilbility from pplied ure nd therefore N uptke up to the first cut. Therefore, the proportionlly higher responses t cut two (T1-T4) were due to the combined effects of residul N from the fertiliser pplied nd climtic conditions on the rte of ccumultion of DM. The sesonl distribution of DM nd N uptke is influenced by temperture nd the timing of N ppliction (Wolton et l., 1971). For perennil ryegrss, this is chrcterised by pek in My followed by decline in June, nd second but smller pek in July (Anslow nd Green, 1967). The pplictions conducted between erly April nd lte My showed proportionlly higher responses t cut one suggesting tht fertiliser-n ws primrily tken up within the first four weeks from ppliction. From pproximtely erly June responses strted to decline in line with the overll decline in rdition. The pek in the responses observed t cut one for T15 mtched well the expected rte of grss growth round this time described by Anslow nd Green (1967); thus, the enhnced responses encountered. A similr effect ws observed t cut two for the ppliction conducted t T13 which responded to the proportionlly higher rtes of growth expected fter the middle of July when the first hrvest took plce. The reltively low responses obtined with ure t T1 nd T2 lso indicted low NUE from ure- N pplied t these timings given tht significnt (P<0.001), positive, liner reltionship ws found between NUE nd β. Therefore, under reltively cold conditions, ure ws less effective compred with CAN for the sme N input s responses (β), nd consequently, NUE, were lower; especilly, when compring the two fertilisers t cut two. This suggested tht some N ws lost to the environment nd to greter extent in ure-fertilised grss. This observtion, however, does not pper to support those of Clrkson et l. (1986) nd Scholefield nd Stone (1995) who suggested preferentil uptke of NH + 4 -N compred to NO - 3 -N by forge crops exposed to low tempertures. Fertiliser pplictions conducted in the summer (T10-T16) led to lower responses to ure-n which resulted in reltively lower NUE compred with the other two fertiliser mterils (0.69, 0.73 nd 0.81 kg N kg -1 N for ure, nbtpt-u nd CAN respectively). The model developed to simulte DMY from N input nd source, nd short-term rinfll nd temperture showed tht predictions cn be stisfctorily mde when the required vribles re within the rnge of vlues reported in this study. This ws confirmed when predicted DMY dt ws regressed ginst tht obtined experimentlly which showed n cceptble fit (R 2 =78%) to the liner model. This model constitutes vluble tool which enbles compring the efficiency of different N sources s ffected by chnges in wether vribles. Comprison of the three N sources Erly pplictions of N in the spring llow for nticiption in the dte of grzing (Blckmn, 1936; McFeely nd McCrthy, 1981) which therefore requires the fertiliser choice to tke ccount of the likely responses to N in this prt of the seson. The review conducted by Wtson et l. (1990) summrised series of contrsting results which indicted tht ure cn be eqully effective s CAN for spring grss production (e.g. Murphy, 1983). 16

17 There is lso evidence which showed reltive efficiencies below nd bove compred with CAN (e.g. Herlihy nd Sheehn, 1977; Chney nd Pulson, 1988) wheres for summer pplictions, efficiencies re usully lower. The results obtined for T1 nd T2 showed tht cumultive responses (β) were lower with ure (c.12 kg DM kg -1 N) compred with CAN (c.16 kg DM kg -1 N). This ws reflected in the clculted N use efficiency which ws 55% higher with CAN compred with ure, the vlue of URY (c.90%) nd reltive N uptke (c.85%). The reltively lower gronomic performnces encountered with ure grees closely with the conclusions reched by Chney nd Pulson (1988) for (erly) spring pplictions of N. Although the uptke of un-hydrolysed ure hd been reported (Mengel nd Kirkby, 1987), its rte of bsorption is reltively lower compred with mmonium-n or nitrte-n (Brdley et l., 1989). Wtson et l. (1990) suggested tht the trnsltion of N uptke into DMY my be less effective with ure thn CAN. Given tht grss crops provide mjor sink for N (Whitehed et. l, 1978; Whitehed, 1995), mximising responses from the N pplied with the fertiliser immeditely following ppliction is n importnt gronomic nd environmentl considertion which llows for tht minerl N to be rpidly removed from the soil-fertiliser nd sequestered into the crop biomss. The verge rte of N uptke between fertiliser ppliction nd first hrvest ws slightly higher with CAN compred with ure. The differences were greter in erly spring (1.1 vs. 0.8 kg N h -1 dy -1 ) nd summer (2.4 vs. 2.2 kg N h -1 dy -1 ). For spring pplictions these were, pproximtely, within ±5% with ll three fertiliser mterils (rnge of 1.9 to 2.0 kg N h -1 dy -1 ). Montemurro et l. (1998) highlighted tht reduced N uptke nd NUE from ure will result in higher soil minerl N which in turn cn increse the risk of N losses to the environment (Bhogl et l., 2003). For pplictions conducted in the spring (T3-T9), reltive yields with ure nd nbtpt-u were, on verge, within ±5% rnge compred with CAN (100%). The use of nbtpt improved the overll performnce of ure which ws more evident with incresed N ppliction rte in the rnge of 25 to 75 kg h -1 (Figure 11 top). Similrly, reltive N uptkes with ure nd nbtpt-u were, on verge, within the suggested ±5% rnge of CAN (except for nbtpt-u t 50 kg h -1 of N) but these tended to be slightly lower thn CAN towrds the summer, especilly with ure. The use of nbtpt enhnced the uptke of ure-n when the N ppliction rte ws incresed within the rnge used in this study (Figure 11 bottom). Ammoni voltilistion increses significntly with the ppliction rte of ure (Overrein nd Moe, 1967) which is one of the min resons for ure to result in lower yields reltive to CAN or AN (Lloyd, 1992; Chmbers nd Dmpney, 2009). This effect hd been shown in erlier studies (Chney nd Pulson, 1988; Murphy, 1983; Vn Burg et l., 1982) nd it cn be the reson for the reltively lower response to ure-n in the rnge of 50 to 75 kg h -1. The differences in the response between fertilisers re expected to be greter t higher N ppliction rtes thn those used in this study (Overrein nd Moe, 1967). It is therefore implied tht ure is reltively less effective N source t high N ppliction rtes, in prticulr, for summer pplictions but with overll lower DMY nd N uptke when used in the spring (Figures 11 top nd bottom respectively). 17

18 Reltive DMY (% of CAN) CAN (100%) Ure (Spring) Ure (Summer) Ure (Autumn) nbtpt-u (Spring) nbtpt-u (Summer) nbtpt-u (Autumn) N rte (kg h -1 ) 110 Reltive N uptke (% of CAN) CAN (100%) Ure (Spring) Ure (Summer) Ure (Autumn) nbtpt-u (Spring) nbtpt-u (Summer) nbtpt-u (Autumn) N rte (kg h-1) Figure 11: Cumultive DMY (top) nd N uptke (bottom) of ure nd nbtpt-u reltive to CAN vs. the N ppliction rte for fertiliser pplictions conducted in spring, summer nd utumn. The error brs show ±5% rnge with respect to CAN (100%) denoted by the dotted lines. Incresing the rte of ure-n to counterct possible yield reductions (reltive to AN) my not be resonble (Lloyd, 1992) since gins in DMY bove the optimum N rte will not be sufficiently high to mtch the yields tht cn be potentilly chieved with AN (Sylvester-Brdley et l., 1982). This is supported by the fct tht mximum yields with the use of ure re expected to be lower thn with CAN (Vn Burg et l., 1982). The reltively lower DMY levels encountered in the grss fertilised with ure did not led to incresed N content in hrvested plnt mteril (N PLANT ); conversely, CAN or nbtpt-u did not show dilution effect (Mrino et l., 2004). 18

19 The differences encountered in N PLANT between-fertiliser tretments (rnge of 2.6 to 2.65%) were not significnt (P>0.05), nd they responded to the expected sesonl vrition in N content in the herbge (Wilkins et l., 2000). Lloyd (1992) suggested tht the reson for this is not differentil DMY between ure nd the other fertilisers but fctors tht reduce N vilbility following fertiliser ppliction; nmely, voltilistion of mmoni. This ws better reflected in the clculted vlues of β nd NUE corresponding to the summer fertiliser pplictions (T10-T16) which showed decline with the N ppliction rte tht occurred to greter extent in urefertilised grss. There ppers to be combined effect of the N ppliction rte nd incresed temperture (towrds the summer) which reduced the efficiency of ure; this cn be demonstrted by simulting DMY with the model. In generl, the rnge of responses (β) encountered in this study (rnge of 10 to 30 kg DM kg -1 N) were within the rnge (from 14 to 29 kg DM kg -1 N) reported in the literture (e.g. Morrison et l., 1980). Men responses for the seson were comprble between CAN nd nbtpt (c.20 kg DM kg -1 N) but higher thn ure (17 kg DM kg -1 N). These exceeded those reported by McFeely nd McCrthy (1981), nd O Donovn et l. (2004) (rnge of 5 to 17 kg DM kg -1 N) but pproximted the men vlue for the seson (23 kg DM kg -1 N) obtined by Morrison et l. (1980). The resons for the reltive enhnced performnce observed in ll mesured prmeters t T5 in the plots fertilised with ure nd nbtpt-u compred with CAN re not cler. R 5 records showed 22 mm but there ws no rinfll subsequent to fertiliser ppliction tht could hve reduced N vilbility from CAN-treted grss; for exmple, by leching. It is possible however tht, given fvourble soil moisture conditions nd temperture, the rte of hydrolysis nd therefore the rte of N uptke hd occurred rpidly in ure- nd nbtpt-fertilised grss. This combined with typiclly high rtes of ccumultion of DM in this prt of the seson (Anslow nd Green, 1967) resulted in higher performnces reltive to CAN. Bsed on the c.v. (%) for the dt corresponding to DMY nd N uptke, it is not possible to indicte tht ure or nbtpt re more vrible N sources thn CAN which grees with Wtson et l. (1990). Bussink nd Oenem (1996) predicted tht in order for ure to be s profitble s CAN, R 3 needed to be in excess of 6 mm (cut one) or 10 mm (cut two) nd tht its ppliction would be unprofitble for lter cuts. Chney nd Pulson (1988) concluded tht DMY losses from ure re likely to occur in ll cut silge nd Wtson et l. (1990) bsed on studies conducted in Scotlnd suggested tht ure-n needs to be bout 10% to 20% cheper thn AN-N to be eqully cost effective. Conclusions 1. A model ws developed to simulte DMY bsed on the combined effects of N ppliction rte nd source, nd short-term rinfll nd temperture. This model is simple, yet effective, tool for ssessing the efficiency of different N sources s ffected by wether vribles. It ids the fertiliser choice nd mngement in grss crops. 2. Ure ws less effective thn CAN for erly spring pplictions s shown by the lower N responses obtined, reltive yield nd N uptke compred with CAN which trnslted into reduced gronomic nd N use efficiencies. Reduced N uptke from the fertiliser-n cn led to incresed soil minerl N which is likely to be lost to the environment by leching or gseous evolution. 3. For spring pplictions, reltive yields nd N uptkes with ure nd nbtpt-u were, on verge, within ±5% compred with CAN (100%); hence, the efficiencies of the three N sources were comprble. The use of nbtpt enhnced the overll performnce of ure; especilly, towrds the summer nd with incresing N ppliction rte. 19

20 4. For summer pplictions, ure ws less effective thn CAN or nbtpt-u both of which showed comprble efficiencies fter erly My. There ws no evidence to suggest tht ure or nbtpt-u were more vrible N sources thn CAN but, overll, cumultive DMY were to be lower. Incresing the ppliction rte of ure-n to compenste for its reltively lower efficiency is not recommended due to greter risk of mmoni voltilistion. This my be done with nbtpt depending on the reltive costs of both fertiliser sources. 5. The verge rte of N uptke between fertiliser ppliction nd first cut ws bout 30% higher with CAN compred with ure in erly spring, nd bout 7% higher in the summer while differences between fertiliser types in the spring were within ±5%. This rte determines the velocity t which N is removed from the soil-fertiliser following ppliction thereby reducing the opportunities for N losses to the environment. Acknowledgements This reserch ws conducted with finncil support from the Deprtment of Agriculture, Food nd the Mrine (DAFM) of Irelnd under the Reserch Stimulus Fund Progrmme. The uthors cknowledge the technicl ssistnce of AFBI (Belfst, NI), Dr J Grnt (Tegsc Ashtown, Dublin), nd field nd lbortory technicl stff t Tegsc Johnstown Cstle. The University of Southern Queenslnd through the NCEA provided finncil support to ttend the 2013 ASABE Annul Interntionl Meeting. References Anslow, R.C. & Green, J.O. (1967). The sesonl growth of psture grsses. J. of Agric. Sci. 68: Bhogl, A., Dmpney, P. & Goulding, K. (2003). Evlution of ure-bsed nitrogen fertilisers. Component Report for Defr Projects NT2601 nd NT2602. Acc. 26 Sept Completed=0 Blckmn, G.E. (1936). The influence of temperture nd vilble nitrogen supply on the growth of psture in the spring. J. of Agric. Sci. 26: Bouwmeester, R.J.B., Vlek, P.L.G. & Stumpe, J.M. (1985). Effect of environmentl fctors on mmoni voltilistion from ure-fertilized soil. Soil Sci. Soc. of Am. J. 49: Brdley, D.P., Morgn, M.A. & O Toole, P. (1989). Uptke nd pprent utiliztion of ure nd mmonium nitrte in whet seedlings. Fert. Res. 20: Bussink, D.W. & Oenem, O. (1996). Differences in rinfll nd temperture define the use of different types of nitrogen fertilizer on mnged grsslnd in UK, NL nd Eire. Netherlnds J. of Agric. Sci. 44: Byrne, E. (1979). Chemicl nlyses of griculturl mterils. An Fors Tluntis, Dublin. Cssmn, K.G., Peng, S., Olk, D.C., Ldh, J.K., Reichrdt, W., Dobermnn, A. & Singh, U. (1998). Opportunities for incresing nitrogen use efficiency from improved resource mngement in irrigted rice systems. Field Crops Res. 56: Chmbers, B.J. & Dmpney (2009). Nitrogen efficiency nd mmoni emissions from urebsed fertilisers nd mmonium nitrte fertilisers. Proc. No.: 657. The Int. Fert. Soc., York, UK. 20