CHAPTER 3 NITROGEN APPLICATION AND CRITICAL NITRATE SOIL SOLUTION CONCENTRATIONS FOR YIELD AND QUALITY OF ANNUAL RYEGRASS

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1 CHAPTER 3 NITROGEN APPLICATION AND CRITICAL NITRATE SOIL SOLUTION CONCENTRATIONS FOR YIELD AND QUALITY OF ANNUAL RYEGRASS 3.1 INTRODUCTION Annul ryegrss is one of the most widely grown irrigted winter pstures in South Afric in the high rinfll res prticulrly in the Ntl Midlnds, the Estern Highveld, the Estern Cpe nd in winter rinfll res of South Afric (Dickinson et l., 24). It hs high nutritionl qulities, pltility, digestile energy, protein nd minerl contents (Theron nd Snymn, 24) nd plys n essentil role in supplying good qulity grzing etween winter nd summer (Eckrd et l., 1994). Nitrogen fertiliser is mjor input influencing yield nd qulity of irrigted nnul ryegrss in South Afric. Current N recommendtions re sed on empiricl reltionships with forge yield, with little focus on psture qulity or losses of N to the tmosphere nd groundwter. The high rtes of N pplied to ensure mximum forge yield (Eckrd et l., 1995), often results in high crude protein (CP) concentrtion, which is ssocited with n increse in non-protein N (Reeves et l., 1996), nitrte toxicity, imlnces in minerl metolism nd metolic disorders (Coome nd Hood, 198; de Villiers nd vn Ryssen, 21). It cn lso reduce non-structurl crohydrtes content, forge intke (Mris et l., 23) nd milk yields (Ts et l., 26), s energy is used to digest excess protein t the expense of milk production. There is high potentil for N losses in psture fed diry production, through the process of mmoni relese nd leching of nitrtes, with urinry N output ccounting for out 65% of N intke for diry cows feeding on highly fertilised psture (Peyrud et l., 1997). One wy to reduce N losses is to reduce CP intke y reducing forge CP concentrtion, which cn e chieved either through reduced N fertilistion or incresing energy intke to lnce for ingested CP (Tmming, 1992; 39

2 Hoekstr et l., 27). For indoor rtion sed diry production, feed composition cn e lnced y mixing diets of herge with supplements. However, due to the lower cost of inputs in the psture sed systems thn mixed rtion system (Gertench, 26), milk production is minly dependent on pstures. Mnipultion of forge qulity of the grzing psture is complex, ecuse high N pplictions re common for chieving mximum forge yield, while high N ppliction cn reduce forge qulity nd energy vlue. For exmple, Nsh et l. (28) reported higher energy yield with n ppliction rte of 3 kg N h -1 cycle -1 even though the highest iomss yield ws otined t 6 kg h -1 per cycle -1. In Chpter two, results showed how N fertiliser pplictions could e reduced using vrious dptive strtegies sed on regulr monitoring of soil nitrte using pssive lysimeter. This Chpter extends the work of Chpter two y investigting the nexus etween fertiliser, iomss production nd qulity in more detil. Specificlly the ojectives of this reserch re to determine 1) response of nnul ryegrss to forge yield nd qulity to N fertilizer ppliction, 2) criticl soil nitrte concentrtions for yield nd qulity nd 3) the nture of the trde-off etween iomss nd qulity prmeters. 3.2 MATERIALS AND METHODS Site description nd crop mngement The experiment ws conducted t Cedr, Deprtment of Agriculture experimentl site (ltitude 176 m, 29 o 32 S; 3 o 17 E) in 27 nd 28. The site is locted in the midlnds of KwZulu-Ntl, one of the min milk producing res of South Afric. It hs men nnul rinfll of 876 mm nd reference evpotrnspirtion of 1511 mm (Tle 1). Totl precipittion over the growing period (Mrch to Novemer) t the study site ws 557 mm in 27 nd 441 mm in 28 (Figure 3.1). Both sesons were drier thn the long-term verge of 611 mm for this period. Monthly men minimum nd mximum tempertures were similr to the long-term men vlues. 4

3 35 Rin Tmx Tmin Jn Mr My Jul Sep Nov Fe Apr Jun Aug Oct Dec Jn Mr My Jul Sep Nov Temperture ( C) Monthly rinfll (mm) Long-term Figure 3.1 Long-term (85 yers), 27 nd 28 totl monthly rinfll nd men mximum nd minimum tempertures for Cedr The experimentl site hs deep, red, kolinitic Hutton soil (Soil Clssifiction Working Group, 1991) with cly lom texture to depth of.4 m, with hevier cly soil from.4 to 1. m. In oth yers, soil core smples were collected to depth of 1 m t the eginning of the sesons. Soil fertility sttus ws determined prior to plnting. Ammonium cette ws used for K, C nd Mg extrction. Orgnic cron nd N were estimted y mid-infrred spectroscopy (Ben-Dor nd Bnin). P mesured with Bry I. Nitrte nd mmonium N were nlysed using 1 M KCl extrction. The soil test results of oth yers were similr nd re presented in Tle 2.2. Bsed on soil fertility sttus 2 kg P h -1 (super phosphte - 1.5%) ws incorported t plnting. Both N (limestone mmonium nitrte - 28%) nd K (KCl - 5%) were top-dressed within two dys fter cutting. The sesonl recommended K (2 kg K h -1 ) ws divided y the expected numer of growth cycles, while the N regime ws determined y the tretment. 41

4 Itlin ryegrss cultivr (Lolium multiflorum) Agriton ws plnted on the 6 th Mrch in 27 nd 25 th Mrch 28 t seeding rte of 3 kg h -1 nd spcing of 15 mm etween rows. A Cmridge roller ws used to fcilitte good contct etween the seed nd soil. Two weeks fter emergence, 2-4-D mine hericide ws spryed ginst rod lef weeds. Fenoxprop-p-ethyle Pum Super ws lso used to control Eleusine indic (L.) Gertn. (goosegrss) which is common invsive weed of irrigted pstures. A drgline sprinkler irrigtion system with delivery rte of 4. mm h -1 nd spcing etween sprinklers of 12 m ws used. Plots were 12 m wide nd 36 m long with order spcing etween plots of 12 m. Ech plot hd its own sprinkler lines nd ws irrigted independently y determining the deficit to field cpcity using the Diviner-2 cpcitnce proe (Sentek, Austrli) to depth of.6 m. Plots were irrigted once week during utumn, spring nd summer, nd once every two weeks in winter. Tretments were refilled to field cpcity except in summer where some room ws left for rin Tretments Experiments were conducted with three levels of N ppliction rtes: (N ), 3 (N 3 ) nd 6 kg N h -1 (N 6 ) in 27. Four fixed rtes of N: (N ), 2 (N 2 ), 4 (N 4 ) nd 6 (N 6 ) kg of N h -1 were pplied in 28. The reson for including four ppliction rtes in 28 ws to ensure tht the yield plteu ws determined ecuse from the 27 dt iophysicl optimum yield ws etween 3 nd 6 Kg of N h -1. For ll tretments, N ws pplied t the eginning of ech growth cycle except for the first growth cycle in 28 when no fertiliser N ws pplied. In oth yers, tretments were ssigned in complete rndomized lock design with three replictions Plnt smpling nd qulity nlysis The psture ws defolited t the two to three lef stges. For yield nd qulity determintion, totl of nine smples per tretment (three from ech plot) were collected from 1 m 2 qudrnts to 42

5 stule height of 5 mm. After tking the smples, the whole field ws hrvested with trctor mower to height of 5 mm. Forge dry mtter (DM: t h -1 ) ws determined y oven drying the smples t 7 C to constnt mss. Smples were milled to pss through.1 mm sieve nd were kept in ir tight ottles until qulity nlyses could e performed. Nitrogen ws determined y Kjeldhl nlysis (AOAC, 2) nd crude protein (CP) ws clculted s N x 6.25 (NRC, 21). True protein (TP) ws determined using the trichlorocetic cid (TCA) precipittion method. Nontrue protein (NTP, i.e. peptides, free mino cid, nucleic cids, mines, nitrte nd mmoni) ws clculted y dividing the difference etween crude nd true protein y 6.25 (NRC, 21). Neutrl detergent fire (NDF) nd cid detergent fire (ADF) concentrtions were determined ccording to Vn Soest et l. (1991) method. Metolisle energy (ME: MJ kg -1 DM) content ws estimted from CP, NDF nd ADF using reltionship developed y Fulkerson et l. (1998) using: ME = ADF * NDF +.84 NDF * CP CP +.1 (ADF) 2 (3.1) Soil nitrte smpling nd nlyses Wetting front detectors were used to collect soil solution for determining the concentrtion of nitrte t different depths. They were instlled t depths of.15,.3 nd.45 m in ech plot. Solution smpling from WFDs ws conducted dy fter n irrigtion or rinfll event. Solutions were kept in cooler ox nd nitrte content ws estimted using pper colour nitrte test strips (Merck KGA, Germny). Only nitrte is considered ecuse it is the dominnt form of inorgnic N (Appendix B1) Clcultions nd sttisticl nlysis Growing degree dys (GDD) were clculted y sutrcting se temperture of 4 o C from dily verge temperture [(mximum + minimum)/2] ccording to Akml nd Jnssens, (24). Dys fter emergence, the length of growth cycles in terms of dys nd GDD re presented in Appendix B2. In pstures, the first week is the most importnt period in terms of N ppliction (Collins nd 43

6 Allinson, 24), nd fertiliser is pplied t the eginning of ech cycle or immeditely fter ech cut. Therefore, mesuring soil N during the first week cn give good estimte of N required for the full growth cycle. Men soil nitrte solution concentrtions of ll depths from ll replictions in the first week were used to clculte the criticl nitrte levels for vrious mesured nd clculted psture prmeters. Biophysicl optimum forge yield ws expressed reltive to the lowest yield tht ws not sttisticlly significnt from the mximum plteu yield for ech growth cycle. In cse of no plteu, reltive yield ws clculted using the mximum yield. Therefore, it ws esier to compre yields etween growth cycles nd sesons. Dt were nlysed using SAS (SAS, 22). Forge yield nd qulity prmeters were nlysed seprtely for ech growth cycle. Where pplicle, significntly different mens were seprted using Tukey s test t the 95% confidence level. Anlyses of vrince nd regression were performed for yield, N nutrition nd crude protein concentrtion using SAS Proc NLIN (SAS, 22). Criticl levels for iophysicl optimum forge yield, optimum (CP opt = 17%) nd mximum (CP mx = 22%) forge crude protein concentrtions (Peyrud nd Astigrrg, 1998) were determined using the Cte-Nelson response plteu (CN) model. Error percentges nd r 2 were used to show the ility of CN model to express the dt (Cte nd Nelson, 1971). According to De Jger (1994), for ccurte model predictions, error should e less thn 2%. 3.3 RESULTS AND DISCUSSION Forge yield Forge yield ws higher in 27 thn 28 (Figure 3.2). This ws due to erly plnting nd lte ending of the 27 seson with longer growing seson nd greter ccumultion of growing degree dys (Appendix B1). Forge iomss produced ws etween 5.9 t h -1 (N ) nd 15.6 t h -1 (N 6 ) (Figure 3.3). The mximum yields were in close greement with the vlues reported y Eckrd et l. (1995) from Cedr, which rnged from 12.5 to 15.4 t h

7 Forge yield (t DM h -1 cycle -1 ) N6 N3 N 63(1) 98(2) 128(3) 155(4) 182(5) 25(6) 232(7) 26(8) 28 N6 N4 N2.5 N 64(1) 99(2) 135(3) 164(4) 19(5) 213(6) 236(7) Dys fter plnting (growth cycles) Figure 3.2 Forge yield of nnul ryegrss under rnge of N ppliction rtes for eight growth cycles in 27 (, 3, 6 kg h -1 cycle -1 for N, N 3, N 6 ) nd seven growth cycles in 28 (, 2, 4 6 kg h -1 cycle -1 for N, N 2, N 4, N 6 ) Biophysicl optimum forge yields (the lowest yield tht is not sttisticlly significnt from the mximum yield) were oserved etween N 3 nd N 6 in 27 nd etween N 4 nd N 6 in 28 (Figure 3.2 nd Figure 3.3). Nitrogen fertiliser ppliction ws not effective in the first 2-3 growth cycles. This could e due to high initil soil inorgnic N nd high rtes of N minerlistion fter tillge in utumn nd in spring. This suggests over-fertilistion for the first 2-3 growth cycles, ecuse frmers usully pply equl mount of N (i.e. 5 kg h -1 cyle -1 ) for ll growth cycles. 45

8 Reduction of N fertiliser for these growth cycles could help increse profitility nd reduce risk of N loss N6 N3 Cumultive forge yield (t DM h -1 ) N 63(1) 98(2) 128(3) 155(4) 182(5) 25(6) 232(7) 26(8) 28 N6 N4 N2 N 64(1) 99(2) 135(3) 164(4) 19(5) 213(6) 236(7) Dys fter plnting (growth cycles) Figure 3.3 Cumultive forge yield of nnul ryegrss under rnge of N ppliction rtes for eight growth cycles in 27 (, 3, 6 kg h -1 cycle -1 for N, N 3, N 6 ) nd seven growth cycles in 28 (, 2, 4 6 kg h -1 cycle -1 for N, N 2, N 4, N 6 ) Forge qulity Crude protein, true protein nd non-true protein In oth yers, CP concentrtions of N 6 tretment were ove the CP mx for ll growth cycles (Figure 3.4). CP opt ws ttined only in few cycles for N nd N 3 in 27, nd in N nd N 2 in 28. At the iophysicl optimum forge growth, CP concentrtions exceed the recommended 46

9 concentrtions for resonle levels of milk production (NRC, 21). In generl, lthough the CP ws dependent on the soil N vilility, N pplictions etween 3-4 kg h -1 cycle -1 generlly produce CP concentrtions etween CP opt nd CP mx. However, N pplictions ove 4 kg h -1 cycle -1 will most likely produce CP concentrtions ove the mximum limit of 22% N N3 N c c c c c CP mx CP opt 1 5 CP (%) (1) 98(2) 28 c c 128(3) 155(4) 182(5) 25(6) 232(7) 26(8) N N2 N4 N6 c c c c d d d c CP mx CP opt 5 64(1) 99(2) 135(3) 164(4) 19(5) 213(6) 236(7) Dys fter plnting (growth cycles) Figure 3.4 Crude protein (CP) concentrtions of nnul ryegrss under rnge of N ppliction rtes for eight growth cycles in 27 (, 3, 6 kg h -1 cycle -1 for N, N 3, N 6 ) nd seven growth cycles in 28 (, 2, 4 6 kg h -1 cycle -1 for N, N 2, N 4, N 6 ) Nitrogen fertilistion hs direct effect on proportion of protein (true or non-true protein). At high N levels the percentge of TP to CP decresed significntly in most of the growth cycles (Appendix B2), which reduces the proportion of true protein while incresing the non-true protein portion (Figure 3.5). 47

10 CP TP NTP CP, TP or NTP (%) N N3 N6 N N3 N6 N N3 N6 N N3 N6 Cycle 2 Cycle 4 Cycle 6 Cycle 8 28 CP TP NTP N N2 N4 N6 N N2 N4 N6 N N2 N4 N6 Cycle2 Cycle 4 Cycle 6 Growth cycles (N rte tretments) Figure 3.5 Crude protein (CP), true protein (TP) nd non-true protein (NTP) concentrtions of nnul ryegrss under rnge of N ppliction rtes for four growth cycles in 27 (, 3, 6 kg h -1 cycle -1 for N, N 3, N 6 ) nd three growth cycles in 28 (, 2, 4 6 kg h -1 cycle -1 for N, N 2, N 4, N 6 ) The mjority of true protein nd non-true protein entering the rumen is roken down to mmoni, which cteri require for synthesising their own ody protein. Ammoni, in excess of tht used y the micro-orgnisms, is sored through the rumen wll into the lood, crried to the liver nd converted to ure, in which the greter prt is excreted in the urine (only smll portion is lost y 48

11 elching). An increse in non-true protein usully leds to nitrte ccumultion in the plnt, where in the rumen the nitrte is converted to nitrite nd then to mmoni. Generlly, when N ppliction exceeds the psture requirement (2-22% CP), the increse in CP ws mostly in the form of nontrue protein, with negligile increse in true protein. True protein only increses up to certin level (2-22% CP optimum for growth), however, ove this CP is stored in non-true protein (lrgely in the form of peptides, free mino cid nd nitrtes) form (Vn Soest, 1994; Mris et l., 23). Animl performnce nd milk production my, however, significntly e ffected when the CP concentrtions significntly drop elow the minimum requirement of 8-11% (Hoekstr et l., 27) Fire Expressing the fire requirement s NDF is superior to ADF ecuse it mesures most of the structurl components of the plnt cell (i.e. cellulose, hemicellulose nd lignin) while NDF does not include hemicellulose (NRC, 211). Therefore, in this study only NDF (Figure 3.6) is discussed ut ADF vlues re lso presented in Appendix B3. Neutrl detergent fire is closely ssocited to rumen fill or intke (Allen, 1996). The slow digestion of the NDF in the herge my hve lrge influence on pssge rte nd therefore the NDF frction is elieved to hve direct effect on rumen fill. Incresed NDF reduces digestiility nd intke y incresing the residence time of the course mteril in the rumen nd therefore, reduce the nutritionl sttus of high producing diry cow. Neutrl detergent fire ws the lest ffected qulity prmeters (Figure 3.6). It ws more ffected y the plnt growth stge of mturity thn y N fertiliser (Peyrud nd Astigrrg, 1998). In generl, the NDF vlues were higher thn the minimum requirements for diry cows of 25-28% (NRC, 21). Lower vlues of NDF usully correspond to high nutritive vlue, however, excessively low NDF vlues thn the recommended requirement cn cuse digestive prolems due to fst pssge through the rumen (Redfern et l., 22). Towrds the end of the seson, the NDFs were high nd my show reduced intke (Hopkins et l., 22). 49

12 6 27 N N3 N (1) 98(2) 128(3) 155(4) 182(5) 25(6) 232(7) 26(8) 6 28 N N2 N4 N (2) 135(3) 164(4) 19(5) 213(6) 236(7) NDF (%) Growth cycles Figure 3.6 Neutrl detergent fire (NDF) of nnul ryegrss under rnge of N ppliction rtes for eight growth cycles in 27 (, 3, 6 kg h -1 cycle -1 for N, N 3, N 6 ) nd seven growth cycles in 28 (, 2, 4 6 kg h -1 cycle -1 for N, N 2, N 4, N 6 ). Horizontl line is minimum NDF requirement for diry cows Metolisle energy Metolisle energy ws clculted from NDF, ADF nd CP concentrtions using n empiricl reltionship developed y Fulkerson et l. (1998). The clculted ME vlues re in the rnge of typicl nnul ryegrss reported in literture (Fulkerson et l., 1998; Meeske et l., 26). In most cses (Figure 3.7), ME contents of ll N rte tretments were within the cceptle rges of 1-12 MJ kg -1 DM (Fulkerson et l., 1998), except end of the seson when the NDF vlues were high. 5

13 14 27 N N3 N ME (MJ kg -1 DM) (1) 98(2) (3) 155(4) 182(5) 25(6) 232(7) 26(8) N N2 N4 N (1) 99(2) 135(3) 164(4) 19(5) 213(6) 236(7) Dys fter plnting (growth cycles) Figure 3.7 Metolisle energy (ME) content of nnul ryegrss under rnge of N ppliction rtes for eight growth cycles in 27 (, 3, 6 kg h -1 cycle -1 for N, N 3, N 6 ) nd seven growth cycles in 28 (, 2, 4 6 kg h -1 cycle -1 for N, N 2, N 4, N 6 ) ME vlues were similr for the first five growth cycles for different N rte tretments (Figure 3.7). However, high N rte tretment tends to produce higher ME towrds end of the seson. This could e due to the depletion of N in the low rte fertiliser tretments which leds to low CP thn the N required for psture growth. Generlly, N ppliction rte hs no effect on ME content (Figure 3.7). This implies tht t the high N rte tretment milk production is likely to e reduced due 51

14 consumption of high CP. The decrese in milk production would e due to higher energy required for metolising the excess CP Criticl soil nitrte concentrtions for yield nd qulity The criticl soil solution concentrtion is concept widely ccepted for interpreting soil nitrte for optimum growth/yield/qulity (Cte nd Nelson, 1971). Vlues lower thn the criticl concentrtions indicte high potentil response in growth to N ppliction. Aove the criticl vlue, the iomss response to n increse in the vilility of N is negligile nd the forge qulity my e reduced. In ll Cte-Nelson (CN) figures, the verticl line which intersects the x xis indictes the criticl soil solution nitrte concentrtion for yield or qulity prmeters (Vn Biljon et l., 28). The CN prtitions the dt into four groups. Horizontl nd verticl lines re plotted to mximise points in the upper right (non-responsive) nd lower left (responsive) nd minimise points t the upper left (over-prediction) nd lower right (under-prediction) qudrnts (Cte nd Nelson, 1971). In Chpter 2, the thresholds for the dptive mngement tretments were somewht ritrrily selected in the knowledge tht they would e improved with experience. In this study, optimum soil solution nitrte concentrtion rnges for yield nd qulity were developed using sttisticl models. Criticl men nitrte concentrtion ws 28 mg L -1 for iophysicl optimum forge yield, 2 mg L -1 for CP opt nd 41 mg L -1 for CP mx (Figure 3.8). These rnges cn e used y trding of forge yield with good forge qulity, whilst lso minimising N leching. All the reltionships etween soil nitrte solution mesured t the first week nd crop prmeters were significnt t 99% level of confidence nd the totl error ws within n cceptle rnge (De Jger, 1994). In the Cte-Nelson model, error I showed tht the prmeters reched optimum while the soil solution nitrtes were elow optimum vlue which my result in yield/qulity reduction ecuse no N fertiliser ppliction is required. On the other hnd, error II showed tht the criticl nitrte level reched optimum ut the crop N ws in deficit, in which cse N would hve een pplied nd would led to leching. 52

15 Reltive forge yield (%) Error I = % Error II = 11.6% CL = 27.6 mg L -1 r 2 =.76** Soil solution nitrte conc (mg L -1 ) CPopt (%DM) Error I = 4.7% Error II = 4.7% CL = 19.5 mg l -1 r 2 =.64** Soil solution nitrte (mg L -1 ) 33 3 CP-mx (%DM) Error I = 7.% Error II = 4.7% CL = 41.3 mg L -1 r 2 =.68** Soil solution nitrte (mg L -1 ) Figure 3.8 Criticl soil solution nitrte concentrtion for iophysicl optimum forge yield, optimum (CP opt ) nd mximum (CP mx ) crude protein concentrtions using Cte-Nelson model. Criticl soil solution nitrte concentrtion (CL), coefficient of determintion (r 2 ), numer of oservtions (n = 43), error I upper left side of the qudrnt (I) nd error II lower right side of the qudrnt (II) 53

16 In Chpter 2 (Fessehzion et l., 211), 5 mg L -1 (men of ll WFD tht responded) ws used. In this study, the vlues re slightly lower thn the 5 mg L -1 (when criticl levels were determined sttisticlly). However, considering the vriility of the pooled dt, growth cycles nd verges of nitrte from the WFDs, rnge of criticl men nitrte concentrtion etween 2 nd 41 mg L -1 could e used s etter estimtes of the criticl nitrte soil solution concentrtion. However, since the nitrte test strips used in the experiment were in rnge of, 1, 25, 5, 1, 25 nd 5 mg L -1, rnge of soil nitrtes etween 25 to 5 ppm would e n pproprite vlue for yield nd qulity whilst lso minimising N leching. Criticl soil nitrte levels developed in this study cn e used in the following strtegies. First, to pply prt of the recommended N t the eginning of the growth cycle nd supplement the rest s necessry sed on the WFD nitrte concentrtion. This cn e more prcticl in winter when intervls of the growth cycles re 35 to 6 dys nd there is enough time to pply N fertiliser once or twice especilly for frmers who fertigte their psture where smll mount of N cn e pplied. Secondly, criticl soil nitrte is guide for how much N to pply for the next growth cycle sed on nitrte soil solution from the lst irrigtion of the previous growth cycle. This would e more suitle during the short growth cycle intervls nd/or when trctors re used for fertiliser ppliction. In utumn, spring nd erly summer when the growth cycles re etween 21 nd 28 dys, there my not e enough time to pply N sed on the nitrte levels from the sme growth cycle. Therefore, soil solution nitrte concentrtions from the lst irrigtion of the previous growth cycle cn e used s guide for the next growth cycle (s reported in Chpter 2). Finlly, criticl soil nitrte vlues cn e used for integrted dptive N nd irrigtion mngement ecuse the WFD shows the depth the wetting front hs pssed nd the concentrtion of nutrients t tht prticulr soil depth. There ws significnt exponentil reltionship etween N ppliction rte nd soil solution nitrte concentrtions collected from the WFDs (Figure 3.9). The reciprocl of the grph would give some 54

17 indiction on the mount of N required to rech trget soil solution nitrte concentrtion for yield or qulity. For exmple, N ppliction of kg h -1 cycle -1 is required to rech men nitrte concentrtions of 25 to 5 mg L -1. To ttin criticl men nitrte concentrtion of 28 mg L -1 (iophysicl optimum forge yield), 2 mg L -1 (CP opt ) nd 41 mg L -1 (CP mx ), N ppliction rtes of 31, 37 nd 43 kg h -1 cycle re required, respectively. Becuse the dt re verged cross smpling dtes over two yers nd the solutions re ffected y plnt uptke, rte of minerlistion nd initil soil inorgnic concentrtions, the reltionship (in Figure 3.9) my not hold for ll growth cycles nd yers. 14 Soil nitrte concentrtion (mg L -1 ) y = e.67x R 2 = N rte (kg h -1 cycle -1 ) Figure 3.9 Exponentil eqution used to show the reltionship etween N ppliction rtes nd men soil solution nitrte concentrtions. Dt pooled from rnge of N ppliction rtes in 27 (, 3, 6 kg h -1 cycle -1 for N, N 3, N 6 ) nd in 28 (, 2, 4 6 kg h -1 cycle -1 for N, N 2, N 4, N 6 ) Nture of the trde-off etween yield nd qulity prmeters Neutrl detergent fire nd ME were the lest ffected qulity prmeters y N ppliction rte tretments. As result, CP ws used for lncing forge iomss yield nd to determine the nture of trde-off etween yield nd qulity. This ws ecuse CP represents the overll forge qulity nd is lso the most ffected qulity prmeter. Forge CP (or N) is the most importnt 55

18 forge qulity prmeter tht cn e esily nlysed. It cn lso e used s good indictor for other qulity prmeters which cn e ffected directly or indirectly y N ppliction. Both metolisle energy yield (MJ h -1 ) for individul growth cycles (Appendix B4) nd sesonl cumultive (Figure 3.1) showed similr trends to forge yield. Similr to forge yield, though not lwys significnt, the highest ME yields were oserved from the highest N rte tretment. As result optimum ME yield ws otined t N rte etween N 3 to N 6 in 27 nd t N 4 in 28 (Figure 3.1) N6 N3 9 6 N Cumultive ME (MJ h -1 ) (1) 98(2) 128(3) 155(4) 182(5) 25(6) 232(7) 26(8) 28 N6 N4 N2 N 3 64(1) 99(2) 135(3) 164(4) 19(5) 213(6) 236(7) Dys fter plnting (growth cycles) Figure 3.1 Cumultive metolisle energy (ME) of nnul ryegrss under rnge of N ppliction rtes for eight growth cycles in 27 (, 3, 6 kg h -1 cycle -1 for N, N 3, N 6 ) nd seven growth cycles in 28 (, 2, 4 6 kg h -1 cycle -1 for N, N 2, N 4, N 6 ) 56

19 Criticl CP concentrtions required for iophysicl optimum reltive yields (which re lso similr for ME yield) developed using the Cte-Nelson model ws 18.1% (Figure 3.11). This vlue is slightly higher thn the optimum CP (CP opt ) concentrtion of 17% ut lower thn the mximum CP (CP mx ) of 22%. Nitrogen fertiliser required for chieving mximum forge yield/me yield nd CP vries mongst growth cycles (Appendix B5) depending on soil N vilility. As result, N fertiliser ppliction for the first two to three cycles not only reduced forge qulity (high CP) ut lso did not improve forge nd ME yields Reltive forge yield (%) Error I = 5.13% Error II = 3.85% CL = 18.2% (CP) r 2 =.71** Crude protein (%) Figure 3.11 Criticl crude protein concentrtion (%) for iophysicl optimum reltive forge yield (%) using Cte-Nelson model. Criticl crude protein concentrtion (CL), coefficient of determintion (r 2 ), numer of oservtions, error I upper left side of the qudrnt (I) nd error II lower right side of the qudrnt (II) Generlly, the current study showed tht the iophysicl optimum N ppliction per cycle (the highest non- significnt forge nd ME yields) ws etween 3-6 kg N h -1 in 27 nd 4 kg N h -1 in 28. Hence, N ppliction rte of round 3-4 kg N h -1 per growth cycle could e iophysicl optimum yield to slightly lower forge nd ME yield, ut with CP concentrtions rnging 57

20 etween CP opt nd CP mx. This my not e pplicle for the first 2-3 growth cycles, where there is excessive CP concentrtions, however, this cn e mnged y considering soil N (s presented in Chpter 2). 3.4 CONCLUSIONS Generlly, for most growth cycles, the highest forge yields were produced when N ppliction rtes rnged etween 3 to 6 kg N h -1 cycle -1, except for the first growth cycles when there ws high soil N crryover. The mount of N fertiliser required for chieving mximum forge yield nd optimum qulity vries widely mong growth cycles depending on soil N vilility. As result, N fertiliser ppliction for the first two to three cycles did not improve forge yield (nd the N nutrition) ut reduced qulity (high CP). Consequently, the current frmers recommendtion (fixed N ppliction rte of 5 kg h -1 per growth cycle) sed on trget yield my give the highest iomss ut not optiml qulity nd hence my not improve niml performnce (for ll growth cycles). It could lso increse N leching s result of high CP which most likely will result in high urinry excretion. Therefore, similr overll niml performnce or milk yield cn e chieved y pplying less N fertiliser nd compensting the reduced yield with n improved qulity of forge (lower CP), while lso minimising environmentl impct. However, frmers who use psture sed system hve only limited options for mnging N y integrting oth yield nd qulity whilst minimising leching, ecuse lncing rtions for psture sed diry production is difficult. Generlly, the current study showed tht the optimum N ppliction per cycle (the highest nonsignificnt forge nd ME yields) ws etween 3-6 kg N h -1 in 27 nd 4 kg N h -1 in 28. Hence, N ppliction rte of round 3-4 kg N h -1 per growth cycle could give iophysicl optimum forge yield (or slightly lower) nd ME yield nd ut with CP concentrtions with the oundries of CP opt nd CP mx. This my not e pplicle for the first 2-3 growth cycles, where there is excessive CP concentrtions, however, this cn e mnged y considering soil N (s presented in Chpter 2). 58

21 The trde-off etween yield nd qulity will depend on the mngement of psture, whether for psture sed systems or indoor rtion sed diry production (feed composition cn e lnced y mixing diets of herge with supplements). For psture sed systems due to difficulty in mnipultion of ll prmeters of psture qulity, trding-off forge yield for etter forge qulity my e required. However, for indoor rtion sed diry production, trgeting mximum iomss yield would e etter ecuse the feed cn e supplemented with low-cost roughges. 59