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Effects of drought stress and sward botanical composition on the nutritive value of grassland herbage.

Byline: Frank KA1/4chenmeister Kai KA1/4chenmeister Manfred Kayser Nicole Wrage-MAlnnig and Johannes Isselstein

Abstract

The predicted increase of drought incidents even in temperate climates might affect not only yield but the nutritive value of grassland herbage as well. It is not yet clear whether species richness or functional group composition could mitigate a possibly negative reaction of the nutritive value to drought. Here we report findings of a study investigating the effects of drought stress species richness (one to five species) and functional group composition (grass forb and legume) on nutritive value (crude protein water-soluble carbohydrates neutral detergent fiber acid detergent fiber) of herbage under semi- controlled conditions in a vegetation hall. Moderate or strong drought was imposed on plants in one growing season and followed by a recovery period. Drought had no or minor immediate or residual effects on nutritive value and there was no interaction of species richness or functional group with drought.

However functional group and seasonal variation distinctively influenced the nutritive value of herbage. It was concluded that under conditions of climate change with drought stress events yield decreases in grassland seem to be by far more important than changes in nutritive value. Copyright 2014 Friends Science Publishers.

Keywords: Crude protein; Water-soluble carbohydrates; NDF; ADF; Functional group composition; Species richness.

Introduction

Producing grassland herbage of a good nutritive value is a prerequisite of efficient ruminant livestock production (Gibon 2005; Hopkins and Wilkins 2006). Herbage of a high nutritive value is more likely to be taken up in high amounts is readily digested and facilitates a high performance of ruminants (Hopkins and Wilkins 2006). The nutritive value of herbage is strongly dependent on factors like grassland management as well as on soil and climatic conditions (Isselstein et al. 2005).For grassland herbage production an adequate water supply is important. Predicted climate change with varying precipitation patterns and frequently occurring droughts may affect herbage production even in temperate climate zones (Alcamo et al. 2007; Hopkins and Del Prado 2007). It has frequently been shown that drought reduces the yield of crops and forages (Ehlers and Goss 2002; Jaleel et al.2009) as well as of temperate grassland (Wrage et al.2009). However the effect of drought on the nutritive value of herbage is much less clear. Wang and Frei (2011) reported an increase in crude protein (CP) concentration under drought stress in a wide range of cash and forage crops e.g. Arachis hypogea Solanum tuberosum Triticum aestivum and Zea mays. In contrast Peterson et al. (1992) found increased CP concentration for Lotus corniculatus and Trifolium pratense as well as decreased for Astragaluscicer. In opposite Seguin et al. (2002) stated just a minor effect of drought on CP of Medicago sativa Trifolium ambiguum and Trifolium pratense. Water-soluble carbohydrates (WSC) have been found to increase in graminoids (Bajji et al. 2001; DaCosta and Huang 2006) and Glycine max (Nakayama et al. 2007) under drought conditions but showed no reaction to drought in forage legumes (Abberton et al. 2002). The reaction of fibre components like neutral detergent fibre (NDF) and acid detergent fibre (ADF) to drought stress is not consistent. Increasing and decreasing concentrations or no reaction of NDF and ADF to drought have been reported for forage legumes forbs and grasses (Peterson et al. 1992; Seguin et al. 2002; Skinner et al. 2004).Species richness and functional group composition may modify reactions of swards to drought and affect the nutritive value. However it is not yet clear whether species richness may enhance (Bullock et al. 2007) or decrease nutritive value of grassland herbage (Bruinenberg et al.2002). Particularly the ratio of grasses forbs and legumes in swards is known to have a marked effect on the nutritive value (Hopkins and Wilkins 2006). Previous drought stress incidents might affect plant physiology even during and after a recovery time - more tolerant plants would resume their functioning while others have undergone severe changes. Mirzaei et al. (2008) reported a shift from reproductive to vegetative growth after a period of drought within a growing season. Van Ruijven and Berendse (2010) and Vogel et al. (2012) found inconsistent effects of species richness in a recovery period after drought. So far it remains unclear whether the nutritive value of a sward during a recovery period after drought would also be modified by species richness.Here we report the results of an experiment conducted under semi-controlled conditions in a vegetation hall with two successive drought stress treatments and periods (moderate and strong) each followed by a recovery period. Species richness varied between one three and five species and we choose three functional groups (grasses forbs legumes).As important parameters for nutritive value of grassland herbages CP WSC and the fibre components NDF and ADF were analysed. CP is essential for nitrogen supply for ruminants; WSC positively influence fodder intake and are important for efficient utilisation of protein; NDF is an estimation of total cell wall (cellulose hemicellulose and lignin) and is inversely related to the voluntary fodder intake; ADF includes lignin and cellulose and is an indicator for the digestibility of the cell wall (Hopkins and Wilkins 2006; Moorby et al. 2006). We hypothesize that (i) drought stress of different intensity will have an effect on nutritive value parameters of grassland herbage during the drought period but also during recovery after drought and that (ii) species richness and functional group will modify the drought response of the nutritive value.

Materials and Methods

Experimental Setup

The experiment was conducted in a vegetation hall at the University of GAlttingen Germany in mid-July 2009 as a randomized block design with four replicates and two factors (sward and drought stress). Five species were selected for the experiments which are common in a wide range of temperate grassland and they have a high nutritive value and mowing tolerance (Dierschke and Briemle 2002). The species are: Trifolium repens L. var. Rivendel (legume) Dactylis glomerata L. var. Donata (grass) Lolium perenne L. var. Signum (grass) Plantago lanceolata L. wild type (forb) and Taraxacum officinale F.H. Wigg. specie agg. wild type (forb). Those were either grown in monoculture in all possible combinations of three-species mixtures and in one mixture that contained all five species.

Experimental Details

In monocultures 1000 viable seeds per m2 for forbs and legume swards and 5000 viable seeds per m2 for grass swards were sown. For the three and five-species mixtures sowing density per species was reduced to one third and one fifths of that of the monoculture swards respectively (replacement design).A homogeneous mixture of 20 kg sand (air-dried sieved to pass a mesh of 5 mm; August Oppermann Kiesgewinnung GmbH Hann. MA1/4nden Germany) 5.5 kg compost (air-dried; Bioenergiezentrum GAlttingen GmbH GAlttingen Germany) and 0.9 kg vermiculite (particle size 8-12 mm; Deutsche Vermiculite GmbH Sprockhoevel Germany) was used as growing substrate per container (round plastic container of 33 cm diameter 42 cm height and a volume of 30 L) and covered with 1.5 kg compost as seed bed. All containers were treated with a rhizobium solution (Radicin Jost-GmbH Iserlohn Germany) to enable nodulation of T. repens roots. No fertilisation and no extra lighting were provided. The pH of the soil (in CaCl2 suspension) as well as the availability of P K (extracted with calcium acetate lactate continuous flow analyser [CFA]) and Mg (CaCl2 extraction CFA) were measured insummer 2011 (pH 7.3; 292 mg P kg-1; 430 mg K kg-1; 364mg kg-1 oven-dry soil).The climatic conditions in the vegetation hall followed a normal seasonal pattern of temperate climates with (mild) frost in winter lower temperatures in spring and autumn and higher temperatures in summer. The conditions were the same for all species and mixtures. Peak temperatures occurred in June and July with maximal temperatures over30C. Temperatures in summer were controlled by ventilation. In winter a heating system was operating when temperatures fell below 0C for more than 24 h. Heating was stopped when the temperature reached 5C. Temperatures were recorded daily at three locations in the vegetation hall.In an earlier paper (KA1/4chenmeister et al. 2012) the germination of the species used in this experiment the establishment of the swards the yields and yield contribution of the functional groups have been studied.

Drought Stress Treatment

In the first full harvest year (2010) swards were subjected to moderate drought stress in spring (mid-April to end of May) and to strong drought stress in summer (early-July to end of August). Water availability was controlled by watering and regular weighing of the containers. Control containers were kept at a water content of 25 Vol. % (-0.03 MPa) and watered once their water content went down to 18Vol. % (-0.3 MPa).Drought stress was induced by stopping watering of the containers for some time after an initial watering of the containers to a target value of volumetric soil water content of 25 Vol.%. For moderate drought stress no water was given until three days after the first stress symptoms (wilting of leafs) appeared on the first plant (-1.5 MPa 10 Vol. %) containers were then watered again (to -0.03 MPa) followed by repetition of the drought phase. To induce strong drought stress the drought phase was extended to five days after appearance of the first stress symptoms (-1.5 MPa 10 Vol.%) and was repeated three times with two irrigations in between. Average Vol. % water content of the containers after the end of the moderate drought period was between11% and 6% and between 10% and 4% after strong drought stress.

Sampling and Measurement

Above ground biomass was harvested two times in 2009 and five times in 2010 (mid-April end-May early-July end-August and mid-October). Shoots were hand-clipped 3-4 cm above the soil surface. Each biomass sample was sorted into species or functional groups (grass species were not separated) dried (60C for 72 h) and weighed. Chemical analyses were done on bulk samples as biomass of some species was found to be too little for analysis.Prior to analysis dried samples were ground to 1 mm and analysed by near-infrared reflectance spectroscopy (NIRS). The spectra were analyzed using the large dataset of calibration samples from different kinds of grasslands by VDLUFA QualitAtssicherung NIRS GmbH Kassel Germany (Tillmann 2010). N concentration of the sampleswas calculated by dividing CP concentration by 6.25. Nyield was calculated by multiplying yield and N concentration. We used coefficients of variation (CV) for every sward in control as well as in the drought treatments to assess the variability of nutritive value over the growing season. CV of nutritive value was calculated by dividing standard derivation of the four periods by their mean.

Statistical Analysis

Statistical data analysis was carried out using Genstat 6.1 software package (VSN International Hemel Hempstead UK) and STATISTICA 9.1 (StatSoft Inc. Tulsa Oklahoma USA). A two-factorial analysis of variance (ANOVA) was calculated for every period and considered the factors sward and drought stress. Least significant differences (LSD values) were used to compare mean values in case of significant treatment effects (P less than 0.05). Additionally we evaluated the relationship between nutritive value parameters and species richness as well as the contribution of functional groups by a linear regression model. The full data set was used for regression calculation except for CP concentration of forbs: here we excluded mixtures with legume the strong effect of legume would have obscured the influence of forbs on CP in mixture with grasses.

Results

Influence of Drought Stress on Nutritive Value

The variations in sward composition from monocultures to three- and five-species mixtures had a highly significanteffect (Pless than 0.001) on all parameters of the nutritive value after both stress and after recovery periods. Moderate or strong drought stress had no significant effect on the nutritive value after stress or after recovery periods apart from ADF in spring 2010. Independent of stress and sward contents for CP ranged between 88 g kg-1 DM and 273 g kg-1 DM (Table 1) and for WSC between 8 g kg-1 DM and 227 g kg-1 DM (Table 2). The fibre components NDF and ADF ranged between 222 g kg-1 DM and 640 g kg-1 DM (Table3) and 175 g kg-1 DM and 355 g kg-1 DM (Table 4)respectively. There were no significant interactions between sward and drought stress for CP WSC and ADF but for NDF. The variability in time for parameters of nutritive value during the growing season as indicated by the coefficient of variation (CV) was significantly different among swards (Pless than 0.001) and ranged between 0.07 and 0.82. Drought stress or the interaction of sward and drought stress showed no significant effect (Table 5).

Influence of Species Richness and Functional GroupComposition

Species richness: Nutritive value did not change with species number: values for highest diversity level (five- species mixture including T. repens) did usually not differ from three-species mixtures that contained T. repens. However species and thus functional groups differed significantly in their nutritive value; functional group composition determined the nutritive value of mixed swards. Apart from the influence of sward composition we found the common seasonal variability in nutritive value. CP concentration (Table 1) and fiber components (Table 3 and 4) increased in summer and WSC (Table 2) was high in spring and autumn.Functional group composition: CP concentrations were especially high in swards that contained T. repens and they varied between 104 g kg-1 DM and 273 g kg-1 DM (Table1). Also forb monocultures and mixtures of forbs and grasses produced high CP concentration up to 199 g kg-1DM while grass monocultures had lower CP concentrations between 90 g kg-1 DM and 147 g kg-1 DM. In contrast to CP grass monocultures and swards with a larger proportion of grasses had higher WSC values of up to 227 g kg-1 DM. Monocultures of dicotyledonous plants and mixed swards with significant contents of dicots were usually low in WSC. For monocultures of dicotyledonous plants WSC concentration varied between 8 g kg-1 DM and 128 g kg-1DM (Table 2). Similarly grass dominated swards were higher in NDF and ADF while swards with larger contributions of forbs and legumes had lower concentrations of theses fibre components. Grass monocultures showed NDF and ADF concentrations between 481 g kg-1 DM and640 g kg-1 DM and 255 g kg-1 DM and 355 g kg-1 DM (Table 3 and 4).

Table 1: Crude protein concentration (g kg-1 DM) of different swards (monocultures and mixtures) with two drought stress

treatments each followed by a recovery period from April to October 2010. Means (n=4) with LSD (5%). Results from an

ANOVA considering the effects sward and drought stress (Control = not limiting water supply)

Sward1###Moderate stress###Recovery period###Strong stress###Recovery period

###Control###Stress###Control###Stress###Control###Stress###Control###Stress

Dg###102###101###103###101###96###101###147###138

Lp###92###90###103###111###113###115###112###105

Pl###100###97###97###88###113###111###166###171

To###168###174###164###168###150###161###199###198

Tr###272###264###223###223###240###224###269###273

LpPlDg###93###94###108###100###103###111###112###112

LpToDg###100###104###111###113###109###113###125###122

PlToDg###118###111###121###103###130###117###176###160

LpPlTo###98###93###116###114###119###120###123###119

TrLpDg###110###104###176###193###145###133###166###155

TrLpPl###140###140###213###211###181###157###180###189

TrLpTo###146###161###208###224###165###171###196###196

TrPlDg###152###133###198###188###130###128###191###190

TrToDg###138###160###195###210###166###148###203###221

TrPlTo###206###198###217###210###173###172###226###225

TrPlToDgLp###150###141###208###198###167###144###176###198

LSD value###19.5###20.5###21.7###24.5

ANOVA Summary###F-ratio###P###F-ratio###P###F-ratio###P###F-ratio###P

Sward###94.14###less than 0.001###96.74###less than 0.001###39.73###less than 0.001###52.42###less than 0.001

Drought stress###0.27###0.607###0.03###0.855###2.91###0.092###0.02###0.899

Sward x Drought stress###1.01###0.448###0.97###0.49###1.08###0.389###0.66###0.816

Table 2: Water-soluble carbohydrates concentration (g kg-1 DM) of different swards (monocultures and mixtures) with

two drought stress treatments each followed by a recovery period from April to October 2010. Means (n=4) with LSD

(5%). Results from an ANOVA considering the effects sward and drought stress (Control = not limiting water supply)

Sward1###Moderate stress###Recovery period###Strong stress###Recovery period

###Control###Stress###Control###Stress###Control###Stress###Control###Stress

Dg###95###105###73###78###79###84###124###116

Lp###218###216###170###147###109###123###214###227

Pl###128###128###88###97###55###62###99###99

To###26###26###18###8###9###15###35###27

Tr###64###74###48###55###71###72###87###88

LpPlDg###195###205###123###152###103###91###198###193

LpToDg###166###180###127###101###90###87###173###174

PlToDg###81###86###46###65###28###42###61###87

LpPlTo###195###206###120###140###77###91###177###195

TrLpDg###191###186###108###80###82###90###146###166

TrLpPl###167###172###68###61###78###86###140###135

TrLpTo###153###142###71###65###64###68###111###122

TrPlDg###103###101###67###63###74###56###99###95

TrToDg###86###80###60###54###53###51###82###75

TrPlTo###56###70###46###54###27###27###75###49

TrPlToDgLp###130###146###61###65###72###75###125###120

LSD value###24.5###28.1###23.7###30.2

ANOVA Summary###F-ratio###P###F-ratio###P###F-ratio###P###F-ratio###P

Sward###87.15###less than 0.001###28.62###less than 0.001###19.65###less than 0.001###48.13###less than 0.001

Drought stress###2.05###0.156###0.01###0.931###0.91###0.342###0.13###0.715

Sward x Drought stress###0.45###0.96###1.34###0.195###0.55###0.907###0.74###0.735

Discussion

The results obtained in the present experiment revealed a considerable variation of data for the different characteristics of the nutritive value mainly related to thedifferent grassland species and the functional groups. Such range of data has also been found in various other studies both under field and controlled environment conditions: Buxton (1996) Harris et al. (1997) and Seip et al. (2011) reported similar values for CP in temperate grasslands

Table 3: Neutral detergent fibre concentration (g kg-1 DM) of different swards (monocultures and mixtures) with two

drought stress treatments each followed by a recovery period from April to October 2010. Means (n=4) with LSD (5%).

Results from an ANOVA considering the effects sward and drought stress (Control = not limiting water supply)

Sward1###Moderate stress###Recovery period###Strong stress###Recovery period

###Control###Stress###Control###Stress###Control###Stress###Control###Stress

Dg###611###606###640###635###610###598###524###527

Lp###520###527###551###556###574###590###490###481

Pl###273###257###335###335###327###300###244###222

To###297###282###334###341###310###317###283###302

Tr###340###324###401###394###366###366###334###322

LpPlDg###535###517###577###545###587###574###497###493

LpToDg###529###524###549###573###557###564###473###488

PlToDg###521###569###518###583###499###552###398###462

LpPlTo###505###502###507###514###528###528###453###466

TrLpDg###510###531###475###484###535###547###452###455

TrLpPl###484###468###446###444###476###492###418###408

TrLpTo###485###459###432###421###476###458###391###382

TrPlDg###510###555###467###486###541###577###397###463

TrToDg###536###482###475###439###479###480###411###391

TrPlTo###307###290###375###363###325###314###335###298

TrPlToDgLp###491###487###453###457###483###515###419###407

LSD value###35.8###32.1###39.9###39.6

ANOVA Summary###F-ratio###P###F-ratio###P###F-ratio###P###F-ratio###P

Sward###136.56###less than 0.001###116.61###less than 0.001###99.81###less than 0.001###65.45###less than 0.001

Drought stress###1.05###0.308###0.25###0.617###1.47###0.229###0.4###0.529

Sward x Drought stress###1.99###0.024###2.04###0.02###1.12###0.354###1.97###0.026

Table 4: Acid detergent fibre concentration (g kg-1 DM) of different swards (monocultures and mixtures) with two

drought stress treatments each followed by a recovery period from April to October 2010. Means (n=4) with LSD (5%).

Results from an ANOVA considering the effects sward and drought stress (Control = not limiting water supply)

Sward1###Moderate stress###Recovery period###Strong stress###Recovery period

###Control###Stress###Control###Stress###Control###Stress###Control###Stress

Dg###350###343###355###354###345###334###274###280

Lp###287###286###302###310###328###331###261###255

Pl###250###246###290###291###287###278###187###175

To###248###241###259###263###273###271###227###235

Tr###257###252###308###302###274###292###247###238

LpPlDg###300###288###325###308###336###334###268###266

LpToDg###302###292###315###329###332###328###264###268

PlToDg###319###339###323###349###324###337###254###264

LpPlTo###289###281###301###296###328###315###259###256

TrLpDg###292###297###297###304###325###328###264###264

TrLpPl###287###278###299###306###311###315###263###259

TrLpTo###288###278###296###291###313###308###261###257

TrPlDg###310###329###305###315###332###339###249###273

TrToDg###324###300###308###295###316###307###269###257

TrPlTo###248###234###293###286###290###276###246###234

TrPlToDgLp###298###289###302###306###311###320###264###256

LSD value###18.5###18.9###19.7###20

ANOVA Summary###F-ratio###P###F-ratio###P###F-ratio###P###F-ratio###P

Sward###42.41###less than 0.001###19.04###less than 0.001###19.58###less than 0.001###19.65###less than 0.001

Drought stress###4.59###0.035###0.50###0.481###0.08###0.779###0.22###0.639

Sward x Drought stress###1.47###0.134###1.28###0.229###0.92###0.549###0.94###0.521

Nakayama et al. (2007) DaCosta and Huang (2006) and Abberton et al. (2002) showed comparable values for WSC for leguminous plants and temperate grasses; our fiber components were in the range of those described by Buxton (1996) Harris et al. (1997) and Seip et al. (2011) for temperate grasslands. We therefore assume that our data are relevant also for field conditions.In the study presented here a significant effect of the sward was found but no or only an inconsistent effect of drought stress on the nutritive value of herbage harvested immediately after the stress period or after a recovery period. In almost all periods no interaction of sward x drought stress was found. However yield reduction in the study was on average 12% under moderate stress

Table 5: Coefficient of variation of crude protein (CP) water-soluble carbohydrates (WSC) neutral detergent fibre (NDF)

and acid detergent fibre (ADF) in different swards (monocultures and mixtures) with two drought stress treatments each

followed by a recovery period from April to October 2010. Means (n=4) with LSD (5%). Results from an ANOVA

considering the effects sward and drought stress (Control = not limiting water supply)

###CP###WSC###NDF###ADF

Sward1###Control###Stress###Control###Stress###Control###Stress###Control###Stress

Dg###0.21###0.17###0.27###0.26###0.09###0.08###0.12###0.10

Lp###0.10###0.11###0.31###0.31###0.08###0.09###0.10###0.11

Pl###0.29###0.33###0.38###0.32###0.15###0.18###0.19###0.21

To###0.12###0.10###0.71###0.82###0.10###0.12###0.09###0.09

Tr###0.10###0.11###0.28###0.26###0.09###0.11###0.11###0.12

LpPlDg###0.08###0.09###0.32###0.35###0.08###0.07###0.10###0.10

LpToDg###0.11###0.07###0.31###0.38###0.08###0.07###0.10###0.10

PlToDg###0.20###0.22###0.52###0.36###0.12###0.12###0.11###0.13

LpPlTo###0.11###0.12###0.40###0.36###0.07###0.06###0.10###0.09

TrLpDg###0.19###0.28###0.40###0.45###0.08###0.09###0.09###0.09

TrLpPl###0.19###0.21###0.43###0.46###0.08###0.09###0.08###0.09

TrLpTo###0.17###0.17###0.45###0.40###0.10###0.09###0.08###0.08

TrPlDg###0.21###0.21###0.25###0.33###0.14###0.11###0.12###0.10

TrToDg###0.20###0.21###0.28###0.26###0.11###0.10###0.08###0.08

TrPlTo###0.14###0.12###0.47###0.42###0.11###0.11###0.10###0.11

TrPlToDgLp###0.17###0.20###0.39###0.39###0.08###0.10###0.08###0.10

LSD value###0.060###0.161###0.037###0.031

ANOVA Summary###F-ratio###P###F-ratio###P###F-ratio###P###F-ratio###P

Sward###17.05###less than 0.001###9.04###less than 0.001###6.82###less than 0.001###13.32###less than 0.001

Drought stress###1.15###0.286###0.03###0.863###0.04###0.837###0.52###0.471

Sward x Drought stress###1.02###0.442###0.65###0.822###0.80###0.679###0.56###0.895

(max. 36%) 22% under strong stress (max. 40%; data not shown). Drought stress had no obvious effect on yields after a recovery period but there was a tendency for smaller yields in stressed swards even after a longer recovery time. This negative effect of drought stress on biomass production is well known (Farooq et al. 2009).Drought stress has been found to increase protein concentration in forage plants or to have no consistent effect (Peterson et al. 1992; Wang and Frei 2011). This might be explained by a delayed maturity or a change in the leaf-stem ratio (Peterson et al. 1992; Buxton 1996). Nakayama et al.(2007) reported declining N concentrations under droughtdue to an impaired N uptake. Seguin et al. (2002) reported no influence of drought on the CP concentration. Although we found a reduced N uptake and so a decreased N yield under drought stress this most likely had no direct effect on CP concentrations as the smaller N uptake can be explained by a reduction in yield.Although we found no significant effect of drought stress on WSC there was a small tendency to increased WSC concentrations; however this tendency might have been obscured by the strong mixture effects. Those effects are due to the varying amount of sward components with either a low or a high WSC concentration when drought is imposed. Also Abberton et al. (2002) explained the absence of drought effects on WSC with the strong impact of plant mixtures. On the other hand significant increases in WSC under drought stress due to osmotic adjustments of plants have often been reported in the literature (Bajji et al. 2001;DaCosta and Huang 2006; Nakayama et al. 2007).The reaction of NDF and ADF to drought stress was inconsistent in our study with no clear trend. Increased decreased or unchanged values were found after a stress period. The botanical composition of a sward has greater effect than drought (Skinner et al. 2004). For forage legumes Seguin et al. (2002) observed small effects of drought on NDF but a higher ADF concentration after drought stress. In contrast Peterson et al. (1992) found a reduction in NDF and ADF values of forage legumes. This might be attributed to an increased leaf to stem ratio and a reduced plant maturity at harvest when drought was imposed (Peterson et al. (1992). In our experiment there was no visible effect of drought on the plant development so that the variation in the fibre concentration of the mixed sowings is more likely to be related to a variation of the botanical composition.There was no interaction of drought stress effects andspecies richness for parameters of nutritive value directly after drought stress or after a period of recovery (Table 14). Also species richness independent of drought had no obvious effect on nutritive value: we found positive negative or no reaction to increasing species number. A positive influence of species richness on the nutritive value e.g. higher CP might be partially explained by an increased probability of T. repens being part of the mixture when the species number increases; the so-called sampling effect (Huston et al. 2000). Bullock et al. (2007) found increased nutritive values in more species-rich swards as well. This was explained by an improved resource use of stands with an increasing number of species and thus more nitrogen being acquired by the sward. In contrast White et al. (2004) found a decrease in nutritive value with increasing species number and explained this with a dilution effect - more plants with lower nutritive value in the mixture. A lower nutritive value with higher species richness was also reported by Bruinenberg et al. (2002) who found a higher variation in plant maturity in species rich swards.No interaction was found between functional groups and drought stress. However nutritive value of swards was significantly affected by functional group composition. The nutritive values of the functional groups in our study are in line with values reported in the literature (Ulyatt et al.1988; Buxton 1996; Marshall et al. 2004; DaCosta andHuang 2006; Harrington et al. 2006; Dragomir et al. 2011; Seip et al. 2011; Lukac et al. 2012). Larger proportions of legume and forbs led to increased CP in all harvests (R2 up to 0.86 Pless than 0.05) while the contribution of grass to the mixture was negative correlated to overall CP concentration (R2 up to 0.63 Pless than 0.001). WSC concentrations in our study depended mainly on the yield proportion of the functional group grass (R2 up to 0.86). The yield proportions of forbs and legume were negatively correlated to WSC concentrations (R2 up to 0.45 Pless than 0.05). NDF and ADF concentrations increased with increasing proportions of grass (R2 up to 0.96 Pless than 0.001). With an increasing contribution of forbs in the mixture fibre concentrations decreased (R2 up to 0.65 Pless than 0.001). The legume T. repens usually had no influence on ADF and NDF in some cases its presence led to slightly lower fibre concentrations. Sanderson (2010) reported that sward composition could be more important for yield and stability than the species number alone. Our results suggest that functional composition of swards is also more important for nutritive value than species number.We found no accumulated effect of drought over the growing season. Variability of the nutritive values measured as CV was not greater in drought stress exposed swards than in the control. Differences between CV of swards with drought stress and control were not more than0.16 while CV over the growing season was up to 0.82 (Table 5). This means that seasonal effects on nutritive values were greater than stress caused by drought. Seasonal growth patterns with a fluctuation in yield of different harvests and changes in CP concentration with varying maturity of grassland plants are well known (Ulyatt et al.1988; Suleiman et al. 1999; Skinner et al. 2004; KA1/4chenmeister et al. 2012). Differences in WSC concentrations depending on harvest date were also reported by Conaghan et al. (2011). With increasing maturity and under conditions of higher temperatures as occurred in our experiment in summer fibre components will increase (Buxton 1996; Suleiman et al. 1999; Bruinenberg et al.2002).In conclusion drought stress may affect herbage nutritive value from grassland but the effect was shown to be quite small or inconsistent in our study. It seems that under conditions of predicted climate change temperate grassland will be more affected by a decrease in yield than by changes in the nutritive value. Furthermore the common seasonal variation of the nutritive value is considerably higher than influence of drought. The response of swards to drought in our study was not modified by species richness and functional group composition. However functional group composition i.e. the percentage of functional groups in the sward had a strong direct effect on CP WSC NDF and ADF. Grass increased WSC and fibre components while it decreased CP. In contrast legume and forbs increased CP and more or less decreased fibre components. According to our results it is concluded that for managed temperate grasslands a balanced sward composition and the time of harvest are largely determining the nutritive value of biomass; this holds true also under conditions of predicted future climate change.

Acknowledgements

The authors thank the Ministry for Science and Culture of Lower Saxony Hannover Germany for the financial funding. Furthermore we thank Dr. Peter Tillmann for support with NIRS.

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Author:Kuchenmeister, Frank; Kuchenmeister, Kai; Kayser, Manfred; Wrage-Monnig, Nicole; Isselstein, Johanne
Publication:International Journal of Agriculture and Biology
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Date:Aug 31, 2014
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