The chemical composition and energy value of high temperature dried grass produced in England

AnbnalFeedScienceand

Technology,

36 (1992)

215-228

215

Elsevier Science Publishers B.V.. Amsterdam

The chemical composition and energy value of high temperature dried grass prodwed in England

D.1. Give&,

Angela

R. Moss” and A.H. AdamsorP

‘ADAS Feed Evaluation Unit, Alcester Road. Slrat/brd on Avon, CV37 9RQ. UK bADAS Nulrition Chemistry Deparfmenl. Burghill Road. Brim4 BSIO 6NJ. UK

(Received 27 November 1990: accepted ;O July 1991)

ABSTRACf Givenr, D.I., Moss, A.R. and Adamson, A.H., 1992. The chemical composition and energy value of high temperature dried grass produced in England. Anim. FeedSci. 1‘cchnol, 36: 215-228. The chemical composition, digestibility in vitro and in vwo and energy value in viva of 22 high temperature dried, milled and pelleted grasses are reported. Prediction relationships between digestibility and metabolisable energy (ME) content and various laboratory measurements were also examined. The dried grasses were produced on live commerctal drymg plants throughout England over 4 years. They comprised I3 samples of early season and nine samples of later season material with 18 samples being bas;d on perennial ryegrass and four on tall fescue: For the oerennial rvearass _ . aicell . _ material there was a sinnificant (PcO.05) increase in the maioritv wall fractions in the later compared with the early season samples although total cell wall content (measured as neutral detergent tibre) did not change significantly. Digestibility measured in vitro and in viva was high but declined significantly (PcO.01) between early and later season. Mean otmanic matter dieestibilitv coeficients in viva for earlv and later season were 0.78 and 0.73. Usinn predicted methane ene& losses the mean ME con& were I I.5 MJ kg-’ dry matter (DM) and 10.3 MI kg-’ DM, respectively. Higher levels of total ash in the later season material significantly contributed to the lower ME contents. The mean measured methane energy loss on a subset of nine samples was 0.061 of gross energy (GE) compared with 0.080 oiGE pred:cted using a published relationship. Four samples of dried grasses based on tall fescue had higher contents of cell wall fractions and lower digestibility and energy values than those based on perennial ryegrass, although small numbers did not allow a statistical assessment oithe differences. The mean ME contents of early and later tall fescue material were 9.2 MJ kg-l DM and 9.4 MJ kg- ‘DM using predicted methane e&gy losses. The best laboratorypredictoroidigestibleorganic matter content oithedry matter (DDMD) and ME was the enzymatic procedure based on neutral detergcnt-cellulase (NCD). The regression relationships were: DOMD (gkg-’ DM)=59.9+0.828NCD ME (MJ kg-‘DM)=-0.59+0.0154

(gkg-‘DM),

R’=90.4%,RSD=l5.2

NCD @kg-’ DM). Rz=80.6%, RsD~0.43

INTRODUCTION High temperature dried grass (HTDG) represents a considerable feedstuff resource within the European Economic Community (EEC) with some 5.8x 105t beingproducedin 1988 (Odot, 1989). TheUnited Kingdom (UK) has the fourth largest production in the EEC producing some 5.7 x IO“ t in 1988 (Odot, 1989). The process of drying fresh grass at high temperature and producing a milled and cubed product brings about several changes which fundamentally affect the nutritive value of the material. These changes have been reviewed by Osbourn et al. (i976). In particular, there is evidence that compared with long material, ground and pelleted HTDG has a much reduced residence time in the rumen and a faster rate of passage through the digestive tract (Blaxter and Graham, 1956). This results in a reduction in whole tract apparent digestibility and despite a reduction in methane energy losses (Blaxter and Graham, 1956) metabolisable energy (ME) content is also reduced, particularly when the HTDG is fed ad libitum (Wainman et al., 1970). The HTDG now available in the UK is produced by a relatively small number of specialist companies who grow and harvest the grass according to strict criteria, taking care to ensure that the cutting interval between grass harvests does not exceed 35 days (see Wilkins, 1985). There is, however, little information on the digestibility and energy value of contemporary HTDG . It may be noted that in some popular literature, high temperature dried legumes (mainly Me&ago saliva) are sometimes incorrectly described as dried grasses. This paper is concerned solely with gramineous species. The purpose of this study was to provide such information and to examine laboratory methods which may be used to predict energy value. Some preliminary tindings have been presented earlier (Givens, 1989). MATERIALS AND METHODS

Dried grasses A total of 22 short-cycle (i.e. a cutting interval ofless than 36 days) HTDG samples were examined. These comprised 13 samples representative of early season production (May to mid-July) and nine samples of later season (midJuly onwards) material. The dried grass was produced over 4 years ( 19851988 ) and by five different commercial drying plants (D l-D5 ). Details of the number of samples by year, season and drying plant of origin are shown in Table 1. All dried grasses were produced from swards of perennial ryegrass (Lolium perenne) varieties except for the four samples from D5 which were produced from tall fescue (Festuca arundinacea). In the majority of cases, thegrass was

ENERGY VALUE OF “lo”

TABLE

TEMPERATURE

DRlED

217

GRASS

I

Number of samples of high temperature plant of origin Year of harvest

1985 1986 1987 I988

dried grasses studied by year, season of harvest and drying

Drying plants involved

Number of samples Early season

Late season

2 L 5 5

0 0 5 4

DI, D2 DI DI , D3. D4, D5 Dl, D2, D3, D4, DS

Di,

subjected to a field wilt, or mechanical squeezing, before being dried (inlet temperatures 800-lOOO”C, outlet temperatures 90-120°C) followed by hammer milling (screen sizes 3-5 mm) and pressing into pellets of 7 mm diameter. Animai studies Each sample of dried grass pellets was fed as the sole diet to four wether sheep (70-80 kg liveweigbt) at a rate (approximately 900 g fresh weight) estimated to approximate to the maintenance plane of nutrition. A mineral, trace element and vitamin supplement was also provided. The diets were fed for an acclimatisation period of 10 days followed by a balance period of 10 days. During the second 10 day period, total collections of faeces and urine were made. Methane energy loss was calculated for all dried grasses according to the relationship of Blaxter and Clapperton ( 1965 ) and in addition for nine of the dried grasses, methane production was measured by placing the animals in open-circuit respiration chambers for two periods of 24 h immediately after the faeces and urine collection periods. Laboratory studies Dry matter (DM) content was determined by drying in a forced draught oven at 100°C for 18 h. Extensive laboratory analyses were undertaken on the dried grass samples. These included proximate fractions, neutral detergent fibre (NDF), acid detergent tibre (ADF) and modified acid detergent fibrc (MADF) , cellulose, potassium permanganate-lignin and water-soluble carbohydrates (WSC). Neutral detergent tibre and ADF were measured on an ash-free basis and hemicellulose was calculated as NDF-ADF. The major minerals calcium, phosphorus, magnesium, sodium and potassium were also determined. The above analytical methods used were as cited by Givens et al. (1989). Digestibility was also estimated by two in vitro procedures, the ru-

218

DI.GI”ENS ETAL.

men fluid-pepsin (RFP) method of Alexander and McGowan ( 1966) and the neutral detergent-cellulase (NCD) method of Dowman and Collins ( 1982). Rumen fluid was obtained from donor sheep fed a diet based on grass hay. Cellulase from Trichoderma virile was used (BDH Ltd., Poole, UK). I.arge batches of enzyme were purchased to ensure that activity was consistent over periods of time. With both in vitro methods the results were expressed as digestible erganic matter in the DM (DOMD). Acid detergent insoluble nitrogen (ADIN) was determined on undried material by-the method of Goering and Van Soest (1970) and was expressed both as grams per kilogram DM and grams per kilogram total nitrogen (TN). Gross energy (GE) was determined by adiabatic bomb calorimetry on the dried grass and also on faeces and urine previously freeze dried onto a polyethylene film.

The effects of year of harvest and season of harvest on both laboratory and in vivo measurements were tested using a general linear model for unbalanced designs (Minitab Inc., 1989) although there were insufftcient data to estimate the yearxseason interaction term. Relationships between in vivo and laboratory measurements were carried out essentially by linear regression analysis using a single prediction variable. For the most important predictor variables the effect of adding a second variable was tested as was the use of quadratic relationships. RESULTS

The results of the chemical analyses are shown in Table 2. No effect of year of harvest was observed and the results in Table 2 concentrate on the comparison between early and later season of harvesting for the perennial ryegrass samples. In the later season perennial ryegrass samples, there were significant increases compared with the early season harvested material in the concentration of all cell wall fractions except lignin, hemicellulose and ADIN, although there was no significant difference in total cell wall contents (NDF). In addition, contents of DM, total ash, acid insoluble ash, magnesium and sodium were significantly increased in the later harvested samples whilst WSC and GE contents of the DM were significantly reduced. The reduced GE in the DM in the later cut material appeared to be due almost entirely to dilution from the higher total ash content. There was no significant difference between early and later harvesting when GE was expressed on an organic matter basis. Whilst there were insufftcient samples of tall fescue dried grass to allow a

ENERGY VALUE OF HlGH TEMPERATURE

TABLE

219

DRIED GRASS

2

The mean chemical composirion of rhe early (ES) and later season (LS) perennial ryegrassand tall fescue high temperature dried grasses studied (g kg-’ DM or as stated, values in parentheses are standard deviations) Comp0lleot

Perennial

Dry matter (g kg-’ Crude protein Crude fibre Total ash Acid insoluble ash Ether extract NDF’ ADFz

fresh%?.)

MADF’ ADIN“ ADIN &k&TN) Cellulose Hemicellulose Lignin wscs GE&(Mlkg-‘DM) GE (MJ kg-’ Calcium Phosphorus Magnesium Sodium Potassium

OM)

ryegrass

Tall fescue

ES (kll)

LS (n=7)

SED (d.f.=16)

Es (n=2)

Ls (n=Z)

907 (I 1.0) 198 (27.8) 216 (21.1) 93 (14.4) 14.7 (6.14) 40.6(0.56) 543 (33.1) 267 (19.3) 252 (24.8) 2.4 (0.73) 80 (22.4) 216(17.1) 277 (22.1) 58 (14.8) 174 (42.0) 18.9 (0.28) 20.8 (0.30) 6.7 (1.02) 3.7 (0.51) 1.5 (0.26) 2.7 (0.79) 27.0 (5.23)

927 (24.0) 201 (20.4) 238 (13.2) III (10.7) 26.1 (8.4) 36.4(11.5) 564 (43.8 291 (22.8)

8.3’ 12.2 8.9’ 6.3* 3.42* 4.0 18.1 10.0’ 10.9” 0.32 9.5 9.0* 13.0 7.1 17.1’ 0.17**

929 181 235 142 61.5 34 557 299 334

933

)

288 (18.7) 2.3 (0.49) 71 (12.9) 238 (21.2) 272 (33.2) 53.7 (14.31 129 (20.2) 18.3 (0.45) LU.b (U.41,

6.7 3.9 2.1 3.7 28.4

(1.19) (0.28) (0.44) (1.04) (4.24)

I

“.I 0.53 0.21 0.16’. 0.43’ 2.36

3.0 103 234 258 73 Ill 17.9 26.8 7.5 3.9 2.4 3.2 23. I

198 251

120 38.8 36 569 292 309 3.0 95 231 277 65 108 17.9 20.3 7.8 3.3 2,6 3.1 26.6

‘Ashed neutral detergent fibre. ‘Acid detergent fibre. ‘Modified acid deterge.lt libre. ‘Acid detergent insol able nitrogen. ~Water-rolubleca~ohydrares. bGross energy. “P
statistical comparison with the perennial ryegrass material, the results in Table 2 consistently suggest the tall fescue to have been richer in the less digestible cell wall fractions, notably crude fibre, ADF, MADF and cellulose. The results also suggest the tall fescue material to have higher total and acid insoluble ash contents and lower concentrations of WSC. Table 3 presents the results of the DOMD measurements made by the two in vitro procedures. No effect of year of harvest was detected. The DOMD contents in vitro were substantially and significantly (PcO.001) reduced in the later harvested perennial ryegrass samples compared with those harvested

D.I.GlVENSETAL.

220 TABLE 3

Digestible organic matter wmtent of theearly (ES) and later season (LS) perennial ryegrass and tall fescue high temperature dried grasses studied measured in vitro (g kg-’ DM, values in parentheses are standard deviations) In vitro method

Rumen fluid-pepsin Neutral detergent-cellulase

Perennial ryegrass ES LS (n=Il) (n=7) 675 (22.4) 778 (27.3)

626 (28.1) 719 (26.1)

Tall fescue SED (d.f.=16)

ES (rz=2)

LS (n=2)

I I.9***

565 636

597 653

13.0”’

early. Additionally the values measured for tall fescue were consistently lower compared with the perennial ryegrass dried grasses. In contrast, the DOMD contents in vitro for the two samples of later cut tall fescue were consistently higher than for early cut material. Digestibility and energy values measured in vivo

The results of the digestibility measurements, energy losses and energy values measured in vivo are presented in Table 4. No effect of year of harvest was observed. Methane energy losses and hence ME values given in Table 4 all relate to predicted methane losses using the relationship of Blaxter and Clapperton ( 1965 ) . Within the perennial ryegrass material there was a substantial and significant (PC 0.0 1 or P-c 0.00 1) reduction in the digestibility of all fractions measured, except crude protein, between the early and later harvesting periods. Digestible energy and ME in the DM and ME in the organic matter were also significantly (PcO.001) reduced in the later harvested samnles. Dried grasses based on tall fescue had consistently lower digestibility values and energy contents than those based on perennial ryegrass, although small sample numbers did not allow a statistical assessment of these differences. There was little apparent difference in the digestibility coefficients or energy values between early and later cut tall fescue for the four samples examined. Table 5 shows the results of the methane energy losses measured using respiration chambers for nine dried grasses together with the values predicted from the equation of Blaxter and Clapperton ( 1965). Also given in Table 5 are the ratios of the ME contents (in the DM) calculated using measured methane losses to ME contents calculated using the predicted methane losses. Measured methane losses were significantly (PcO.001) lower than the predicted values. The measured values also exhibited much more variability (coefficient of variation, 10.2%) than the predicted values (coefficient of

ENERGY VALUE OF HlGH TEMPERATURE

TABLE

221

DRIED GRASS

4

Digestible organic matter content, digestibility coefftcients and energy values of the early (ES) and late season (LS) perennial ryegrass and tall fescue high temperature dried grasses measured in viva (values in parentheses are standard deviations) Perennial

Tall fescue

ryegrass

ES (n=lt)

LS (n=7)

SED (d.f.=16)

::=2)

LS (n=2)

DOMD (g kg-’ DM) Digestibility cocff~cients Organic matter Dry matter Crude protein NDF’

707 (19.8)

t i53 (17.8)

9.2***

579

601

0.78 0.76 0.73 0.82

Gross energy Energy losses (proportion

0.75 (0.020) ofGE)

0.73 0.71 0.70 0.77 0.69

FCCCCS

Urine Methane’ Energy values ME’/GE4 MF;. IE4 DE (MJ kg-‘DM) ME (MI kg-‘OM) ME (MI kg-’ DM)

(0.019) (0.020) (0.082) (0.020)

0.25 (0.021) 0.057 (0.010) 0.083 (0.001)

(0.022) (0.019) (0.028) (0.034) (0.024)

0.30 (0.022) 0.050 (0.019) 0.080 (0.001) 0.57 (0.026: 0.81 (0.027) 12.7 (0.30) II.6 (0.49) 10.3 (0.50)

0.61 (0.017) 0.81 (0.012) 14.1 (0.42) 12.7 (0.37) 11.5(0.35)

‘Ashed neutral detergent fibrc. ‘Predicted imm Blaxter and Clapperton I965 1. ‘Metabolisable energy, calculated using predicted methane ‘Digestible energy. **Pco.ol; ***P
0.010** 0.010*** 0.032 0.013** 0.010***

0.68 0.63 0.66 0.69 0.65

0.68 0.65 0.67 0.70 0.65

0.010***

0.35 0.056 0.077

0.35 0.049 0.077

0.007 0.0007*** u.o1w** o.OQ9 0.18**’ 0.20*** o.zo***

0.51 0.80 Il.5

0.52 0.8 I I I.6

10.7 9.2

IO.6 9.4

(

losses.

variation, 2.9%). The effect on the ME contents of the lower than predicted methane losses was such that ME contents calculated using predicted methane losses were on average 1.035 times lower than when measured losses were used. This factor varied relatively little. Relationships between digestibility and energy values and Iaboratory measurements Table 6 shows the significant (P-=0.05) linear relationships between DOMD in vivo and various laboratory measurements. The NCD procedure accounted for most of the variance (90.4%) followed by RFP and MADF. The relationship between DOMD in vivo and NCD is shown graphically as Fig. I. This method seemed to cope equally well with dried grasses based on both perennial ryegrass and tall fescue. Water-soluble carbohydrates and the remaining cell wall fractions provided substantially poorer relationships with

D.I. GWENS ET AL.

222 TABLE

5

Measured methane energy losses (as a proportion a comparison with values predicted from Blaxter Sample

Harvest

of GE) for nine high temperature and Clapperton ( 1965) hnethane

season’

Predicted methane loss

energy loss I 2 3 4 5 6

L L E E E L

0.064 0.050 0.070 0.066 0.057 0.062

7 8 9

L L L

0.065 0.055 0.059 0.061 0.0062

Mea” Standard deviation SED (d.f.= lb) ‘E, early; L, late. *Metabolisable energy 3Metabolisable energy

dried grasses and

IME, ME,’

0.078 0.077 0.083 0.083 0.080 0.081 0.081

1.028

0.077 0.081 0.080 0.0023

1.032 1.039

1

1.054 .026 1.026 1.041 I.038 1.028

I

1.035 0.0093

0.0022***

(MJ kg-’ (MJ kg-’

DM) DM)

calculated calculated

using measured

methane

loss.

usingpredictedmethaneloss.

***P
6

Linear relationships betwen digestible organic laboratory measurements (S kg-’ DM)

matter

c”ntent

Independent variable (X)

Regression

equation

NCD* Rumen fluid-pepsin MADF WSC’ ADF (ashed) Crude tibre CellUlDSe NDF cashed)

Y=59.9+0.828X Y=27.3+0.998X Y=1012-!.243X Y=536+0.9OOX Y= 1050- 1.350x Y= 1020- 1.534X Y=983-1.390X Y=993-0.586X

in viva

(Y, g kg-’

Accountable variance’ (0,) 90.4 76.8 73.4 56.0 41.6 39.1 27.5 12.6

DM)

and various

Residual standard deviation 15.2 23.7 25.4 32.1 37.6 38.5 41.9 46. I

‘Adjusted for degrees of freedom. ‘Neutral detergent-cellulase DOMD. ‘Wawr-soluble carbohydrates.

DOMD in vivo. There was no significant relationship with lignin content. The prediction of DOMD in vivo from either RFP or NCD was not improved by the addition of a second predictor variable or the use of a quadratic model. There was a significant (PcO.01) linear relationship between the apparent

223

Fig. 1.Relationshipbetweendigestibleorganic matter content measured in viva and by neucral detergent-cellulase. TABLE 7 LinearrrlaIionshipsbetween metabolisable energy’content ( Y, MJ kg-’ DM) and variouslaboratory measu~emenls (g kg- ’ DM or as stated) Independent

NCD’ Rumen fluid-pepsin MADF Gross energy (MS kgwsc4 ADF (ashed)

Accountable variance2

Regression equation

vatialiie (X)

’ DM

)

Crudetibre Cellulose

Y=-0.59+0.0154x Y=--1.82+0.0195X Y= 16.9-0.0224X Y=- 16.3+ 1.4557X Y=8.34+0.016lX

Y= 17.5-0.0243X Y= 17.0-0.0276X Y= 16.0-0.0233X

(%)

Residual standard deviation

80.6 76.4 61.3 58.2 45.6

0.43 0.47 0.60 0.62 0.71

34.3 32. I 18.8

0.78 0.80 0.87

‘Using predicted methane energy losses. IAdjusted for degrees of freedom. ‘Neutral detergent-cell&se DOMD. “Water-soluble carbohydrates.

digestibility coefficient of crude protein although ADIN accounted for only about

(ADCP) and ADIN (g kg-’ TN) 33% of the variability. The relation-

ship established was: ADCP=0.756-0.000775

ADIN &kg-’ TN); R*=33.0%, RSD=0.031

Table 7 presents the significant (PC 0.05 ) linear relationships between ME in the DM (using predicted methane energy losses) and various laboratory procedures. The NCD method gave the best relationship with ME (accountable variance 80.6Oh) frllowed by RFP and MADF. The relationship between ME and NCD is also shown in Fig. 2. Gross energy in the DM accounted for some 58% of the variability in ME and this appeared to be almost entirely

224

Fig. 2. Relationsllip between netabolisable gestible organic matter content.

D.I. OWENS ET AL.

energy content and neutral detergent-cAdase

di-

due to the variability in GE introduced by differences in total ash content. Total ash and acid insoluble ash accounted for, respectively 60.6% and 5 1.4% of the variability in the GE content of the DM. The prediction of ME from RFP and NCD was significantly improved by the addition of GE as a second predictor variable. The adjusted accountable variance values increased from 80.6 to 88.5% for NCD and from 76.4 to 8 1.5% for RFP. DISCUSSION

One of the main aims of a frequent and regular cutting regime is to maintain a leafy vegetative pasture which should tend to provide a fairly constant content of structural polysaccharides throughout the cutting season. The present results (Table 2) suggest that this was achieved in terms of cell wall content and hemicellulose, although concentrations of cellulose, ADF, MADF and crude tibre were significantly increased in the later season perennial ryegrass material. The results also strongly suggest that the tall fescue grasses have higher concentrations of cell wall fractions and lower WSC contents than the ryegrass. This is in agreement with the findings presented in the reviews by Bailey (1973) and Smith (1973). Van Soest ( 1982) suggests that ADIN contentsgreater than either 3 g kg-’ DM or 150 g kg-’ TN are indicative of heat damage. Most of the results from the present study were lower than these threshold values indicating that the high temperature drying process had caused little heat damage .to the feedstuff. It is noteworthy, however, that despite the relativeiy narrow range of ADIN contents, ADIN was still signiticantly related to apxient digestibility of crude protein. The relationship obtained was similar in fc&mto those quoted by van Soest ( 1982). The results suggest that even low ADIN contents may have some biological meaning in terms of dietary nitrogen availability.

The significant increases in magnesium and sodium contents of the later season ryegrass material (Table 2) followed theclassical pattern described by Fleming (1973) although the very high later season values observed by Fleming were not recorded. The increased content of certain cell wall fractions in the later season perennial ryegrass samples was reflected in significantly lower digestibility values when measured both in vitro (Table 3) and in vivo (Table 4). Metabolisable energy content was also lower in the later season ryegrasses. The digestibility of GE in the perennial ryegrass samples (Table 4) agreed well with the values obtained for early (0.74) and later cut (0.71) dried, ground and pelleted grass by Coelho da Silva et al. ( 1972). They were, however, somewhat lower than reported for spring(0.77) and summer (0.72) growths of fresh perennial ryegrass dominant swards by D-1. Givens et al. (unpublished work, 199 1) , which may be a reflection of a reduction in digestibility brought about either by drying at high temperature or by the grinding process as described by Blaxter and Graham ( 1956). More recently, ‘Thomson and Cammell ( 1979) reported a mean reduction in GE digestibility from 0.57 in chopped luceme to 0.54 in the dried, milled and pelleted product, although the effect of processing on the digestion of legume cell walls is reported to be much less than for grasses (Osboum et al, 1976). The literature does not appear to contain any recent measurements of the ME content of HTDG which may be compared with the present findings. In the earlier studies of Wainman et al. ( 1970), mean ME contents of 11.5 MJ kg- ’ DM and 10.8 MJ kg- I DM were recorded in first and third harvests of dhed, ground and pelleted perennial ryegrass. These values compare well with those recorded in the present study, bearing in mind that the values of Wainman et al. ( 1970) used measured methane energy losses, whereas the results in Table 4 were calculated using predicted methane losses. Using the information given in Table 5, the mean ME contents for early and later perennial ryegrass samples in Table 4 ( 11.5 MJ kg-’ DM and 10.3 MJ kg- i DM, respectively) would be equivalent to approximately 11.9MJ kg-’ DM and 10.7 MJ kg-’ DM, respectively if methane energy losses had been measured. The present results, therefore, indicate that contemporary HTDG based on pcrennial ryegrass is likely to have a high ME content early in the season, although this is likely to be somewhat reduced in later harvests. It imay be noted that in the present results the lower ME content in the later season material was in part due to the increased total and acid insoluble ash {contents and hence lower GE contents of the DM. The increased ash values were presumably a result of soil contamination and it would seem important that all possible means to reduce soil contamination during harvesting are investigated. The present results also strongly suggest that HTDG produced :from tall fescue is likeiy to have substantially lower digestibility and energy values than those produced from perennial ryegrass.

226

D.I.CWENS ETAL.

The findings of Blaxter and Graham ( 1956) that grinding and pelleting of grass results in lower energy losses as methane may partly explain the lower measured than predicted methane energy losses observed in the present work as the prediction equation of Blaxter and Clapperton ( 1965) was based on a mixture of diets. Although not a direct comparison, other studies in this laboratory (D.I. Givens et al., unpublished work, 1991) measured in fresh herbage an average methane energy loss of 0.070 of GE compared with 0.061 of GE in the present study. The present results have therefore shown that methane energy losses are lower than would be predicted and are probably lower than would be obtained from the original fresh grass. Measured methane loss was reasonably constant (0.06 I 2 0.0062) and the use of this value when calculating ME from digestible energy content and urinary energy loss would seem to be reasonable. All of the present measurements in vivo were made at maintenance and whilst Blaxter and Graham ( 1956 ) and Van Es and Van der Honine. ( 1973 ) have shown GE digestibility ;o be reduced from milling and pelletingby about 6% at maintenance, much larger reductions of between 9% (Van Es and Van der Honing, 1973) and 15% (Blaxter and Graham, 1956) have been recorded when animals have been fed at hi&r planes of nutrition. Van Es and Van der Honing ( 1973) using lactating dairy cows concluded that the ME content of milled, pelleted grass was some 8% lower than the non-processed material. Despite this reduction in ME, they observed that the net energy (NE) value of the milled product was essentially unchanged from the original fresh grass. This phenomenon has been reviewed by Osboum et al. ( 1976) who concluded that the enhanced efficiency of utilising ME in milled, pelleted dried grass was most likely related to the enhanced absorption of amino acids relative to energy from volatile fatty acids. This improved efficiency of ME utilisation and hence maintenance of NE content following milling and pelleting has led some workers (Barber et al, 1984) to increase predicted ME contents of HTDG by 8% as a means of compensation. At the time, this seemed a reasonable practice for extension purposes, but subsequent data suggested that similar compensations should also be applied to other feeds where the efficiency of utilisation of ME is known to be affected by various factors (e.g. grass silage vs. fresh gras,, grasses vs. legumes) which are also not yet accounted for in the rationing system used in the UK (Ministry of Agriculture, Fisheries and Food, Department of Agriculture and Fisheries for Scotland and Department of Agriculture, Northern Ireland 1984). The fact that laboratory methods which estimate digestibility provided substantially better regression relationships with DOMD and ME is in agrecment with recent studies on fresh grass (Givens et al., 1990a,b). In the present work, the enzyme-based NCD procedure provided extremely good relationships with DOMD (Table 6; accountable vanance, 90.4%) and ME (Table 7; accountable variance, 80.6%) and should provide a useful basis for pre-

ENERGYVALtiEOFHlGHTEMPERATUREDRlEDGR,SS

221

dieting the value of unknown samples based on both perennial ryegrass and tall fescue. Barber et al. ( 1984) also reported prediction relationships for HTDG based on NCD. These equations which accounted for some 58% and 52% of the variability of DOMD and ME, respectively, are substantially different to those obtained in this study and their use or the present samples would have led to substantial underprediction of DOMD and ME (mean underprediction, 63.3 g kg-’ DM and I .5 MJ kg-’ DM respectively, for early season perennial ryegrass). The reason for the difference between the two sets of relationships is not entirely clear, although the HTDG used by Barber et al. ( 1984) were obtained from only two sources and predominantly from one harvest year. The present study has shown that contemporary milled and pelleted HTDG based on perennial ryegrass is likely to have a high ME content (about 11.9 MJ kg-* DM) early in the season and that this is likely to decline later in the season despite a rigorous cutting regime. Minimising soil contamination appears to be an important factor in maintaining high ME contents. The results also indicate that a considerably poorer quality product is likely if tall fescue is used instead of perennial ryegrass. The study has shown that the enzymebased NCD procedure can provide :.ln accurate laboratory prediction of energy value which will allow producers to introduce quality control procedures at reasonable cost. ACKNOWLEDGEMENTS

The authors are grateful to the technical staff of the ADAS Feed Evaluation Unit for valuable assistance and also to the ADAS Analytical Chemistry Departments at Wolverhampton and Cambridge for analytical support. This study was jointly funded by the Briti. !I Association of Green Crop Driers who also provided the samples and the Ministry of Agriculture, Fisheries and Food.

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