The metabolisable energy requirement for maintenance and the efficiency of utilisation of metabolisable energy for lactation by dairy cows offered grass silage-based diets

ELSEVIER

Livestock Production

Science 51 (1997) 141-150

The metabolisable energy requirement for maintenance and the efficiency of utilisation of metabolisable energy for lactation by dairy cows offered grass silage-based diets T. Yan *, F.J. Gordon I, R.E. Agnew ‘, M.G. Porter, D.C. Patterson The Agricultural

Research institute

’

of NorthernIreland, Hillsborough, Co Down, Northern Ireland, BT.26 6DR. UK Accepted 15 April 1997

Abstract Between 1992 and 1995, a number of lactating dairy cows (n = 221) were offered grass silage-based diets in a range of feeding experiments and subjected to gaseous exchange measurements in calorimetric chambers at the Agricultural Research Institute of Northern Ireland. The objective of the present study is to bring together the energy metabolism data from all these cows and use these in a range of regression techniques to estimate the metabolisable energy (ME) requirement for maintenance (ME,) and the efficiency of utilisation of ME for lactation (k,) for lactating dairy cows given grass silage-based diets. The ME, and k, were estimated by linear regression of milk energy output, adjusted to zero energy balance (E,(,,), against ME intake or ME available for production (ME,), or multiple regression of ME intake against metabolic live weight, milk energy output and energy balance. The derived ME,,, from these equations ranged from 0.61 to 0.75 MJ/kg”-75, with a mean of 0.67 MJ/kg”.75. This mean value is proportionately 0.40 higher than that of 0.48 MJ/kg’,” calculated using the equations of Agricultural and Food Research Council [Agricultural and Food Research Council, 1990.

Technical Committee on Responses to Nutrients, Report Number 5, Nutritive Requirements of Ruminant Animals: Energy. Nutr. Abstr. Rev. (Series B), 60: 729-8041, and the mean live weight and ME/gross energy ratio (qm) obtained in the present study. Similarly, the derived k, from these equations varied from 0.61 to 0.68, mean of 0.65. This latter parameter is the same as that derived from the regression of E,(e) a g ainst ME,, when the ME, was taken as 0.67 MJ/kg’.“. This finding for k, is the same as that of 0.65 predicted from AFRC (1990) using the mean qm of the diets in the present study. 0 1997 Elsevier Science B.V. Keywords:

Calorimetry;

Dairy cow; Energy utilisation; Grass silage

1. Introduction The metabolisable energy (ME) requirement for maintenance (ME,) and the efficiency of utilisation

* Corresponding author. ’ Also members of staff of the Department of Agriculture for Northern

Ireland and The Queen’s University

of ME for lactation (k,) are two key parameters in the calculation of the energy requirements of dairy cattle. For many years the ME, has been estimated by measuring the fasting metabolism of non-lactating cattle with, for example, Agricultural Research Council (Agricultural Research Council, 1980) using

of Belfast.

0301.6226/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO301 -6226(97)00065I

these

published

late the ME,

data to develop

for dairy cattle.

equations

to calcu-

This approach

was

142

T. Yan et al. /Livestock

Production Science 51 (1997) 141-150

further supported by the Agricultural and Food Research Council (Agricultural and Food Research Council, 1990) and suggests an ME,,, of 0.48 MJ/kg0.75, when using a live weight of 573 kg and ME/gross energy (qm) of 0.65, i.e., the means of these from the present study. Alternatively, the ME, for dairy cattle can be estimated by regression of energy outputs against ME intake. Within the literature, there is a wide range in the ME,,, values for dairy cattle estimated from non-fasting calorimetric data. For example, Vermorel et al. (1982) reported an estimated ME, of 0.51 MJ/kg”.75 for lactating Holstein/Friesian cows offered maize silage, while Grainger et al. (1985) recorded a higher ME, of 0.79 MJ/kg0.75 for dry pregnant Friesian cows given fresh grass although a proportion of the latter estimate may have reflected the increased requirements due to pregnancy. Miinger (1991) noted that estimated ME,,, values differed with different breeds of lactating cows offered maize silage and hay or a fresh grass-clover mixture (ME, values of 0.53, 0.56 and 0.47 for Holstein/Friesian, Jersey and Simmental cows, respectively). Even computing large sets of non-fasting data, each pooled from a large number of individual calorimetric experiments, the estimated ME, values for lactating dairy cows varied from 0.49 to 0.64 MJ/kg”.75 in a number of studies (Table 11. The k, has often been determined in calorimetric studies by assuming a ME,,, value which is deducted from ME intake (MEI) to provide the ME available for production (ME,), and then relating this to adjusted milk energy output (Et(,), k, = E,,,,/ME,). Alternatively it can be determined using a range of regression techniques relating energy output to MEI.

Table 1 Summary calculated

Using the former method and the equations of Agricultural and Food Research Council (1990) to calculate ME, for dairy cows offered grass silage-based diets, Unsworth et al. (1994) reported a mean k, of 0.56 averaged from 4 calorimetric studies. This value was within the range of 0.58 and 0.52 reported by Gordon et al. (1995a) with data from calorimetric and production studies respectively. These derived k, values from the above two series of experiments were proportionately 0.14, 0.25 and 0.12 lower than those predicted from Agricultural and Food Research Council (1990) respectively. In contrast, the derived k, values for lactating dairy cows by regression techniques as presented in Table 1, while variable, and ranging from 0.60 to 0.67 with a mean of 0.63 (s.d. 0.026), are much higher than those reported earlier from studies in which ME,,, is assumed. These latter k, values were also positively correlated to their corresponding ME, values (R* = 0.77, p < O.OS>, indicating that k, is greatly influenced by the accuracy of the ME,. The reason for the major difference between Agricultural and Food Research Council (1990) predicted k, and those reported by assuming ME,,, or regression techniques could reflect either an inaccuracy in the equations of Agricultural and Food Research Council (1990) for calculation of ME,,,, or the fact that low k, values are obtained with grass silage-based diets. In the literature a further method has been used to calculate the k,. In this method k, is derived as a proportion of net energy (NE) as total MEI, where NE is the sum of milk energy, retained energy multiplied by a constant and NE requirement for maintenance (NE,). Using this approach, and assuming NE, of 0.293 MJ/kg”.75, a number of stud-

of the ME requirement for maintenance (ME,) and the efficiency of ME utilisation by a range of authors using regression techniques and pooled calorimetric data

for lactation

(k,) by lactating

dairy cattle,

Reference

Scale

Forage

Method

ME,,, (MJ/kge7s)

k,

Moe et al. (1970) Van Es et al. (1970) Van Es (1975) Unsworth et al. (1994) Hayasaka et al. (1995) Mean s.d.

350 trials 198 trials 1148 cattle 13 trials 53 trials

Lucerne hay/grass hay Hay/silage A range of roughages Grass/silage Hay/silage

Multiple regression Linear regression Linear regression Linear regression Multiple regression

0.51 0.49 0.49 0.64 0.59 0.54 0.067

0.64 0.62 0.60 0.67 0.64 0.63 0.026

T. Yan et al. /Livestock

Production Science 51 (1997) 141-150

ies have reported that k, was close to 0.60, with a range from 0.57 to 0.63 (Kirchgessner and Mliller, 1989; Kirchgessner et al., 1989; Windisch et al., 1991; Gadeken et al., 1991). Between 1992 and 1995 across a number of experiments, lactating dairy cows (n = 221) were offered grass silage-based diets and subjected to gaseous exchange measurements in calorimetric chambers at this Institute. The objective of the present study is to bring together the energy metabolism data from all these animals and to use regression techniques to estimate the ME,,, and k, values for lactating dairy cows given grass silage-based diets.

2. Materials

and methods

2.1. Animals and calorimeters A total of 221 Holstein/Friesian lactating dairy cows was subjected to measurements of their energy metabolism in indirect open-circuit respiration calorimeters at the Agricultural Research Institute of Northern Ireland between 1992 and 1995. The animals were drawn from a number of experiments (Carrick et al., 1996; Ferris et al., 1997; Gordon et al., 1995a,b; Gordon et al., 1997; Yan et al., 1996; C.S. Mayne, personal communication; A. Cushnahan, personal communication), and in each experiment a 6-day diet digestibility and nitrogen (N) balance study was carried out prior to each animal being transferred to calorimeters. The cows remained in the calorimeters for 3 days with measurements of gaseous exchange being made over the final 48-h period. The animals were at a range of lactation stages and of various genetic merits. The mean values, and standard deviation (s.d.1, for lactation number, milk yield, days of pregnancy and live weight of the animals during the calorimetric measurements, are presented in Table 2. Seventy-nine of the 221 cows were pregnant during the calorimetric measurements and mean stage of pregnancy for these pregnant cows was 46 days (s.d., 36.0). All equipment, procedures, analytical methods and calculations used in the calorimetric studies were as described by Gordon et al. (1995b). Calibration tests on the calorimeters were carried out using oxygen

143

Table 2 Data on animal, feed and energy utilisation

in the present study

Minimum Maximum COWS Milk yield (kg/d) Live weight (kg) Days of pregnancy Lactation no.

a

Silage composition Dry matter (g/kg) PH NH,-N/total-N (g/kg) Crude protein (g/kg DMJ Gross energy (MJ/kg DM) Ash (g/kg DM) Acid detergent fibre (g/kg

Mean

s.d.

3.2 385 0 1

49.1 133 138 9

23.1 573 13 3.2

8.25 61.4 26.5 1.72

168 3.60 44 106 17.8 66 301

398 4.19 120 193 20.0 105 435

226 3.86 79 146 18.7 85 357

60.8 0.176 18.5 26.5 0.56 11.0 35.5

7.47 0.18 0.12 0.54

24.54 1.oo 0.37 0.74

16.99 0.58 0.24 0.65

3.460 0.217 0.056 0.033

DM)

Feed intake Total DM intake (kg/d) Silage DM/total DM intake ADF intake/total DM intake ME/GE ((1,) Energy utilisation Gross energy intakes (MJ/dJ Energy outputs (MJ/d) Faeces Urine Methane Heat production Milk Energy retained (MJ/d) “Seventy-nine measurements.

137.3

460.3

3 16.1

65.43

25.6 5.1 7.6 79.5 11.1 -88.2

133.3 27.6 29.9 182.8 141.2 70.6

75.8 13.5 21.0 125.1 76.0 4.6

18.52 4.02 3.83 20.84 26.39 25.54

of 221 cows were pregnant during the calorimetric and the data presented here are for all 221 cows.

free nitrogen (certified 99.998%) and chemically pure carbon dioxide (certified 99.995%) supplied by the British Oxygen Company, Guildford, Surrey, England. In these tests, the gas analysers were set up using gas mixtures prepared with a cascade of Wosthoff pumps in addition to a specially prepared compressed gas mixture supplied by BOC Special Gases Division. The whole system tests based on gravimetric cylinder losses and recovered volumes allowed the outputs from the temperature and humidity sensors, the electronic barometer, the flowmeters, and the carbon dioxide and oxygen analysers to be validated. Over the past 4 yrs the recoveries of both gases have been consistently in the range of 99101 %.

144

T. Yan et al. /Liuestock

Production Science 51 (1997) 141-150

2.2. Food

total DM (ADF/DM) during the calorimetric surements are also presented in Table 2.

A total of 21 perennial ryegrass silages was offered in these experiments. These silages were spread over primary growth and first and second regrowth, and had a range of dry matter (DM) concentrations and additives. All silage were well preserved, and the mean and range in composition are presented in Table 2. Thirty-six of the 221 cows from one experiment were offered silage as the sole diet, but otherwise in all other experiments the cows (n = 185) were offered silages with a range of proportions of concentrate from 0.18 to 0.70 (DM basis). The concentrates used in each of the studies included a mineral/vitamin supplement and some of the following ingredients: Cereal grains: barley, wheat or maize By-products: maize gluten meal, molasses beet pulp, citrus pulp or molasses

sugar-

Protein supplements: fish meal, soya-bean meal The concentrate portion of the diet was offered either in a complete diet mixed with the grass silage, or as a separate feed from the silage. All cows were offered either silage or the complete diet ad libitum. Total DM intake, the ratio of ME/gross energy (qrn) and the proportion of acid detergent fibre (ADF) in

mea-

2.3. Data analysis With the pregnant cows the ME required for pregnancy was subtracted from the ME intake (MEI) (Agricultural and Food Research Council, 1990). The milk energy output adjusted to zero energy balance (E,& was calculated from milk energy output (Et) plus positive energy balance (Es) or E, minus 0.84 X negative Eg (Agricultural and Food Research Council, 1990). Using varying combinations of these data the ME,,, and k, were estimated using the following range of regression techniques. E l(O)=aMEI+b

(1)

Elcoj = a ME1 + b( 9, - 0.65) + c

(2)

E,(,) = P, [ 1 - EXP( - P,( ME1 - &))]

(3)

MEI=aMW+bE,+cE,

(4)

MEI=aMW+bE,+c(+E,)+d(-Es)

(5)

El(,) = a ME, + b

(6)

where MW, ( + Eg) and ( - E,) are respectively metabolic live weight, positive Eg and negative Eg, and ME, is energy available for production (ME, =

Table 3 The linear and multiple regression equations, and the derived Ml3 requirement for maintenance (ME,) and the efficiency of ME utilisation for lactation (k,) obtained from present data set (all relationships are significant (p < O.OOl), the figures in brackets are s.e. of the coefficients, except in Eq. (3) where 0.050 is the s.e. of 0.66) Equations

R2

ME,,, MJ/kg”.75

k,

E I(O)= 0.65~o.oz,)MEI - o.435(0.048) E I(O)= 0.6l~o.ozqMEI + 0.93,,,4, X (qm - 0.65) - 0.372Ca,04,, E 1Coj= - 6.94[1 - EXF’( - (- 0.088XMEI - 0.66~,,,,,~))1

0.90 0.90 0.89 0.92 0.92 0.88

0.67 0.61 0.66 0.75 0.75

0.65 0.61

(2)

0.68 0.68 0.65

(3) (4) (5) (6)

MEI = o~75(0.0Z6)Mw + 1.48~0.032+? + 1.09~0.037,Es MEI = 0.75~o.o,s,MW + 1.48~0.03s$i + [email protected],s) (+Es) E I(O)= O%oos,~,

+ l.O8,.a,z,(-Es)

Es-energy balance (MJ/d). + Es-positive energy balance (MJ/d). -Es-negative energy balance (MJ/d). E,Co)-adjusted milk energy output to zero energy balance (MJ/kg”.75 in Eqs. (l)-(3), Et-milk energy output (MJ/d). MEI-ME intake (MJ/kg0.75 in Eqs. (l)-(3), and MJ/d in Eq. (4) and Eq. (5)). ME,-ME available for production (MJ/d). MW-metabolic live weight (kg’.“). q;ME/GE.

and MJ/d

in Eq. (6)).

(1)

T. Yan et al./Liuestock 1.6

^

7

and for this reason a section of the present paper is now devoted to this aspect.

Eiiui = 0.65 ME1 -0.435

1.4r

145

Production Science 51 (1997) 141-150

‘.C@l, n=zzL . .

-

f.-:

3. Results

.-L. *

-*

-5

-

..y

s

. i

.-3. .c_ --_ a._.

-.-

--. : i_

.-/

0.0

r’.

.a

.*

. . .-.

;-

:

. . AZ--_ : -

c 3.0

0.0 ME

intake (MUkgO”)

Fig. 1. The linear relationship between metabolisable energy (ME) intake (MEI) and adjusted milk energy output (E,o,l.

ME1 - ME,,,). The value of 0.65 in Eq. (2) was the mean qrn of the diets for the 221 cows. Eq. (2) (Van Es et al., 1970) is intended to eliminate the effect of q,,,. Eq. (3) was developed by Cammell et al. (1986) in which the P, is taken as ME,,,. Eq. (5) (Moe et al., 1970) is similar to Eq. (4) but the former divides energy balance into two components, positive and negative. Within each regression method the model explored accounted for the following factors: experiment, parity, cow genetic merit and silage composition, and continuous variables: milk energy output, lactation days and silage GE intake as a proportion of total GE intake (silage-GE/total-GE). In all the regression methods the only factor reaching significance (p < 0.05) was experiment and hence the regression equations presented in the present study relate to the model in which this factor is removed. Within the continuous variables silage-GE/total-GE was the only one to reach significance ( p < 0.05)

The data on energy intake and outputs of the cows, recorded during the calorimetric measurements, are presented in Table 2. The data represented a very wide range of GE intakes and correspondingly large differences in energy outputs from faeces, urine, methane and heat production. The milk energy output ranged from 11 .l to 141.2 MJ/d and energy balance from - 88.2 to 70.6 MJ/d. The linear and multiple regression equations developed using the data, and the estimates of the ME, and k, derived from these, are presented in Table 3. As stated previously, the effect of the experiment factor on these equations was significant (p < 0.05) and hence has been removed. For all the equations the proportion of variation accounted for by the variables is very high, ranging from 0.88 to 0.92, and all relationships are highly significant ( p < 0.001). The standard errors for estimates of ME,,, and k,, are also relatively small in all equations. The estimated ME,, value, derived from Eq. (1) (also shown in Fig. 1) as the ME1 equivalent to zero &,), was 0.67 MJ/kg’.“. This value was marginally higher than those of 0.61 MJ/kg0.75 derived from Eq. (2) when the effect of q,,, was eliminated as suggested by Van Es et al. (1970), and 0.66 MJ/kg0.75 derived as P2 from Eq. (3) as used by Cammell et al. (1986). However, the estimated ME, value (0.75 MJ/kg”-75) derived from both Eqs. (4) and (5) when MEI was computed against metabolic live weights, milk energy output and energy balance, was slightly higher than those reported above. The mean derived ME,,, from Eqs. (l)-(4) was 0.67 MJ/kg’.“.

Table 4 Effect of silage GE intake as a proportion of total GE intake on the linear relationship between adjusted (MJ/kg0.75) and ME intake (MEI) (MJ/kg0.75) and the derived ME requirement for maintenance (ME,) utilisation for lactation (k,) (all correlationshius are sienificant ( I) < 0.0011) Silage GE/Total

< 0.50 0.5 I-0.99 1.00

GE

n

Equations

R2

ME,

99 86 36

&or = 0.6’& o.osir MEI - 0.365(,., a 061) E,(o) = O+,,,. a 029) ME’ - 0.434,,,, 0.050) E,(o) = 0.636x. o 0x4) MEI - 0.465,q.e. o osar

0.80 0.85 0.89

0.59 0.68 0.74

MJ/kg”.75

milk energy output (E,(,)) and the efficiency of ME

k, 0.62 0.64 0.63

(7) (8) (91

146

T. Yan et al. /Livestock

Production Science 51 (1997) 141-150

1.6 Q.50: E,~o$62MEI.O.365 1.4 --

0.516.99:

4. Discussion

R2-0.80, n=99

E,~~~=00.64h4!31-0.434 R’-O.SS, ~86

1.00: E,i,@63MEI-0.465

RQ.89

, n=36

i 0.0

0.5

1.0 ME

1.5

2.0

2.5

3.0

intake (MJ/kgO.“)

Fig. 2. The effect of gross energy intake (GEI) as a proportion of total GE1 ( < 0.50 ,-, f 0.51-0.99, o---; 1.00, +---) on the linear relationship between metabolisable energy (ME) intake and adjusted milk energy output (EICo,).

In previous studies in the literature researchers have recognised the difficulty in experimental procedures when using heterogeneous materials, such as ensiled grass, in calorimetric studies intended to estimate the ME, for cattle. Hence most researchers have used fresh or dried forages. However, in many parts of North America and North Europe grass silage is the main forage source for cattle during late autumn, winter and early spring. The present data set is therefore relatively unique and also very important for the industry. In addition the data set represented a very wide range in milk production, and in particular included some very high milk yields from high genetic merit cows (cow genetic index 1050, Swanson, 1991). 4.1. Metabolisable nance

The estimated k, values, derived from Eqs. (1) and (2), were 0.65 and 0.61, respectively. These values were less than that (0.68) obtained from multiple regression Eqs. (4) and (5). Using a mean ME, value of 0.67 MJ/kg”.75 (mean from Eqs. (l)-(4)) to calculate the ME available for production (ME,), the estimated k, value was 0.65, both when averaged from the individual k, values (Elco,/MEp) for each of the 221 cows (s.d. 0.106), and when derived from the linear regression equation of EICol against ME, (Eq. (6)). The mean derived k, value was 0.65 from Eqs. (1) (2) (4) and (6). The silage-GE/total-GE ratio, as a continuous variable, significantly influenced (p < 0.05) the linear relationship of EICoj against MEI. The overall data were therefore divided into three sub-sets according to silage-GE/total-GE ratios, i.e., proportionately below 0.50 ( < 0.50, n = 99) between 0.51 to 0.99 (0.51-0.99, IZ= 86) and 1.00 (silage only diets) (1.00, n = 36). The results, as presented in Table 4 and Fig. 2, indicated that the derived ME, values were significantly increased (p < 0.01) with increasing silage-GE/total-GE ratios (0.59, 0.68 and 0.74 MJ/kg0.75, respectively), while the derived k, values were similar across each of the ratios (0.62, 0.64 and 0.63, respectively).

energy

requirement

for mainte-

Agricultural Research Council (1980) summarised 8 sets of observations on fasting metabolism of steers and dry cows (n = 88) published between 1927 and 1974, and reported a logarithmic relationship between the fasting metabolism and live weight of the animals. Based on this finding, Agricultural Research Council (1980) proposed a logarithmic equation to calculate the net energy (NE) requirement for maintenance (NE,) for dairy cattle, and a linear regression equation based on q,,, to calculate the efficiency of ME utilisation for maintenance (k,). These two equations were further supported in the review by Agricultural and Food Research Council (1990). Using these relationships, and the mean live weight of 573 kg and q, of 0.65 from the present study, the calculated ME, would be 0.48 MJ/kg ’ 75. A number of studies have statistically estimated ME, by regressing EICoj against MEI, or ME1 against metabolic live weight, milk energy output and energy balance (Table 1) from large sets of non-fasting calorimetric data each pooled across experiments. Using these techniques Moe et al. (1970) Van Es et al. (1970) and Van Es (1975) recorded estimated ME,,, values of 0.51, 0.49 and 0.49 MJ/kg”.75 respectively, which are close to the value of 0.48 MJ/kg”.75 calculated using the methods of Agricultural and Food Research Council (1990).

T.

141

Yan et al. / Licestock Praduction Science 51 (I 997) 141- 1.50

However, two recent studies by Unsworth et al. (1994) and Hayasaka et al. (1995) reported much higher ME,,, values of 0.64 and 0.59 MJ/kg”.75, respectively. In the present study, the estimated ME, values varied from 0.61 to 0.75 MJ/kg0.7s (Table 31, using the different regression techniques, with a mean ME,,, value of 0.67 MJ/kg0.75. This mean is proportionately 0.40 higher than that (0.48 MJ/kg0.751 of Agricultural and Food Research Council (1990) using the mean live weight and q, data from the present study. Grainger et al. (1985) also obtained a high ME, value of 0.79 MJ/kg”.75, using linear regression of Eg against ME intake, for dry pregnant Friesian cows (n = 20) offered fresh grass, although some of this effect may have reflected the increased requirements due to pregnancy. Within the literature, however, there are other calorimetric studies which have shown, also using regression techniques, that ME,,, values for lactating dairy cows ranged from 0.47 to 0.58 MJ/kg0.75 (Vermorel et al., 1982; Patle and Mudgal, 1977; Miinger, 19911, when maize silage and hay or a fresh grass-clover mixture was offered. The high ME, value obtained in the present study could reflect a number of issues. It is probable that the lactating cows in the present study had higher metabolic rates than the steers and dry cows used by Agricultural Research Council (1980) to set up the equations to calculate ME,,,. The cows in the present study were lactating, often at high levels, and always offered diets ad libitum, while the steers and dry cows used by Agricultural Research Council (1980) were in fasting state and prior to the fasting studies were offered diets at maintenance levels, or at low planes of nutrition. In a comparison of the ME, values for lactating and dry dairy cows, Moe et al. (1970) statistically analysed the energy metabolism data obtained in their laboratory using multiple regression of MEI against metabolic live weight, milk energy output and energy balance, and reported that the ME, for the lactating dairy cow (n = 350) was proportionately 0.21 higher than that for the dry dairy cow (n = 193). Also in view of the major proportion of oxygen consumption which is attributable to the gastrointestinal tract (Reynolds et al., 1991), and the impact of ad libitum forage feeding on the tract as a proportion of live weight, it is conceivable that ME,,, could be considerably higher

in the present study involving ad libitum feeding of grass silage-based diets. In addition, with fasting metabolism studies the plane of nutrition offered to cattle before fasting has been reported to influence fasting metabolism and hence the ME,. For example, Birkelo et al. (1991) in a study over five seasons recorded, on average, proportionately 0.07 and 0.14 higher fasting HP and ME, (p < 0.05) respectively for steers, offered diets at a 2.27 X maintenance level rather than a 1.20 X maintenance level prior to fasting. Similar results were also reported by Flatt and Coppock (1963), Smith and Mollison (1985). The high proportions of grass silages offered in the present study could be a further reason for the high estimated ME,,, value. Grass silage varied from 0.18 to 1.00 of the total diets, average of 0.58 (s.d. 0.217). When the overall data set (n = 221) was divided into three sub-sets, according to silageGE/total-GE ratios (Table 4 and Fig. 2), the derived ME,,, values were significantly increased with increasing proportion of silage in the total diet. The positive relationship between ME,,, and silageGE/total-GE ratio could reflect an increased oxygen consumption attributable to the larger gastro-intestinal tract (Reynolds et al., 1991) normally associated with silage-based diets. Regression of HP/ME against the silage-GE/total-GE showed a positive and significant ( p < 0.001) relationship (R* = 0.261, indicating that the proportion of ME directed towards HP increased with the proportion of the silage DM in the total diet. The logic of this might be that the NE,,, value is increased and/or the k, is reduced as the proportion of silage DM in the total diet is increased. The latter hypothesis would be supported by the relationship of the proportion of methane energy output (Ecu,> over GE intake (Ecu4/GE), which increased with the proportion of silage DM in the total diet (p < 0.001). The higher ratio of ECHJ/GE would indicate a lower ME intake, if the energy outputs from faeces and urine remain unchanged, and hence through its effects on q, could Research Council (1980) reduce k,. Agricultural have proposed that q,,, is positively related to k,. 4.2. Eficiency for lactation

of utilisation

of metabolisable

energy

In the present study the k, value was estimated using a number of regression techniques. The value

148

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Production Science 51 (1997) 141-150

of 0.68, derived from multiple regression of ME1 against metabolic live weight, milk energy output and energy balance (Eq. (4) or Eq. (5)), is marginally higher than those (0.65, 0.61 and 0.65) estimated from both linear regression of E,(,) against ME1 or ME, (Eqs. (l), (2) and (6)). The latter values were similar to that (0.65) averaged from the individual k, values for each of the 221 cows, when the mean ME, value of 0.67 MJ/kg0.75 was used. The overall mean k, value across the different methods used in the present study is 0.65, which is close to those of 0.67 reported by Unsworth et al. (1994) and 0.64 recorded by Moe et al. (1970) and Hayasaka et al. (1995). This value is the same as that (0.65) predicted from the equation (k, = 0.35q, + 0.42) of Agricultural Research Council ( 1980) when the mean q,,, of 0.65 was used. However, all these k, values developed by regression techniques are much higher than those obtained from production and calorimetric studies using the equations of Agricultural and Food Research Council (1990) to calculate the ME,,, . For example, Gordon et al. (1995a) using this approach reported average k, values of 0.58 and 0.52 from calorimetric measurements and production data respectively, which were considerably lower than that (0.65) predicted from Agricultural Research Council (19801, when grass silage-based diets were offered to lactating dairy cows with a range of genetic merits. Unsworth et al. (1994) also reported that the average k, value, similarly calculated from calorimetric data with grass silage-based diets in four experiments, was proportionately 0.14 lower than that predicted from Agricultural Research Council (1980) (0.64 vs. 0.56). In the present study, when k, was calculated from the data for each of the 221 cows using the ME, calculated from Agricultural and Food Research Council (1990), the mean k, at 0.53 was much lower than that of 0.65 predicted from Agricultural Research Council (1980). However, if the mean ME, of 0.67 MJ/kg0.75 obtained in the present study was used, the calculated k, would be 0.65, which is the same as that predicted from Agricultural Research Council (1980). This would therefore support the view that the low k, values reported by many authors when using silage-based diets may have been a direct reflection of the ME, used in the calculations and that this may have been in error.

4.3. The problems relating to the fasting metabolism The fasting metabolism has widely been accepted as a basis of ME, for cattle. The present study and some previous studies, as discussed previously, have resulted in low k, values (E,(,,,/ME,) when using this approach to calculate ME,. The ME,,, value derived from the fasting metabolism (Agricultural and Food Research Council, 1990) may therefore have underestimated the maintenance energy requirement for cattle offered grass silage-based diets ad libitum in feeding studies. A number of studies in the literature has indicated a range of problems relating to the fasting metabolism (Marston, 1948; Webster et al., 1974; 0rskov and MacLeod, 1990; Chowdhury and Orskov, 1994). These issues merit consideration. Following long periods of restricted nutrition, fasting for a prolonged period can result in deamination of amino acids from tissue protein for the supply of essential glucose, and consequently increase N excretion in urine. This can induce a number of metabolic disorders of the animal, such as hypoglycaemia, hyperlipidaernia, hyperketonaemia and hypoinsulinaemia (Chowdhury and 0rskov, 1994). Such effects have been shown to affect fasting heat production. For example, Chowdhury (19921, using the intragastric infusion technique in sheep, reported that heat production was lower than fasting heat production when volatile fatty acids (VFAS) and casein were intragastrically infused to provide ME and N up to 0.289 MJ/kg0.75 and 1.5 g/kg”.75, respectively. Ku Vera et al. (1989) noted that intragastric infusion of glucose at low levels, in comparison with fasting, considerably reduced urinary N excretion and in most cases heat production in Friesian steers. Ku Vera et al. (1987) also reported that a mixture of VFAs given to lambs at about proportionately 0.2 maintenance energy was sufficient to decrease fasting N excretion in urine to that observed at maintenance energy. Chowdhury and Orskov (1994) therefore suggest that during measurement of the energetic efficiency of different feeds, animals should be give one third of maintenance energy, rather than fasting. A further problem relating to fasting is that the fasting metabolism varies with the nutrition levels prior to starvation. Marston (1948) reported that the fasting metabolic rate of sheep was linearly related

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to the plane of nutrition prior to fasting. Ferrell et al. (1986) reported that fasting heat production (MJ/kg0.75) o f growing lambs was significantly increased with increasing nutrition planes prior to fasting from below maintenance to high nutrition levels. A recent study with dry dairy cows (Yan et al., 1997) also noted a proportionately (0.40) higher fasting heat production (MJ/kg0.75) than fasting metabolism predicted from Agricultural and Food Research Council (1990), when diets were offered at near ad libitum prior to starvation. These results may be due to increasing weight of digestive tract, blood flow rate and oxygen consumption per unit of weight (e.g., liver) with increment of feeding levels (Ferrell et al., 1986). Webster et al. (1974) reported that predicted fasting metabolism was not in line with actually measured fasting metabolism when k, and the efficiency of ME use for fattening (k,) were calculated from the ratio of ME/GE, and concluded that fasting metabolism was not a good basis to predict energy retention in steers. Using fasting metabolism as a basis (Agricultural and Food Research Council, 1990) to determine maintenance energy and predict energetic efficiencies has always shown low k, value with forage diets as discussed previously. This may be attributed to fasting inducing metabolic disorders, and more importantly restricted nutrition prior to fasting reducing the maintenance metabolic rate. Therefore it would be realistic to measure the maintenance metabolic rate following a period of ad libitum feeding rather than restricted nutrition (e.g., maintenance), whether measured as the fasting metabolism or heat production plus urinary energy output when the cattle are offered one third of energy for maintenance (Chowdhury and Brskov, 1994). Alternatively, if the maintenance metabolic rate is measured in fasting after restricted nutrition, the difference in the fasting metabolism with the ad libitum feeding prior to fasting should be considered.

5. Conclusion A number of linear and multiple regression techniques were used to estimate the ME,,, and k, values for dairy cattle offered grass silage-based diets. The derived ME, ranged from 0.61 to 0.75 MJ/kg0.75,

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with a mean of 0.67 MJ/kg0.75, which is proportionately 0.40 higher than that of 0.48 MJ/kg”.75 calculated from Agricultural and Food Research Council (1990). Similarly, the derived k, ranged from 0.61 to 0.68, with a mean of 0.65, which is the same as that of 0.65 predicted from Agricultural Research Council (1980).

Acknowledgements The authors wish to thank their colleagues at the Agricultural Research Institute of Northern Ireland for access to the data from the calorimetric experiments used in the present study.

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