R .E. Agnew, T. Yan Livestock Production Science 66 2000 197 –215
203
1990. The increase in NE cost for grazing would
For individual calorimetric studies, k has often
m l
be greater 0.30–0.40 under extensive grazing con- been determined by assuming a ME
value which is
m
ditions, since the animals would spend more time deducted from ME intake to provide the ME avail-
eating and walking Langlands et al., 1963. In able for production ME , and then relating this to
p
addition, lactating dairy cows grazing on pasture are adjusted milk energy output E
, k 5 E ME .
l 0 l
l 0 p
usually required to walk a distance twice a day for The k
estimated from this approach is thus in-
l
milking. The energy costs for walking are suggested fluenced by the accuracy of ME . Using this meth-
m
to be 2.0 or 28 kJ kg liveweight per km for od, and ME
of AFRC 1990, k values have often
m l
horizontal and vertical movement, respectively for been estimated to be low 0.50–0.58 with data of
cattle ARC, 1980. Brody 1945 suggested a dairy cows given diets based on either grass silage
proportionately 0.08 increase in maintenance energy Unsworth et al., 1994; Gordon et al., 1995; Yan et
requirement for each km of walking. al., 1996; Ferris et al., 1999b, or maize silage or
NRC 1988 recommend a proportionately 0.10 whole crop wheat Beever et al., 1998; Sutton et al.,
increase in maintenance allowance for cows grazing 1998a,b, 1999. However, the above k values could
l
on good pasture and up to 0.20 on sparse pasture. be increased 0.59–0.65, mean of 0.62 by using a
0.75
SCA 1990 suggest that increases in maintenance ME
of 0.62 MJ kg , as obtained from Eqs. 1
m
requirement for grazing cows are in a range of and 2. The latter mean k 0.62 is similar to that
l
0.10–0.20 in intensive grazing conditions, to approx- 0.64 predicted from AFRC 1990 k 50.35?ME
l
imately 0.50 for animals grazing extensive, hilly GE10.42 using ME GE obtained in these studies.
pastures where they walk considerable distances to A further method has been adopted by some
preferred grazing areas and to water. However, these workers mainly in Germany, in which k is derived
l
allowances are not included in the current UK ME as a proportion of NE of total ME intake, where NE
system AFRC, 1993 and there is a requirement to is the sum of milk energy, retained energy multiplied
address this deficiency in the future. by a constant and assumed NE . However, the
m
efficiency calculated using this method is a mixture of efficiencies of ME utilisation for lactation k and
l
3. Efficiency of ME utilisation maintenance k .
m
3.1. Methods of estimating the efficiency of ME 3.2. Effects of dietary nutrients on the efficiency of
utilisation for lactation ME utilisation for lactation
The efficiency of ME utilisation for lactation k The composition of a diet can shift the microbial
l
can be determined using a range of regression population in the rumen and consequently influence
techniques on large sets of calorimetric data. In the the production of VFAs. In general a high fibre diet
literature two regression equations have often been produces VFAs with a high proportion of acetic acid,
used; i.e., the linear regression relating milk energy while a concentrate diet normally generates more
output adjusted to zero energy balance to ME propionic acid. It has been well documented that
intake, and the multiple regression relating ME VFAs produced in the rumen can influence milk
intake to metabolic liveweight, milk energy output composition, i.e. molar proportions of acetic and
and energy balance. Using these two methods the butyric acids are positively related to milk fat
present review would suggest an average k of 0.66, concentration. The rumen VFAs can also alter energy
l
as presented in Eqs. 1 and 2, derived from partition between milk and body tissue. A number of
calorimetric data of dairy cows drawn from 42 feeding studies e.g., Flatt et al., 1969a; Sutton et al.,
studies from across the world. This value is within 1993 and infusion trials e.g., Ørskov et al., 1969;
the range of k values 0.60–0.67, as presented in Huhtanen et al., 1993 have demonstrated that in-
l
Table 2, which were derived from regression tech- creasing propionic acid proportion can result in more
niques reported in a number of studies with dairy energy partitioned into body tissue and less into
cows. milk. It has been suggested that this effect would
204 R
.E. Agnew, T. Yan Livestock Production Science 66 2000 197 –215
occur when the absorption of propionic acid exceeds the capacity of the liver to handle it Ørskov and
MacLeod, 1990. Some propionic acid can thus enter the peripheral circulation and stimulate the pancreas
to secret insulin, so leading to utilisation of energy for tissue synthesis and depression of the cow’s milk
yield and milk fat production Ørskov and MacLeod, 1990.
The effects of VFAs produced in the rumen on energy utilisation have been studied extensively in
sheep and steers. The results are not conclusive, with some workers reporting a lower efficiency of utilisa-
tion of energy derived from acetic acid for body tissue synthesis, while others noting equal efficien-
cies between acetic and propionic acids Tyrrell et al., 1979; Ørskov and Ryle, 1990. The research on
Fig. 2. Relationship between forage proportion and k calculated
l
the influence of VFAs production on k with lactat-
l
with the ME of AFRC 1990 using calorimetric data of dairy
m
ing dairy cows is limited, but there is no evidence
cows published since 1976.
indicating a relationship between k and the molar
l
proportion of acetic or propionic acid produced in the rumen. For example, the infusion of different
proportions of acetic and propionic acids into the Increasing fibre concentration in diets can increase
rumen of lactating dairy cows has shown no effect the ME
of cattle as discussed previously. It there-
m
on k Ørskov et al., 1969 or heat production
fore suggests that the lower animal performance with
l
Ørskov and MacLeod, 1982. The infusion of acetic high forage diets may be a reflection of a higher
acid either showed little effect on heat production of ME , leaving less energy available for production,
m
dry dairy cows offered forage-only diets, although rather than that the high forage diet results in a lower
the efficiency of utilisation of additional energy from k . This would be suggested by the findings of Yan et
l
infusion of acetic acid varied with composition of al. 1997a who showed that increasing dietary
basal diets Tyrrell et al., 1979. proportion of grass silage increased ME
but had no
m
The k obtained using the linear regression tech- significant effect on k in lactating dairy cows.
l l
nique on large sets of calorimetric data also indicates little differences with forage proportions in diets. In
3.3. Effect of cow genetic merit on the efficiency a single experiment Flatt et al. 1969b observed
of ME utilisation for lactation similar k values when alfalfa proportions in diets
l
were increased from proportionately 0.2 to 0.6. Yan Dairy herds in the British Isles have been undergo-
et al. 1997a pooled data from 221 cows across ing a period of rapid increase in cow genetic merit
experiments and reported that k values were not since the mid 1980s, with Coffey 1992 reporting
l
influenced significantly by the proportion of grass current rates of genetic gain of proportionately 0.013
silage in the diet. The data experiment mean used per year in milk fat plus protein yield for the indexed
to develop Eqs. 1 and 2 have been used by the population in UK and Ireland. This increase is
present authors to examine the relationship between approximately 4.5 kg year of fat plus protein yield, a
forage proportion in diets and k calculated using value which is about 62 kg year of milk yield when
l 0.75
either ME of AFRC 1990 or 0.62 MJ kg
assuming milk of standard composition Agnew et
m
derived from Eqs. 1 and 2. The result indicates al., 1998. It has been reported extensively that gross
no relationship between forage proportion and k energetic efficiency milk energy output as a propor-
l
values when using either the former ME Fig. 2 or
tion of ME intake is higher with high than low
m
the latter. genetic merit cows during early, mid and late
R .E. Agnew, T. Yan Livestock Production Science 66 2000 197 –215
205
lactation e.g., Grainger et al., 1985b; Gordon et al., with which ME is utilised for lactation between
1995; Ferris et al., 1999b. The magnitude of this Holstein and Jersey cows.
increase in the efficiency was higher in multiple The discrepancy in effect of cow genetic merit on
lactation than first lactation cows Veerkamp et al., gross energetic efficiency and k may partially be
l
1994. However, when tissue energy retention and derived from the difference in ME
between high
p
ME are taken into account, the partial efficiency for
and low genetic merit cows. The main factor may
m
lactation k appears to be similar between high and however be because high genetic merit cows have
l
low genetic merit cows. The difference in milk the ability to shift the partition of ME absorbed, i.e.
energy output between high and low genetic merit more into milk and less into body tissue. A number
cows could almost entirely be explained by differ- of long term feeding studies have demonstrated this
ences between genotypes in energy intake and tissue effect Grainger et al., 1985a; Gordon et al., 1995.
energy retention Grainger et al., 1985b. When using An example of this is illustrated in Fig. 3, as reported
the equations of AFRC 1990 to estimate ME , by Veerkamp et al. 1994 and Veerkamp and Em-
m
neither Gordon et al. 1995 nor Ferris et al. 1999b mans 1995, which showed that high genetic cows
detected any significant difference in k between
had a higher gross energetic efficiency and a lower
l
high, medium and low genetic merit cows. Veerkamp energy retention than low genetic cows during the
and Emmans 1995 in a literature review also first 26 weeks of lactation. Lamb et al. 1977
indicated little difference in the partial efficiency assessed the first lactation of 289 daughters of 17
Fig. 3. Effects of cow genetic merit of high — and low - - - on condition score, tissue energy retention and milk energy output as a proportion of Me intake [mean of high and low concentrate levels from Veerkamp et al., 1994; Veerkamp and Emmans, 1995].
206 R
.E. Agnew, T. Yan Livestock Production Science 66 2000 197 –215
Holstein sires when the animals were fed according tively by Armstrong and Blaxter 1965, Aguilera et
to milk yield and also concluded that the daughters al. 1990 and Rapetti et al. 1998. When using
of high genetic merit bulls used less nutrient intake regression techniques on calorimetric data of lactat-
for body gain and more for milk production. The ing dairy cows, Flatt et al. 1969b and Van Es et al.
difference in partition of ME between high and low 1970 found that ME was utilised no less efficiently
genetic merit cows is associated with insulin con- for concomitant tissue retention than for milk secre-
centration in blood. During early lactation insulin tion. The above findings indicate that during lacta-
concentration was significantly higher in the plasma tion the efficiency of ME utilisation for concomitant
of low genetic merit cows, that were in energy tissue retention would appear to be similar to that for
surplus and gaining body weight, than in high lactation, although Moe et al. 1970 reported that
genetic merit cows that were in energy deficit and ME was more efficiently utilised for concomitant
losing liveweight Hart, 1983. The difference in tissue retention than for lactation in dairy cows.
insulin concentration disappeared when the animals stopped lactating Hart, 1983.
3.4.3. Efficiency of utilisation of mobilised tissue energy for lactation
3.4. Energetic efficiency for liveweight gain or The efficiency of utilisation of mobilised tissue
from mobilised tissue energy for lactation energy for lactation was recommended by ARC
1980 to be 0.84. This value was obtained by Moe 3.4.1. ME use for production during lactation vs.
and Flatt
1969 from
regression analysis
of during dry period
calorimetric data n 5126 of lactating dairy cows in It has been suggested that the utilisation of ME for
negative energy balance. There has however been milk production adjusted to zero energy balance
little new information available in the literature on k in lactating cows is more efficient than that for
l
this aspect since then. During the period from 1992 tissue retention k in dry cows ARC, 1980. In a
g
to 1998, a large number of lactating dairy cows were recent study at this Institute a forced drying off
subject to
gaseous exchange
measurements in
procedure was adopted to study how the efficiency of calorimetric chambers at this Institute, of which 127
ME changed with physiological state of the animal cows were in negative energy balance. These 127
Yan et al., 1997b. A reduction in milk energy of 1 data were used to develop similar multiple regression
MJ day with lactating cows was found to be associ- equations to those adopted by Moe and Flatt 1969.
ated with an increase in tissue energy retention of The equations are presented as below
0.82 MJ day with dry cows. When relating to ME
p
ME intake2ME , the efficiency k for lactating
m l
cows can be proportionately 0.09 higher than that MEI 5 1.474
E 1 1.067 E
0.0552 l
0.0891 g
k for dry cows. A similar reduction 0.11 with
g 2
1 0.745
R 5 0.85 3
dry dairy cows was also reported by Moe et al.
0.0309
1970 using multiple regression techniques on calorimetric data of dairy cattle n 5543.
E 5 0.578 MEI 2 0.749
E
l 0.0216
0.0467 g
3.4.2. ME use for lactation vs. for concomitant
2
2 0.359
R 5 0.90 4
0.0330
tissue retention Yan et al. 1997b observed an increase in tissue
energy retention of 0.96 MJ day associated with a where MEI, E and E are, respectively ME intake,
l g
0.75
decrease in energy of 1 MJ day for milk synthesis in milk energy output and energy retention MJ kg
. lactating dairy cows. This supports the proposal of
The values in brackets are standard errors. The ARC 1980 that the efficiency of ME utilisation for
derived efficiencies of mobilised tissue energy for concomitant tissue retention is 0.95 times than for
lactation are, respectively 0.72 and 0.75 with a mean milk production. Similar results of 0.96, 0.91 and
of 0.74, which is proportionately 0.88 of that ob- 0.99 with lactating goats were also obtained, respec-
tained by Moe and Flatt 1969.
R .E. Agnew, T. Yan Livestock Production Science 66 2000 197 –215
207
4. Energy value per unit of liveweight change Bath et al., 1964; Reid and Robb, 1971; Tamminga
