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
et al., 1997. Protein mobilisation is stopped at about Energy value per unit of liveweight change for
4 weeks after calving, while fat mobilisation is not lactating dairy cows is fixed in NRC system and each
stopped at 8 weeks Tamminga et al., 1997. This of European systems, but varies with a range be-
may be due to a considerable hypertrophy of both tween 19 and 30 MJ kg of liveweight change. A
gut and liver during early lactation in response to number of recent studies have reported different
increased feed intake Reynolds and Beever, 1995. energy values within the above range Chilliard et
The composition of mobilised tissue fat and protein al., 1991; Gibb et al., 1992; Tamminga et al., 1997.
can thus differ during the first 4 weeks of lactation, The reason for this discrepancy may be attributed to
i.e. the ratio of fat-to-protein would increase. If the the effects of body condition and lactation stage.
water content in the mobilised tissue is assumed to Body condition factors have been recognised to
remain unchanged, the energy value per unit of influence the energy value of liveweight change.
liveweight loss would be higher as lactation pro- These factors include gain or loss of body fat or
gresses. On the other hand, energy balance is not protein, replacement of body fat with water and
always related to liveweight change of dairy cows. changes in gut fill. The lack of precision in these
Beever et al. 1998 reported that high genetic merit factors can result in errors in ration formulation and
cows were still in negative energy balance at 20 the prediction of animal performance, especially in
weeks of lactation, but liveweight of the animals was early and late lactation when liveweight change of
maintained after 5 weeks. This can be partially dairy cattle can be large. It may not be realistic in
explained by the findings of Tamminga et al. 1997 practice to distinguish the effects of gut fill on gain
as changes in the water content in mobilised or loss of liveweight and how much water is
liveweight. The total water loss was proportional to contained in the weight of change. However, a
the mobilised liveweight during the first 4 weeks of number of studies have related the energy value of
lactation. Afterwards the animals retained water in liveweight change to body condition. In a serial
their bodies and liveweight loss was very small, but slaughter study with Holstein–Friesian cows, Gibb
energy loss was still high Tamminga et al., 1997. and Ivings 1993 reported that the body fat and
Energy value per unit of mobilised liveweight thus energy contents of animals were positively related to
increased substantially from 1 to 8 weeks of lactation their liveweight and condition score and body protein
Tamminga et al., 1997. content was positively related to their liveweight.
The fixed energy value for liveweight change in Fat-free mass of Holstein cows remains similar
NRC system and European systems is therefore during the dry period, early and late lactation stages,
incorrect and can result in errors in practice, espe- while water content of fat-free mass is greater during
cially for high genetic merit cows which have a high the dry period and early lactation than late lactation
liveweight loss during early lactation. There is a stages Andrew et al., 1994. These relationships are
need to develop more appropriate measures of reflected in the Cornell net carbohydrate and protein
energy status in dairy cows. If liveweight change system Fox et al., 1992. In this system the energy
continues to be used as an index of tissue energy concentration per condition score is linearly and
change, it must be related to both body condition and positively related to body condition score and
stage of lactation. liveweight of dairy cattle. In Australia a linear
regression equation for dairy cattle has been de- veloped to relate the energy value of liveweight
5. Validation of the UK ME system and other
Kategori