Validation of the UK ME system and other

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

change to body condition score SCA, 1990. energy systems Another factor affecting energy value per liveweight change is stage of lactation. During early The data used in the present validation are the lactation dairy cows can mobilise both fat and experiment mean data n 542 derived from the protein from their body reserves for milk production, literature, used to develop Eqs. 1 and 2, as but protein mobilisation decreases faster than fat described previously for references see Appendix 208 R .E. Agnew, T. Yan Livestock Production Science 66 2000 197 –215 1. The energy systems examined include the UK 1 2 ] ME system AFRC, 1990 and 1993, the Australian MSPE 5 O A 2 P 5 n ME system SCA, 1990, the American NE system ] ] 2 2 2 2 2 NRC, 1988 and the Dutch NE system Van Es, MSPE 5 A 2P 1 S 1 2 b 1 S 1 2 R 6 P A 1978. AFRC 1993 is a working version of AFRC 1990 which adds a proportionately 0.05 to total where n is the number of pairs of values of A and P ] ] ME requirement predicted from AFRC 1990. The compared n 542; A and P are, respectively, the 2 2 Dutch NE system was chosen to represent a range of means of A and P; S and S are, respectively, the P A the NE systems based on the principles proposed by variances of A and P; b and R are, respectively, the Van Es 1975, e.g. used in France INRA, 1989, slope and correlation coefficient of the linear regres- the Netherlands Van Es, 1978, Switzerland Bickel sion of A on P. The three components are thus due to ] ] and Landis, 1978 and Germany Deutschen Land- mean bias A2P, line bias the deviation of the wirtschafts Gesellschaft, 1982. The American NE slope and random variation of the slope. In order to system NRC, 1988 does not provide any method to compare the accuracy of prediction between ME and calculate intake of NE for lactation NEL from its NE systems, the MSPE is expressed as the mean ]] Œ ¯ ME intake, so the observed NEL intake in the prediction error MPE 5 MSPE A . present validation for the NRC system is based on The results of the present validation are presented the equation of Moe 1981 NEL50.697 ME2 in Table 3 and Fig. 4a and b. The MPE was much 0.877, MJ kg DM. The predicted NEL requirement higher with AFRC 1990 than the other three in Van Es 1978 and NRC 1988 is a sum of systems Van Es, 1978; NRC, 1988; SCA, 1990, predicted NE and observed E and E . The pre- indicating that AFRC 1990 is the least accurate at m l g dicted ME requirement is a sum of predicted NE predicting total energy requirements of lactating m and observed E and E divided by relevant energetic dairy cows. The predicted error of AFRC 1990 was l g efficiencies. In both the ME and NE systems E was largely derived from an under-prediction of total g ] ] treated as, if E was positive then it was added; if E energy requirement mean bias, A2P, which re- g g was negative then it was subtracted after multiplica- sulted in a large proportion of mean bias over total tion by either 0.80 Van Es, 1978, 0.82 NRC, MSPE. However, when a proportionately 0.05 was 1988, or 0.84 AFRC, 1990; SCA, 1990. added to total ME requirement predicted in AFRC The model, used to compare those energy systems 1990, as suggested by AFRC 1993 currently for actual energy ME or NE intake A and adopted in UK, total energy requirements were predicted energy requirement P, is the mean-square predicted with a similar accuracy to NRC 1988 and prediction error MSPE as described by Rook et al. SCA 1990. Van Es 1978 had a marginally higher 1990. The MSPE is defined as Eq. 5 and can be MPE than NRC 1988, SCA 1990 and AFRC regarded as the sum of three components Eq. 6 1993. The higher prediction error for Van Es Table 3 Prediction precision of different energy feeding systems in calorimetric data of dairy cows published since 1976 n 542 Systems Energy intake MJ day MSPE MPE Proportion of MSPE Actual Pred. Bias Bias Line Random ME system AFRC 1990 181 171 9.9 219 0.082 0.45 0.01 0.54 AFRC 1993 181 179 1.4 129 0.062 0.01 0.07 0.92 SCA 1990 181 182 2 1.6 116 0.060 0.02 0.13 0.85 NE system Van Es 1978 110 106 3.6 56 0.068 0.23 0.03 0.74 NRC 1988 112 111 0.5 43 0.059 0.01 0.07 0.92 R .E. Agnew, T. Yan Livestock Production Science 66 2000 197 –215 209 likely to be the ME . This can be demonstrated in m the validation of SCA 1990 which uses the same k l as AFRC 1990 but a higher ME by adjusting it m with total ME intake. There is no basis to compare the validation between ME and NE systems, since in the 2 NE systems k is assumed to be same as k to m l calculate NEL. However, this assumption is not biologically correct, because research evidence has indicated that k was higher than k . The difference m l between k and k is calculated to be 0.12 or 0.08 in m l Van Es 1975 or ARC 1980 when ME GE ratios are from 0.55 to 0.70. The assumption that k is m equal to k in both Van Es 1978 and NRC 1988 l could thus cover the under-prediction of NE used in m these two systems, because the metabolic rates, as discussed previously, are similar between Van Es 1978, NRC 1988 and AFRC 1990. This bias would be larger for late-lactating and dry cows because the ratios of ME MEI is higher at these m two stages. Nevertheless, the more accurate predic- tion of NRC 1988 than Van Es 1978 mainly reflects that the former adds an activity allowance of 0.10.

6. Conclusions