Asher, A., Newe-Ya'ar Research Center, Beef Cattle Section, Department of Ruminant Science, Institute of Animal Science, Agricultural Research Organization, PO Box 1021, Ramat Yishay 30-09500, Israel, Israeli Center for Interdisciplinary Research in Chronobiology, University of Haifa, Haifa 31905, Israel
Shabtay, A., Newe-Ya'ar Research Center, Beef Cattle Section, Department of Ruminant Science, Institute of Animal Science, Agricultural Research Organization, PO Box 1021, Ramat Yishay 30-09500, Israel
Haim, A., Israeli Center for Interdisciplinary Research in Chronobiology, University of Haifa, Haifa 31905, Israel
Aharoni, Y., Newe-Ya'ar Research Center, Beef Cattle Section, Department of Ruminant Science, Institute of Animal Science, Agricultural Research Organization, PO Box 1021, Ramat Yishay 30-09500, Israel
Miron, J., Department of Ruminant Science, Institute of Animal Science, Agricultural Research Organization, PO Box 6, Bet-Dagan 50250, Israel
Adin, G., Department of Cattle Husbandry, Extension Service, Ministry of Agriculture, PO Box 28, Bet-Dagan 50250, Israel
Tamir, A., Faculty of Medicine, Technion-Israel Institute of Technology, PO Box 9649, Haifa 31096, Israel
Arieli, A., Department of Animal Science, Faculty of Agriculture, The Hebrew University of Jerusalem, PO Box 12, Rehovot 76100, Israel
Halachmi, I., Institute of Agricultural Engineering, Agricultural Research Organization, PO Box 6, Bet-Dagan 50250, Israel
Moallem, U., Department of Ruminant Science, Institute of Animal Science, Agricultural Research Organization, PO Box 6, Bet-Dagan 50250, Israel
Orlov, A., Newe-Ya'ar Research Center, Beef Cattle Section, Department of Ruminant Science, Institute of Animal Science, Agricultural Research Organization, PO Box 1021, Ramat Yishay 30-09500, Israel
Brosh, A., Newe-Ya'ar Research Center, Beef Cattle Section, Department of Ruminant Science, Institute of Animal Science, Agricultural Research Organization, PO Box 1021, Ramat Yishay 30-09500, Israel

Thirty-five lactating dairy cows throughout weeks of lactation (WOL) 16 to 30 were used to determine optimal time needed for reliable measurement of performance variables, and to classify the cows into high-, medium-, and low-efficiency groups. Individual performance variables [body weight (BW), dry matter intake (DMI), and milk production] were measured daily with a computerized monitoring system. Body condition was visually scored weekly and used to calculate retained or depleted body energy as a result of fat content change (REF). Milk composition was analyzed weekly. Body weight, DMI, and total recovered energy (RE), which represents energy in milk production plus REF, were summarized weekly. Efficiency was calculated as RE/DMI and as residual feed intake (RFI; i.e., the difference between actual and expected DMI), which was calculated from multiple linear regression of DMI dependence on BW0.75 and RE. Unexpectedly, it was found that BW did not affect DMI and RE/DMI. Changes and relative changes in phenotypic coefficient of variation and correlations among data from shortened tests ranging from 1wk (WOL 16) to a sequence of 15-wk tests were used to determine optimal test period durations for 5 traits: BW, DMI, RE, RE/DMI, and RFI. Traits were fitted into a mixed model with repeated measures. For each week, the traits were summarized as a sequence of cumulative data, starting from WOL 16 and cumulated over periods that increased in 1-wk steps up to WOL 16 to 29. Weekly cumulations were compared with those for entire test period (WOL 16 to 30). Consistency of each cow's efficiency classification as high, medium, or low was tested by the total-agreement procedure; the kappa index P-value was used. Throughout WOL 16 to 30, the effects of increasing test period duration on between-animal coefficient of variation differed with respect to the various performance variables and RE/DMI: it tended to change with respect to BW, did not change with respect to DMI, and decreased with respect to RE and RE/DMI. In conclusion, compared with a 15-wk study, a 2-wk study can classify RFI and RE/DMI to 3 efficiency levels, with an individual correlation coefficient of 0.6. When the study was carried out over 3wk or more, the lowest significant index of the classification was P<0.004, the lowest individual correlation coefficient was 0.65, and its lowest significance was P<0.01. The current study indicated that the insignificant effect of the BW of dairy lactating cows on their DMI should be validated in more studies. © 2014 American Dairy Science Association.