The introduction of large numbers of European dairy cattle in the Arab world in the last decade, and up till now, aims at increasing the animal protein output of these countries to face the animal-protein gap in human diet. Available reports and research on the performance of these cattle under the adverse conditions in their new habitat, feed, health and management beside the hot summer weather, show clearly that the productivity and reproductivity of these exotic breeds deteriorate, that is when compared to the European standards of performance (see AOAD 1983, for a comprehensive review of cattle production in the Arab world). The stressful environment may arrest the full expression of the exotic genotypes. Also, animals of relatively high milk yield, that is higher than a certain limit dictated or tolerated by the environment, may be rejected earlier, and consequently will have shorter longevity compared to those of lower production. This would imply developing a negative relationship between milk yield of the earlier lactations and both longevity and reproductive efficiency. Negative correlations were reported by several investigators (CAR-MAN 1955; EVERETT, ARMSTRONG and BOYD 1960;SPADLING, EVERETT and FOOTE 1975) between lactation yield and reproductive efficiency. GILL and ALLAIRE (1976) found that each increment in yield of first lactation above a certain level leads to negative differences in herdlife.In this work comparisons are made in each lactation between the means of milk production, lactation length and average daily milk yield, of cows staying on the farm to produce the next record and of those discarded after completing that particular lactation. If no selection pressure is exerted on milk yield and no relationship exists between milk yield and herdlife time, no differences between trait means are expected. Records on Friesian, Jersey, Pinzgauer and buffalo are utilized. Data and analysisThe data utilized in this work comprise four sets of records including information on actual milk yield and the number of days in milk (from which a third trait was calculated: the average daily milk yield):1. The Frzesiun recoids: Records commencing within the period from 1975 till April 1982 inclusive were collected on the Friesian herds of three Libyan farms (in Tripoli, Benghazi and Sebha). A detailed description of these data will be published soon by two of the present authors (M. A. M. and A. A. N.) and their coworkers. The total of 3318 records used was distributed among the first five lactations (in their respective order) as follows: 1511, 823, 457,322 and 205. Sebha farm did not contribute to the records of the fifth lactation. U.S.

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The present study investigated environmental factors affecting birth (BW), weaning (WW) and yearling body weights (YW) and estimated the heritabilities and genetic correlations of these traits in a Barki sheep flock maintained in the north western coast of Egypt. Body weight data of 1046 animals (542 females and 504 males) progenies of 163 sires and 557 dams obtained from 1963 to 2005 were used. Data were analyzed using a model composed of location, year of birth within location, sex of lamb and age of dam. Moreover, heritabilities as well as genetic and phenotypic correlations were estimated using multiple traits animal model. Barki males had significantly heavier body weights than females at birth (3.64 kg vs 3.46 kg), weaning (19.56 kg vs 18.44 kg) and at 12-month of age (32.05 kg and 30.02 kg). Year of birth within location showed highly significant effects on the studied body weights. Furthermore, age of dam revealed highly significant effect on birth weight, where older dams tend to produce heavier lambs than those of younger ones. These results clearly indicate that corrections for the studied environmental effects are necessary to increase accuracy of direct selection for BW, WW and YW. Heritability for weaning weight was found to be higher (0.41) than BW (0.32) and YW (0.30). Therefore, selection based on WW would probably be more effective compared with BW or YW. Hence, heavier body weights at weaning are worth to be considered in the Barki sheep flock if the improvement of mutton production is the objective.