RESUMEN
Twenty four healthy lactating Baladi cows [local breed] and Friesian x Baladi cows [1[st] group] and their calves [2[nd] group] were studied during summer and winter seasons in North of Nile Delta of Egypt to investigate the effects of summer and winter conditions on the production efficiency. Milk yield [kg] during summer was lowest in the two breeds of cows when compared to values observed during winter. Lactating period was longer in crossbred than Baladi cows. Milk yield/day was higher in Friesian x Baladi than that in Baladi cows during both summer and winter. Milk components [fat, protein and total solids] were higher during winter than during summer at all times of testing, while an opposite trend was observed in milk lactose and solid non-fat [SNF] in the two breeds. Birth weight of calf and calf/dam percentage in Baladi breed was lower than the crossbreds during the two seasons. Breed of dam exerted a pronounced effect on birth weight, where purebred Baladi calves averaged 22.2, 24.8 kg which were lower than that crossbreds [31.0, 31.5 kg] during summer and winter, respectively that is not a maternal effect only it included the individual genetic effect as well. The calf/dam weight percentage in crossbred scored the highest percentage [7.36 and 7.32%] while the purebred Baladi calves means were 6.04 and 6.48% during summer and winter, respectively. The average weight of the animals in the two breeds increased as the animal grew with age; the increased weight during winter was greater than that during summer. The daily average growth rates during winter and summer were [0.487 vs 0.563 kg] and [0.475 vs 0.515 kg] in Baladi and crossbreds, respectively. In both breeds, live weight of calves at six months of age showed that, the calves which were heavier at birth [crossbred calves] were also heavier at six months of age. The means of calf weight at six months which were born during winter were greater than those born during summer. On the other hand, dam weight at calving, birth weight of calf, weaning weight for calf, average daily gain, lactation period and total milk yield were studied using records for Baladi and their crossbred cows covering the period from 1998 till 2007. Data were analysed for the effect of season and breed on milk production and growth performance