This study is based on 1015 live single calves brod by artificial insemination in a herd of Ayrshire, Friesian, Jersey and two-breed and three-breed cross cattle derived from them. Birth weights, subjected to a least squares analysis, show little effect of year or season of calving. Males were 6·0 lb. heavier than females; second and later parity calves were 6·6 lb. heavier than first parity calves.
Over the range of breeds and crosses studied, heredity was the principal determinant of mean birth weight. No evidence of heterosis was found— means for crossbred calves, however, differed in reciprocal crosses. They deviated from the mean of the parents in the direction of the maternal weight at birth. If 0 is the mean birth weight of the dam's breed or cross and 0 of the sire's breed, the mean weight of offspring is closely estimated by
This expression may not apply to the crossing of breeds differing much more in weight than the Jersey and Friesian breeds. In such crosses it is likely that small dams depress foetal growth more than large dams promote it. The maternal influence on birth weight in cattle as expressed above is in both qualitative and quantitative agreement with the maternal influence as derived from egg transfer studies in sheep.
It is a pleasure to record the authors' heavy obligations to many friends for help during the course of this long experiment. The Milk Marketing Board, in the persons of Dr J. Edwards, C.B.E., Mr T. O'Sullivan, and their colleagues at the A.I. centres made the breeding programme possible. Much time has been saved and much statistical computation cut short by the help afforded us at the Rothamsted Statistical Department by Dr F. Yates and his staff, particularly Mr J. H. A. Dunwoody. Responsibility for the animals on the farms fell at first upon Mr W. S. Biggar, and from 1950 upon Mr J. S. Tavernor. They were aided by many of whom mention must be made of Mr H. L. Williams, Mr R. Pringle and Mr R. Simpson. In the preparation of this paper the authors have been much influenced by the work and criticism of their colleagues.