Factors influencing average milk yield of herds at two levels of production.
An experiment was started in 1950 designed to find out what proportion of the difference in milk yield between herds is genetic in origin. Sixty-five pairs of twins were split at birth equally between farms having high-yielding herds and farms having low-yielding herds. A previous paper on this experiment reported on the growth of the twins, the present paper on milk yield and on management practices in the herds. Twenty-four pairs of twins survived to complete one lactation and these provide an estimate of 24%±18% for the genetic part of the difference between the herds. Aspects of rearing of the twins and of management of the herds were examined. Twins in the high-yielding herds were served and calved earlier than those in the low-yielding group but they had a longer interval between service and calving. Differences between average heifer yields and average cow yields were from one-and-a-half to two times greater in the high- than in the low-yielding herd group. There were no obvious quantitative differences in feeding practice between the two groups although differences in body size suggest that twins in the high group were done better. For the age at first service there was a positive rank correlation of 0·30 between body size differences and subsequent differences in first lactation milk yield. There was no single factor of management or environment which appeared to be the cause of the yield difference between the herds.
Genetic differences between bull-breeding herds.
In the analysis of the causes of variation between cows of a given breed in milk production, differences between herds pose some difficult problems. The nature of this variation is of importance from several points of view in the discussion of breed improvement. The situation is complicated by the fact that the herds of any breed type do not form a uniform group but can be classified roughly into a sort of ‘social structure’ (Robertson and Asker, 1951). The discussion of variation between all herds of a given breed type can then be broken down into the variation between strata and that between herds in the same stratum. From the analysis of production data from herds using artificial insemination (Robertson and Rendel, 1954; Korkman, 1953) there is evidence that the genetic component is a small proportion of the total variance in these herds; and also that in Great Britain the genetic difference in yield between the dominant group of herds (which provide many of the bulls used in A.I.) and the herds using A.I. is small. This is in agreement with studies on breed structure. In this paper we shall discuss the genetic differences between members of the small group of important herds which dominate each breed. These usually supply a high proportion of the bulls used. If we have good estimates of the breeding values of such bulls from the production records of their progeny, we can then examine the different breeding herds according to the breeding value of the bulls bred in them. Evidence of this type has been presented for Jersey bulls in New Zealand (Castle, 1952) but without statistical analysis of the results, although inspection suggests that there are definite differences between studs.
Migration of pedigree Ayrshire cattle in Great Britain
Migration of pedigree Ayrshire cattle in the British Isles is estimated for 1927 and 1946 from samples of herd-book registrations. Most migration is from Scotland, particularly Ayrshire, to England, but considerable numbers were also transferred between regions within Scotland or England. A larger proportion of bulls than of cows was moved in this way. Only a small proportion of animals had two parents bred in the same herd as themselves. If the males and females registered from England in 1946 were the sole parents of the next generation, the latter would still have more than half Scottish blood. The amount of movement of Ayrshire cattle shown by these data virtually precludes genetic differences between regions, but not perhaps between herds. Other implications of movement are discussed.