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.
Breed structure in the pedigree Ayrshire cattle population in Great Britain
Ayrshire Cattle Herd Books for the years 1927, 1946 and 1947 were examined to study the breed structure. Extended pedigrees of a sample of registered females showed that more than half their genes had come from eight herds. Most of the genetic contribution of these herds comes through the bulls they breed. Herds were divided into: breeders' herds (a), breeders' herds (b), multipliers' herds (i), multipliers' herds (ii), and herds registering males only. The function and numerical importance of each group is shown. The position of any one herd in the breed hierarchy is unstable. Changes in the extent of specialization of herd function between 1927 and 1946 are described. The Ayrshire breed structure is discussed in the light of its implications on the effective size (N) of the population and on the selection of bulls for breeding.
A Selection Index for Fat Production in Dairy Cattle Utilizing the Fat Yields of the Cow and Her Close Relatives
Summary An index for intraherd selection for fat production was derived, utilizing the fat yields of the cow and those of her dam, daughters, maternal half sisters, and paternal half sisters. Statistics to construct the index were computed from 23,330 lactation fat yields of 12,405 Jersey cows on HIR test. The intraherd statistics needed to construct the index were: repeatability, 0.412; correlation between maternal half sisters, 0.073; correlation between paternal half sisters, 0.120; and heritability, 0.201. Herd differences accounted for 39% of the total variance. Within herds the year-to-year variation in things which affected all cows alike accounted for 8% of the variance. Progress to be expected by using the index for selections would generally be about 1.10 to 1.15 times faster than by making selections on the cow's own performance alone. The exact ratio for a specific situation would depend on the number of records on the individual, on the number and kinds of relatives, and on the amount of information available about each.