SUMMARYA numerical model of male and female replacement processes in a dairy cattle population was constructed in order to describe the long-term genetic and financial consequences of selection amongst bulls in AI.Changes with time in the relative contribution of different generations of offspring to the total genetic improvement of the population were described. It was shown that the genetic change resulting from a single selection process amongst the males eventually approaches an equilibrium many years after selection is practised. Owing to the delays involved in the progeny test, individual males made no genetic contribution for a period of 10 years and a small but constant contribution thereafter.The costs of bull testing and selection, and the monetary benefits obtained from it, were estimated. Current selection procedures proved to be highly profitable at prevailing market prices. Capital invested in the testing programme was recovered within 2 to 5 years of the first offspring calving.The effect of changes in milk prices and feeding costs on monetary returns was examined. It was concluded that sire testing programmes are likely to prove unprofitable only under extremely adverse economic conditions.

A numerical model of AI selection procedures has been constructed in an attempt to assess the efficiency of current selection methods and to examine possibilities for their improvement. The results indicate that (1) present selection methods are relatively efficient in their utilization of test resources, but much less efficient in their usage of tested bulls. Additional investments in bull testing are likely to profit the dairy industry. (2) The main effects of improved bull utilization are (a) to permit increases in selection pressure with no increase in cost over current selection procedures and (b) to extend the limits to profitable selection. (3) The adoption of alternative stud management systems, in which bulls are slaughtered early in life, are likely to require excessively large investments in semen processing and storage.

Insulin, thyroxine and tri-iodothyronine concentrations were measured in samples of blood from 175 Friesian bulls, aged between 2 and 7 years, which were being progeny-tested for milk production. Least squares regression coefficients were estimated for these hormone levels on time of day and time after feed at sampling and age of bull. Correlations between hormone levels and Improved Contemporary Comparisons of the bull, a measure of his daughters' milk production, did not differ significantly from zero, suggesting that simple spot estimates of levels of these hormones in bulls would not supply useful predictors of daughters' production.

Five hundred and ten Danish Red male calves slaughtered for the production of veal at live weights of approximately 200 kg were used to estimate carcass composition.Regression coefficients expressing the relationships between live weight and carcass components indicated that weight at 6 months of age consisted mostly of muscle development in the breast and hindquarters.Variation in all of the more important growth and carcass attributes was very small, which suggested that selection responses would be difficult to obtain.There was no apparent correlation between growth rate and the muscle, fat and bone components of the carcass. The best single indicator of the muscle content of the carcass was the muscle content of the pistol cut (r = 0·79), which itself involved complicated dissection procedures. The longissimus dorsi area provided relatively little information, and none of the body measurements, apart from the leg circumference, were strongly related to carcass composition.The predictive value of many combinations of traits was examined. The best of them accounted for 90% of the variation in muscle content, but when the pistol cut was excluded, none of the multiple correlation coefficients exceeded 0·8.

SUMMARYA numerical model of male and female replacement processes is developed and used to construct a matrix which specifies the numbers of lactations of different parities expressed in a given time period by daughters of bulls used in artificial insemination (AI).Populations resulting from newly established AI breeding programmes contain an unduly high proportion of offspring by young bulls. Subsequently an equilibrium is reached in which offspring of young and tested bulls are present in proportions determined by the relative usage of the two types of sire and by differences in replacement rates amongst their progeny. Once an equilibrium has been reached the numbers of progeny sired by bulls lacking a progeny test proof is extremely small.The use of the technique to solve a number of specific planning problems is demonstrated.