Literature review and introduction
farmer's net profit from beef production depends on the amount he produces, the price he receives for it and the costs of producing it (Fig. 4). The amount of beef is related to a readily measurable characteristic, liveweight per unit age. This is in turn a function of birthweight, rate of gain and mature weight. The price he receives per kg depends on carcase quality which is a function of the percentage and quality of beef in the carcase; percentage beef tends to increase with rate of gain in liveweight and therefore with liveweight per unit age. Production costs fall into feeding costs, a function of feed intake per kg gain (which also influences percentage beef) and other costs which are closely dependent on duration of fattening and therefore likewise on liveweight per unit age. Thus liveweight per unit age is related to all the important characteristics for beef production and can be used in breeding.
Information on these traits in beef cattle can be derived from data collected under similar conditions in one testing station or from field data of cattle kept in different places. My research was based on field data.Material and methods
For individual selective breeding, data were first collected from young AI bulls (Table 5). The inheritance of traits was studied by comparing figures for daughter groups from some of these bulls (Table 6). Comparison between the daughter groups indicated the value of such a progeny test. Son groups from some of the AI bulls were reared on a testing station (Bergström, 1969). Observations were only on measurements, weights and carcase characteristics as judged on the hoof (Fig. 5).
Any trait in cattle is the result of interplay between genotype and environment (Fig. 6). To assess genotype, cattle must be compared at the same age and in indentical environments. All values were corrected for age (Fig. 12, Table 9) and comparisons were confined to each breed (Meuse-Rhine-Yssel or Dutch Friesian) and to each sex. For heifers data were also corrected for stage of pregnancy (Fig. 14, Table 15). Calculations were made within each AI station, year and season. Liveweights of the heifers were mostly calculated from heart girth and spiral girth.Results and discussion
The liveweight of cattle at any age depends on the shape of the growth curve which is governed by the adult liveweight and the age at maturity. The age at which a certain percentage of adult weight is reached is called rate of maturity (Figs 2 and 12). There is a positive relation between adult liveweight and age at which it is reached (Table 43, Fig. 20). Other body measurements show similar relations with rate of maturity but each curve is specific (Table 19, Fig. 20) so that their relationships change with age (Tables 18 and 24). At any age the less mature characteristics are more infuenced by environment than more mature ones (Tables 1, 2, 27 and 30) so that relationships between body measurements also depend on environment.
Muscle, bone and fat also have specific growth curves and their proportions change with age (Figs 15, 16 and 21). Bones mature first, then muscle and finally adipose tissue. The infuence of genotype on them incraeses with maturity (Tables 27 and 30). The liveweight and muscular development at any age depends on the value it will reach at maturity and on the rate of maturity. The relation between liveweight and muscle development is positive. In view of the positive relation between mature liveweight and age at maturity, the relation between age at maturity and adult muscle development is also positive (Table 43, Fig. 23). Rate of maturity in liveweight and muscle development can easily change with environment.
At maturity bulls are heavier, taller and more muscular than cows (Tables 16 and 21, Fig. 17). In general, heifers mature earlier than bulls and develop less muscle so that they reach slaughter condition earlier (Tables 17, 18, 23 and 24). In my field data this was not true because bulls were kept on the AI stations with more intensive feeding and better enviromnental conditions than the heifers which were kept on farms. Meuse-Rhine-Yssel cattle reached adult liveweight on average later than Dutch Friesian (Tables 17, 19, 23 and 42) but adult liveweights and muscle development were greater in the Meuse-Rhine-Yssel cattle (Tables 16, 18, 20, 21 and 24). Because of the difference between the breeds in rate of maturity, these traits do not clearly differ in young cattle. Differences between sires in rate of maturity did not recur in their daughters (Table 41). There is indeed a positive relation of liveweight and muscle development in sires aged 940 days with these characteristics in their daughters at 400 days (Tables 40, 41 and 45). Between the characteristics in the sire and those of the sons intensively reared at one testing station to one year of age, the relation seems to be inverse (Table 46). This suggest an interaction between genotype and fattening system (Tables 45 and 46). In field conditions the genetic potential to reach a certain mature liveweight seems important. With intensive feeding according to Eveweight to an age of one year, rate of maturity seems important.
The traits rate of maturity, mature liveweight and muscle development could be used in selection for beef production (Figs 25 and 26). Simultaneous improvement in these characteristics by selection is difficult because of the age at maturity cannot be lowered without lowering adult liveweight and muscle development. Selection on mature liveweights and muscle development is quite easy with data from field material. Rate of maturity is not an easy trait for field observations. An increase in mature liveweight favours an extensive system of fattening, in which final liveweights are high. This selection, however, result in later maturing cattle less suitable for fattening to low final liveweights in an intensive system. Intensive selection for beef will restrict the selection possible for milk. This compromise restricts beef characteristics in Dutch cattle; it is not really practical to develop separate lines within the breeds for different purposes.
The best response can be expected from selection of the breeding bulls. To improve beef traits, those bulls should be kept for breeding which have been sired by bulls with good beef characteristics. Before they are put into use for breeding at one year of age, these bulls should already show good beef characteristics. This avoids selection for late maturity. Data collected later for these bulls should govern the further use of them, especially in the use of them as sires for the next generation of breeding bulls. Final conclusions about any bull can also be derived from progeny data. This is certainly more time-consuming and information will only apply to the system of fattening used.