Genetic relations between milk and beef characters in dual-purpose breedsFrom an article in Spring 1964
It is estimated that about 70% of beef stores bred in Britain are out of dairy cows. Some 30-40% of them are by beef bulls, so that about 40-50% slaughtered for beef are of pure dairy breeding.
To see if it was possible to obtain any estimate of the genetic correlation between milk and beef characters from commercial (as distinct from experimental) cattle, an investigation was carried out recently by the ARC Unit of Animal Genetics of Edinburgh. This was reported in the February issue of Animal Production by Mr Ian Mason of the Institute of Animal Genetics.
The records were obtained from private herds of purebred cattle in England and Wales which were producing milk and also rearing steer calves for beef. All the herds were milk recorded and were either pedigree or grading up. All were using their own bulls, but an occasional animal got by AI was included. All steers were artificially reared on the bucket or on nurse cows, and not on their own dams. Most of the animals were fattened on grass and slaughtered in the autumn. No show cattle or any specially treated were included in the analysis.
Since the steers were reared under commercial conditions they were sent to slaughter when considered sufficiently fat. This led to a wide variation in age and weight at slaughter. In order to get a rough measure of rate of gain, therefore, the liveweight was divided by the age at slaughter (to the nearest week) to give the ‘liveweight-for-age’.
Of major economic importance is the carcase weight-for-age. This measure has the advantage of including the influence of dressing-out percentage and of being measured much more accurately than liveweight. However, it was subsequently found that liveweightfor-age and carcase weight-for-age were very close correlated and gave almost the same results in the genetical analyses.
The table shown at the end of this article lists the averages for these characters in the Red Poll and Dairy Shorthorn breeds and shows the total number of steers investigated.
The characters finally chosen for genetical analysis were; liveweight-for-age and carcase weight-for-age (as measures of rate of gain), age at slaughter and dressing-out percentage (a rough-and-ready indication of carcase quality, ie amount of fat and flesh).
Milk yields of female relatives of nearly all the steers were obtained. The minimum aimed at was the first lactation milk yield and butterfat percentage of their dams. Wherever possible, similar records were obtained for maternal half and full sisters. Only lactations made in the steer’s herd were used. Normally the 305-day lactation record was used, although for a few early years only the 365-day yields (or complete lactations) were available. These were reduced to 305 days using appropriate correction factors.
Milk yields of paternal half-sisters were also obtained. Those used for calculating heritability of first lactation yield and butterfat percentage were from the same herds and years as the steer records. ‘Contemporary comparisons’ of the sires were supplied by the Milk Board.
In order to eliminate variation due to differences between herds and between years, all analyses were done within herds and within half-years.
Animals born within the same six months (Oct-Feb and March-Sept) and on the same farm were counted as contemporaries and contemporary comparisons of the (natural service) bulls used were calculated for the liveweight-for-age, carcase weight-forage, age at slaughter and dressing-out percentage of their steer progeny. Variance between contemporary comparisons yielded heritability estimates for these four characters of 0.27, 0.38, 0.08 and 0.05 respectively.
The estimate for heritability of age at slaughter was low. Age was used in this investigation in the hope that it might give some indication of rate of maturity (ie fattening ability). However, says the report, it is difficult to believe that rate of fattening is not heritable—the successful selection for early maturity in the British beef breeds shows the opposite. It therefore seems more likely that age is a poor index of fattening ability in this material.
The report states that it seems safe to conclude that the genetic correlations between the beef and milk characters investigated cannot be large—whether positive or negative. This, adds Mr Mason, is very satisfactory from a practical point of view in a country of dual-purpose cattle where a large proportion of beef comes, directly or indirectly, from the dairy herd. However, he continues, in view of the possibility that the higher standard errors conceal small negative genetic correlations it would be advisable at some time to consider the beef characters of dual-purpose bulls. But the milk characters must be given priority.
It is surprising, states the report, that no positive correlation was found between milk yield and weight-for-age. Insofar as weight-for-age is a measure of either size or efficiency of food conversion, a positive connection would be expected. In future work of this subject, concludes the report, it seems important to use large numbers of animals, to maintain them under conditions better controlled than on a commercial farm and to measure characteristics which enable body size to be distinguished from fatness.
Breed averages for growth and carcase characters of all steer recorded: