Section 14 — Animal Genetics and Breeding 



they are in correlation with the content of lean 

 meat and the content of high prized cuts in the 

 carcass. The importance of this criterion has 

 been discussed in numerous publications. The 

 fact, that longer and leaner pigs are prefered 

 nearly all over the world, can not be without 

 influence on the distribution of animals with 

 different numbers of vertebrae. 



The material to be discussed comprised 

 822 pigs of German improved Landrace (ver- 

 edeltes Landschwein) in the years 1954 and 1955, 

 all of them animals of the old type of German 

 pigs. 



The amelioration with boars and sows of 

 Dutch or, if we look further back, Danish origin, 

 began later. Therefore the material can be seen 

 as sufficiently uniform. The distribution of 

 thoracal and lumbar vertebrae is presented. A 

 comparison of body length and number of 

 vertebrae shows that length is in dense corre- 

 lation with vertebrae numbers. 



The figures show that the average length of 

 vertebrae is nearly the same in all classes of 

 vertebrae numbers. It is to be stated that pigs 

 with a higher number of vertebrae are longer in 

 the average. A selection with the aim of in- 

 creasing the number of vertebrae must be 

 successful in increasing the length of the animals 

 and if length is connected with lean characteris- 

 tics, also in improvement of quality. Practical 

 breeding is following this trend. 



14.51. Selection for High and Low Fatness in Duroc 

 and Yorkshire Swine. H. O. Hetzer, 

 W. R. Harvey and W. H. Peters (Beltsville, 

 U.S.A.). 



Individual selection based on backfat thickness 

 at a live weight of 175 lb has been carried in 

 both upward and downward directions through 

 7 generations in Duroc swine and through 5 

 generations in Yorkshire swine. By generation 

 7, b.f.t. in Durocs averaged 2.01 in the high-fat 

 line, 1.22 in the low-fat line and 1.50 in a 

 randomly selected control line. The correspond- 

 ing values for the 5th generation Yorkshire lines 

 were 1.43, 1.15 and 1.28 inches, respectively. 

 Realized heritabilities computed as the regression 

 of generation means on cumulative selection 

 differentials were 0.64 ± 0.06 and 0.42 ±0.07 

 for high- and low-fat Durocs and 0.36 ±0.12 and 

 0.35 ±0.05 for high- and low-fat Yorkshires. The 

 significantly slower change in the low-fat than 

 in the high-fat Duroc line (P < 0.05) indicates 

 that the response in the two lines was asymmetri- 

 cal. Heritability of b.f.t. in the Duroc and 



Yorkshire control lines was estimated at 0.53 ± 

 0.07 and 0.32 ±0.09 from the intra-group 

 regression of offspring on mean of parents. These 

 values agree very well with the corresponding 

 averages of the realized values and indicate that 

 Yorkshires are less variable genetically for fatness 

 than Durocs. Litter size and litter weight 

 differed little among the Yorkshire lines but 

 declined in both the high- and low-fat Duroc 

 lines. Post weaning daily gain tended to be 

 higher for low-fat than for high-fat Duroc pigs 

 (1.44 vs. 1.38 lb), while the reverse was true for 

 high- and low-fat Yorkshires (1.38 vs. 1.29 lb). 

 Feed requirements per 100 lb gain were sub- 

 stantially lower for low-fat than for high-fat 

 lines in both breeds with control lines inter- 

 mediate. Low-fat Durocs produced substantially 

 higher yields of lean meat and less fat than 

 high-fat Durocs. Low- and high-fat Yorkshires 

 showed similar though somewhat smaller differ- 

 ences. 



14.52. Genetic Change of Backfat Thickness in the 

 Danish Landrace Breed. Charles Smith 

 (Edinburgh, Great Britain). 



Data from the Danish progeny test reports 

 were used in an attempt to measure the genetic 

 change in backfat thickness in the Danish 

 Landrace breed from 1952 to 1960. Over this 

 period the average backfat thickness of tested 

 pigs fell from 34.2 mm to 28.5 mm, a change of 

 two standard deviation units. 



The method used to measure genetic change 

 depends on the difference in performance in 

 two or more years of progeny from particular 

 sires of dams. Environmental differences between 

 the years are avoided by measuring performance 

 relative to the year mean. However, allowance 

 has to be made for selection among parents on 

 the basis of their first set of progeny records, 

 through adjusting the initial records by theoreti- 

 cal regression factors. The genetic change is 

 then estimated as a function of the difference 

 between the adjusted first progeny records and 

 the records of subsequent progeny groups. 



Separate estimates of the genetic change in 

 backfat thickness were calculated in this way 

 from the progenies of sires and of dams at each 

 of the three stations. These were in general 

 agreement and indicated that there was some 

 genetic improvement in backfat thickness in the 

 Danish Landrace from 1952 to 1960 but that not 

 all of the observed change was genetic change. 

 The overall estimate of the genetic change 

 was — 0.15 ±0.10 mm per year and this repre- 



268 



