Section 14 — Animal Genetics and Breeding 



grade at weaning, live slaughter and carcass 

 grades were small. 



Calves sired by crossbred bulls averaged 13 

 days earlier in date of birth than those sired by 

 purebred bulls. 



14.44. The Relationship between the Performance of 

 Mothers of Dairy Bulls and the Progeny 

 Tests of their Sons. L. K. O' Conner (Thames 

 Witton, Great Britain). 



The purpose of this study was to obtain infor- 

 mation on the relationship between the perform- 

 ance of mothers of bulls and the progeny tests 

 of their sons, which would be useful in develop- 

 ing a practical method of pedigree evaluation 

 for young dairy bulls. 



The data consisted of the progeny tests of 736 

 Friesian bulls with contemporary comparisons 

 based on a weighting of 10 or more, and the 

 305-day lactation records of their 640 dams. 

 Where possible, the first five lactation records of 

 the dam were included. The differences between 

 the dams' lactation records and the appropriate 

 herd average, and the contemporary comparisons 

 of their sons, were used in the milk yield anal- 

 yses. In the case of fat percentage, the dams' 

 own records and the first lactation daughter 

 averages of their sons were used. 



For milk yield and for fat percentage, regres- 

 sions of sons' progeny test on dams' rating were 

 calculated for each lactation of the dams sep- 

 arately, and for averages of increasing numbers 

 of dams' records up to five. Estimates of the 

 repeatability of the dams' milk yield and fat 

 percentage were also obtained. It was found 

 that whilst the regressions for fat percentage 

 were in good agreement with those expected 

 in theory, the regressions for milk yield for sec- 

 ond and subsequent lactations of the dam were 

 less than expected. 



A full report of this work will be submitted 

 for publication in Animal Production. 



14.45. A Genetic Study of Feed Efficiency in Holstein- 

 Friesian Cattle. R. D. Plowman, N. W. 

 Hooven Jr. and W. R. Harvey (Beltsville, 

 U.S.A.). 



This study was conducted to determine if 

 genetic differences existed in gross efficiency of 

 feed conversion among progeny groups and if 

 feed efficiency could be estimated from a portion 

 of the complete lactation. Efficiency in this paper 

 is defined as: 



Fat Corrected Milk Yield 



Therms of Estimated Net Energy Consumed 



The data consisted of 584 lactation records 

 completed on 332 cows. Eighteen sire groups 

 were represented and 254 daughter-dam pairs. 

 The variables considered were: FCM yield, 

 feed efficiency, body weight, body weight change 

 and therms of estimated net energy consumed. 

 Results from the first lactation analyses indicated 

 significant differences among the sire groups in 

 feed efficiency and body weight. Significant 

 positive correlations were found between feed 

 efficiency and FCM yield. Body weight and 

 body weight change were negatively associated 

 with feed efficiency. Heritability and repeata- 

 bility estimates of feed efficiency are similar 

 to those of milk production. 



Correlation between the third 60-day period 

 and the total lactation feed efficiency ranged 

 from 0.88 to 0.94 for different seasons of the 

 year. Corresponding correlations for the sec- 

 ond one hundred days as compared to the 

 total were 0.92-0.96. It seems possible to predict 

 feed efficiency from part lactation data. 



14.46. Effects of Crossbreeding and Crisscrossing on 

 the Birth Weights and Gestation Periods of 

 Dairy Cattle. R. W. Touchberry (Urbana, 

 U.S.A.). 



The data for this study consists of the birth 

 weights and gestation periods of 1205 calves 

 arising from various crosses and crisscrosses of 

 the Holstein and Guernsey breeds. The calves 

 were born over a period of twelve years and are 

 categorized into five generations. The first 

 generation consists of purebred Holsteins and 

 Guernseys and reciprocal crossbreds and was 

 produced by mating unrelated Holstein and 

 Guernsey heifers to Holstein and Guernsey 

 sires. The second generation consists of Hol- 

 steins, f-Holsteins, f-Guernseys and Guernseys 

 and was produced by systematically mating the 

 fiist generation females to purebred Holstein 

 and Guernsey sires. The third, fourth and fifth 

 generations were produced in a similar manner 

 by systematically mating the females of the 

 second, third and fourth generations to Holstein 

 and Guernsey sires. For the first generation data, 

 crossbreds were 2.8 ± 1.6 lb heavier than 

 purebreds, calves out of Holstein dams were 

 18.7 ± 1.6 lb heavier than those out of Guern- 

 sey dams, while those by Holstein sires were only 

 6.9 ± 1.61b heavier than those by Guernsey sires. 

 Crossbred calves were carried 0.54 ± 0.78 days 

 less than purebreds, those out of Guernsey dams 

 were carried only 3.2 ± 0.76 days more than 



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