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Effects of Inbreeding, Selection, Linecrossing and Topcrossing in Swine. II. Linecrossing and Topcrossing¹

University of Wisconsin, Madison

Abstract

Results are presented from an experiment in which inbred lines of swine were tested in linecross and topcross combinations in an experimental breeding herd and in which inbred and linecross boars were compared with outbred boars in two-sire farm herds. Data from 270 litters farrowed in 4 years were studied in the first part of the experiment. The farm top-cross tests involved a total of 1648 litters in 93 herds over an 8-year period. The data from 228 litters in 14 of these herds were analyzed separately because the effect of topcrossing could not be separated from that of crossbreeding.

These results showed that linecrossing resulted in a recovery of the vigor lost during inbreeding, but showed no average advantage for the linecrosses over outbred controls. This latter is in contrast to results obtained in some other experiments which have shown linecrosses to be superior to outbred purebreds.

Topcrosses by inbred boars of predominantly Landrace breeding were significantly heavier at 5 months of age than non-topcrosses in the same herds. The actual difference was 11 lb. No other topcrosses by inbred boars showed any significant advantage in viability or rate of gain.

Topcross gilts by inbred boars were consistently superior to their controls in size and weight of litters raised to 5 months. Results from 27 herds showed significant increases for the topcross gilts of 1.0 pig and 156 lb. in these two traits, respectively.

There was an indication that inbred and linecross boars differed in their performance in topcrossing. Progeny of two-line cross boars showed a significant advantage in viability and quite large increases in pig and litter weights, whereas progeny of inbred boars of the parent lines showed no increase. In productivity comparisons, the gilts by inbred boars were superior to their controls, whereas gilts by linecross boars were not.

More general implications of these results and of the effects of the inbreeding and selection practiced in the development of the lines will be considered in a subsequent paper.

¹ Paper from the Department of Genetics No. 678 and the Department of Animal Husbandry, University of Wisconsin, in cooperation with the Regional Swine Breeding Laboratory, AHRD, ARS, U. S. Department of Agriculture. Published with the approval of the Director of the Agricultural Experiment Station.

² Present address: Department of Animal Husbandry, University of California, Davis, California.

³ The authors wish to acknowledge aid from the following: The Quebec Agricultural Research Council, for a scholarship to the senior author; Oscar Mayer and Company, Madison, for a grant in partial support of the project; Dr. C. R. Henderson of Cornell University, for providing the method of analysing the farm topcross data; A. Gerbitz and M. Lehr, University Farm herdsmen;R. M. Durham and C. S. Bernard for help in collecting the farm topcross data, and the farmers in the state who cooperated in providing these data.