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The Effects of Mild Inbreeding on a Herd of Holstein-Friesian Cattle¹

Iowa Agricultural Experiment Station, Ames

ABSTRACT

The effect of mild inbreeding on size, type and production in a herd of Holstein-Friesian cattle was analyzed. The measures used were intra-sire correlations and regressions of the various traits oh inbreeding.

The inbreeding of the individual itself resulted in an average decrease of about one-eighth of a pound in birth weight for each increase of 1 per cent in intensity of inbreeding. The inbreeding of the dam had a slightly smaller effect than this on the birth weight of the calf. Both values border on statistical significance.

For the five body measurements and weight at 6 mo. and over, an increase of 1 per cent in inbreeding never resulted in a decrease of more than 0.5 per cent and usually only about 0.1 per cent of the average of the respective measurement. The results indicate that the shape of the growth curve changes as the intensity of inbreeding changes. It appears that inbreeding slows the rate of growth at early ages but permits rapid growth to continue longer, so that mature size is not decreased and may even be increased. Although not all of the coefficients are significant, it is concluded that, in general, increased inbreeding resulted in a smaller size at least to 2 yr. and, for some of the later-maturing measurements, perhaps up to 4 yr. of age. A tendency for larger size with increased inbreeding was indicated at 5 yr.

Inbreeding had no certain effect on type ratings. A non-significant negative regression coefficient of type rating on inbreeding suggests a slight tendency for inbreeding to be detrimental to desired type.

Production decreased as inbreeding increased. The regression of butterfat production on inbreeding was –4.5 lb. per 1 per cent inbreeding. This coefficient is significant at the 1 per cent level.

It was possible to practice enough selection for production, by using bulls from the best producing cows, to counterbalance the effect of inbreeding and to raise the genetic ability for butterfat production approximately 40 lb. during the 12 yr. studied. It appears that if a breeding plan is followed in which the increase in intensity of inbreeding is less than 2 per cent per generation, it should be possible to practice enough selection to counterbalance the decline in production expected to result from inbreeding.

Inbreeding did not result in the birth of any physically abnormal calves.

The numbers were too small and the intensity of inbreeding too low for any prediction of the effect of really intense inbreeding.

¹ Journal Paper no. J-1725 of the Iowa Agricultural Experiment Station, Ames. Project no. 1053.

² Now Professor of Animal Husbandry, Michigan State College, East Lansing.