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

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Abstract
In a herd which had used home-bred bulls mostly since 1930 and wholly since 1935 the effects of this mild inbreeding were studied by computing the intrarsire regressions of the animal''s measured characters on its inbreeding coeff. Birth wt. (available on 370 calves) declined about 1/8 lb. for each additional 1% of inbreeding in the calf and nearly as much more for each additional 1% of inbreeding in the dam, both these values barely bordering on statistical significance. For wts. and 5 linear measurements at ages of 6 months and over (176-65 females at each of these ages), the inbreeding lowered the growth rate slightly at the younger ages but permitted it to continue longer, so that substantially the same mature size was reached by both inbreds and outbreds. The regression of type on inbreeding was negative but not significantly so. The observed regression was of such a size that 25-30% of inbreeding would lower the official type rating only 1/3 of a grade. An increase of 1% in inbreeding was accompanied by a significant decrease of 4.5 lbs. of fat in a 10-month lactation. No distinct abnormalities were observed, but few animals inbred as much as 25% had yet been produced.