Some Effects of Inbreeding and Evidence of Heterosis Through Outcrossing in a Holstein-Friesian Herd

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Abstract
The analysis of the effect of inbreeding on yeild in the University of California Holstein-Friesian herd is based on the records of 164 cows. Daughters of 22 sires are represented; however, the majority are the progeny of 3 sires (100A, 100B, and 100D). The pooled intrasire regression coefficients of production on inbreeding indicate significant declines of -209.8 lb. of milk, -4.9 lb. of butterfat, and -131.0 lb. of fat corrected milk (FCM). A significant increase in butterfat test of +0.008%/degree increase in inbreeding was found to accompany the decline in milk yield. A significant reduction in height of maximum daily yield occurred as inbreeding increased. No significant inbreeding effect was demonstrated on the time required to reach maximum production or upon the slope of the lactation curve. It would thus appear that no important change in the shape of the lactation curve accompanied the depression of yield with inbreeding. There is some evidence that inbreeding may have a somewhat lesser effect on production in ranges up to 0.20 than in ranges above 0.25. Significant differences were found between sires in the response of their daughters to inbreeding. One of the sires (100B) remained in active service for 13 years. His 43 inbred daughters showed a marked decline in production. Twenty-one of them, averaging 5,366 lb. of FCM, were outcrossed to a third unrelated, ibred sire (100D). The 26 resulting daughters from this outcross averaged 10,910 lb. of FCM. This yield was significantly greater than that of the inbred dams and of any group or estimate representing the production capacity of the herd prior to the outcross. The increase noted in the outcross daughters is interpreted to be due, at least in part, to heterosis.