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Effects of Inbreeding and Selection in a Closed Guernsey Herd ¹

Department of Animal Science, Iowa State University, Ames

ABSTRACT

The effects of inbreeding, linebreeding, and selection were measured in a small, closed Guernsey herd which was a replicate of an experiment with a larger herd of Holstein cattle. Inbreeding ranged up to 31%, with an average of 6.4% for all cows included. Selections were primarily on an index based on milk fat production and type of the individual and its close relatives. Each analysis included approximately 90 cows. The intra-sire regressions of production on per cent of inbreeding were –36 and –51 lb of milk, –1.7 and –2.3 lb of milk fat, and +.001 and +.002% test. The intrasire regressions of weight in pounds on per cent inbreeding were – 0.3 at birth, – 0.7 at six months, –1.5 at one year, –1.9 at two years, –1.4 at three years, –4.5 at four years, and –3.2 at five years. The intrasire regressions of production on coefficient of relationship to a superior foundation cow were –.6 lb of milk fat, –21 lb of milk, and +.006% milk fat test. The estimated heritabilities of the production traits were .12 ± .21 for milk yield, .22 ± .24 for milk fat yield, and .46 ± .27 for milk fat test. The total genetic improvement per year expected from the selection of parents, expressed as a per cent of the herd average was .33% for milk, .57% for milk fat yield, and .40% for milk fat test.

¹ Journal Paper No. J-4856 of the Iowa Agricultural and Home Economics Experiment Station, Ames, Iowa. Project No. 1053. This research was part of North Central Regional Project NC-2.