Four inbred and two single trait selection lines of Shorthorn cattle were established. Each of four foundation sires, which had been selected on individual performance or progeny testing, were mated to enough unrelated cows to produce thirty-two daughters. Sixteen from each set of the thirty-two daughters were randomly assigned to one of the four inbred lines and of the remaining sixteen, eight were assigned to the Type line and eight to the Growth line. The foundation sire of each inbred line was bred to his daughters and granddaughters for as long as he was serviceable and then replaced by an inbred son. The selection criteria were growth rate from birth to about twelve months of age in the Growth line and conformation score at the same age in the Type line.

The performance of the lines was evaluated from the 1962-66 calf crops. The average level of inbreeding of the calves born in these years was 0.40, 0.37, 0.27 and 0.23 in the S-1, S-2, S-4 and S-5 inbred lines, respectively, essentially zero in the Type line and .06 in the Growth line. The preweaning traits were birth weight, average daily gain, 205-day weight and grade at weaning. After the data were adjusted for age of dam and sex of calf, three statistical methods were used to adjust the data for the effects of inbreeding.

From 1964 to 1967 bulls from each of the inbred and selection lines were mated to unrelated females. Two bulls from each line were used in the test herd of about 120 cows. The cows were randomized to the sire lines each year. The preweaning traits of the topcross calves birth weight, average daily gain, 205-day weight and grade at weaning were adjusted for sex of calf and age of dam effects.

The topcross line means for the preweaning traits were regressed on the means of the lines themselves. The line's own performance was a good predictor of topcrossing performance for preweaning traits. The best method of adjusting for inbreeding was based on regression coefficients estimated separately for each line and the adjustment made to the average level of inbreeding of the four inbred lines. The best predictions of topcrossing performance were made for birth weight and grade at weaning. The Type line showed the greatest deviations between original line performance and topcrossing performance for average daily gain and 205-day weight. After weaning the male and female calves, from the lines themselves and the topcross lines, were separated. The male calves were put on postweaning performance tests, and the replacement heifers were fed growing rations. The topcross line means for postweaning weight of the males and females and the postweaning grade of the males were regressed on the means of the lines themselves. The line's own performance, adjusted for inbreeding, was a good predictor of topcrossing performance. The adjusted line means of the males were better predictors of postweaning weight than the adjusted line means of the females.