Multiple Shifts for Testing Cockerels

DURING the first 8 years of selection at this laboratory for genetic resistance to leucosis combined with other traits of economic value, it was our practice to leave each male in his breeding pen throughout the entire hatching season of 8 to 10 weeks. By 1942 it was evident that, while marked improvement had been made in the first three years, progress thereafter had been almost nil. It was equally clear why improvement had stopped. We had tried each year to find enough good proven sires to head at least 25 percent of the breeding pens. During the first 5 years, the average number of pens used for the two resistant lines together was only 13.2, but, among the 10 cockerels or so tested annually, it was seldom possible to find 3 or 4 proven sires that could be used again without considerable misgivings. In 1940 to 1942, the average …