In poultry and dairy cattle heterosis is occasionally found in crosses of different strains of the same breed (Hutt and Cole, 1952; Johansson, 1939; Nordskog and Ghostley, 1954). Limited data on the milk production of reciprocal crosses between Dutch and American strains of Friesian dairy cattle being available in Israel, an investigation was carried out to determine whether crossing these two strains resulted in a heterotic effect on milk production.

Natural scrapie can develop in sheep at any age later than about 18 months old. In order to formulate a realistic policy for experimental investigation of the disease as it occurs naturally, it is necessary to understand what factors might affect the incidence in sheep of different ages. Any attempt to use data from commercial sheep flocks to draw conclusions about the age distribution of scrapie cases must make allowance for the fact that the population at risk is being culled on commercial criteria and incurs losses by death from various causes.

Animal Production, Volume 6, Part 1, p. 53

In Table 5, the identification numbers of twin pairs should read 3, 4, 1, 2 (and not 1, 2, 3, 4)