The world population will increase from nearly four billion in 1974 to a projected figure of seven billion by the end of this century. 1-3 In the meantime, world food production is lagging behind human requirements by a greater margin each year, particularly in the economically developing countries, in which the population growth rate is the greatest. 4 It is apparent, therefore, that there must be a marked improvement in animal and plant productivity if the critical shortage of food in many regions is to be overcome.