Let’s suppose that your main objective is to increase the
genetic potential for milk production. If this is the primary
goal, and it probably should be, then you should select solely
for milk production. If you put emphasis only on milk production in a breeding program, then you will make maximum progress for this trait. If, however, you select for two traits at the same time, you have lowered your progress in milk production (the primary trait) to 71 percent of the maximum.Likewise, if you select for 10 traits, you have seriously reduced your potential increase in milk production to only 32 percent of its maximum.