The size of the genetic improvement from generation to generation depends on the technique used to select animals for breeding. Introduction of new selection techniques made it possible to select the best animals for breeding more accurately and efficiently. Especially the introduction of reproduction techniques like artificial insemination (AI), that made it possible to have (very) large numbers of offspring per sire and to select only the very best males for breeding. In females techniques like embryo transplantation (ET) or ovum pickup followed by in vitro fertilization have made it possible to produce much larger numbers of offspring of excellent females than with normal reproduction techniques in this species where normally only one or few offspring per year were feasible. The wide use of these reproduction techniques resulted e.g. in a steady increase of the milk production per cow.
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In the figure above on the left, you see the increase in milk production in the Netherlands between 1945 and 2022. The increase until 1970 is much less steep than that from 1990 onwards. Reasons for this are many, but important ones are, in addition to the application of reproduction techniques, the introduction of more accurate techniques for estimating breeding values, introduction of automatic milking and the free stall instead of the tied stall, and better quality of the diet. In the graph on the right is the phenotypic trend compared to the genetic trend in the period 1995 – 2022. You see that the increase in phenotypic (= realized) milk production in that period is very similar to the estimated increase in genetic potential for milk production: in both cases approximately 2000 kg. The fact that both trends have approximately the same slope indicates that environmental factors such as management, feeding, housing have been supporting, and not limiting the expression of the genetic improvement.