The usual answer to this question applies to all breeds. Reduction in stature can be achieved by selection of AI bulls whose progeny are genetically smaller than genetic average for the breed. A caveat is that research with Holsteins at the University of Minnesota, often referred to as the "big cow/little cow study," showed that over approximately 30 years of selection for reduced size, the little cows mated to the smallest sires (actually the sires with most negative proofs for stature) in the breed actually increased in stature over time. This is because the entire breed was increasing in stature, and so the females selected to be smaller were smaller with respect to the breed's average but not smaller than their female ancestors.
To select for smaller individuals in a breed, use proofs for stature, available from the various breed societies, and select for bulls with low or negative proofs for stature. Single trait selection, ignoring all other traits, will produce greatest response but may result in using bulls inferior for many other economically important traits. A better approach is to select a group of bulls with higher rankings for total economic merit on a comprehensive index like Net Merit, and then practice secondary selection for low proofs for stature. The breeding goal in the question may not be achieved for several generations or may not be achievable at all if bulls that decrease in stature are not available through AI.
For the Jersey breed, there are also miniature cattle available. Rabbit-eyed Jersey, Guinea Jersey, Barnyard Jersey, and Island Jersey are names used in various locations. The American Miniature Jersey Association & Registry is where the stud book is maintained; they may be contacted at 308-665-1431 or by e-mail at rarebreed@bbc.net. See also the Web site for American Miniature Jersey Association & Registry, LLC, at http://www.miniaturejerseyassociation.com/.
