01719naa a2200229 a 450000100080000000500110000800800410001910000150006024500870007526000090016252010990017165000370127065000160130765000110132365000100133465000150134465000170135965000100137665300170138665300140140377300720141715201092023-04-27       1977    bl uuuu     u00u1 u    #d1 aRAE, A. L.  aGroup breeding schemes in sheep improvement in New Zealand.h[electronic resource]  c1977  aGroup breeding schemes involve the screening of high-producing animals from contributing flocks into a central (nucleus) flock. The nucleus flock is used for further evaluation of the ewes and to supply rams to contributing (and often other) flocks. Most of the schemes which have developed since 1967 have the major objective of improving reproductive rate in Romney. Perendale and Coopworth sheep. Maximum rates of genetic gain are achieved when j_ to 10% of the group population are in the nucleus and 40 to 50% of the female replacements are from contributing flocks. With this structure 10 to 15% faster genetic gains will result from an “open” nucleus scheme, compared with a closed flock, when a trait is measured in both sexes. For a trait measured only in ewes an “open nucleus” scheme gives a 15 to 25% faster genetic gain. The success of group breeding schemes in New Zealand has been partly due to their concentration on simple criteria of productivity for screening ewes. The effect of limiting the inbreeding in the nucleus flock appears unimportant in most circumstances.  aBreeding and Genetic Improvement  aNew Zealand  aRomney  aSheep  aCruzamento  aMelhoramento  aOvino  aGenetic gain  aSelection  tProceedings New Zealand Animal Productiongv. 37, p. 206-212, 1977.